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Unregistered
18th May 2009, 03:03 AM
Should bottom of boards (and surfaces of fins) be polished (shiny) or "mat" finish ??

Think there`s a few schools of thought. Seen arguments for both. Anybody knowdefinitive answer ?? Evidence ???

Unregistered
18th May 2009, 05:53 AM
I don't know, but I've polished my Drake R13 70 NR S Flex and I'm having more spinouts problems after that.

Farlo
18th May 2009, 05:31 PM
Matt finish makes water wet the hull, which in turn reduces friction. Several products have been proposed to reduce friction, but none works really better than water itself. Also they tend to increase weight while sanding (one popular way to get matt finish) does not. You can see that a surface is properly wetted when water forms a thin continuous film on it rather than individual drops. A well wetted fin will resist spin-out better. However be carefull not to alter the shape when sanding your board or fin.

Unregistered
20th May 2009, 11:58 PM
Matt finish makes water wet the hull, which in turn reduces friction. Several products have been proposed to reduce friction, but none works really better than water itself. Also they tend to increase weight while sanding (one popular way to get matt finish) does not. You can see that a surface is properly wetted when water forms a thin continuous film on it rather than individual drops. A well wetted fin will resist spin-out better. However be carefull not to alter the shape when sanding your board or fin.

hogwash.

read this - " http://www.mothboat.com/CMBA/Building/notwax.htm "

Farlo
21st May 2009, 04:20 AM
No evidence that boundary layer is thicker on wet-sanded hull. Moreover when planning most of this doesn't apply. And you'd better keep water stuck to your fin, friction drag being second order. Does Lockeed Martin also sell wax?

Unregistered
21st May 2009, 03:03 PM
No evidence that boundary layer is thicker on wet-sanded hull. Moreover when planning most of this doesn't apply. And you'd better keep water stuck to your fin, friction drag being second order. Does Lockeed Martin also sell wax?

go back and read it again , it debunks your comments.

Unregistered
21st May 2009, 03:37 PM
Bethwaite in "high performance sailing" writes about a test( page 233) he did with 2 rudder blades with NACA 0009 sections. they were prepared identically to 1200 grit finish then one was buffed to mirror. The dull (sanded) one apparently generated 23% less cross flow lift force. Apparently this tallies with expectations from theory.
This then rather suggests that polished is better

can anyone explain how the drag will be lesser over a sanded rougher surface than over a polished one?
I can believe that there is a permissable level of roughness depending on the boundary layer thickness, which would mean once sanded to a certain smoothness there isnt much benefit in going smoother(ie polished) but to say that polished is worse than sanded? i havnt seen any evidence of this.

leysenkr
21st May 2009, 06:19 PM
Irregularity causes vortexes like a swimsuit with sharkskin. Vortexes causes less friction than smooth surfaces. Smooth surfaces cause a vacuum and this is the friction in the water. But irregularity doesn't mean that every irregularity is good. What is smooth? What looks like smooth can look very rough under a microscope.

Unregistered
21st May 2009, 07:38 PM
how do smooth surfaces cause vacuum?
and what surface finish causes the right vortex's to reduce friction?
and how does the presence of vortices reduce the friction and where on the foil? leading edge, somewhere after seperation bubble?
how about polished leading edge to delay seperation, then sanded somewhere after to aid reattachment of the seperation bubble?

Unregistered
21st May 2009, 07:44 PM
Its a fantastic answer (the link from earlier post) If its read properly it does answer the question.
I`m polishing fins and bottm of board. !!!

Farlo
21st May 2009, 10:40 PM
Well I read it again and the author gives no evidence that boundary layer is thicker on a wet-sanded surface. Moreover where is that thickness measured from? Top or bottom? Regarding friction drag, matt is of course rougher than polished but does it make a sensible difference? We're not talking about mountains here. The question is more about wet or not, and how the hull will glide on a mix of air bubbles and water. But go polish everything. I'm quite curious to hear if you go any faster. Hello Tiesda, do you have an idea?

mike
21st May 2009, 11:11 PM
FYI on board bottoms:

http://www.boardlady.com/fast.htm

She says don't polish.

Unregistered
21st May 2009, 11:33 PM
To quote Marchaj in aero and hydrodynamics of sailing - page 342

"In general conclusion it appears that the roughness of foils surface is always harmful, and the labour expended on perfecting the finish on the forward third of the chord of a rudder or fin keel is never likely to be wasted."

before this he comments that polishing the leading edge is a good thing but that you can pretty much do as you please with the rear third of the chord and it makes not much difference. He compares a highly polished surface with a surface sprayed with 180 carbundurum.

so thats bethwaite and marchaj and a guy from lockheed martin.... versus some comments from internet forum posters..........

Unregistered
22nd May 2009, 12:49 AM
does it make any difference?
the bottom line is that it does not.
the bottom line is martin van meurs doing 50 knots at southend on a fin that a few minutes before he had run into a mussel bed with- it was scratched and battered and hanging and yet he did 50 knots on it.

If you have a polished fin you can then keep it clean easily, with a wipe over- if you wet sand it it will pick up dirt and crap easily, and hence you will forever be having to wet sand it- so polished is probably easier to look after. But do whatever it probably makes little difference.

The lockheed martin guy talks about the hobie tri foiler and cavitation- i think he is mistaken to call this cavitation, i think its ventilation. He made that mistake so he might easiliy have made others.

Unregistered
22nd May 2009, 12:52 AM
For those with any R&D in their blood-

Easy test
Take a good fin that works for you, and wax one side and not the other. Try it and see GPS and all. Let us know what you find.
Remove the wax with Acetone.
Better yet polish and then wax. But then you may need to resand

leysenkr
22nd May 2009, 01:05 AM
The bottom line is, first tune the sailor. If you do the test yourself, the difference will be 99,99% chance due to yourself and not the polished or sanded fin, board. So R&D conclusion will be that the variance will be bigger on the sailor then the sanding or waxing issue.

Unregistered
22nd May 2009, 01:38 AM
leysenkr can you explain your theory about vortices?
I thought the sharkskin idea was to reduce vortices disturbing the boundary layer, not to create them.

also the lift and drag of varying surface roughness foils is easily tested in a tank- its been done and referenced by Bethwaite who says that polished is better.

what more do you want?

Farlo
22nd May 2009, 03:18 AM
A planning hull does not exactly behave as a foil in a tank. Even analogies with two-phases flow in pipes can be misleading because boundaries are not stable. You will notice that most pro-model boards have matt finish. You may call it marketing BS but I can assure you that if you polish your guitar you won't play faster.

leysenkr
22nd May 2009, 03:26 AM
A smooth surface is good and for this example the reference. Some specific roughness might be better at a certain speed. But on another speed, this exact roughness will slow down more than a smooth surface so it will be difficult to gain more speed. You need the exact speed to obtain the optimal gliding for a certain roughness.

