There's a good discussion of the theory of jibing centerboards on the Website, though without detailed advice on converting an existing board to a jibing one.
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Ed. Also check out the article on converting a non gybing to a gybing centerboard on the Montreal 505 fleet web site. The article is at URL: http://www3.sympatico.ca/dellandrea.molimard/505/jibing.htm. |
As always, the free advice below is guaranteed worth the price (but it's worked for us.) Others on the WorldList are urged to comment, criticize and add. (If there's a boat in your fleet with a good jibing board from one of the major builders, bribe the owner to take it out and give you a chance to look and measure before you start cutting yours.)
1. With the board full down, mark a horizontal line on each side, about 1/4"-1/2" below where the bottom of the boat is. (Boats vary in the amount of trunk length between the bottom and the board pin.) Remove the board, and take off all hardware. On the upper part (the part that goes in the trunk) mark a vertical line on each side parallel to the leading edge and about 2-1/2" forward of the trailing edge. Put masking tape aft of these vertical lines and below the horizontal lines. (Above and forward of these tapes is the first place you cut.) Mark a centerline on the forward edge of the board, above the horizontal line.
2. Measure the inside width of the trunk and make a rectangular jig consisting of two long and two short pieces of 1" wood, tacked or (preferably) screwed and glued together, so the jig is longer than the board is wide and exactly the same inner width as the trunk.
3. With a belt sander or plane, taper each side of the marked-off area forward of the vertical tape in a smooth, flat wedge such that it tapers from full width at the vertical line to about 1/8" or less at the forward edge (the glass will make it thicker.) Fair the tapered area into the rest of the board below the horizontal line. Be sure the wedge area is perfectly flat; a useful tool is a fairing board made of a new, coarse belt-sander belt cut apart and glued or stapled to a long, flat piece of 1" wood. (I glued one to each side of a 3"-wide board, one coarse and one medium. Belts are cheaper than centerboards.) Do not cut aft of the vertical line (don't grind the tape.)
Leave allowance for the glass you'll be putting on, and leave the surface slightly rough for good bonding.
4. Move the vertical tape to the forward side of the vertical line, and taper the 2-1/2"-wide aft area (thinner aft) exactly parallel to the forward taper on the opposite side. Here's where you use the trunk-width jig; cut the aft taper such that you can flop the jig back and forth and it'll touch the board simultaneously all along each aft wedge surface and the opposite forward wedge surface, pivoting at the angle formed by the vertical lines. Again, leave allowance for the glass you'll add.
5. Relieve the existing glass down to the wood for several inches below the horizontal line, in a broad taper, to give a bonding surface for the new glass you'll add. Provide a smooth transition to the uncut board below the jibing wedge, such that the board stops jibing at about 20-25 degrees up (others who know more may have a different opinion on that angle.) The trunk jig helps here too (we mounted an extension on it, like sides of a short, shallow trunk, with holes where the pivot pin would go so we could swing it around the pin and tell where the jibing became blocked.) Since the spot that blocks the jibing takes a lot of friction as the board comes up under load, make it smoothly rounded and leave room for a little extra glass there.
6. Glass both sides of the entire modified area, using epoxy resin (NOT polyester) and paying particular attention to the area near the horizontal line (which takes all the bending moment when you're righting the boat after the dreaded C-word has occurred.) Add some carbon fiber there if you have it. You'll want at least two layers of glass, maybe three in the high-stress area near the horizontal line. Have the weave of the cloth run horizontally/vertically, so the fibers will be in straight tension, and squeegee out as much excess resin as possible for the strongest possible laminate. (Use unidirectional cloth if you have it.) Be sure to wrap glass around the leading-edge side of the wedge and fair it in smoothly (can use bias strips here.)
7. Using the fairing board, finishing sanders, sanding blocks and the trunk jig, finish and smooth the entire area. Be sure all transitions are smooth, and that the angle at the vertical-line location (between the long forward and short aft tapered areas) is clean and straight. (The horizontal cross-section of the top of the board is a parallelogram, with the forward sides much longer.) Remember that coarse sandpaper is for removing material, while fine sandpaper is only for removing scratches from coarse sandpaper.
8. After a final thin coat of epoxy, sand, fair, fill, sand, paint, sand, paint, wet-sand, etc. as necessary. Put extra coats on the friction areas at the bottom of the jibing wedge, and let all paint dry very thoroughly. (Awlgrip primer's the favorite.) Any high spots will wear rapidly, so make the flat surfaces perfectly plane. Check all the way up and down with the trunk jig to be sure the finished shape will "flop" correctly with minimum clearance. Make sure the board hole is a slightly loose fit on the board pin; drill it out and bush it with a hardwood dowel, Teflon or other plastic rod if necessary. (Since there now will be axial as well as rotational movement, be sure to seal the inside of the hole against abrasion and water entry.)
9. Check the inside of the trunk to make sure there are no rough areas; fix them if so, removing the board gasket for access if necessary. The lower part, just above the bottom of the trunk, must be perfectly smooth and straight (perhaps with a slight outward taper in the lower 1/4" or so.) After waiting as long as you can stand for everything to cure and harden (leaving the board in the sun in the back yard for a week, turning it over each day, may speed the process) replace the hardware and reinstall your new board with a wicked grin. (Avoid compressing the trunk with a too-tight pin nut; use soft sealing gaskets instead.) Don't tell your fleet-mates what you did.
10. Be sure to keep the boat footing so the board can lift you to weather; you'll be aiming lower but going higher. Trying to pinch long-term with a jibing board is suicide, since the board loses lift; you have to keep the speed up. Be careful judging laylines, or you'll find yourself overstanding more often. To spare all your work from an untimely end, if you're trying to get the boat back upright after a mishap, put your feet on the INNER end of the board (don't bounce on the tip) and lean out horizontally, holding to a line from the gunwale or trap wire. Bouncing won't help; smooth and steady brings her up. (Yeah, you may fall in the water. Would you rather make a new board?)
Hope this (and the inevitable comments and/or corrections from the rest of the list) helps.
--Dave Eberhardt, USA 6570 "Carte Blanche" etc.
1. Work out the position of the jybing fulcrum, and mark it.
2. Measure the thickness of the case, with some clearance.
3. Work out the finished thickness of the leading and trailing edges of the head, and also the thickness of the opposite edge if you were to project from the fiinished thickness through the fulcrum. Make up four strips of Aluminium sheet of the appropriate thickness, plus, with holes in them to screw to the front and trailing edge parallel to the fulcrum line. You will need some packers. Add an offset of say 10mm from the edge of the sheet to the holes.
4. Put a set of long rails on the bottom of your router, and fit a big side and face cutter bit. Set the bit the offset amount (10mm) from the sliding surface of the rails.
5. Turn router on, run the rails over the strips screwed to the leading and trailing edges. You have to swap the LE and TE strips at least twice, depending on how you set them up.
6. The setup is slow, but the cutting is fast and accurate.
Nigel.
AUS7982