more on centerboards . . .
Posted 04 February 2012 - 06:31 AM
I ripped a couple scrap 2x6 boards up to 1 1/4" square pieces and glued them together with the grain close to vertical. When dry I scraped the glue globs and hit both sides a quick lick with a belt sander and cut the centerboard blade profile. Now for the fun part - shaping. In the past I used a drawknife, planes, and belt sander to get the rough shape done, but this time I had a plan.
I drew an NACA 0010 foil full scale, then using my parallel rule drew 1/8" spaced lines and sketched areas to be removed from the 1 1/4" x 12' rectangle. My router depth gauge takes two turns for 1/8" - perfect Then I mounted a 3/4" dado bit, the edge guide, set the edge guide for the first cut and off I go. Oh yeah, also marked a stop point so the centerboard part that is always in the case wouldn't get tapered. Numerous readjustments later, reset edge guide according to my foil sketch, and dial two turns to cut a groove 1/8" deeper, I have one done. Flip board and repeat. Even though lots of resets were involved the whole process took less than an hour. Would have been less if I hadn't paused for coffee sips.
Now I had a centerboard with 3/4" wide grooves separated narrow peaks which at original board thickness. My drawknife whacked those off in minutes, few more minutes with a plane and the board is ready for final shaping.
Posted 04 February 2012 - 03:11 PM
Posted 05 February 2012 - 06:05 AM
This duckworks article offers another variation on the router theme:
This is another version of the plywood rudder.
Yet another scheme I've considered, but have never seen attempted is to glue thinner laminations of wood into the foil shape. Basically, this process would have you calculate the width of thin (1/4" or less) stave sections along a series of points corresponding with your % of chord location, and pre-cut them to that width. Before you cut them, you put them in a stack and drill holes through the centerline of each, so when you are ready to assemble and glue up your stack, you just drop them onto a couple small dowel rods. The holes align the staves into an instant foil shape. Knock down the protruding shoulders and you are done. Never tried it, because again, it looks like a lot of work measuring and cutting all those staves to exact widths.
Yet another method is to simply staple or glue the shape of the foil onto the end of the rudder, use that as your guide and sight down the edge. Just chew it down to match the shape. Simple, but crude and potentially, not very accurate, depending upon the abilities and eyesight of the termite doing the chewing.
An improved version of that is to mark the foil shape you want to use on a thin piece of plywood and cutout the foil, tacking the foil to the end of the rudder as above, but also use the remainder of the piece you took the foil from as a guide to slide up and down your foil.
An improved version of that, and one Michael Storer uses in his plans is summarized here:
Scroll down until you see his description of the foils. Aside from the simplified foil shape (have never tried it, so can't vouch for it, other than to say if he is using it, it must work well), the template process described here has a lot going for it. Much more to this than meets the eye.
First, instead of a constantly curving foil shape as with the 00xx foils, this shape has a flat spot on it in the middle. That allows the board to remain flat while you are shaping it. It does not rock back and forth. Second, the template is sized to match the final width of the board, exactly. Before you start shaping, the board blank is planed to it's final desired thickness, exactly. The template has a centerline marked on it with curves for the foil, but the template extends beyond the centerline of the board exactly half the width of the board. So when shaping, the board is laying flat on a flat surface and the template, when the board is shaped properly, will also register and sit flat on the same surface the board rests upon. What this means is the foil is being shaped perfectly aligned with the centerline of the foil. It won't be off any.....no twist out of alignment foil shape as might be the case with foils shaped by eye. You just keep working the template to the board until it will slide up and down it perfectly.
I suspect something similar could be devised for the NACA 00xx foils as well and about as easy a way to for a home builder to accurately shape a board or rudder as I've run across.
Lastly, the thickness of the board seems to be a point of debate. As I understand it, the final thickness represents a tradeoff between lift and drag. The fatter and thicker the section, the more lift sailing to weather, but will also create more drag off the wind on a run. Does this still apply with a CB that you can easily lift? Why not go with a sturdy, fat, high lift board in the 12% to 15% of chord range and pull the stick on a reach or run?
Posted 05 February 2012 - 08:24 AM
That is version of a hot wire foam cutter. Neat. But I doubt a foam core CB or rudder is going to hold up. Only way I see to take advantage of something like this would be to take the foam core and use it as a model. Glass over it to tool a mold. Then wax up your molds and use it to make a thicker fiberglass rudder or CB. Layup each glass half separate, then maybe put some wood strips in as a core stiffener, and join the two haves. I suspect something like this is done for the vast majority of commercial built CB's and rudders on production boats. Depending on the thickness of the glass, they may well hold up. I suppose it would work, but seems like a lot of trouble to go to for a "one of project.
Posted 05 February 2012 - 09:42 AM
As for airfoil thickness? At the intended recreational speed of travel, anywhere near NACA 0010 is just dandy. And only because the board needs to have enough beef to support me if I need to stand on the end to right a capsize. Perfect foil? No need to get all nervous over that. Leading edge should be round, trailing edge not knife sharp but squared off maybe 1/8" - 1/4" thick. Main reason, don't want a knife edge down there if you're fumbling around with your legs in an attempt to stand on board to right a capsize. Front half of foil should be a smooth curve, no slabside flat spots, and aft section near a straight run to the trailing edge. No need to get overly picky, just grind it til it looks fair, glass it, coat it with whatever - I use graphite filled epoxy, rig it up and go.
Posted 05 February 2012 - 09:58 AM
Posted 05 February 2012 - 10:11 AM
Method #1: 1/64" aircraft plywood was wrapped around wing, yes it will bend around LE, and adhered with water base contact cement or epoxy. Epoxy requires cure time, so I would insert glued assembly back into the foams parts hot wired off the wing and weight it down. Done.
Method #2" Same as above but using wide pieces (I had 12" x 48" balsa in stock) of 1/16" balsa wood.
Method #3: The coolest. Requires a large dead smooth surface (Formica or glass). Wax surface, stretch out 3/4 oz fiberglass and saturate glass with epoxy resin, then set a sheet of blue foam on it and weight down. When cured, hot wire a n 1/8" sheet off the bottom and one has a thin sheet of blue foam with a fiberglass side to wrap around the wing foil.
A foam core centerboard needs to be strong enough to stand on (capsize remember). My plane wings had spars. Before sheathing I would slice the foil at the thick part and insert a full depth spar made out of sitka spruce or balsa and glue it back together. One could do that witha cb as well. Plus add a few strips of carbon fiber along with a few layers of fiberglass to beef it up. Result, a composite centerboard.