My centerboard experiment

[mustangermatt]

In order to achieve the foil section I wanted in the centerboard, I am building it 1.4" thick instead of 1". This gives me a NACA 0010 foil which is right in the middle of what is recommended for slow moving foils (like sail boats).

I have also altered the construction considerably. I am using foam cored fiberglass with one strongback running through the widest part of the board. This will insure that the localized compression where the board rides against the trunk can't crush the structure, not provide any real stiffness laterally. That is the job of the fiberglass and core.

I have the wood parts done and the foam shaped and in place. After work tomorrow I will start laying up the glass skins. Hopefully I will also get some pictures up tomorrow.

At this point I am hoping this is worth the extra work :-)

[Gordy Hill]

I added lead weight to my centerboard and really liked the difference it made.

I'm not sure I see the advantage of using foam.

Somehow a boyant centerboard doesn't strike me as a good thing.

Gordy

One good turn usually gets most of the blankets.

[Scott Dufour]

Sounds like a cool technique; you're way beyond me with this stuff. Do you have plan to see how much of a difference it makes? Are there boats in the area you can compare to? (The science teacher in me rears its ugly head...)

[Bill Heil]

that board may not be strong enough, there is quite a side load on c'boards and your one structural member may not be enough. happy experimenting!!

[mustangermatt]

The bouyancy is minimal, somewhere in the neighborhood of 8 lbs, there isn't that much foam in there :-) I did toy with the idea of weighting it but decided that would affect beaching etc. If the bouyancy is noticable I will probably cut a hole and glass a slug of lead in there. None of the one designs I read about using foam core carbon fiber boards mentioned any problems, but...

The one "structural" member isn't supposed to keep the board from breaking. The only load it is intended to take is the crushing load right at the hull where the board meets the edge of the trunk. The fiberglass skins are the actual structural members the same way your hollow aluminium mast uses the wall as structure. The foam and wood simply keeps the skins/wall in position to bear the compression loads as the board tries to bend. I have seen masts constructed this way (foam cored fiberglass) with a two inch cross section support a 180lb man standing on it. The mast deflected about twenty degrees and could have bent a lot further without breaking. The biggest broblem I see structurally will be too "soft" a board that might spill water the way the unstayed masts spill air in a gust. But then again I could be wrong and the board may break. Then I will build one the Graham says to do it in the first place ;-)

Joe Nelson is building a core sound 20 about twenty miles north of me I think. I am hoping to compare my boats performance to his and get an idea if this was worth it. It has been a lot of extra work :-)

I got the first layer of 6 Oz glass on tonight, sorry but I am too tired to go back out to take pictures :-(

[Gordy Hill]

Matt,

I sail in very shallow water. After contacting the bottom, a weighted centerboard will swing back down a bit on it's own, giving, maybe 6 - 8 inches of exposure automatically as soon as there's enough depth.

A bouyant board stays up in the trunk even after you're in deeper water. I found myself continually "searching" for the bottom or realizing I'd been sailing in deeper water for some time with the board up when it could have been down.

Your expirement is very interesting. I'm not sure the differance will be enough to notice. Graham's design is very efficient. I'd continually outpoint very capable boats.

Gordy

[mustangermatt]

I don't have much shallow water around here. And when it does come up it does it fast. Big ugly rocks and all :shock:

But that is another point in favor of the weight.

[mustangermatt]

Got the last layer of glass on the skins. Standing in the middle of the board (190lbs) I got virtually no deflection. VERY tough :-) It is not as light as I originally thought it would be. It has three layers of six ounce glass each side and four on the leading and trailing edges. I think this thing is going to work out well.

[Roger Peterson]

Let's see some photos! How heavy did it turn out? I built mine from the plans and it turned out heavier than I thought it would as well. I was going to install a lead weight at the end to help sink the board but now that it's built, I don't think it's worth the effort.

Roger

[Gordy Hill]

Roger,

I used a three inch hole saw and cut a hole in both my centerboard and rudder blade. I taped the hole on one side and filled it with epoxy and lead shot, over filling a bit to account for shrinkage.

It made a noticeable difference.

I finished the centerboard and rudder blade with epoxy with graphite. I sanded it down to 400 grit and they looked like carbon fiber sections of the space shuttle.

Gordy

[Roger Peterson]

Hi Gordy,

I coated the inside of the centerboard trunk with graphite and epoxy over the fiberglass and "Boy Howdy" is it ever tough. It's bullet proof! I was planning to do the final coat on the centerboard and rudder with the graphite as well. I was so impressed with the graphite surface of the trunk, I might fill the glass weave of the boat bottom with the graphite/epoxy mix as well. It is a mess to sand if you don't connect the shop vac to the sander.

I forgot about using lead shot in epoxy for weighing down the centerboard. Much better than playing with molten lead... I'll give it a go and see what happens. I willing to take the easy way around the hot lead issue.

[mustangermatt]

I finally got around to posting a picture.

This is after the last coat of epoxy , but no paint yet.

I did another strength test today, I put the board on the edge of a bench and had my tenage daughter stand on one side while I stood on the other (had to offset it just a little :-)

In short it held a little over three hundred pounds without flexing. I am pretty comfortable that it won't break :-) I will be using the same technique for the rudder.

I am going to test sail the boat before I decide whether or not to add weight.

[attachment over 4 years old deleted by admin]