DIY sail design

Hi all,

I'm working on a 14' sailing canoe project and am getting to the point where I'm beginning to lay out the sail. I figure I'll use a poly tarp sail as a test-bed and it will become a pattern for a Dacron sail later on.

Now, what I'm really unsure of is how much camber to incorporate into the sail.
The sail has a silhouette similar to a junk sail or full-roach main except it's not what I would call high-aspect.

The hull was originally a 14' pirogue (flat bottom), now with with a deck, outriggers, leeboard and rudder/footpedals. The rig is a sprit/gaff with no boom. I have a cheek block at the masthead that allows me to haul the sprit vertically to give the mast and extra 39" of height. As it is, I have it laid out with the luff at 56" to the gaff jaws. The foot will be about 66" and the overall sail area about 26 square feet.

Is it practical to try and come up with a hybrid sail shape that will allow me to raise the sprit vertically to flatten the sail when I'm working upwind and lower it as a gaff off the wind ?

That was my motivation for not using a boom (though I did make one) - I figure it will give me more flexibility in sail shape using just a sheet and maybe a whisker pole if I need help to keep it from collapsing.

Any and all opinions are welcome

 

I've been watching this thread to see if any real sailors showed up: I'm not an expert but I can theorize, and since you haven't had any responses I'll put in my oar.

It sounds like you have a gunter rig (or possibly a batwing rig) and you are trying to simulate the mast bending that is used for tuning a bermuda rig. As I understand it, mast bend is usually done to increase sail fullness in light airs by increasing camber and hence the lift of the sail profile. The middle of the mast bends backwards; you'll be going the other way so the extra fullness may introduce vertical rather than horizontal camber. I don't think that will increase lift but directly downwind it may stop more air. Slackening the gaff may allow more sail twist; depends how you have attached the gaff. However, I don't see why it shouldn't work.

I suggest you post a sketch of what you plan to do. Then if you don't get more knowledgeable responses, try googling for gaff and gunter rig tuning to see what other folk are doing.

One caveat though: you are likely to get more information from folk with bigger boats. The vertical wind shear is at its greatest near the surface and will affect your boat far more than a larger one, so whatever they say about sail twisting may apply even more to your case.

Either way, good luck!

 

This may be found interesting.: http://outriggersailingcanoes.blogspot.com/2009/04/gaff-rig.html

 

This also may be useful in defining terminology.: http://en.wikipedia.org/wiki/Gunter

 

A different perspective

Beg to differ a little.

Actually, most sails have draft induced via a curved luff which fits into a (mostly) straight mast. As the mast bends backwards, the curve of the mast comes closer to the curve in the sail luff, reducing draft and flattening the sail. This is why 5o5s and the like all recommend cranking on the mast bend in higher winds. Flatter sails generate less lift, and can keep a boat right side up in breeze. Watching "Higher and Faster", a DVD on skiff sailing and boat tuning will open your eyes a lot. If you live near an active dinghy club, you may be able to borrow a copy. Look at photos of 5o5s and Contenders in breeze and on light wind days - the difference in mast bend is easily apparent.

Balanced lugs, gaff rigs as well as gunters generally don't respond to mast bend as an active control - the relatively short spars are not long enough in relation to the luff of the sail to be a factor. Luff tension, foot tension and vang (on a gunter) are the primary controls. Loose footed sails are less controllable than curved foot sails that fit into a boom track, where you can at least try to place curves where you want them.

Given the original poster's outline, I'd contribute that the more control you have over the airfoil you present, the better chance you have of performing upwind, and at any point of sail other than dead downwind. Boomless designs generally are very limited in the control department - there are good reasons why rigs have evolved to the current state of the art, and reasonable upwind performance is a major factor. The better you control the airfoil, the more chance you have of achieving success.

Many simple batwing style canoe sails are effective only downwind at best. The other side of the coin is that one of the absolute best upwind boats on the water is the International Canoe - optimized for upwind destruction of much higher performance designs.

It really depends on your objectives with the boat - if it is for relief from paddling fatigue, a simple sail will suffice - but if you are truly lazy and want to get upwind fast, I'd consider a little more development and experimentation.

Good Luck,

CutOnce