Polished might be better in that specific test. But the sailor will influence the endspeed much more than a polished or non polished something.

Unregistered
22nd May 2009, 05:19 PM
Production costs for polished finish are higher.(Especially with fins) Difference is perhaps that small that for 99% of sailors its irrelevant but knocks £50 or so off board.

Perhaps Martin Van Meurs might have done 50 .5 knots had he had a polished fin ??? Who knows ???

Evidence is there . It definitely suggests polished is best. Theories are wrong though ! (Sometimes)

Farlo
22nd May 2009, 06:14 PM
Evidence is that windsurfers are not waxing their boards, and theories which suggest that polished is best are not relevant for planning hulls, unless you find one.

Unregistered
22nd May 2009, 06:24 PM
Evidence is that windsurfers are not waxing their boards, and theories which suggest that polished is best are not relevant for planning hulls, unless you find one.

HAHAHA
hulls or fins?
if the only evidence is that windsurfers arent doing it then you have no argument.
plenty of arguments put forward in favour of polishing, referenced as well. The arguments in favour of sanding seem spurious so far- unreferenced. It would seem that the sanded is better theory is based in lala land as opposed to argued reason.
If you have an argument that suggests sanding is better than polishing can you explain it?

Farlo
22nd May 2009, 07:03 PM
Takes less time. Windsurfers are lazy. Your references are not relevant for planning hulls. And you'd better keep water stuck to your fin to limit spin-out, friction drag being of second order.

Ken
22nd May 2009, 10:34 PM
Interesting discussion.

The only theory I can support is the sanding of fins to minimize spin out. Faster or slower I can't say, but there does seem to be better control with sanded fins (formula and slalom).

When fins do get dinged or scraped, I sand to remove the abrasions. A half polished, half sanded fin doesn't sound like a good thing, so if my fins come unsanded (rarely), then they get sanded before use.

Regarding boards, I sail them the way the come, some polished, some with very fine sanding. I can't tell any difference.

What do the America's Cup boats do. If anyone has the money to research the fastest hulls, it's them. Polished? Sanded?

Klint
23rd May 2009, 12:38 AM
Here is what Carbonart advice you to do every now and then with your board to maintain top performance. I wonder if even the top PWA sailors do this, or do they hand their boards in to some bottom finish guru ?

http://www.carbonartwindsurf.com/Performance/Maintenance

Farlo
23rd May 2009, 01:30 AM
Clean surface is fast surface... not bad. Let's take it this way: planning is due to hydrodynamic lift. Water is pushed down as the board goes forward. Flat or concave are more effcient than vee shapes because they trap water better. But air is also trapped with water. A wetted hull will roll over air bubbles with minimum interaction while a non wetted hull will stay in touch with air longer and "drag" the bubbles. However at some stage water comes in touch with this hull, otherwise it would not plane. The additional surface energy needs to be taken somewhere. As we can assume the water/air ratio under the hull to be consistent, this energy is given back somewhere. I tend to believe it causes additional spray. Does it make a significant difference? I'm not sure.

Unregistered
23rd May 2009, 05:12 AM
Windsurfing is full of spsuedo scientists.
To my mind the original question was answered way back (in link)
Amazing how many sailors cant read !!!

leysenkr
23rd May 2009, 01:45 PM
What do you read in the link unregistered because it seems that you are the only one who is concluding that polished is definitely the best thing to do. NO one can conclude anything about it. It looks like you are the smartest one here. Maybe you can explain it to us.

Unregistered
23rd May 2009, 06:22 PM
Martin obviously crashed his fin on the way back to the carpark.

Go polish your fins and try. But make sure you bring some 600 paper to fix it afterwards. You will soon tire of the spontaneous spinouts.

Unregistered
23rd May 2009, 07:35 PM
martin crashed his fin first. Then he pushed it into the sand bank to remove the worst of the debris. then he caught a gust and went over 50 knots with a fin that was really lessed up.
Polished sanded? hehe dont think he cared!

so why is sanded good for a fin? why do you get bad stall? earlier in the thread i said about the reatachment of flow. it can be argued that sanded will be better rearwards on the chord in order to promote reatachment after the seperation bubble. Forwards of the seperation bubble polished will be good in order to delay seperation in the first place. Sanding is all about giving more friction for the water to stick to, good for a turblent flow section maybe? not so good for a lamina flow section?

Farlo
23rd May 2009, 08:09 PM
"Sanding is all about giving more friction for the water to stick to". Finally we get there. But the difference in friction drag between a matt or polished fin is of third order, while keeping water stuck is essential. Now if you want to polish the first third of the chord and sand the rest... have fun, but you will spend less time on water.
By the way, do you really polish your board and fins (and guitar)?

Unregistered
23rd May 2009, 08:57 PM
"The viscous nature of air or water causes it to "stick" to the surface over which it flows; thus the velocity directly on the surface is zero for any velocity of the main air or water stream. Put into our terms, as our hull speeds through static water (water with no velocity) at 10 kts, the water molecule right next to our hull “sticks” to the hull and is “dragged” along at the same 10 kts. Proceeding above the surface, the velocity gradually builds up to free stream velocity (the velocity of the stream if the surface wasn’t present at all) at some distance above the surface. In our sailing case, where it is the surface that’s moving through the fluid, as one looks at molecules of water in increasing increments of distance from the hull, they gradually go from 10 kts, to 0 kts.. This area between the surface and the point where velocity reaches that of the free air stream is called the boundary layer. The reaction to the retardation of the flow velocity within the boundary layer is called skin friction drag.The thicker the boundary, the more drag."

Quote from link

Salient point

"The thicker the boundary the more drag"

The rougher the surface the thicker the boundary !!!

Surfaces should be polished.

Nex time you board a plane have a look at wing`s surface. Shiny ! Smooth not rough!!!

Suppose they`ve got it wrong too !!!

Farlo
23rd May 2009, 09:41 PM
"In extensive tank testing for world-class racing boats, it was established 40-some years ago that a glossy surface has substantially more drag than a matte one. In tests I was involved in for an America’s Cup boat, we found that simply sanding a glossy bottom with 600 grit paper, reduced the surface friction by about 5% at ½ hull speed, i.e. at about 4.5 knots. Instead of sticking to the glossy bottom, the water molecules would be “tripped up” by the minute ridges left by the sandpaper. This slight turbulence would reduce the thickness of the film of water being moved along with the vessel (the “boundary layer”), and thereby reduce the overall drag.

Surfboards are traditionally finished super-glossy-shiny. Windsurfers, on the other hand, have long since followed the lead of sailboat racers and taken the gloss off the bottoms of their craft."

