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Old 14th June 2011, 10:37 PM   #1
Farlo
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Default Fin size in relation with board/sail

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/...d-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?

Last edited by Farlo; 15th June 2011 at 12:02 AM.
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Old 15th June 2011, 12:19 AM   #2
Ken
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Farlo,

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.
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Last edited by Ken; 15th June 2011 at 12:22 AM.
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Old 15th June 2011, 12:51 AM   #3
Floyd
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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) ????????
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Old 15th June 2011, 12:53 AM   #4
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PS Wrote mine before Ken`s appeared. !! I was meaning other way to look at it from Farlo`s question.
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Old 15th June 2011, 02:25 AM   #5
BelSkorpio
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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
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Old 15th June 2011, 01:05 PM   #6
COACHG
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Quote:
Originally Posted by Farlo View Post
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.

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Old 15th June 2011, 02:36 PM   #7
Farlo
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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).
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Old 15th June 2011, 05:41 PM   #8
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Question Failed engineer ???

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.
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Old 15th June 2011, 07:26 PM   #9
Farlo
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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.
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Old 15th June 2011, 09:24 PM   #10
Ken
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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!
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