Surfing & Windsurfing/Optimising the sail for light wind
I am mostly subplaning in light winds ( 12 knts) because of the prevailing conditions here, but even my largest sail, a Severne Focus 8.5 is barely enough to get me subplaning in this light air. I’d probably need a 9.5 or larger, but I’d not be comfortable because large sails are harder to manoeuver and to uphaul than smaller sails. My board is a Bic Core 293, with a 460 cm Gun Sails mast ( 55%, constant curve).
In these conditions, I’d like to maximize the low wind end of my sails by keeping the DH to a minimum, in combination with the OUTHAUL tension.
Unfortunately, modern shortboards and sails are designed primarily for strong winds and planing and, as such, they usually require a strong DH. If I try to go easy on the DH, then the sail won’t rotate because the DH is still too small to pull the batten off and away from the mast. So I have to increase the DH and this defeats my purpose.
In this regard, I hear of two possible solutions :
1. use a Hard Top mast , because for the same DH tension, the top leech would remain tighter than with a softer mast, while the softer bottom section of the mast would bend enough to release the batten for rotation.
2. I also read in one of the forums that one way to deepen the profile in the lower sail would be to stiffen the batten above the boom at the leech end, by inserting a 2nd shorter half- batten in the pocket. This would further flatten the sail there and by doing so , the center of effort of the sail would be shifted even further towards the front ( luff side), with a deeper profile.
The rationale behind this, from what I understand, is that with a curvature from luff to leech across the sail ( as opposed to a more tapered shape ) the aerodynamic air flow along the windward and leeward sides would separate at the trail end of the sail, causing a curl-back of the flow along the leeward side. This would add back some pressure to it, which would reduce the pressure differential necessary for the lift ( or the motion).
3. I also thought of shortening the main two battens by a few cm at the luff-end ( and restitching the pockets) so that they would not stick too much to the mast with a small DH. This way the sail would rotate properly, even with a small
I'd appreciate your oipinion on thgis
I wonder if you or anybody can comment on all this.
In 12kts wind there are two things that are going to get you planning, knowing how to pump the board on to a plane and having enough sail to power up the board.
An 8.5 should be enough sail. Adjusting the DH and OH to get max light wind power is just a matter of adjusting these to the lower range of the manufacturers recommendations. You can change mast tops and mess with the battens but then you are changing what an expert sail designer has spent years perfecting. Will it help?, maybe, maybe not. I say, why mess with a good thing.
You seem to read and understand a lot about the mechanics of windsurfing. With your set up, you should be able to plane in 12kts. If the average wind is 12kts then there will be times that it is a bit lower and a bit higher. On the lower end, anyone will have trouble staying on a plane.
What I would recommend is that you apply your studies more to learning about 'pumping onto a plane'. What I am deriving from this and other ?'s from you, understanding how to pump the board and sail together to get on a plane will help you tremendously when sailing light winds.
I have perfected my techniques to where I am usually on a sail 0.5-1.0 sq mtr smaller than all the other sailors. Near top speeds a smaller sail will continue to power due to apparent wind speed continually increasing. With a larger sail in the same wind, there comes a point when the sail drag prevents the apparent from providing more power and it basically stalls (kind of like setting cruise control) and you cannot get any more board speed.
So, learning to plane with your 8.5 will actually be more benefit to you than changing the mast, battens, etc. and as good or better than going to a larger sail. Do some serious research on using your legs to pump the board and also pumping the sail. If you are still having trouble, check with your local shops for an advanced instructor and take a couple of advanced lessons. Explain what your concerns are and where you feel you need help. It may seem expensive or you may feel that you don't need instruction, but please trust me when I say it will help. An advanced instructor will observe your techniques and help you adjust to get maximum power from the board and sail. Learning the subtle body movements to get the board planing will help you for the rest of your sailing life. Small expense for huge gains.
Hope this helps.
Keep on sailing,