Windsurfers got it wrong.

When theory doesn't help, it's time to practice.
See you later, I've got something to sand.

Unregistered
23rd May 2009, 11:02 PM
"Sanding is all about giving more friction for the water to stick to". Finally we get there. But the difference in friction drag between a matt or polished fin is of third order, while keeping water stuck is essential. Now if you want to polish the first third of the chord and sand the rest... have fun, but you will spend less time on water.
By the way, do you really polish your board and fins (and guitar)?

finally we get to where YOU think it is but the truth is less well understood
another interesting read here

http://www.sailtoronto.com/racetraining/bottom.pdf

note it is NOT thought that sanding is a method of keeping a layer of water attached to the hull. It is thought that this is the case whatever the surface finish, polished or sanded.

further read this
https://secure.seabreeze.com.au/Forums/topic.asp?TOPIC_ID=43204

interesting, basically the surface finish is going to be dependant on the section of the foil and the conditions you are sailing in. If its a turbulent flow section, sanding seems to be in order- if its a laminar flow section in smoother water then polish away. Again interestingly there doesnt seem to be a logical argument for wet sanding a hull in preference to polishing a hull.....

Farlo
24th May 2009, 05:43 AM
Except that "America's Cup sailors are wet sanding their hulls to 600 grit finish" [Sailing World]. They probably got it wrong too. A carefully wet sanded hull can be as smooth hydrodynamically as a polished one, with no sensible difference in friction drag. Anyway we won't convince each other. Let's hear what others have to say.

Unregistered
24th May 2009, 03:58 PM
Except that "America's Cup sailors are wet sanding their hulls to 600 grit finish" [Sailing World]. They probably got it wrong too. A carefully wet sanded hull can be as smooth hydrodynamically as a polished one, with no sensible difference in friction drag. Anyway we won't convince each other. Let's hear what others have to say.

americas cup boats dont go as quick as a windsurfer, permissable roughness will be different and as you rightly pointed out earlier, windsurfers in general are planing hulls.

Darbs713
25th May 2009, 03:33 PM
Personally, i want water to flow over the board with as minimal resistance as possible, so i polish the board.
The fin however, i want water to stick to it to reduce spinout. For this reason, the fin gets sanded with a fine wet sand

mim
26th May 2009, 05:37 AM
Here is what Carbonart advice you to do every now and then with your board to maintain top performance. I wonder if even the top PWA sailors do this, or do they hand their boards in to some bottom finish guru ?

http://www.carbonartwindsurf.com/Performance/Maintenance

Hi,

I really do not understand the CarbonArt. I was on Defiwind in France where they presented the whole quivery of slalom and speed boards and none of them was polished.

My opinion is definetely not polish the fin, and if you would like to than only on a certain area. And about the hull...I am for wet sanding. The reason is simple...polishing might work as well, might even be better or about the same...I do not actually care. The point is that it is too difficult to keep it in that state...the same for sanded surface...the advantage on roughened surface is that the small dirt or fat does have a smaller effect than on a pollished surface...that's what I think about it.

PS: Please tell me about one production board that is delivered with polished bottom...I would like to now...and do not mention CA...because thay are definetely sanded!!!

Ciao Michal.

Unregistered
26th May 2009, 01:42 PM
Hi,

I really do not understand the CarbonArt. I was on Defiwind in France where they presented the whole quivery of slalom and speed boards and none of them was polished.

My opinion is definetely not polish the fin, and if you would like to than only on a certain area. And about the hull...I am for wet sanding. The reason is simple...polishing might work as well, might even be better or about the same...I do not actually care. The point is that it is too difficult to keep it in that state...the same for sanded surface...the advantage on roughened surface is that the small dirt or fat does have a smaller effect than on a pollished surface...that's what I think about it.

PS: Please tell me about one production board that is delivered with polished bottom...I would like to now...and do not mention CA...because thay are definetely sanded!!!

Ciao Michal.

Carbon art boards used to come with a high gloss bottom my new CA speed boards have a sanded finish.

mim
26th May 2009, 03:13 PM
Carbon art boards used to come with a high gloss bottom my new CA speed boards have a sanded finish.

That is what I saw in France.

PS: a pitty that i did not know it while i was there I would ask the guys why they changed from pollished back to wet-sanded surface...

Unregistered
27th May 2009, 07:15 PM
from a carbon art press release

"The Pro Model Slalom board is constructed with a higher carbon content and like all CA Pro Models, has a special Teflon™-like polished bottom"

source - http://www.boards.co.uk/news/article.asp?id=3317

Ken
27th May 2009, 11:19 PM
"

Nex time you board a plane have a look at wing`s surface. Shiny ! Smooth not rough!!!

Suppose they`ve got it wrong too !!!

I wonder why some commercial plane wings have small tabs on top of the wings near the leading edge. Clearly, they are there to create turbulence for some reason - to compensate for the "smooth" surface?

I don't have the answer, but somone out there may.

NetWastrel
28th May 2009, 11:04 AM
the little thingamewatsit sticky outy bits on the front of plane wings are: Vortex Generators. The idea behind these is to create a small, but powerful vortex which guides the boundary layer onto the wing surface, which does two things. 1) it delays separation (stall) which is good for takeoff and low speed handling, and 2) it keeps the flow attached to the control surfaces (aelerons etc) keeping them effective.

The cost in terms of drag is minimal since they are so small compared to the rest of the wing, and only affect small parts of the span, effectively creating vortex fences. This also reduces crossflow effects where say separation on one part of the wing would normally spread to the adjacent parts. Its a little like trying to pull a sticker off something. The vortex generators create extra sticky points that keep the boundary layer attached, so the separation is less likely to spread to adjacent spanwise sections.

Cool solution for a plane. Not so useful for a windsurfer.

Ken
28th May 2009, 10:12 PM
NetWastrel,

Good explanation - Thanks. This may explain why sanded fins have less spinout, with the rough surface generating mini vortexes allowing the water to stay attached to the fin surface.

Erik Loots
29th May 2009, 06:23 PM
Polished surface faster??? I bet when you can control it a polished fin would have a tiny little bit less drag. I think you will barely notice this difference. My CA speed was polished, but a sanded a few spots now it is sanded and no difference.

A polished fin is easy to ventilate, this means the liftpotential of the fin suddenly decreasing by a huge factor. When going @ low speed 30-35kn, I feel only more drag, and most people will spinout before they notice.

On cavitation speeds 45-50kn ventilation??? That is the rumor about hydroptere and they got up to 60kn, so far I understood they also had a lack of lift& less drag.

On a windsurfer I don't think it is possible to do that all. You don't have a crew... You have trimsail/ keep course/ keep balance/ keep board steady/ keep finpressure all in one time. I use max 600 grid sandpaper.

