Question for J.Dillon

[Dennis M]

JD, a couple of years ago you gave me the dimensions to Carrianne's mainsail (did you use Culler's 99sqft spritsail as a model from Skiffs & Schooners on p. 112?)and drew it to scale today. Why? I'm thinking of converting Parker's Small Ohio Sharpie to a spritsail ketch. Anyway, after drawing it up and doing the math to figure out the COE, I found I was looking at a 106sqft spritsail rather than a 100sqft as I expected. I used the following dimensions:

luff 11'1"
Foot 10'
Head 7'
leach 15'1"
Diag throat to clew: 13'5"

So, I have a couple of questions: Is my geometrical draftsmanship off?; My math?; or is the sail really a little larger than 100sqft?

Thanks

PS: I drew up Culler's sail from the drawing of his Good Little Skiff in S&S and it came out to 72sqft instead of 70. As my son would say, "What is up with that?" [img]redface.gif[/img]

[Todd Bradshaw]

The sail listed above is 101.25 sq. ft. If the luff is drawn plumb, it's C.E. is 88.72" above the level of the tack corner and 48.34" aft of the luff edge. Peak angle is about 47 degrees.

Of course, the actual area of the real sail will be slightly different. The leech would be hollowed about 2.75" to prevent flapping, creating a small decrease in area, the foot would likely be rounded about 3"-4", which adds a bit of area back and the luff would be rounded by an inch or so, adding a little sail area. The head would normally be cut straight. For design purposes though, it's 101.25 sq. ft. If you want one that's 100 sq. ft. on the nose, multiply the linear dimensions you already have by .99385.

P.S. Boat designers, even some of the really good ones, often make errors in sail plans - both measuring errors and sometimes some really serious sail construction errors. Many use rather quick and dirty measurement or estimating methods to generate sail area figures. Usually they're accurate enough for practical purposes, but they may not be exactly correct (though it won't likely affect the way the boat works and those guys have enough other stuff to worry about that does, so we cut them some slack). In general though, your usually much better off giving your sailmaker the dimensional plan and letting him figure out the construction details for the sail than you would be by trying to get him to follow a designer's version. Some plans are very basic - just the major measurements. Other designers like to design every square inch of the boat and also the sails, all the way down to sail corner patch shape, threadline orientation, fabric paneling, etc. Some of them shouldn't....

[ 03-27-2005, 03:13 AM: Message edited by: Todd Bradshaw ]

[Dennis M]

Thanks, Todd. I appreciate it.

Dennis

[Dennis M]

Todd, btw, if I want the sails to accommodate a sprit boom, what would be, in your estimation, the minimum (or perhaps optimal) angle of the run of the foot of the sail from tack to clew?

[J. Dillon]

I'm glad Todd answered this one for me.

When I designed and built the boat I just liked the looks of the sprit rig and used Culler's rig for his 18' boat as a model for mine. I gave the drawing to my sailmaker and let him have a go at it. Never figured exactly how much area it had.

Wish it had more in light air and less when it comes on to blow.

Any way, added a jib for the light air and take it in and or a reef when it blows.

As to the angle of the sprit I would suggest expermenting and see which location is the best.

JD