Appendices -- Useful Supplemental Information

SPRIT BOOMS AND THEIR KIN

My home waters, the Chesapeake Bay, were once populated by all sorts of working craft sporting sprit-boomed sails: flattie and deadrise skiffs, sharpies, log canoes. Not four-cornered spritsails, whose peak is held up by a sprit, but triangular sails with a boom angled down to the clew from somewhere up the luff.

Howard Chapelle’s American Small Sailing Craft has lots of examples. Contemporary designers like Phil Bolger, Ruell Parker, and Karl Stambaugh have drawn many little boats with sprit booms.

The forward end of sprit booms is usually hung in a snotter, a line rigged up to hold the butt of the sprit at the desired height on the mast, which can be adjusted to thrust the sprit aft, thus changing the draft of the sail.

The simple elegance of sprit-boomed geometry is pretty impressive. A sail with a sprit boom retains its shape no matter whether beating, reaching, or running, without recourse to gadgets for “tweaking.” Because of this geometry the clew can’t lift and spill wind -- that triangle of cloth below the boom serves as a vang. (Persuading the sail to vang itself is pretty neat. I wonder who did that first?) You can let the sheet fly and the sail will weathercock harmlessly while you discuss possibilities. With the butt of the boom higher, there’s less hazard to the crew during tacks and jibes. Finally, consider that because the sprit boom doesn’t have to resist bending -- it only functions in compression -- it can be considerably lighter than a normal boom. All adds up to elegant geometry.

There is one big disadvantage, though. The sail is deformed when the sprit is to leeward with the wind pressing the sail against it, which of course it will be at least half the time. On two-masted boats, it was traditional to ship one sprit to port and one to starboard to equalize the disadvantage.

Keen sailors hanker for the same good sail shape on both tacks. Enter the wishbone boom (wishboom?) Two curved sprits, joined to create a space between, where the sail can always assume its natural shape. As often happens, the cure of one fault introduces another: not only is the wishboom twice as heavy and bulky as a straight sprit, it must encircle the mast and be somehow suspended there in a bridle that keeps it from tipping out of horizontal. In larger craft, like the Freedom 40 my wife and I once cruised in, heavy wishbones can be daunting or even dangerous.

The most highly evolved and elegant morph of all, my favorite kind of small-craft boom, which has all the merits and none of the faults noted above, is the half wishboom. It can be almost as light as a straight sprit, and has less windage than a full wishbone. It has to be carefully made (laminated) to minimize compression bend (good place for some carbon fiber), since any bend in a wishboom means a shortening of the distance from tack to clew, and therefore a fuller sail in stronger winds, the reverse of what is wanted. The “draft” of a wishboom spar might be about 10% -- one foot in 10, located 45% aft of the forward end. Unfortunately, the half wishboom introduces its own fault: not being balanced by its mirror image on the other side of the sail, it wants to fall down on the job. Fortunately, there are cures for that, one of which is shown in the photo -- a rigid collar on the mast that holds the half-wishboom horizontal. The sail can no longer be flattened by a snotter, so a clew outhaul (red in the photo) is led forward along the outside face of the boom to a jam cleat. Alongside the outhaul is a leech reef pendant (white), sharing the same fairleads and jam cleat. A lanyard passing through a reinforced grommet in the leech passes under the front end of the boom, to lift the collar to the desired height when the sail is raised.



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