Cheers,

Erik
www.speedsurfingblog.com

Unregistered
29th May 2009, 07:35 PM
Have you seen the glider in Thomas Crown Affair; glstening in sun, including rudder and tail plane.Super smooth and shiny. !!!

Micro ground is just another case of psuedo science BS.(Which starboard dominate BTW)

Mind you there`s no way any sailor could ever tell difference !!

Unregistered
30th May 2009, 01:00 AM
After checking a new Deboichet, it looks as though it may be sanded with 220 or 320 on the horizontal plane. Not even close to polished or 400 grit.

Since this is a major brand, there must have been some testing?

Also why not vertical sanding?

Anyone with knowledge?

Farlo
30th May 2009, 03:24 AM
Some twenty years ago Olivier Augé, one of the top speed French windsurfer at the time, advised about how fins should be sanded. He said to sand horizontally near the head and turn vertically near the tip. This was supposed to optimize water flow. Personally I don't think it makes any difference as long as the grit is fine enough.

Come on unregistered #46 (I guess it is a psuedo).
You didn't even told us if you really polish your board, fins (and guitar).

Unregistered
30th May 2009, 05:49 PM
Why is our sport so full of BS and self qualified "experts" quoting stuff from years ago and failing to grasp salient points from studies done by "real" engineers ???

Nasa;air craft manufacturers etc etc (according to some) have got it all wrong !! Cut outs decreasing drag; micro grinding and loads of other BS is playing straight into manufacturers hands. Its unbelievable !! Novelty has replaced development in WS. We read it on here; or in some corrupt magazine so we believe it; buy into it and then promote it !!! Crazy.
Every study (modern) says polished is better for foils/planing surfaces !!

BTW what`s guitars got to do with it ???

Farlo
31st May 2009, 03:45 AM
Well I guess that if you polish your guitar you will play faster. Exemples and studies you have found are not relevant for windsurf. Also if you look around you will find that much more boards/fins have matte finish than polished. So you may think that most windsurfers and boards manufacturers are idiots, but as you never told us if you really polish your board and fins (and guitar) nobody knows on which side you are. BTW I'm an engineer too but I tend to forget it when windsurfing.

Unregistered
31st May 2009, 05:20 AM
farlo if you are an engineer then please tell me what bridges you designed- i will make sure not to drive over em or walk under em.

reading back through your posts and the various things you have said does not inspire confidence in your "qualification"

are you a heating engineer? or a fluid dynamist? or an electronics engineer? what is a real engineer?

I know many people who i went through University with who call themselves engineers and it has always frightened me.

Unregistered
31st May 2009, 05:36 AM
farlo chris lockwood says
"I've done my fastest runs on fins with polished finish"

and
"So the answer on surface finish is "it depends". If you dont need to induce early transition, then I cant see why anything but polished is desirable on any surface. For windsurfing fins the 400-600 finish is less susceptible to ventilation. I've tested this on several occasions by going out on a polished fin for an hour or so (having plenty of ventilation problems), back to the beach, wet sanding, and going out again to find the fin completely stable. As for drag, a fin that has partially separated flow a lot of the time is still going to have more drag than a turbulent flow section which is not separating. I havent noticed any difference in drag between polished and wet sanded."

the latter quote should lead you to ask questions(as an engineer) about your fin sections. especially in their tolerance towards recieved turbulence and the sections used at what location , root to tip.
perhaps the reality is that your position re micro ground is unsustainable without serious modification, further that your assertions have been erroneous and error strewn. There are gems of wisdom in your posts, however as an engineer you are far too sure of yourself. Every single engineer i know who is competent is far more circumspect in their beliefs and far more open to new or different information than you appear to be- are you sure you are an engineer? in what?

Unregistered
31st May 2009, 04:50 PM
Word engineer comes from Latin for injenuity; perhaps we ought to try and use some rather than blindly following accepted questionable wisdom ?

I suspect modern take on subject leans towards polished surfaces; dont see many around and to be honest this idea that rough (sanded) fins helps prevent spinout is in sailors mind .There`s no scientific evidence to suggest this and on my part no experimental either. I cant really tell difference.(None shows up on GPS; other things have more effect) (But you can get a lovely polished finish on epoxy/G10 and carbon fins with autosol and a buffing machine)

Floyd
31st May 2009, 06:25 PM
Eric
How does a polished fin increase possiblities of ventilaion ?? Years ago we used to use forefins (and fences/ wings on fins) Theory was they produced a line of turbulence (at forefins tip) which prevented air getting down to fin. They certaily worked on some boards ;which in some case were bordering on unsailable becuse of spinout. Their effect was pronounced and I could "see" mechanics involved but a "mat" fin doing same ??? Cant see that at all !!! (Fins at time were mat finish and spinout was appaling ; a much bigger problem than nowdays)

And
Cavitation
Do we really work with pressures to promote true cavitation. I used to race small boats (with big engines!). Cavitation (and aeration) was always a problem. Aeration did little damage to props (none) but even stainless props showed pitting(on negative pressure side) of blades after cavitation. In all my years of sailing I`ve never seen any cavitation damage to fins.(Its over 40 years now !!!)
And surely epoxy would show daage easier than stainless ???
Does a windsurfer even at 50k reduce pressure on windward side of fin enough to make water boil ??? I dont think so !!! I suspect we only ever get aeration; ie air "sucked" down to fin.(Its actually pushed but thats another argumen )

I agree with unregistered .(all 3 of them ??) Polished bottom and fins is way forward. Evidence is their.

Farlo
31st May 2009, 07:54 PM
OK maybe my posts were exagerated. There is no more evidence that matte is better than polished because there is no conclusive study related to windsurf. There is wind today; I will polish one fin, try for one hour and sand it back again after I get spinouts. Maybe new profiles are far more efficient than a few years ago. Last time was complete disaster. My engineering skills are not in polishing for sure.

Farlo
1st June 2009, 03:02 PM
Worse than ever. Guys you teached me a great lesson. I definitely need to improve in polishing. Open to new ideas? Hmmm... Is polish any new idea? From the 30's maybe. There is one in post #47 with Deboichet fins being sanded horizontally. Note that we are no more discussing matte vs. polished, but grooves. Select is nearby, I will find out what they think about it. Maybe my sandpaper will be for sale after that.

Unregistered
1st June 2009, 03:58 PM
I have a c3 fin with a polished finish which i maintain with a buffer every so often. It doesnt spin out, its a very secure fin. Back in the day we used to have slots in our fins and forefins in order to stop spin out!

