8 replies to this topic

[Scott Dufour]

Hey guys,

I've started on the Princess Sharpie 26 (Hull #3). Just the fiddly bits like the centerboard and rudder. The centerboard pivot calls for the ol' epoxy bushing trick, and I've already poured it. But now I'm thinking that I should have used something other than straight epoxy- maybe something thickened with silica or milled glass for strength. That centerboard is about 5 feet long and the tip's weighted with 35lbs of lead. Any advice if I should drill out the epoxy and add something thickened? Or is that over-engineering?

[tom151]

Hey guys,

I've started on the Princess Sharpie 26 (Hull #3). Just the fiddly bits like the centerboard and rudder. The centerboard pivot calls for the ol' epoxy bushing trick, and I've already poured it. But now I'm thinking that I should have used something other than straight epoxy- maybe something thickened with silica or milled glass for strength. That centerboard is about 5 feet long and the tip's weighted with 35lbs of lead. Any advice if I should drill out the epoxy and add something thickened? Or is that over-engineering?

Definitely drill it out.
Any of the recommended structural extenders will make the difference.
Fumed silica, milled fibers, Cabosil, even wood flour if nothing else.

[Ray Frechette Jr]

You are looking at 50 lbs of lead at tip. And the pin is just a shade under 1/2 diameter.

As Graham only speccs resin bush and has specced beefier pin and bushing diameter I suspect it is fine as is.

The lead is only to make the board negatively bouyant once immersed.

Bigger point is to have that board resting on a support on the trailer going down the road rather than being supported by blocks and rope.

[Tom Lathrop]

Definitely drill it out.
Any of the recommended structural extenders will make the difference.
Fumed silica, milled fibers, Cabosil, even wood flour if nothing else.

Plain epoxy with cabosil as a thickener (it is not a structural additive)has been used very well in loaded pivot points for years with no problem. Milled fibers would strengthen the bushing but might increase friction and wear on the pin. Fumed silica and cabosil are the same thing.

[Scott Dufour]

So one Tom says drill it out, one Tom says maybe, and Ray says it's fine as it is. (And it is 50 lbs, not 35. I don't know how I got 35 in my head.) Ray, are you thinking that there's just not that much force on that pin because when the board's up, it's just got the upper half of the wood weight, and when it's down, it's in the water with neutral buoyancy?

A related question falls out of this: silica thickened epoxy certainly seems harder, but I've also found it very brittle. I'm guessing a bushing would rather be elastic than brittle...

[hokeyhydro]

Yes, fumed silica AKA cabosil resin mix is rather hard and somewhat brittle. Adding flock (cotton or ?) to the mix makes it more shock resistant, in my experience, which was building outboard race boats. We called the flock "dryer lint" of "bellybutton lint." For a bushing I might be tempted to add graphite. powder to the mix.

[Ray Frechette Jr]

I usually trust the Designer. He generally gets it right in the case of this designer.

Everytime I look to improve on his design I come away feeling his way is better.

If you have the epoxy bushing poured I would leave well enough alone rather than drilling it out and redoing it with a different mix.

Yep when board is down, it will be down in the water and the bouyancy of the board will offset much of the weight.

When board is retracted and on trailer I would want some sort of tray to rest board on under trailer so you don't have continual shock with every bump in the road on the block and tackle and uphaul line. So stress on the pivot pin will be moot at that point.

[Joe Anderson]

I looked through my Gougeon Brothers book. I did not see any reference to centerboard bushings, but they have a long section on techniques for mounting fasteners. They over drill the the hole and fill with epoxy then redrill leaving a 1/8 to 1/4 collar. Another technique is to embed the bolt in epoxy.

Sometimes they are using unthickend epoxy sometimes high density filler. In one paragraph they seem to be using the filler only to make installation easier on non horizontal surfaces.

They are testing the fasteners to failure both and shear and tension.

I did not see any mention of other additives.

Probably more important than additives is that you get an accurate, well mixed batch of epoxy.

[Hirilonde]

The only load on the centerboard bushing is compression, the trunk (case) handles all the other loads. The only other considerations I can think of is as mentioned before: wear/friction and impact. I don't see how any filler can help any and I can see how it might hinder (friction wear and cracking due to impact as the filler making it brittle).