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Old 11th September 2006, 09:07 AM   #11
Ian Fox
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Default RE: what fin size for a 6.8m on a S type104

For sure the whole thing is a complex area, mystique and not.

It's pretty clear that (for many years) I have been laboring the point that even amonst "similar" fins (ie: slalom) that typeX 32cm does not equal typeY 32 cm, and that in the end it comes down to a significant number of variables (inc rider style, as well as sail tuning, local conditions) that really make the difference between good and great.
Yes, agree, these are the micro level details - but what most here want.

Granted, there are some customers who simply require the macro knowledge (29 vs 36 etc) - although on average the guys looking for fin tuning info on these forums are more often seeking the micro tuning aspects (the "pro" level details found only by endless hours of on water time, racing, testing and comparing - something that is difficult for many normal customers to achieve but likewise welcome knowledge to share.) Yes, to some extent adding area specs to the dimensions could help (although a quick test of a Drake70NR with a similar area Drake40 Shallow will demostrate that area alone does also not do it). OK, macro example, but you got the idea. FW fins for F160? all 70cm, all similar area -but performance differences are quantum. Undertsanding could easily be described by non pros as mystique. Next one can be wave fin vs slalom (such as are both possible on say ST104..)

For the discussion, we had some years ago a very detailed "fin matrix" in preparation, it became a bit of an obsession to deliver the info accurately. In the end, it became too complex to be practical and precise, and, well, here we are..

No, doesn't mean it was a failure. More a work in progress .

Cheers ~ Ian
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Old 13th September 2006, 03:29 AM   #12
Dolf
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Default RE: what fin size for a 6.8m on a S type104

Ian,

We seem to have some disagreements, some misunderstandings and some agreements that you seem to see as disagreements. I will try to group my response accordingly.

First of all you say that most forum users want ?micro level details?pro level details? I am not in that category. My objective is to make my ?good? level of wave sailing look the best that it can, given my advancing years, heavier weight and limited skills. I have until now found this forum an excellent medium for my objective and until recently Starboard had equipment that was perfect for me. It seems that Starboard?s gear is becoming more technical and advanced and specialized which I would have thought is away from the major market of guys like me. If the forum is also shifting to the ?pro? level then perhaps I should shift to the Hot Sails Maui Forum. It is your forum, but for now I will continue.

I agree that fin design, manufacturing, and selection is a complex area. I personally know that, as I had the assistance of some terrific experts to help me design and build my own line of Aero big wave fins. Within this complex field there are some basic, simple and straightforward scientific based principles. Let?s compare it to boards. It is a basic decision if I want a wave board, a formula board or a speed board. The same goes for the basic shape of a fin. I do not use my engineering degree to sort that out. Secondly I make a basic decision on what size of board I want. This used to be measured in cm of length and Starboard changed the industry by switching to litres of displacement. (The two are generally related but ?scientifically? this is more accurate.) This is perhaps a slightly less basic but still not a terribly hard to understand decision based on flotation and resistance. The same goes for fins. To prevent spin out for a given sailor weight, sail size and average anticipated speed, one needs a proportional fin area. Any more than that area gives unnecessary drag and, after a point, instability of the set up.

I have a simple guide to required area which is not as you state ?macro knowledge (29 vs 36 etc)?. It is NOT based on fin LENGTH, that is my whole point; it is based on fin AREA. And the answer is simple. Minimum required fin area (cm2) = 0.53 x sail size (m2) x sailor weight (kg.). It covers 95% of your market. The additional 5% ?pro? market could be covered by adjusting the 0.53 constant to suit the individual and speed if they wanted to. It is my experience that by far most fin issues are spin out caused by insufficient fin area, typically for heavier guy on bigger sails in lighter winds; and secondly by instability caused by oversized fins, typically for lighter guys on smaller sails in higher winds. I am convinced, based on significant personal experience that starboard would significantly enhance the sailing pleasure of a huge segment of their market by helping these people.

What do you mean ?(although a quick test of a Drake70NR with a similar area Drake40 Shallow will demonstrate that area alone does also not do it)? What does it not do? Are you being silly about my first point of selecting a beginner fin vs. a racing fin? Is it not obvious that first you pick the appropriate type of fin. Then you need the right area. If these two fins have the same area then they both meet that criterion. Am I missing something or are you being sarcastic?