Unregistered
1st June 2009, 04:02 PM
re cavitation- a lot of sailors talk about cavitation, some fin makers/designers talk about cavitation and yet i have never heard of cavitation damage to a fin, or heard the distinct noice of cavitation and i have never heard of anybody hearing it.
and yet people still talk of the cavitation barrier to windsurf speed.
like sanding and polishing? lots of opinions plenty of bull si*t

Floyd
1st June 2009, 06:26 PM
Unregistered
You ought to register. You are one of few posters who recognises BS. (Mind you its easy enough to find in windsurfing)

I`m pretty sure windsurfers never experience cavitation. (We could just see evidence of it on 14inch prop putting 260 Bhp into water !!!)

Farlo
1st June 2009, 10:11 PM
Who talked about cavitation here (except the article from Lockeed Martin)? There have been some references to ventilation, yes. Anyway you made me reconsider a few assumptions. For instance perfect design (polished?) which has the best performance but in a narrow range, versus optimal design (matte?) which is less sensitive to real world conditions, manufacturing tolerances and abuses. There is something with the grooves on Deboichet's fin. Hopefully we'll get the end of the story. And yes, you should register. We need controversial spirit.

Unregistered
1st June 2009, 11:04 PM
Eric Loots did in post 45 !!!

Unregistered
1st June 2009, 11:20 PM
registering wont make what i have to say any more or less meaningful. Too much attention is paid to such things, judge what is writen by its own merits.

quote "For instance perfect design (polished?) which has the best performance but in a narrow range, versus optimal design (matte?) which is less sensitive to real world conditions, manufacturing tolerances and abuses."

no argument there. surface finish is entirely to do with the sections and intended uses.

Having read all of the references made in this thread ( some by myself) i can safely agree that the definitive answer has not yet been arrived at re fins.

Something i forgot to mention, a friend of mine who's technical knowledge and abilities in fluid dynamics i respect tremendously was genuinely surprised by bethwaites results with his rudder blades, thinking that wet sanded would be "smooth enough" for the boundary layer and that the marked increases in performance that bethwaite found were quite startling. In light of this i do suspect bethwaites methodology, to be reassured i would like to know that bethwaite reversed the surface finish on the 2 foils and repeated the experiment in order to ascertain if the findings were due to differences in the boards as opposed to surface finish, however he doesnt mention this in his book so i cannot be sure.

Further the same fluid dynamist also said to me that about the worst thing you could do would be to use a silicone polish on foils, he seemed to think that silicone interfered with the boundary layer in some negative way( not smoothness but chemically)- i havnt had a chance to quiz him further on this but i will at some point in the future. With that in mind i also doubt the voracity of the Lockheed martin guy who talks about using silicone polishs on his boat hull.
No answers- just more questions.

Floyd
2nd June 2009, 12:03 AM
Registering wont alter what you say but it will alter wether its listened to !

People (rightly or wrongly) do take more notice of what`s said when they can attach an identity to it.Even an imagined one. Strange but a fact of life !!


Its been a good thread anyhow !!

Farlo
2nd June 2009, 02:55 AM
Interesting. Eric Loots mentioned cavitation just to say it was not likely to happen. Perfect design versus optimal design? I will certainly reconsider that. The grooves on Deboichet's fin may be a real breakthrough. Finally we got to a point where it's not so matte vs. polished, but wettable vs. non wettable (silicon). Ask your friend about surface energy. Sanding is a cheap way to make things more wettable, but not the only one.

A pity you won't register.

NetWastrel
2nd June 2009, 12:11 PM
The term "cavitation" is used to describe the formation of bubbles in the water associated with a reduction in pressure. At what point this effect occurs, and its effect on the performance of a hydrofoil are two areas of ongoing research. But there is much that has been verified through thorough tank testing and observation.

The cavitation associated with propellors is an extreme case. One where the speed of the tip and the load of the tip is extreme compared to those encountered by a windsurfer. It leads to highly ignorant statements such as "there was no erosion therefore cavitation can not exist". Well, I cant see the curve in the earth therefore it IS flat.

There is a further bit of nonsense circulating regarding the instant and catastrophic ventilation (spinout) that would occur if cavitation did happen. Unfortunately, a well known and well respected hydrodynamicist, (none other than Tom Speer) long ago (early '90s) speculated that this would be the case. Therefore it must be true! Right? His comment has been regurgitated on boating/windsurfing forums since.

Some googling into the subject reveals its not necessarily the case. There are too many variables at play to say that "if the foil cavitates it will therefore ventilate too".

Fortunately for most windsurfers its not an issue since the effect on current fin designs starts above 45kts.

Unregistered
2nd June 2009, 03:11 PM
The term "cavitation" is used to describe the formation of bubbles in the water associated with a reduction in pressure. At what point this effect occurs, and its effect on the performance of a hydrofoil are two areas of ongoing research. But there is much that has been verified through thorough tank testing and observation.

sort of, cavitation is the formation AND COLLAPSE of bubbles due to the pressure drop, the water is vapourising.
The reports of cavitation "onset" creating a wall to speed by some speedsailors give no other evidence than the wall to speed- which can be legitamately explained by board drag. in a lot of cases the same people who are talking about cavitation only explain the wall, and do not report any sudden loss of fin grip, audible signs of cavitation, weird stability problems as the flow starts to boil, damage to fins etc. Personally i dont think windsurfers are operating at the required speeds/pressures for cavitation yet. who says the effects on current fin designs starts at 45 knots? and what is the evidence?

If the foil cavitates what is happening? the water is vapourising, the "flow" has gone? what will be the effect if not sudden loss of grip? why is there no evidence of the shockwaves that would occur with the collapsing of the bubbles? no noise no vibration?

i note from your comments that you dont put up the counter argument other than do a google search, thats not terribly reassuring....

Unregistered
2nd June 2009, 03:15 PM
and furthermore from http://www.globalsecurity.org/military/systems/ship/hydrofoil-limits.htm

"Fundamentally, hydrofoils differ from aerofoils in that two fluid phases are possible across a hydrofoil. The two phases include a liquid phase and a gas phase. The liquid phase is water and the gas phase is water vapor or air, either separately or in combination. When the gas phase present is predominately water vapor, the hydrofoil is cavitating. When the gas phase present is predominately air, the hydrofoil is said to be ventilating. If no gas phase is present, the hydrofoil is referred to as subcavitating. "

Unregistered
2nd June 2009, 07:00 PM
And your point ???

Are we saying cavitation does occur (on WS fin ) or not ???

There`s loads of things that dont show any evidence of happening and they dont occur either !!!

Cavitation NEVER happens on WS. We just dont handle powers/pressures/speeds likely to cause it !!!

Its just another BS baffles brains issue.

