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Thom V
Administrator
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Posted: Wed Mar 29th, 2006 02:30 pm |
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"The swing‑up centerboard has one great advantage: it can ride up and over shoals and obstacles on the bottom. Its main disadvantage is the pin, which calls for holes in the board and trunk, sometimes allowing water to go where it is not wanted, and it is sometimes a source of wear and corrosion. The daggerboard has no pin and allows a much lighter, and simpler, plywood trunk, but it lacks the swing‑up feature, which may be a necessity for those who sail in shoal waters. That will include most of us, on occasion.
After spending too much time wrestling with the problem I have finally devised an arrangement without the pin, that will have the advantages of the pinned board and the simplicity of the daggerboard. It has advantages over both. There is no loose board to stow and no slot to let water inside, sometimes on the seat of the pants. It has no metal parts, and the board cannot escape, but is easily removed, if necessary.
The board is positioned by two lanyards, the up‑lanyard and the down‑lanyard. These are attached to the board and positioned so as, respectively, to raise the board and hold it in the raised position, and to lower the board and hold t anywhere from full up to full down. The lanyards are 3/16" dacron braid. They are led inside the boat, handy to the crew. A piece of 1/2" dowel hitched to each lanyard serves as a handle and a stop to limit extension. The up‑lanyard belays on a jam cleat, and the down‑lanyard is attached to a bungee which holds the board down while allowing up swing if it strikes.
The board is made from a pine board, 3/4" thick, No. 2 (cheaper, but not cheap) or for larger boards, fir stair tread stock might be used. It is high grade lumber about an inch thick. The board is sheathed with fiberglass cloth and epoxy for strength, and resistance to splitting and water absorption.
The top of the board is cut to two circular arcs, with the radius equal to about one third board width on the trailing edge corner and about two thirds on the leading edge corner. The moment arm for the up lanyard is about equal to the forward corner radius, giving an advantage for a given board width. The board rotates (more or less) about its center, which I call "The Phantom Pivot". This is about the center of the part of the board that stays in the trunk. Two corks (from wine bottle) in 3/4" holes near the center protrude enough to prevent rattling and provide some slight resistance to board movement.
The trunk is a closed box except for the bottom slot, with only two small holes in the top for the lanyards. The holes will be shaped and treated so as to minimize abrasion of the lanyards. A plastic thru‑deck fairlead, such as Schaefer 76‑02 might be used. Elimination of the pin saves considerable weight in structure, heavy timbers ordinarily used are not required. It is bonded and taped to the bottom plank. It makes an easy retrofit, particularly on plywood boats with flat bottoms.
The board is made from common pine lumber, 1"nominal, actually about 3/4" net, or for larger boards, fir stair tread stock might be used. It is high grade lumber about an inch thick. Bond for width if necessary. That board is sheathed with FG cloth and epoxy, hard to do in one operation, but easy in two. Use about ten ounce cloth cut on the bias. It will conform to the shape better and there will be no raveling at the edges. Clamp the edge of the forward side in the vise. Sheathe the after side about 3/5 of the width, then sand the edges and repeat for the forward side. Use about 10 oz. cloth cut on the bias. Add cloth doublers about 6" diameter at the lanyard holes. Seal the wood inside of all of the holes with epoxy. The 3/16"braid lanyards go thru 2 holes in the trunk top and into the holes in the board, where they end in a figure 8 knot. Corks in holes near the center of rotation bind lightly in the trunk, causing only slight restraint of rotation.
The trunk is assembled before installation. Determine space available as shown. The "BOX" (marked with small triangles), allows 1 1/4" at top and ends for attachment to frames or thwarts. The side panels are 1/4" plywood (3/8" is OK). Face grain should be vertical. if trunk size permits. The insides of panels and top and end pieces are sheathed with FRP before assembly. Parts should extend at least 1 3/4" outside the box dimensions and be cut to 1 1/2" after assembly, reduced to 1 1/4" at thwarts and frames. The 3/4" x 3" top side pieces go on after the ends and top are trimmed. Note how the bottom extends thru the bottom plank for trimming, after installation. A good sized fillet of epoxy and filler and several layers of tape and epoxy inside attach the trunk to the bottom. The thwarts are bonded and screwed to the trunk. After taping to the inside bottom it is made flush on the outside. After the outside of the boat is sheathed, the corners are rounded and sheathed with tape extending about 1" or more inside and about 2" on the bottom. The inside trunk width and the slot thru the bottom plank are sized to give the board about 1/16" to 1/8" clearance laterally. The board should move freely with minimum clearance and no binding except for the corks. A little extra clearance is easily corrected with more sheathing on the board, but binding is a serious problem."Attachment: Dobler Centerboard .gif (Downloaded 82 times)
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