View Full Version : Fin size in relation with board/sail

14th June 2011, 10:37 PM
Hi all, after reading ten pages of "keep going upwind" one question reminds in my mind. If you look for fin size advice, for instance in Select's selector (http://www.select-hydrofoils.com/en/selector/speed-slalom-race/) you find that the ideal fin for a 7.2 mē sail is 35 for a 60cm, and 45 for a 80cm wide slalom board, while it would be 25cm on a 90L wave board!!

I understand it has to do with OFO and the lift required to extract the wider board from water, however the difference is huge. Simple logic would say that you need a certain amount of lift to resist the sideways force of a given sail whichever the board width. One reason could be that you go much faster with a narrow board so you get the same lift from a smaller fin (which in turn makes you go faster) but I still wonder. Roger, Ken, Coachg, others, do you have an explanation?

15th June 2011, 12:19 AM

While the width of the board plays a role in the fin size, the width of the tail probably plays a more important role as well as how the board is to be used. The slalom boards have wide tails which means that they can carry larger fins than freeride or wave boards with narrower tails. Wave sailors/boards generally are NOT interested in speed, but they are interested in quick turning, and a small swept back wave fin does it best.

Slalom boards jibing at speed and in chop turn best with a more vertical, longer style fin. The radius of the turns is pretty large compared to a wave board, in order to maintain a plane) Smaller fins would be faster, but if you go too small, then the likelihood of spin out increases. Also, quick acceleration and planing out of each jibe is critical and the larger fins help significantly.

I looked at the select charts and they seem like an excellent guide for what to buy for the various types of sailing, but as you can see, there are a lot of variables.

You are right, if 5 different sized boards have the same size fin (25cm), they all offer the same lateral resistance to keep the board from sliding sideways. Let's say the smallest of the 5 boards is a speed board that is very narrow and is only used in a straight line, off the wind. The 25 cm fin might be too big because the lift could be too great and cause the board to "turtle" or flip over. The largest board of the 5 might be formula board. Since formula boards are designed to go upwind and downwind, they need large fins (70cm) for maximum upwind performance. With a 25 cm fin, it may be fine off the wind but the huge sails and lateral force on the board would make it a huge challenge to get upwind with any great success.

As you can see, how the board is used plays a huge role on fin size and design. Also, sail size is equally important since a 4.0 sail will have limited lateral force on the fin compared to a 12.0 sail.

So you have to consider the following in determining fin size:

1. Board size (volume, length, width and tail width)
2. Board design (speed, wave, slalom, freeride, freestyle, formula)
3. Board use (the board my be designed for wave use, but many will use it as a freeride)
4. Sail size (depends on board in use, wind speeds, type of sailing and sailor's skill level)
5. Sail use (downwind slalom or beam reaching or upwind)
6. Sailor skill level (beginners and novices generally use smaller fins since planing is secondary to learning the basics, plus short fins work better in shallow water)
7. Weight of the sailor (heavy guys need large fins since they can produce more leverage)

All this is pretty simplified and there is a lot more when you look at each fin regarding it's cord length, foil, flex, length and how much is it swept back vs a vertical profile.

I don't know if I have helped or not, but maybe some others can help out with a better explanation.

15th June 2011, 12:51 AM
I havent got the answer but I think its sort of other way around. Smaller boards just dont seem to work with big fins.When I`ve experimented its not been railing or problems getting planing thats been issue its been during gybes. Board just dropped straight off plane with too large a fin mid gybe;tail sinks and you always end up flare gybing. I think this scenario is something to do with larger higher drag fin needs width to maintain planing when off the power ???

Theres more to it than board needing width to act as extra leverage on big fin.(Which is obviously part of it) ????????

15th June 2011, 12:53 AM
PS Wrote mine before Ken`s appeared. !! I was meaning other way to look at it from Farlo`s question.

15th June 2011, 02:25 AM
To explain this in an easy way, you need to understand the primary function of a fin.
The primary function of a fin is to prevent the board from moving sideways, i.e. giving it a direction of movement, i.e. giving it grip on the water.

Now, for the same course, the same sail size, the same wind force, the same weight of the rider, the wider board needs a bigger fin than the narrower board, simply because the weight of the rider is devided over a larger surface, hence less downward force per surface area, hence less grip of the board, hence more grip & steering action needed from the fin, hence a larger fin.