Unregistered
2nd June 2009, 07:24 PM
i dont think it is happening on a windsurfer, there is no evidence to support the theory that cavitation happens on a windsurfer even at 45-50 knots.

NetWastrel
2nd June 2009, 11:05 PM
There is plenty of evidence. You just have to do some work to figure it out. Or just give in to "human nature" by taking the lazy/ignorant option: convincing oneself that it is not a problem when one does not understand, is a tempting alternative to finding out that it is a problem.

Here is a hint: Cpmin of a NACA64A010 section running at 1 degree AOA is -0.38... how fast will it go before cavitating at that angle? My calc puts in bang on 44kts. What did you work it out to be?

Oh, you were too busy looking for burnt patches on the fin...

Unregistered
2nd June 2009, 11:22 PM
There is plenty of evidence. You just have to do some work to figure it out. Or just give in to "human nature" by taking the lazy/ignorant option: convincing oneself that it is not a problem when one does not understand, is a tempting alternative to finding out that it is a problem.

Here is a hint: Cpmin of a NACA64A010 section running at 1 degree AOA is -0.38... how fast will it go before cavitating at that angle? My calc puts in bang on 44kts. What did you work it out to be?

Oh, you were too busy looking for burnt patches on the fin...

ok, so you think it should cavitate at 44 knots. That is what your theory is telling you. Now what proof do you have that it does cavitate at 44 knots? You say it should- well does it? Im afraid it is not enough for you to say that it should, you need to bring objective evidence to bear otherwise al you are is hot air.....

Farlo
3rd June 2009, 12:39 AM
Cavitation, if any, would not be a good way to depolish a fin. I prefer sanding ;-)

NetWastrel
3rd June 2009, 07:54 PM
unregistered #71,

Well its quite simple... for the foil to be generating a steady lift in a normal single fluid steady condition, the Cpmin needs to go as low as -0.38 for 1 degree AOA. At ~44kts this corresponds to an atmospheric pressure of 101kPa. At 0deg AOA (ie generating NO lift) atmospheric pressure matches Cpmin at a slightly higher speed over a large portion of the fin surface. So what happens? Certainly not "nothing" as you are implying. Most of the tank test data that exists in the public domain shows the onset of cavitation at speeds around where the Cpmin matches the freestream equivalent (which will be close to atmospheric). Its really not hard to do a google... see it as some homework which I'm not going to do for you.

Its a bit harder to find out at what speed your fin is going to cavitate without some information on the expected pressure distribution of the profile, over the operating range. This is easy if you know how to drive xfoil (or some verified equivalent) or have some data tables. There is no quick answer if you havent got experience with it.

I'm afraid my objective calculations and research into the topic dont agree with your subjective speculation. Plenty of study for you to do before you understand the basics it seems.

Unregistered
3rd June 2009, 08:01 PM
what evidence do you have that it cavitates at 44 knots? not theory that it should, evidence that it does.

Unregistered
3rd June 2009, 08:52 PM
what happens when cavitation starts on windsurfer?

Unregistered
4th June 2009, 12:58 AM
It would spin out !!!

Dont think a windsurfer would ever operate at angle of attack of 1 degree. Poster spouting calcs making some serious blunders. At 44 knots likly AoA would be at least 6; (At best) look at your VMG and compare with where you think you are going.(Study videos of any speed runs !!!

At 1 degree fin would never generate sufficient lift !!! Do your calcs and see !!!

AoA of 1 degree is ridiculous !! (Think you know it is too but you are bending things to suit argument !!!)

And assuming theory is correct why is there no evidence ??? Not a jot. We get aeration (or stall from overloading/too great AoA) long before onset of (true) cavitation.

BS baffles Brains again. Its us windurfers,we love it !!!

Half read some journal; half know some hydrodynamic equations and bobs your uncle. Its proved; we do "cavitate".. BS

Floyd
4th June 2009, 01:24 AM
In pumps (and with hgh speed propellors) onset of cavitation is temperature dependant.Colder the fluid the later the onset of cavitation. I`ve not achieved the speeds where anyone is presuming cavitation might occur but I`ve never heard anyone who has achieved 44 knots + say anything about getting less spin out in cold water; which would be case if cavitation was happening; wouldnt it ???

On the damage to props issue I must admit the damage was generally on what I thought to be high pressure side.Apparently the bubble forms on low pressure side but then smacks into high pressure side of opposing prop. (as bubble collapses) So perhaps even if cavitation was occuring it would not damage our fins. ?? !!! Not saying it does happen but offering an explanation as to any lack of physical damage ???!!

I dont think it happens but ????!!!!

Unregistered
4th June 2009, 04:38 AM
what are the effects of cavitation on a windsurfer.

What happens when cavitation occurs?

spin out?
audible signal?
damage to fin?
nothing?
what is it?

plenty of windsurfers regularly sailing low 40 knots, plenty more regularly sailing high 40 knots- what have they reported that corroborates the THEORY that cavitation is occuring at those speeds? i have never heard anything said in the speedsailing fraternity that corroborates it other than the speed wall and the speed wall is explained by the board drag.

realistically you can mathematically model till the cows come home but at some stage you need to reference back to reality and prove the model. That requires evidencing and referencing your findings next to the model. What is the evidence that windsurf fins are cavitating around 45 knots?

Unregistered
4th June 2009, 04:42 AM
I'm afraid my objective calculations and research into the topic dont agree with your subjective speculation.

what makes your "calculations" objective? given that the calcs quoted are far from complete

and what makes my "speculation" subjective? given that you dont know what i am basing my views upon?

Erik Loots
5th June 2009, 06:07 PM
I might think from a more practical perspective... All speedspots I sail have limited space to accelerate. In other words your fin should deliver its performance quite soon otherwise your acceleration kicks in at the end of the speedstrip.

When looking to pure topspeed I think it won't be faster in most cases. This is also why polished fins are not recommended by me. An easier and more faster entry of the speedcourse is way more important than maybe 1-2% more drag in my eyes.

Maybe some profiles work polished better than others, my profiles (between 9.5 and 10% thick) never felt right totaly polished. I have missed many good gusts with too slippery fins, just by overloading. In the end when I did everything right with a polished fin, the big reward wasn't there...

Maybe on longer speedstrips with a natural curve a very good skilled windsurfer can take little advantage. But the conditions should be near perfection. This doesn't happen often, certainly not when the speeds are between 45-50kn.

http://feeds2.feedburner.com/SpeedsurfingBlog.1.gif (http://www.speedsurfingblog.com/)

Unregistered
5th June 2009, 07:32 PM
Eric
There isnt a shred of evidence to say a "rough" fin gives better initial acceleration and ultimately more drag . Infact evidence is mounting suggesting exactly opposite. Perhaps we have learnt to sail with "rough" fins ???