You are more knowledgeable than I on the micro level details on the next levels of fin selection which I will not even attempt to go into. With the exception of some very poor design and manufacturing that I have found on too many fins on the market, it is beyond my interest, needs and wants. If you want to design a complex ?fin matrix? to aid in fin selection for the ?pro level? of the windsurfing market I wish you luck on such a daunting task.
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Old 13th September 2006, 08:41 AM   #13
Ian Fox
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Default RE: what fin size for a 6.8m on a S type104

Hi Dolf,

It's a nice windy day today, just about to start a fun session with my friends. (true, so I'll be brief..) The wind is blowing 15-20 with gusts to 25, the water is moderate chop, the sailing style is fun B&J blasting, a few fun moves and a few straight line drags when it's on. It's not "pro" stuff, just good pure "sport" sailing.

I'm fully intending to use the funnest gear I can choose for the session, from what I know works, based on numerous previous sessions in such conditions.

My equipment selection, (and not modified in any way prior to any fin size formula debate) is :

ST93 / NC-X 6.5m / Freeride 280 (it's the one jammed in the board)
(aprox fin area = 220cm /or equiv performance size to a moderate AR 30-31cm slalom)

Using the Dolf formula, my "minimum fin size requirement" will be :

"Minimum required fin area : 327cm2 = 0.53 x sail size (6.5m2) x sailor weight (95kg.)"

So according to the minimum formula I would need to take a minimum of 320+ cm2.
That's equivalent (in relating measured area) to a 37cm slalom in moderate aspect ratio - or even longer in higher AR race/slalom designs. (i've a moderate 37cm race/slalom with 11cm base chord right here - actual surface area 316cm2/49sq")

I've been easy on your numbers, and not tweaked area/s, AR etc on my side.

The difference in the result is (with respect to your contributions, and no sarcasm) : quantum.

30/31 vs 37/38cm. (~220cm2 vs ~320cm2 in same style/AR etc)

In fact, as far as most of this Forum's users are concerned, that's the whole game plus change.

I haven't run the numbers on a bunch of other combos, simply what I'm going to use today in "normal" conditions - and surprisingly close to the subject example of this thread (fin size for ST~6.5m).

We do run a number of Forums, with the subtle but logical intent of offering different levels of information/discussion. This one happens to be in the AskTeam section, so the majority of the readers/questions/answers are intended to be of an advanced level, the target audience in AskTeam is unlikely to be your 95% market figure. It's far more likely (on average) to be our advanced sport/intermediate/local/national racer types guys (far less than 95% of market) but also short of the Pro level (as pros simply know they have to test for themselves, on their gear, in their conditions to get their optimal individual tuning edge). In most cases, our users are appreciative of being able to access (somewhat) personalised product specific and accurate detailed, info and advice. Especially with boards like ST that subtly deviate from "typical" sail/fin capacity vs board volume/dimension parameters (as explained in earlier posts in this thread and elsewhere).

Anyway, the wind is up and I'm outta here. Hope you have a great day.

Cheers ~ Ian
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Old 13th September 2006, 11:54 AM   #14
steveC
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Default RE: what fin size for a 6.8m on a S type104

Boy things appear to be heated a bit!

Really, I think that Dolf has raised a vaiid point about fin area. Apart from recommendations, it would be good to better appreciate fin area, and particular, its planform when talking about fins. Depending on use, the shape of things can be varied. Information is power, so let's invest a bit more sensibily. And that should include the "Team" too.

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Old 13th September 2006, 01:05 PM   #15
Ian Fox
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Default RE: what fin size for a 6.8m on a S type104

No emotive heat from my side, all pretty clear logic.

But no shortage of investment from my side either.

I agree, it should all be simple. In theory. In practice, maybe not.

Area and formulas are great things - when they are used correctly and when they work.

Above is a good example. Take it as just that. Not more.

As we knew from our fin matrix project (itself just a series of factors, variables and formulae),
to do it properly truly is a complex or daunting task.

An accurate fin formula would indeed be a welcome attribute ~ for all of us.

So yeah, let's go for it. Who's up for the challenge ?

