Question for Todd

[J. Dillon]

http://img231.imageshack.us/img231/7638/carriannegettingherjibwba7.jpg

Been thinking of making new robands for Carrianne for the 09 season. The existing ones allow the sail to be a few inches away from the mast. My reasoning at the time was to keep the sail back from the eddies created by the mast and improve air flow to the sail.

My question is if I make tighter or smaller robands bringing the sail closer to the mast would this improve sail performance or make no significant difference ?

JD

[Todd Bradshaw]

It's hard to say. I can see where moving the sail closer to the mast might allow less pressure leakage through the gap, increasing performance slightly, but it's true that it might also increase the amount of the luff area affected by the eddies behind the mast.

Whether or not this is bad for performance is another question. I must admit that I have worried much less about mast turbulence ever since I happened to see a cross-section of a dragonfly wing. It had it's bones, spars or whatever bug people call them right along the front edge, forming some big, sharp ridges at the leading edge, followed by the airfoil and looked like this.

http://webpages.charter.net/tbradshaw/!UNTITLE%20copy.jpg

How this might translate to a sail and mast, I really don't know, but I certainly wouldn't have any reservations about moving the sail closer to the mast to see what happens. A couple of temporary telltales, taped on, right at the luff or the aft side of the mast, might be interesting to watch. They might get sucked through the slot from leakage and it should be fairly obvious if they are constantly living in disturbed air.

Also keep in mind that, assuming the boat is pretty well set up and pretty well sailed, small rig adjustments which might instantly add as little as a half-knot increase in boat speed are fairly rare. You may not notice any speed or efficiency change at all. Our trimaran used to sail up in the 18-20 knot range with a good breeze. I put Technora radial sails on it and all manner of sail-shaping gizmos, yet once it was up to speed, the increases from carefully fine-tuning were far more often in the .1-.2 knot range than something you could really see or feel.

[J. Dillon]

Thanks Todd for your thoughtful response.:)

I think I'll keep it the same but experiment a little by cinching up on the roband by applying a cinch knot drawing in the sail to the mast a little and see what happens.

I'll have wait for next season. :(

JD

[paladin]

a "cuff" on the leading edge of the wing is a stall delay device that prevents wing stall at low airspeed...sometimes called "slats" that are spring loaded to stay at the normal wing form for speed, but when the aircraft is flying at certain angles and speeds, the cuffs slide forward to lower the apparent airspeed over the wing creating lift and lowering the operational stall speed.

[J. Dillon]

If I remember correctly the TBM--TBF Grumman Avenger of the USN during WW2 had such a "cuff"

JD

[Yeadon]

Personally, I'd like to see some more photos of Carianne underway. Lots more photos.

As far as the sail issue, I have a similar sail set-up with robands. I just try to get them loose enough to reef easily, yet tight enough to keep the sail set as well as possible. Not extremely scientific, but effective enough for a little boat like mine.