Extremely put, if a board would be as narrow as a blade of a knife, it would not need a fin at all.
But it would also sink :) lol

15th June 2011, 01:05 PM
Simple logic would say that you need a certain amount of lift to resist the sideways force of a given sail whichever the board width.

The way I see it a wider board creates more drag. So using a given sail the wider board will need more lift to overcome its greater drag so it needs bigger fin.

However, I think BelSkorpio has a neater way of saying it if not as simple.


15th June 2011, 02:36 PM
Thank you all. There is probably a relation with wetted area as Belskorpio says, although I would think that the larger surface has more grip/drag whatever the pressure (static or dynamic). The analogy with a blade suggest that the narrow board will resist more with its leeward rail, until it reaches a certain speed. Once fully planing, a few more knots may explain that you get similar lift from a smaller fin, while the sideways force of the sail does not increase in proportion.

Ken, I also found the selector pretty useful so far (but I use mostly Select fins).

15th June 2011, 05:41 PM
When a board planes we assume it planes on the surface.It doesn`t.It planes just beneath it, in a process called "momentum exchange". Water is moved by the board.When sufficient is being moved and enough momentum generated by its movement to balance the static load. (Weight) planing starts to happen.At low speeds large amounts of water are moved off slowly.At faster speeds smaller amounts of water are moved off much faster.Balance must be maintained against the unchanging static load so board planes on smaller and smaller area but moving water away quicker and quicker.

A wider board will initially move larger amounts of water;its why it planes earlier, but once planing it will also (at equal speed) sit shallower than its narrower counter part.(It will be moving a wide but shallow cross section of water) In other words at any given speed the narrower board will plane deeper and hence have more lateral grip consequently needing a smaller fin.

(But dont bring water line length into it !! It complicates the issue but with boards the water line length is more or less (relatively) equal. (compared to larger craft)

If anyone doubts board does not plane under surface go water mono skiing and look how deep ski sits;it cuts a deep groove.

15th June 2011, 07:26 PM
For sure the board will stay in touch with water. The immersed length of the rail may play a role too, but I guess that at some point the resistance to sideways force will come predominantly from the fin. I assume that the smaller board will go a bit faster, sit at the same level on water (in average) and get similar lift from the fin and immersed hull. If you look at various sizes of the same model board, the small ones do not seem to need thicker tails because these would cruise deeper (to the contrary). That's OK for me, thank you all.

15th June 2011, 09:24 PM
Thanks for the added info. I like BelSkorpio's and Unregistered's comments.

Another factor in narrow vs wide boards is the lift wide boards get from air under the board once planing. This decreases the wetted surface and causes the board to ride shallower, all suggesting the need for a larger fin.

That is until you reach the "lift off" point and you become air-born. I have done this on my formula board a few times on an overpowered reach, by turning upwind a bit in an effort to slow down. Big mistake!

Failed Engineer
15th June 2011, 10:59 PM
The argument about boards riding on a cushion of air is I`m afraid just another popular myth.

Its easy to see why when you look at the maths. Board load in region of 200 pounds ??? or more ? Riding on around 10 square inches of water/board (can be less can be more) Required air pressure in region of 20 pounds per square inch to affect things??? Two things
a) Those sort of pressures would never be generated under hull.
b) Even if they were water would simply give and move to contact board elsewhere.

Concaves on hulls are to channel the water not air.

It could easily be argued aerated water is worse for planing on; aerated water does not have the same weight/densitry so would contribute less in momentum exchange. Again its why boards plane slightly earlier in salt water.Board slightly more bouyant and water slightly denser so contributes more in momentum exchange. Board planes deeper over white water .(aerated) (feels great though)

Air is introduced into cavitating props/foils/pumps for different reasons. (to induce cavitation; but thats another headache) Pressures in that are way higher than a board would ever experience.

15th June 2011, 11:03 PM
Let's also make a difference between "drag" and "grip".

I would define "drag" as a negative aspect that is caused by the wetted surface and also the fin surface, which slows us down in the (intended) direction we want to go.

