center of effort in multiple headsails

[dsuursoo]

i settled on a cutter(true cutter with a more centrally placed mast) for my adventure tri.

two headsails normally(three possibly, not sure yet)

using howard chappelle's book on design has me figuring out the center of effort between main and headsail, and main and staysail.

but how do i figure out how they interact to produce the true center?

the centers of effort don't line up nice and neat, like i hoped might happen.

i'd have to raise the peak of the staysail and lower the luff of the headsail to get the centers to line up, but i'd still have to figure the overall CE!

oy. doing it the 'right' way instead of the 'i'll fark about till it works great' way is some real skullsweat, i tell you.

but i want to minimize the trial and error stage this time, because my life's going to depend on this one.

i'll dig out a cad program and get a drawing up, if anyone likes, but not this instant.

[Gashmore]

Try this: Multiply the distance from the bow to the center of effort of each sail by it's area. Add them together and divide by the total sail area to get the horizontal CE. Do the same for the height to get the vertical CE.

[diwebb]

Hi,
the "center of effort" normally referred to is actually a misnomer and is actually the center of area. The actual forces on a sail are almost impossible to measure and the actual center of effort will vary with the set of the sail, wind strength etc. Most designers of multiple headsail rigs use the center of area of the foretriangle to calculate the center of effort of the sailplan. It does not seem to make a lot of difference what number of headsails are used provided that they fill most of the foretriangle. Overlaps on the multiple jibs do not seem to make too much difference as they are usually only used in lighter airs, and are reefed ,changed or furled in heavier airs.
My advice is to use the center of area of the fortriangle and not worry too much about trying to calculate each sail separately.

There is also a graphic way of calculating the center of effort of combined sails. First draw a line between the center of area of the two sails. Next draw a line at right angles to this line from the ca of the first sail of a length representing the area (say five inches represents 50 sq ft etc)of the second sail. Then on the opposite side draw a line at right angles from the ca of the second sail with a length representing the area of the first sail. Then draw a line between the outer ends of these two lines. Where this line crosses the original line between the two ca's is the center of area of the combined sails. Sounds a lot more complex than it really is but it is accurate and saves all of the brainwork!!!
David.