i guess mathews post about cavitation and fins made me think of this.

i more powered is required to push past fin drag how can sails change to do this.

how do you get more power (deeper profile) and still keep the drag low?

do sails need to become really high aspect?

Tis not about more power young palewan, tis about leverage.

If more power you want, let off downhaul or outhaul you must.

Too much power pull you over the front, it will.

Longer legs or heavier chest faster you will make

I'm not sure we need that much more power... the land sailing guys are already capable of 120+kph (60kn) - maybe the sails are already good enough... I suspect that some of the regular 45+ guys are going faster simply because the sails are better - better control during gusts, better efficiency, better foil stability...

However, water is about 800 times denser than air - it doesn't take much of anything touching the water to cause a huge drag - the big picture says that we should be focusing on reducing drag somewhat. Looking at old speed board designs, the only major difference is that they were longer (but that is ignoring the advances in rocker line, nose lift, etc....), so fins are a major talking point.

Mind you, these guys could also be going faster simply because they are getting more practice... or everyone is becoming an American....

Its the bow and archer analogy - A bow can only be as strong as the arm of the archer. Likewise the power in the sail is constrained by the weight of the sailor. There's no point in making more power than you can hold down. Hence the focus on reducing drag, and eating more pies.

My focus, being a shortarse, has been on getting power lower in the sail so as to be able to direct it where I want it, that is; forwards drive. HA sails tend to have a higher C of E so more lift your weight has to counter and less drive forward. This is one of the main reasons for loose head sails in my opinion. It drives the C of E lower, but you still must have enough power from the lower part of the sail.
If running close to the wind one should benefit from a flatter sail profile as the apparent wind over the sail is very much higher. Flatter profile = less drag. Deep off the wind a fuller profile helps produce the power needed from the lower apparent wind.
HA, flatter entry sails may work better in light wind though where the apparent wind is always well forward and much stronger than the true wind.

My vertical lift sail experiments have stalled because of the need to maintain mastfootpressure as a vital part sailing a modern board.
Starting from scratch with a kite and linked foils under ski boots.
How about putting wheels on a sailboard to reduce drag?. L.Ron Hubbard tried this , but maybe he was ahead of his time. (where dat tong in cheek emoticon?).

Does anyone know an easy way to tell the height of the centre of effort of a sail?
I guess you could fly a rigged sail like a wing and it will give you a rough idea but the eccentricity of the weight will skew it.

Maybe you could stand a rigged sail on a smooth surface or mount it on a skate sailor so the weight is supported but the foot is free to move and then find the height of the support required to stop the bottom sliding away. You might need to jury rig a line from the booms to the mast through the assumed c of e.

hmmmm... You could fix a line from the clew and run it right around the front of the mast and fix it to the clew on the other side so it passes through the assumed c of e. To hold it at the chosen height you could run a line along the mast from the mast tip to the boom front.....

If you want a sail with a low C of E, I suggest you look at the 08 & 09 Naish Stealths, they have the deepest draft under the boom you will ever see. Loaded up as the mast bends and the sail twists, the leech tightens up rather than going floppy, with the drive so low the whole sail is working producing power. The days of being lifted in a gust are long gone, its just solid drive forwards and sore thighs after a hard days sailing. Only peculiarity is you have to release downhaul tension to get the drive to move up in the sail and the board to fly more or move the mast foot a long way back. They are a whole new sailing experience but very effective in the hands of vertically challenged sailors!

Not being at the same technical understanding as the rest of you excuse what maybe a dumb question.
When the sail gets overpowered it spills out the top. Are we wanting to hold that power to get more drive into the sail?
Would a framed sail shape hold the power better? We create a frame on the front of the sail with the mast and at a point at the end of the boom. Would "Framing" the leach or foot provide any benefit to powering the sail more?

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Old Salty said...

Not being at the same technical understanding as the rest of you excuse what maybe a dumb question.
When the sail gets overpowered it spills out the top. Are we wanting to hold that power to get more drive into the sail?
Would a framed sail shape hold the power better? We create a frame on the front of the sail with the mast and at a point at the end of the boom. Would "Framing" the leach or foot provide any benefit to powering the sail more?

What you are suggesting is a recipe for uncontrollable overpowered sailing.
Sails used to be like that - with a tight leach and shape all the way up. Apart from being unstable (which was not the fault of the tight leach) they were powerful but tuggy, and had very poor range. Sail twist is a pretty elegant way of allowing a sail to vary its virtual size to suit the wind. It's elegant because it allows you to tune your sail with downhaul (more downhaul = smaller sail) and allows the sail to actively react in gusts to do the same thing automagically - all while keeping the centre of lift in pretty much the same place. As I understand it the flat floppy top panel just acts as a wind driven lever to open the sail below it. It may also do the service of smoothing out sail tip turbulence and lowering drag but I don't really know.

I remember reading a post years ago about the design of a sail's entry curve. The poster referred to the design of some ice sailer sails. They have to work at much higher speeds than windsurfing sails.

The idea was that for the first 30 cm or so the sail was flat, then it blended into a more or less conventional draft curve. This is meant to achieve better flow attachment. The air stream is somewhat perturbed right at the entry by the mast. The flat section gives the turbulent flow a chance to settle a bit before being further stressed by draft curve.

He was experimenting with achieving this by stiffening the first 30cm of the bottom 3 battens of a conventional camber induced sail. I never saw any subsequent report but the theory seems valid to me - not that I'd know.

I wonder about reducing drag in a sail. At first blush it would seem sensible to reduce all the surface roughness and any features that impinge on the air stream - things like batten pockets, batten bulges, seams, boom cut out, clew patches, that sort of thing.

However, because the flow over a sail is turbulent, I wonder if there would be any advantage to a hypothetical dead smooth sail ?