I remember seeing a youtube vid on this site a while back - it was a camera mounted on a formula board looking down at the fin - the fin was flexing big time to leeward.

so... just wondering what effect that could have on efficiency and performance - ie as the fin flexes away does it lose performance (drag increase, increased chance of cavitation)?

For a pure speed unidirectional run (ie sandy pt..) then could it be beneficial to have a fin that was curved to windward and as it is loaded up it straightens out?

Been thinking about it for a while so I got out the pencil and calculator




A bit fuzzy, I'll take them to speed fortnight, but it's interesting. To get the approximate forces involved you take moments about a fore 'n axis through the centre of lift of the fin. It's a windsurfer beating hard into the wind but the effect should be there on looser points of sailing.

Assumed the centre of lift of the sail is 1.8 metres above the base. The sailor is 80 kg and 1.8 metres tall. Gets his C of G 1.35 metres horizontally from the fin. So, balancing moments, the sail lift is 54.57 kg.

If the sail is raked at 20 degrees to windward that 54.7kg resolves to 51.3kg horizontal and 18.57 kg vertical. That's weight off the board!

So the fin has to provide 51.3 kg of lift horizontal to balance lateral forces.

If we now cant the fin, we'd have to use a slightly bigger fin. But at 20 degrees cant, like the sail the numbers are impressive. A 6% bigger fin providing 54.7 kg of lift still gives the 51.3 kg horizontal lift but takes an impressive 18.7 kg of vertical lift off the board.

So we relieve the board of providing 18.7 kg of lift but are only requiring the fin to do an extra 3.3 kg of lift!! And fins are more efficient than hulls at producing lift anyway.

It's got to be better to lean both that sail and the fin to windward. Get your foils working closer to along the same line. Unless of course you want the fin to work on both tacks then you have to have a flexy fin or one that's got calculated looseness in the box

Ian K said...
Assumed the centre of lift of the sail is 1.8 metres above the base. The sailor is 80 kg and 1.8 metres tall. Gets his C of G 1.35 metres horizontally from the fin. So, balancing moments, the sail lift is 54.57 kg.

If the sail is raked at 20 degrees to windward that 54.7kg resolves to 51.3kg horizontal and 18.57 kg vertical. That's weight off the board!

hmm... I dont see it. The 18kg vertical weight of the sailor, is being absorbed by the vertical component of the lift in the sail (assuming optimal sailing/trimming) - none of it is pressing down on the board. You can feel this when you beach-start -> with strong wind, there is enough vertical lift for the body AND the board to be lifted off the water.

So the fin has to provide 51.3 kg of lift horizontal to balance lateral forces.

If we now cant the fin, we'd have to use a slightly bigger fin. But at 20 degrees cant, like the sail the numbers are impressive. A 6% bigger fin providing 54.7 kg of lift still gives the 51.3 kg horizontal lift but takes an impressive 18.7 kg of vertical lift off the board.

So we relieve the board of providing 18.7 kg of lift but are only requiring the fin to do an extra 3.3 kg of lift!!

And fins are more efficient than hulls at producing lift anyway.

There is about 10-15kg of dead weight in the board and rig - some vertical fin lift will help here, but I think the flex of the fin more than covers it.

Note that the fin is being dragged through the water (800 times more dense), it must be more efficient simply because of this; the board on the other hand (ideally) is touching the water with about the same surface area (*insert laughter* !!). So, I'd expect that the board is more efficient that is being credited for.

It's got to be better to lean both that sail and the fin to windward. Get your foils working closer to along the same line. Unless of course you want the fin to work on both tacks then you have to have a flexy fin or one that's got calculated looseness in the box

Not so sure... if you look at the world-cup sailors, most of the guys are not leaning to windward all that much. Granted, they are exceptional at efficiency, so their windward rake would be reduced - but still, I think your diagramming is a little out on this one.


There is one major reason why you dont want a huge amount lift from your fin. As Ian said, the efficiency is higher, so any change to forces involved, needs to be compensated by having better control. Anyone who has sailed formula (and I know Ian has), will have experienced the "over-powered fin causes airborne board" syndrome, ie: a little extra lift (say from the change in AoA in a gust) will cause the fin to compensate faster than us humans can.

Fins (generally) are more efficient - but if that was the only consideration, we would only be sailing on foils. Ideally you want a "bit more lift", not "how much flight time can we hold"...


As for the original question, on a clear day with a bit of beam-reaching, you can see the fin from the top of the board, ie: I would expect that the shot of Sam Parker where he is jumping his formula board with his fin bent at 20deg., is not all that abnormal for formula fins.

mathew said...

Ian K said...
That's weight off the board!

hmm... I dont see it. The 18kg vertical weight of the sailor, is being absorbed by the vertical component of the lift in the sail (assuming optimal sailing/trimming) - none of it is pressing down on the board.

I should have said " That's 18 kg of lift the board doesn't have to provide."

We're considering the external forces and moments on the system. The "system" in this case is the board, rider and rig. Forces internal to the system don't need to be considered. There are only 3 external forces - aerodynamic, hydrodynamic and gravity. If the fin and sail are both upright the only way to get vertical equilibrium is if the hull provides all the 100kg - or whatever the sailor and the kit add up to. (Forward lift and drag forces don't come into the equations looking along the board axis)

So, I'd expect that the board is more efficient that is being credited for.

Not sure how efficient the board is - my guess is based on the fact that foiling moths go better than planing moths.

Not so sure... if you look at the world-cup sailors, most of the guys are not leaning to windward all that much. Granted, they are exceptional at efficiency, so their windward rake would be reduced - but still, I think your diagramming is a little out on this one.

Probably right, you might start crashing into waves @ 20 degrees - might be other ergonomic or control issues . Shouldn't change the conclusions on fin angle though.

While you keep the board level the 20 deg fin cant shouldn't go more than the 18 kg of lift. That'd just free it up a bit. You'd still want to retain an amount of hull pressure on the water for the control advantages that hulls have.

I'd always felt that formula board take off was initiated by the sail lift - ie leaning back too much in a gust and sheeting out a little = a good vertical component of sail lift.

Could we say that formula boards with flexy fins are already a foiling/planing hybrid?