I would define "grip" as a positive aspect that prevents us from going in a unintended sidewards direction.
The key parameters are the wetted surface in combination with the sharp leeward rails and the fin.
Wider boards give us less grip so more fin action and fin surface is required. Did you ever try a formula board with let's say a 40cm fin. I did, it dances around on the water.

It's logical that there's a strong correlation between "drag" and "grip". (same parameters are involved)

Then last but not least, I consider "lift" as a secondary function of the fin.
You always have it, if you want it or not. It's a side effect of the fin. And in most cases it is a positive one because it helps us planing. But like with many advantages, they quickly can become a disadvantage if you have too much of it. Over finned boards will try to fly off the water, causing instability and lack of control. Much of this can be countered by skilled riders of course. Well, I don't want to go into this any further, because there is already another thread of 9 pages about this. :)

15th June 2011, 11:22 PM
My take on fins has only two aspects. Lift and drag. The lift isnt against gravity its against leeward movement.(But drag has two aspects.the drag induced by causing lift and parasitic (frictional) drag.???)

We picture a fin going straight through the water but it doesn`t. It goes very slightly sideways.(Its why they need very rounded and not pointed leading edges)

That sideways movement (caused by leeway itself caused by the wind) transforms the "skeg" into a foil ,generating a force against leeway.(Imagine the foil going through the air with the leading edge higher than the trailing; it becomes a wing and generates lift.If it was level (ie leading edge same height as trailing edge it would not produce lift.(dont forget our fins are symetrical unlike a plane wing)
If you tow a board in a straight line the fin produces no lift; it only produces lift when made to "crab" through the water.Towing in a straight line only has drag.(and only parasitic drag)

WS is great..

16th June 2011, 12:02 AM
This would also explain why you can't go below a certain fin length on a wide board, because the hull has less "grip". However I think that there is a speed at which the board is still immersed but water release is so fast that only the hull is wet and not the rail (of course not when crossing chops). Unfortunately it is on the other side and we can't see it.

16th June 2011, 12:06 AM
Yes that sums it up.

16th June 2011, 03:07 AM
Failed Engineer,

I am not sure who said boards are riding on a cushion of air, not me, but I do believe there is "lift" from air passing under the board.

I bow to your expertise, but if I am moving at 25+ knots of boards speed off the wind on my Formula board, I can keep it on the water and in control without too much effort. However, if I am on a beam reach at slightly slower speeds and then turn into the wind, lift off can be a serious problem unless I add significant downward force to the boom. Any sheeting out or taking weight off the mast foot and disaster strikes. Board speed is slower, but the chances of lift off are higher. The only thing I see is increased air flow under the board causing it to take off.

What am I missing?

16th June 2011, 03:30 AM
Ken, I know what you're talking about. I've had the same experience several times.

My thought is that when you turn into the wind, there comes a lot of pressure on the fin, causing it to bend more and creating more "foil" action resulting in a lot of lift.

16th June 2011, 04:00 AM
There must be some aerodynamic lift under board; perhaps not between hull and water but certainly under hull out of water. Perhaps when rig is unloaded this lift is allowed to have an effect. Afterall even in only 15 knots of wind apparent wind hitting hull will be at least equal to board speed (probably more?) I`d have to look at vectors but I`d suspect in 15 knots wind with 20 knots boardspeed (easily achievable on Formula) there must be at least 22 knots apparent wind hiiting hull. That would certainly have a lifting effect.
And turning into wind would initially increase apparent wind !!

16th June 2011, 09:56 PM
A board sitting on the beach not moving and not attached to a rig can achieve lift off the ground if there is enough wind. Lift from air moving under the board is REAL I have seen too many boards moving down the beach in big gusts. I am always careful how I set mine down when not attached to the rig.

Yes, the fin does bend in the transition from reach to a higher point of sail, and it does contribute to the lift, but the key is air moving under the board. Remember, the board speed is dropping as the board turns upwind, but the apparent wind is increasing.

Chop hopping is essentially the same, air moving under the board as the board moves into the air with significant apparent wind. You only need a small wave to create a space for the air to move under the board and bingo - LIFT. It's not just the speed of the board over a ramp, although this plays a big role in getting air.

You get bigger jumps when the wind increases, but the board speed and chop size remain the same.

