Nylon sheave

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Nylon sheave

Postby ManInBlack on Tue Jun 29, 2021 9:52 am

I have been working on a PocketShip plans build here in Thailand, and I'm fairly comfortable and confident in the woodwork and the fiberglass/resin work. But I'm concerned about my ability here to source the additional sailing hardware. I'm talking to a few large sailing equipment and parts suppliers here and they aren't so forthcoming about being able to source the various namebrand hardware listed in the build manual and the various kits supplied by CLC. They aren't even offering alternatives.

So I've mostly been building from the manual and hoping I could figure out how to source what I need to before I get to that point.

I've finally stumbled into a sourcing problem and I wanted to ask for advice. On page 108, there is a nylon sheave for the centerboard. (Picture attached.) The text says "The sheave should be nylon, marine grade, 2" in diameter, and between 1/2" and 5/8" wide." I have a local machinist that will machine a piece of nylon to these specs for me for about $10. The biggest question I have is about the words "marine grade". Is there such thing as "marine grade nylon"? I've provided a few pictures of the sheave from other builder blogs to the local machinist, and he seems confident he can build exactly that. Is this an acceptable way to proceed? Are there concerns about this?
aIf I need to change this sheave later to something more standard in the future, will I just be able to unscrew it and replace it, or is it somehow entombed in the centerboard trunk in such a way that it had better last forever? (The pictures I've seen show pockets cut out to make it lighter, or probably just use less material and make it cheaper, but if I am machining it, should I just make it solid except for the screw hole and the slot for the rope at the end?)

Any suggestions would be greatly appreciated.
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Re: Nylon sheave

Postby [email protected] on Tue Jun 29, 2021 10:08 am

The Schaefer sheave has hollowed-out portions because it is molded. Molded plastic items can't have big variations in their thickness.
I bet a solid sheave made on a lathe would work just fine. Most marine items have to be resistant to UV radiation from sunlight, and that isn't an issue here.

For my build, I made 3-d printed shims to go between the sheave and the trunk walls, instead of using the plywood bits from the kit.
They are round, so they didn't have to be glued in place. In your case, maybe you can get the pulley made almost as wide as the trunk slot, so that no separate shims are needed.
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Re: Nylon sheave

Postby TassiePete on Fri Jul 09, 2021 4:14 pm

It is just the sheave for the centerboard and has very little load on it. Almost anything will do here, unless it rusts. Nylon, stainless steel, bronze, ...
Even the size is not critical, as long it fits and the line can't jam. You might be able to 'liberate' a sheave from an old block.
Alternatively, you can do without it altogether. This is what I did, and it works very well and doesn't cost anything.
The 3 mm spectra line is fixed to the underside of the cover, so that the cover is the 'handle' to pull the center-board up.
The line has knots for up, half-way and full-down.

Other than that, don't worry too much about substituting bits and pieces, e.g. any good blocks of the right size will be fine, whether they are Ronstan, Harken, Seldon, ... Most hardware is available by other manufacturers, and if the specs are close, it will be fine.

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Re: Nylon sheave

Postby ManInBlack on Tue Jul 13, 2021 11:53 am

Thanks so much for all the advice. We want ahead and had the local machinist build us one on his lathe.
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