Sunday, March 14, 2021

A Real Sharpie

Our Panama skiff was very handy, and I had been reading Chapelle; thus, when we arrived in Alabama, with its big reservoirs on the Tennessee River, I decided to build a sharpie. Real sharpies have two masts and are over 20' long. I designed a sharpie 20 1/2' long by 6' wide (4' beam at the chine). In order to achieve considerable flare to the sides while keeping a fairly upright bow, I used conic projection. One apex below the bow and another amidships and lateral to make the transition front to aft. Again, I was able to calculate all lengths, offsets, and angles beforehand, thus not needing a strongback building form. However, handling sheathing panels over twenty feet long was challenging just from their size. The same mathematical techniques were used in shaping the sail panels as had been for the hull. To achieve balance between two sails and a pivoting centerboard, I used temporary mast steps, which could be adjusted, until after some trial runs; then I bonded them in place permanently. If you look at the shadow of the straight boom on each sail, you can see an almost perfect airfoil curvature, reflecting the shape of the sail. However, I made one big error. I was unable to find good information on the proper draft-to-chord ratio for sails, so I guessed. I used a draft-to-chord ratio of about 1/14 and later discovered that a 1/9 ratio would have been more appropriate. Thus, my sails were never able to develop the power that they should have, and the boat was not as fast as its potential.


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