17th June 2011, 06:19 PM

Agree boards blow down beach but not when you are stood on them !!!

Water is 400 times heavier than air;stands to reason any water effects on board are going to have about: well 400 times more effect !

Unreg was talking about planing; air effects on planing are minimal but fair point windage on front of board/under hull especially when unloaded could effect (does effect) the board.

Once board leaves water any wind will only want to blow board down wind (once board has lost momentum).The wind could only produce lift if there is some force opposing it.ie your fin or once in air your momentum.

Wind alone can not produce lift.Its a common misconception. A kite needs tethering; a hanglider needs rising air. (It dives at a slower rate than air (wind) is rising to gain height) Its why a hanglider needs a hill or a tow. (Hill makes wind rise;the tow opposes the wind to produce lift)
Take away the opposing force and fairly quickly you are coming down.
Not sure about wind producing lift on board whilst in air; if it does its only through board/riders own momentum ; which would produce some lift with or without the wind.Would a thrown boomerang go higher on a windy day ??????

Fin only produces lift against gravity whilst not vertical. If it were horizontal with correct AoA all its lift would be against gravity but whilst vertical no force helps planing and the lift from flex of fin would be minimal.(ie flex is only 10 degrees or so and only over last few inches.)
Fins do not help planing at all.Not directly.Towed behind a boat a fin less board would plane at same point as a finned one.Fin is there to stop leeway;this produces motion which produces planing.
But when railing quite a bit of fin lift helps planing but the board as a planing surface is less efficient when angled ???

Windsurfing is a mixture of aerodynamics;hydrodynamics;the itereaction between the two and lots of BS for good measure.!!!!!!!!

17th June 2011, 11:08 PM

The board on the beach was not a good example, I grant you that.

Lift does come from two opposing forces and the boards momentum is one force when combined with the air under it, provides lift. Once the momentum drops, so does the lift and board.

The fin, converts lateral resistance to forward momentum as you say, which in turn allows the wetted surface to be reduced as the speed increases, which increases the apparent wind under much of the board. I really don't know how much lift if any is happening here, but it seems that there has to be some.

Wide boards with big fins tend to tail walk or take off more quickly than narrower boards with smaller fins. The difference is speed. They all will tail walk or take off at some point. When a fin is too large for the speed and board, it wants to "lift" to the surface, after all it is a foil. This in turn causes the board to become unstable and tail walk, which I assume is a result of air passing under the board and the fin trying to raise to the surface. If the fin always remained perfectly vertical, there probably wouldn't be much instability, but that is not the case in choppy water. Your example of towing a board with or without a fin is only valid if the fin remains perfectly stable and vertical, which would probably never happen.

I know we are going around in circles here, but nevertheless, I have enjoyed the discussion and am always eager to learn and or debate and issue.

Good sailing.

18th June 2011, 01:42 AM
Agree with you about all points except tail walk.

Tail walk is a result of board producing more lift than load;it lifts up;lift drops ;it drops . In effect board is too wide and it can no longer reduce its wetted area to a point where balance would be achieved. Its why boards with high tail rocker (ie wave boards) do not tail walk. They just carry on reducing wetted area; planing on a smaller and smaller area. Its exactly why cut outs were introduced; a way of reducing area at rear of board but still maintaining width for leverage on fin.There is no way on earth a vertical fin can produce lift against gravity.It must be inclined to produce actual lift. Bigger incline ;the more the lift.(ie at 45 degrees lift would be distributed half against lift ;half against leeway..
And even when inclined it can only produce lift in one direction.ie it must try and flip board.It needs an angle of attack to produce any force.Inclined away from wind it would pull back under(actually aiding gravity!)(ie if inclined down wind it produces negative lift !!!);inclined to wind it would try and climb out of water. ie both instances rotating board in same direction !!! Sounds crazy but its how it is !!!(look up Bruce foils)
My take on things anyhow.

18th June 2011, 09:36 PM
We are now at the point where I can only speculate, and have little knowledge to support anything else I could say. I will accept your take on things. Another good discussion.

I appreciate you "cut out" explanation, it helped me to better understand the design benefits.

Good sailing.