Cheers ~ Ian
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Old 14th September 2006, 04:37 AM   #16
Dolf
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Default RE: what fin size for a 6.8m on a S type104

Hi Ian,

I have seen you sailing and from my perspective your idea of ?just good pure sport sailing? is high speed blasting and running circles around everyone else on the ocean. Is it fair to say that you are not just good but unbelievably fast; can we say a difference of ?quantum?, in the top 1% of the windsurfing elite? If I (or the other 99 or 95 or 80 or whatever % of us windsurfing mortals) used your set up, I would be spun out and lying in the ocean as I would not have the fin area for my skill level to get me up to speed.

We know that required fin area is inversely proportional to the square of the speed. If you go twice as fast you can use a fin up to one quarter the area. Conversely if you go twice as slow you need four times the fin area. This is why so much of the forum discusses techniques to get up to speed before loading up the fin. So for you the coefficient seems to be 0.36 rather than my ?normal? 0.53. (As an aside it seems that you are sailing 21% faster than me which is the square root of the ratio of coefficients) But your specific coefficient should hold for your different combinations of sail sizes, boards etc. So if you did not have your vast experience and intuitive sense you could use your specific coefficient in the formula to pick equivalent fin sizes for all your different gear combinations. Also the few other sailors in your league or race could use the same coefficient as they are in the same speed range. So the formula works even for the advanced users of the AskTeam forum, but I concede that it would not be as valuable a tool for this level of sailor. It is the other 95% of your total market that I often see struggling with grossly incorrect fin sizing that would benefit immensely from some initiative with basic guidance from Starboard and that is where the formula is easy with a constant coefficient and a complex matrix project is overkill.

Have a great day.
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Old 14th September 2006, 04:09 PM   #17
Ola_H
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Default RE: what fin size for a 6.8m on a S type104

Dolf, we have been discussing this before and I can see your point: I promised once to measure my fins and start to take notes of what sizes I used for different sails, but haven't gotten around to do it. In any case I'm sort of with Ian here from personal experience with wave and crossover boards. My main argument why I think (bi-) linear formula (even with a "personal coefficiant) is to simply is that in by definition locks the fin size to the sail size. I tend to look at sail-board-fin as a kind of holy trinity and I reguarly trim the feel (and performance) od a certain board-sail combo by changing fins (both fin type but also fin size). If we accept your formula as a basis, anyone who want to take full advantage of a boards "crossover appearance" will still have to have a set of coefficients for different types of sailing and then we're allready on our way into matrix land.

Actually, while I'm a fairly competent sailor on wave baords, I'm totally new to slalom boards and even with my new iSonic 101 and (my only) freeride sail of 7.5 sqm, I've so far have had equally fun experineces with the Slalom pro 40 and 34 cm fins. The choice for me depends on if I want to go for max downwind speed, and struggle a bit more upwind or just enjoy the (for me) fantastic upwind capability of the 40cm fin but loose a tad of top end speed maybe. I don't think this is an elitist way of thinking from my side, I think even many "casual users" wants to have the chance to use their boards to the fullest and fin choice is a part of this.

In any case, publishing the area of the Drake fins can't be a bad idea. Then people can do with this information what they want.
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Old 14th September 2006, 08:02 PM   #18
Dolf
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Default RE: what fin size for a 6.8m on a S type104

Ian and Ola,

I understand your points and do not think that you are looking at this in an elitist way, rather from the perspective of very advanced sailors who are having some memory lapses of your struggles before you joined the ranks of the super elite of windsurfing. You can turn my formula into a matrix indeed for the expert sailors if you like, that is if the experts would bother because by and large they don?t need a formula. (But it would work if they did. Every expert can develop his own coefficient and they could go to the extreme of having 2 or 3 coefficients for different aspects of windsurfing). It is the rest of us struggling mortals, the vast majority of your customers, who would benefit tremendously from a guideline. Once they advance from the point of needing to turn the guideline into a matrix they can do so or throw the formula away.

I use myself as a measure. I do not deviate from my formula as I am simply not good enough to appreciate the benefits of doing so when I have so many more urgent issues to deal with on the water. I consider myself a good sailor, maybe in Maui slightly less than that, but probably in the top 10%. Frankly Ola, casual users don?t often change fin sizes for a given sail, they are lucky if they have one fin per sail in their quiver. More often they are grouping sails around one fin size. I am rare amongst casual sailors to have 11 fins for my 11 sails. And even if they did, it does not change the benefit of the formula as providing a personal standard around which modifications could be made to suit unique situations.

I know that there is no disrespect from either side and we are all genuine in our efforts to promote our sport but I hope that you can walk in my shoes and appreciate that I see your position on this issue as stubborn to the point of being insensitive.

