Otimun draft distribution

Is there an optimum draft for a high roach mainsail and a fractional rig?

How much draft and where?

 

That would depend on windspeed, amongst MANY other things.....

 

...Not to mention what you are optimizing for.

 

Yes, too little information.

It is actually for a 1/20 scale version of the Luna Rossa.

The mainsail is 1.5 mts tall, 48.5 cm at the foot.

Guess I thinking on a set of sail optimized for little wind, sometimes gusty, force 1 or 2 kind of thing.

Gues draft possition should be at or around 50% of the chord... but thats an non-educated guess...

Kind of shooting in the dark...

 

... and this could be more useful than that TP-52 thread....

 

In general you want the entry to be round, and the exit flat, so I think the draft should be in the 25% to 40% range. This might change, though, for mainsails flown in combination with large genoas, esp. on masthead rigs.

It's important that the entry angle matches the apparent wind. See
http://www.sabresails.com/article1-2.htm
http://www.sabresails.com/article1-4.htm

Tender vessels sailing in heavy winds benefit from flatter sails, but also sails that can be opened at the leach and allowed to twist some.

 

It has been repeatedly demonstrated with model sails that vertical camber distribution is considerably more important than horizontal distribution. In general depth of camber percentage of chord should increase smoothly toward the head of the sail. Emphasis on the word; smoothly.

Small sails of the RC model variety demand great care in construction. Precise sail blocks are normally used for assembly. Broad seam tolerance of less than half a millimeter is essential. Guessing at mast bend will give you a clue about luff curve allowances. If a fore stay sail is used the luff curve will be negative and in accordance with your best guess at jib stay sag. Competitive model sails are assembled with sail blocks that are segments of a true circle. In theory, that will put the max camber at 50%, but it seldom works out that way. Most of the model sailors believe that 40% is about right for horizontal distribution of max camber. The circular block segments seem to produce sails that work out that way.

Sorry, didn't mean to make a big deal about a basic question.