20th June 2011, 10:43 PM
Fins on sailing craft

The use of angled foils for powered craft is relatively easy.They provide reducing SA with speed and hence greatv efficiency and high top speeds. This is far from the case with sailing craft. Generallyt for recreationaL purposes (eg Moth foiling;windsurfers) a T foil is utillised which gives better efficiency (lift/drag) to a planinbg hull but does not reduce its SA with speed (as angled foils would) Hence no better top speed than a "normal" windsurfer.

If angled foils are tried as used let say on a hydro foil (ie foils angled outwards) the down wind foil will ,rather than give lift , pull the hull under with negative lift.(because of leeway/slip angle) If foils are angled inwards (not ideal;as craft rises its doing so an a reducing width base ) the windward foil will now give negative lift !! This can (and is) utilised in foiling cats/trimarans; where the windward foil rather than lifting craft is used to counteract leaning forces. (As in Hydroptere.Windward foil holds hull down; leeward foil gives lift and a T foil on thge rudder gives added lift to balance craft fore/aft)
I beliueve when inward angled foils are utilised with the windward one opposing heeling forces and the downwind giving lift they are known as Bruce foils.

It can be seen from this that when a foil (our skeg) on a windsurfer is angled downwind it will under its own forces try and return to vertica(Negative lift)l. As it angles upwind (railing) it will satrt producing true lift against gravity.

21st June 2011, 12:13 AM
Ha ha, nice argument. Do you agree that over a certain speed the hull may be wet but not the rail, and the grip would come predominantly from the fin? Just for fun...

21st June 2011, 12:37 AM
What ever speed you are doing there is always some displacement into the water and therefore the rail must be in contact to some extent.IMO it appears rail is exposed but probably water has been shed away but even if it has it would be playing it part. (if it sprays off at high speed trhe momnentum exchanged would be massive)

Putting it simply board never just touches surface ; even at really high speeds (way higher than board is capable of) there would still be a channel of some description.Without it no momentum exchange can take place.

21st June 2011, 12:42 AM
But yes I think you are correct that faster the board the more the relative contribution the fin makes.After all the board is planing higher but never so high to just "skip" over surface. (As it appears to do)

21st June 2011, 01:37 AM
Ah, but a board in chop with enough speed does "skip" over the surface with no displacement for a short time. However, I agree with your point. If you watch speed sailing runs that Antoine Albeau and others have made on very flat water, the boards don't seem to skip as you say, and it looks like displacement is consistent.

When they do "blow up" or leave the water, the crash usually begins with a spin out. Small fin with too much force. However, I do believe that at some point, it's possible for the apparent wind to get under the board and lift it off the surface of the water. This isn't too likely given the narrow, short boards plus the heavy sailors with weight jackets all designed to keep the board on the water to prevent "take off" (and to allow the use of larger sails).

21st June 2011, 05:20 AM
But Ken the only reason it bounces is because of water its moved !!! Newtons third law;action and reaction !!! Failed engineer explains it well !!!

PS Dont think he`s talking about bouyant displacement;he`s talking about water displacement through motion !!

21st June 2011, 01:52 PM
Yes we all agree that there is water displacement at high speed and the lift comes primarily from that. There might be phases where the hull doesn't touch water and air speed may help, but it won't last. Now depending on surface tension and sharpness of the rails, water may not have time to wet the edges but will rather be sprayed around. And depending on its outline, the tail of the board may have no grip at all due to the way water is released. IMO this is more likely to happen when going downwind over chops.

21st June 2011, 02:48 PM
Energy involved breaking surface tension is hardly ever mentioned but it must have a good deal of effect especially on flat water.Not sure wether it would be beneficial or otherwise ???? Who knows.

There is no trully scientific way of explaining all effects of boardshape or if there is I cant find it ?? Think its why its a mixture of science and art ?? (a black one ?)

Pretty sure a lot of "developments" are just marketing gimmicks but some work ?? Air tubes from F2 was a good one ?? Remember lots of years ago some builder putting round dints all over base of board ?? (Golf ball ???) Think SB have done their fair share too ??? Original Hypersonic ???

Perhaps failed engineer knows ???????????

21st June 2011, 08:42 PM
I think we all agree.

28th June 2011, 03:46 PM
What is the board size and sail size? I've heard that smart fins are great.