Erik Loots
5th June 2009, 08:40 PM
The theory is 100% spot on! No doubt, I am talking about human limitations.
In theory when the windsurfer is loading the fin perfect and pushing the board just on the surface it will give less drag

But hey, I can't do that, OK I am not the fastest windsurfer in the world. I am 99.9% of my time in awfull speedconditions. With a polished fin I don't have any margin, every mistake means End of speedrun. Now I don't have time nor patience nor money for such sailingstyle.

Also the gear especially the sail has some comprimizes in the run up for high speed, especially with my lower weight, the moment of inertia doesn't work out 100%. When your fin has no room for mistakes this means you have to easy down and NOT put pedal to the metal. Windsurfing is sooo dynamical, this fact has to be considered when going for worldrecord speed. But also when just freeriding or lower speed.

I do believe we windsurfers can go beyond 50kn, but it takes very good training (especially mental). Me and many other try to get the holy grail, but it is just such a long road.

Unregistered
6th June 2009, 01:19 AM
Its always worrying (or should be) when anyone claims "theory" is 100% spot on.

First of all thats why its called "theory" and not fact....

Secondly; history is littered with in theory"facts" going wrong. Titanic and icebergs; space missions falling to earth in fireballs etc etc.

In theory large bumble bees cant fly !! In theory dolphin max speed is 20knots etc etc.

AND there is NO EVIDENCE to support your 100% right theory anyhow. Much of the "current" theory says the opposite !!!!

I`m not a slow sailor (but I`m not that fast either) (Vmax 42 knots) and I`ve been sailing 20 years +. I can not tell difference in polished to mat fins !!! Spin out or top speed !!! (Currently using polished but not waxed) (ie naturally shiny !!!)

Diffence between polished/mat is at best neglible.But try using ventilated fin. You CANT spin the damn things out (Well you can but its a real effort !!!) But they are slower.(I think, not 100% certain though, NOTHING is 100% certain, well nearly nothing !!!)
Get my drift ???

Try getting someone else to put your fins in Eric. See if you can actually tell ???!!! Are you 100% certain you could !!!!

As some other poster mentioned perhaps you learnt to sail with "matt" fins. (Think we all did !!!)

Farlo
6th June 2009, 08:03 PM
This morning I met Stephane and Samuel Moscher, founders and managers of Select. Both told me that they preferred by far wet-sanded over polished. This is based on tank tests they did 15 years ago and since confirmed by continuous experimentation. Their explanation is basically the same as in post #3. A matte surface is easier to wet and a wet surface behaves better in water. Difference in friction drag is negligible or in favour of wet sanded. Interestingly, Select is also wet sanding high performance fins horizontally with 240 grit. Samuel Moscher said that phenomenons such as cavitation might start occurring at 45~50 Knts, but this was just his intimate conviction.

About fins I trust those guys more than any old or modern theory, or comparative tests I could do by myself. Stephane Moscher said that this would probably apply to hulls as well but he had no experience or data about it. Of course someone may invent a surface finish (diamond-like carbon?) that will make it polished and wettable and fast and controllable in a broad range etc, but apparently it has not happened in windsurf yet.

Unregistered
6th June 2009, 08:33 PM
Did Stephane and Samuel mention how much cheaper it is to not bother with final polishing of fins ???


NASA must be etting it all wrong these days !!!

Farlo
6th June 2009, 08:55 PM
Stephane just said it was not any kind of sandpaper. I forgot to mention sanding horizontally from head to tip. Manufacturing cost certainly plays a role in choosing a certain design, but Deboichet has taken a similar option, and I wonder how Kashy fins look like, haven't seen any. Price should not be a limitation there.

Unregistered
6th June 2009, 10:19 PM
did the select guys tell you why their fins always hum and vibrate and need the trailing edge sanding down? or why i had 3 of the viper speed fins of 28cm length breaking at the base. or why i used to have to buy 4 or 5 fins to find a good one? or why as soon as they try a tectonics or a c3 people change and dont go back to select?

Farlo
6th June 2009, 10:53 PM
Well they are making fins every day for years and I guess they are more qualified than you and me. Many people I know are happy with their products. Certainly other brands are doing excellent fins, so check whether they deliver them matte or polished. You will learn much more than by reading articles from Lockeed Martin.

Unregistered
6th June 2009, 11:12 PM
So because mat finish fins sell best they must be best ??? Sort of self fullfilling prophecy.???

Suspect Lockheed etal know a damn site more about aerodynamics/hydrodynamics than any fin manufacturer. Even those claiming to have done tank testing 15 years ago !! Think that says it all !!!!
BTW select fins are not really a good exmple to hold up;

It mght well be mat fins spinout later/ have better lift drag ratios/offer less drag etc etc but FACT is there just isnt any factual evidence to suggest any of claims made !!! Its another part of WS folklore ; like cutouts reducing drag ? When acually they increase it !!!! We just play into the BS. (Cut outs do offer advantages but NOT reduction in drag as normally claimed but thats another argument !!!)

If claims made for "mat " fins are true why do we not see use in commercial fields ??? (Eg Hydrofoils ; aircraft wings ? ???

Farlo
7th June 2009, 01:33 AM
It seems that we are rounding in circles. This is what the guys at Select told me and I've nothing to add. I'm not impressed by references to hydrofoils or aircrafts or guitars which do not really apply to windsurf. Now it could very well be that a given fin performs better when polished. If you have one keep it preciously. I'm leaving this post, bye.

Unregistered
7th June 2009, 02:28 AM
Guitars .... Nothing to do with it !

Aircraft........ Some connection ??

Hydrofoils ...?? Isnt that another word for our fin/skeg ??? A water foil ???? A fin is simply a Hydrofoil rotated 90 degrees ????

You might be going in circles ???

Unregistered
7th June 2009, 02:29 AM
farlo you are correct, circles.

The only definitive thing to come form this is that there is no definitive answer to the original question.

In my experience as someone who has a polished fin that does not spin out(a c3) and also lots of sanded fins, some that spin out, others that dont, it doesnt seem to make an awful lot of difference if you polish or sand and this would tally with theory that says after a certain level of smoothness any smoother doesnt really do anything extra( its inside the boundary layer)

Of far more importance seems to be the quality of the section.

so unless something new comes to the party im outa here to.

Erik Loots
7th June 2009, 06:30 PM
It would be very intresting to see if Anyone goes a X.X% faster or experienced less drag with polished fins. I didn't sorry, I have experimented it myself.

Also I don't want to spent to much time with such minimal improvements. But if anyone can make a nice testreport we can publish it on my website: www.speedsurfingblog.com

I decided comming period NOT to try out much little things.

