Select fins have a fin selector on their website if that helps.

burkey 1 said...

a rough guide is sail size x 5 + - 2 depending on conditions = approx fin size for that size of board

I went through all my sail and fin combos, this worked amazingly well, nice.

the bigger the sail the further back on the board the force will act in a lateral direction, this puts more sideways force on the fin even though the total power gained from the sail could be the same as the power generated by a smaller sail in more wind.

from that I can't see why a small sail needs a small fin but a big sail should require a big fin

Ian K said...

I can't see why fin size has anything to do with sail size. Fin size should only depend on the board width and the type of sailing you're going to do. Whether you want to go upwind well or maximise your speed off the wind. I can understand you might want a smaller fin in windier, choppier conditions to keep your windward rail wetter and the board slower. I can understand that If you expect to go faster with a small sail then a smaller fin makes sense. It can still generate the required lift at the faster speed, but mostly we go slower as sail size gets smaller because of the chop.

The fin can only see the water going past, it has no idea what's going on up top.

I think it's an old wive's tale that fin size should match sail size. Took out a 5.3 the other day with a 36 cm fin - it was fine, it matched the 64 cm board width.

What's anybody's logic that a small sail needs a small fin?

+1

There is definitely a correlation between the style of riding, the board shape, the centre of effort from the sail, the speed of travel and the angle to wind etc, but sail size is still independent of the fin size. To me fin size and shape is all about board trim.

Ian K said...
The fin can only see the water going past, it has no idea what's going on up top.

This is what I used think i.e. same board speed should require same size fin regardless of the sail used ... but the direction of the force up top changes (for different wind speeds but matched board speed) and this in turn must be balanced by the fin. So in a way the fin does "know". Compare the case of a board holding a reaching course at a speed=20knots, for the case of wind=15knots compared with in wind=30knots. For the first case you effectively point higher into the "apparent wind" and therefore require more lift force from the fin (for same course and speed), either you set the fin at a higher angle of attack (but keep spinning out) or you increase fin area. So fin size (area) is related to wind speed because of the change in apparent wind direction.

An idea anyway...

Of course sailsize and finsize is related. There are many boards where you don't feel the difference as much, however most people will notice it, when you give them a proper fin-size.
My 80cm wide board works with a 44cm fin and 7m2 sail, and 48 and 8 sail?
But not the other way around. There is no way to sail comfortably with 7m2 sail and 48 cm fin, and likewise with 44cm an 8m2....

jonesmb said...

Ian K said...
The fin can only see the water going past, it has no idea what's going on up top.

This is what I used think i.e. same board speed should require same size fin regardless of the sail used ... but the direction of the force up top changes (for different wind speeds but matched board speed) and this in turn must be balanced by the fin. So in a way the fin does "know". Compare the case of a board holding a reaching course at a speed=20knots, for the case of wind=15knots compared with in wind=30knots. For the first case you effectively point higher into the "apparent wind" and therefore require more lift force from the fin (for same course and speed), either you set the fin at a higher angle of attack (but keep spinning out) or you increase fin area. So fin size (area) is related to wind speed because of the change in apparent wind direction.

An idea anyway...

That's a good point!

I think what your getting at is the lift/drag ratio of the sail plays apart, in the sideways force, that must be counteracted for the board to track in a straight line. Maybe

In your example, the board is travelling 20knots across the wind, in the former case there is 15knots of wind which if not for the friction between the water and board would send it downwind at 15knots. In the later case the wind the force is 4 times greater trying to send you downwind at 30knots.

Unless newton is a liar in both cases you have to counteract that force.

The lift/drag ratio of a foil increases, as the angle of attack is increased, and then it reduces before the maximum amount of lift is reached. From memory the max L/D occurs around 4 degrees for 'normal' foils.

This would account for there being an optimum fin size for a given sail size. Or perhaps it should be 'there is a optimum fin size (with its lift/drag ratio) for a give sail size to give an ideal (lift/drag ratio) for the sail. But its obviously very complex to work it all out. I can't

Something else that isn't immediately obvious: your board doesn't travel straight where it's pointing, but crabs sideways a bit. This gives your fin an angle of attack to the water, otherwise you won't get any usable force from it.

I'm not sure about the physics of how the full dynamics work, but I do know that on bigger size sails I need a bigger fin, otherwise I get too much spinout and find it too hard to go upwind. As the wind picks up and I go down a sail size, I can get away with smaller fins. I tend to notice this most on my wave board, for what it's worth (sweet spot on a 5.2 is a 23cm fin)

Roar said...

the other factor that i also important is the velocity of the fin thru the water.

Fins work better as you pick up speed. Smaller fins requite a greater velocity before they start to work. They also reach a point where more speed will actually start to cause more drag,

In 15 knots with a 9 , sail you need a big fin to get the lift working early. sure you can put a smaller fin and it will work fine once you get up to speed but when accelerating or thru the luls it will tend to slide sideways.

so what we really need is an ajustable fin that retracts up into the board as we pick up speed :P

I think Anders had a retractable fin system setup back in the day

Trousers said...

kecksoff said...

Has anyone a link or info on matching fin sizes to sail sizes - or even a general rule of thumb?

i have no idea whether your question has been answered because every time i open this thread i find myself mesmerised by your avatar...

Wicked Trousers! Loved the photo as soon as I saw it & have had it as my avatar forever now!
I'm having an absolute ball turning back the closck to my high school physics days with the banter - it is amazing how lazy I have become - "caarn guys, just give me a link" where I would have said as a youngun' "here you go fellas, I've built a parabolic graph!'

yes it is +2 for lighter conditions just to help with getting upwind

joe windsurf said...

for the larger sails - heavyweight + larger boards
it is better for me to use 5*sail+4
example 10.0 --> 54.0 - use 53
---------8.5 --> 46.5 - use 48
---------7.0 --> 39.0 - use 40

james douglass used 0.1*C13^2+6.46*C13
which is 0.1 times the sail squared + 6.46 * the sail

10.0 -- 55 for freeride
08.5 -- 48
07.0 -- 40
which is pretty much 5 * sail + 5 !!!

I am a maths teacher and really find all that unnecessary, I use whatever feels best, and if it doesnt feel right I try something else then remember it.

For example today my 35 Type R Black Project worked a dream and didnt let go once with a 7.6m tush x15. Yet my 37cm Select S11 kept slipping out upwind. Same board, conditions told me to go a bit bigger due to chop but the smaller one worked a lot better.

Does anyone want to buy a select S11 37cm ;-)

I confess to following those equations in the past but have found that the brands and the sailing style to be just as important.

One brand will work for someone and be a totally failure for someone else. I find select tricky, black project amazing and have a C3 venom that is my speed fin which on the 2 occasions I have used it never lost grip once.....but I am not as fast as most.