I am off for a few days but I think that I have said all that I can say.

Have a terrific day.
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Old 15th September 2006, 11:43 AM   #19
Del Carpenter
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Default RE: what fin size for a 6.8m on a S type104

Thanks to each of you for the good discussion. I'm one of those average (or less) skilled guys with quite a few more sails than fins. Dolf's formula interests me as a general guideline. It was presented as a formula for determing the "minimum required fin area" needed. Dolf, do you have a formula for determining the maximum useful fin area? And does the length of the board change either formula? My knowledge of the physics involved is minimal. My intuition tells me there are formulas that apply. And my intuition also says the differences between my 372 cm long Equipe and my much shorter X-186 should mean there are significant differences in the formula's I should apply to each board. (I don't remember the X-186 length, I think its about 270-280 cm.) Maybe overall length doesn't change the formula but planing surface length would? And wouldn't the board width in the fin area also change the formula? Again this is just my intuition, I have no theoretical or applied experiments to back up those ideas.

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Old 15th September 2006, 05:30 PM   #20
Ola_H
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Default RE: what fin size for a 6.8m on a S type104

Dolf,

I'm trying not to be insensitive since I'm a mathemathician and hence I like both numbers and structure. I would love a formula to help me understand these thing better and to give better advice. However, there has always been something about your linear formula that has been bugging me. Often when I see a formula or some other type of "explanation" I get this feeling "right - thats it". Since I've had no record of the areas of my fins I have not been able to "see though" your formula from an experience point of view and I neither really see why it would hold theoretically (maybe it does (within limits) but I just do see why).

So, I just took out a few fins and compared their area, the Slalom pro 34, Drake Natural 24 and Drake Style 20. They all pretty much have the same area (around 220cm2, +-2cm2 or so, the Style being a tad bigger).

I don't have very much experience with the slalom pro except that is worked pretty good with a powered up 7.5 freeride sail. OK, upwind was not excellet, but still oceans better than any wave board with any wave fin.

The Drake Natural 24 is a fin I use a lot, usually with 5.3 in onshore conditions or with 5.5-5.6. Its noth a nice wave fin but also a nice freeride/B&J fin for say a Kombat and 5.3. In that setting it could easily handle 4.5 too.

The Drake Style 20 can be used with 5.3 or even bigger, but it then feels very small and very freestyle specific. For general sailing (on freestyle, crossover or wave board), I would prefer the Style 22 for 5.3. With the style 20 you need a fair bit of technique to go upwind.

So, what's my point? Well, first, using the Style 20 would be totally useless with a 7.5. I would say its easier to get planing with the SlalomPro 34 and a 7.5 than with the Style 20 and a 5.3 (different winds and different boards, but you get the idea) and the difference would be even bigger if you less good a sailor.

Even if you compare the Style 20 and Natural 24 on the same board (Kombat, fx) with, say, a 5.3, I consider the Natural 24 to be very easy to use and friendly also to beginners with that sail size, while the Style 20 is demanding to sail and a beginner would not have a very good time on that fin.

Using the 0,53 coefficient with my 70 kilos puts the (max) sail size for both fins at just below 6.0. Thats "super expert freestyle only area" for the Style and personally, despite being quite good with small fins, I wouldn't be totally comfortable in most conditions on the Natural24 with a 6.0 either (confirming my thought that these wave fins sails small, not only given depth but also area).

So, whenever you don't stay with the same type of board, sail and fin, it seems to me it get kind of hard to use the formula to predict fin size. However, it may well be that the fomula is still excellet for predicting how much you shoul increase fin size when you get a new bigger sail (fin type, board and sail type staying fixed).

Best regards,

Ola H.

PS: The other day I read an article about scientists that had measured the activity level of each of the 30.000 humen genes on 100 patients with a certain form of cancer. By using mathematics, they could from this very sparse data get a relevant visual representation where you could indentify two discinct groups of gene expressions. This predicted that there was actually two distinct forms of cancer and htis turned out to be clinically relevant (one of the groups tunred out to live longer than the other, statistically speaking). My point with theis ps is that given enough data, it would be possible to come of with lots of results on which fin that works best. After all, we would be dealing with less than 30.000 variables and could probably fins more that 100 people willing to report data to us. Not sure I'm willing to coordinate this though...

DS
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