BTW I Changed from Tectonics to Select and improved my PB. The pre 2007 selects are sometimes not that clean. But I have measured all my 2009 fins and difference wasn't measureble <0,1mm

Yes I learned with mat/sanded fins, it works...

Cheers,

Erik

Unregistered
7th June 2009, 09:26 PM
erik are you sponsoed by select?

Erik Loots
7th June 2009, 09:44 PM
I was on tectonics which I still really like, one day I tried a select fin from a testrider. I felt in difficult conditions the Select giving a more constant performance. Tectonics sometimes threw me of on higher speed when wind suddenly increased. Now I see big names in the Netherlands using the tectonics and they do crash more often (compared to me)

I asked in my local shop about those select fins, they offered me a nice deal for select fins.

The best thing about select they can make anything. With tectonics you have basicly only one very good speedfin. I tried a few days ago the new Select S09 fin, what a performance!! definitly a winning slalomfin, very good steering!. You don't hear much but even some worldcuppers use &got some select fins.

Yes some select fins make a noise, but if you look to all other aspects it is a very fast foil. about 50% of my foils I finish myself, its not needed in most cases. It gives a good feeling, when the fin is totally checked :)

Cheers,

Erik

Unregistered
7th June 2009, 11:46 PM
To whom it may concern

The laminar flow of polymer solutions is exploited by animals such as fish and dolphins, who exude viscous solutions from their skin to aid flow over their bodies while swimming. It has been used in yacht racing by owners who want to gain a speed advantage by pumping a polymer solution such as low molecular weight polyoxyethylene in water, over the wetted surface of the hull. It is however, a problem for mixing of polymers, because turbulence is needed to distribute fine filler (for example) through the material. Inventions such as the "cavity transfer mixer" have been developed to produce multiple folds into a moving melt so as to improve mixing efficiency. The device can be fitted onto extruders to aid mixing.

The example above leads to another possibility:

Microgrind the wetted area ( board or fin ) then spray it with a semi viscous solution such as Icex

Unregistered
8th June 2009, 12:17 AM
Talking to bloke reckoning to be an hydrodynamist. (I know him from uni days)
Anyhow his take on it was ;
Polished less drag but !!!
You aren`t comparing like with like once a fin is "rough".
Rougher builds up thicker boandary layer ; dependant on speed and viscosity of fluid. The thicker boundary layer effectively increases thickness of fin throughout its chordlength.This makes fin potentially less susceptable to changes in angle of attack and more tolerant of higher angles of attack.(If surface flow is maintained)In effect it behaves more like a thicker (more rounded leading edge)fin.

He also suggested thicker boundary layer "might" delay onset of cavitation/aeration but wasn`t sure !!!???

I`ll phone him back and ask about additives as in last post !! Plenty of additives in coast around us already though !!!

Perhaps we`re all right !!!!

Erik Loots
8th June 2009, 05:28 AM
He also suggested thicker boundary layer "might" delay onset of cavitation/aeration but wasn`t sure !!!???



Intresting... it would confirm the feeling I have got about the finish of a foil...

Farlo
6th November 2009, 04:16 AM
Just seen nanostructured surfaces that are completely non wettable. I wonder what the effect can be on lift/drag. It must be different because air get trapped into the structure so there is virtually no boundary layer.

sailquik
7th November 2009, 08:18 PM
Xfoil can be used to calculate cavitation speeds for windsurfing foils. These calculations are as I understand it correlated with actual tank tests. The speeds calculated correlate very closely with those 'hit the wall' experiences some top speed sailors have had and which can be observed in their trackfiles. (eg. Chris Lockwood in the canal a couple of years ago).
There is no sudden loss of control when cavitation starts to affect a windsurfing fin. Spin-out does not occur. Ventilation is a different phenomenon. There might possibly be some vibration effects but if you can feel that when rattling over corrugated chop at 45+ knots you are indeed a very sensitive new age man. ;-)
Xfoil calculations have shown that AOA of windsurfing fins at 45-50 knots are indeed very small and in the range of 1 degree. Assy foils have even less AOA.
The amount of force required to propel a windsurfer at 50 knots is actually quite small. Most of that force we feel is sideways force. The resultant force is only a fraction of that. I have decided it should be fairly easy to experimentally find the order of that force simply by towing my speedboard behind a boat with a spring scale attached to the tow rope. A project for the next calm day.
So we don't usually have an excess of power to 'push through' any extra drag created by the onset of cavitation, however minor. Damage to the fin surface may not be apparent because the inception of cavitation is probably not severe enough to do that damage. Damage requires power and we don't have any to spare. It is all used up by the increase in drag caused with the incipient cavitation which makes the 'brick wall'.
Those of us that speed sail here at Sandy Point are quite familiar with the feeling that you have hit a 'wall' in some situations. It's not board drag though because on the next run after one of the many variables or conditions have changed, you suddenly find that 'wall' gone. Same board, same fin, different conditions. Maybe the wind had swung a bit more broad and increased so one can now unload the fin slightly. Some of us think we can tell the difference when water turbulance conditions change. There are many things we can probably never quantify but we can get a 'feel' for them with experience and objective clear thinking from careful observations. There are about a dozen guys here who probably get more time over 40 knots, often in quite marginal conditions (less than 30 knots of wind) than just about anywhere else in the world. Some patterns definitely emerge and many of them confirm the theory from the science.
Sanded v's polished: Working with extremely accurate and optimized speed fins we have tried both. The jury is still out which means the difference is most likely minimal, or at least very hard to pin down in an objective way due to constantly varying conditions. Most of the sailors here go to great lengths to carefully prepare their fins to be as true to design, fair and blemish free as possible. Even those that 'sand' their fins use very fine wet and dry paper (1000-1200 grit) I have had some PB's on mirror finish polished fins and have also done the same after grinding the tip through the sand on the bottom at high speed. (I try to avoid that though if possible! ;-)) Often, there are combinations of factors like the water state, wind angle and the big one: wind strength that over ride the dulled, less than theoretically optimized fin for a PB run. Who knows if we may have gone 2 knots faster on that run if we hadn't 'naturally wet and dry'd' the fin though??

Unregistered
8th November 2009, 11:07 PM
And to continue this unanswered question

WHICH WAY SHOULD YOU SAND- PERPENDICULAR TO THE BOARD OR PARALELL??

Farlo
9th November 2009, 07:02 PM
Some twenty years ago Olivier Augé, one of the top speed French windsurfer, advised to sand horizontally near the head and turn vertically near the tip. This was supposed to optimize water flow and probably coped with the knowledge and technology of that time (remember the speed record was around 30 Knts). Frankly I wonder if sanding direction makes any sensible difference when the grit is fine enough. Select and Deboichet now sand their fins horizontally from head to tip, so the current (logical?) option seems to be parallel to the board.