chainplates: round holes vs square holes

[benjiwoodboat]

talked to three non-sailor machinists this week about my cracked SS chainplates. All of them said the same thing: you'd be better off with round holes. both chainplates had cracks at the first bolt. what are the thoughts on this? carriage bolts are nice on the lines and humans, but does it make sense to drill round holes and use hex head bolts? are there solutions im not aware of for this?

http://lh4.ggpht.com/BenEriksen/SAt6...jpg?imgmax=640

[Brian Duff]

square holes suck

make em round

your buddies are spot on

use hex heads bolts with no threaded portion bearing shear load and lightly round the upper corners of the hex head, then fully polish the bolts.


chainplates are seldom installed where lines or humans can contact them, normally ones with carriage bolts are on the outside of the hull, ie Westsail 32

[benjiwoodboat]

right, glad you agree. I have all external chainplates on my BCC, so looking for a suitable solution for hex heads.

[Quickstep192]

Could you get the clean look you want and use round holes by using button head socket cap bolts?

Here's a link to a picture. I'm not necessarily advocating buying them at Amazon, they just had the best picture. I've seen them often at car shows. They're very popular for use in antique car restoration when a polished surface is desired.

http://www.amazon.com/Stainless-Stee.../dp/B00137FGGW

[benjiwoodboat]

Quickstep, that's an interesting concept. Any thoughts from some other more experienced folks about that idea? Anyone have any personal experience on a boat with that style head?

[Brian Duff]

Sure, that bolt is used on many hans christian- as long as its a structural head and has an unthreaded shank long enough for the chainplate and hull, your fine- check out mcmaster.com for fasteners and next day delivery on the east coast.

[benjiwoodboat]

Thanks Brian -- will investigate this option.

P.S. I got a RFQ with you guys for my new rig...

[SV Papillon]

I just mounted a new bobstay tang and used carrige bolts as I didn't want a hex head and most pan head fastners tend to be threaded the whole length or hard to find otherwise. As long as they are long enough to get something on to tighten. The ones I put in were rather large as they were pre-existing in the stem 3/4 x 16 so the fit allowed them to be tightened without having to hold the shank.

Jake

[billknny]

Some of the chainplates on my boat are mounted with what are essentailly flat head machine screws in a countersink. What makes them unusual is that the head has an undercut on the taper, making for a very large head, looks good, has held up without a prolem for 30 years.

You can have great fun flipping through a McMaster-Carr catalog for just the right bolt to meet your needs...

[Sean MacLeod]

My only problem with flat-head fasteners, especially those with a wider head than normal, is that any torque applied to the head of the bolt (as tension on the chainplate would seem to do) gets more leverage than on a normal pan or hex head bolt causing the likelihood of breakage to go up. One of the nice thing about hex or pan head bolts is the head is not in line with the load, all it is doing is not allowing the end of the bolt to go through the plate.
The other thing about flat head bolts is that, because of the inclined plane in line with the load, the force exerted by the plate actually pushes out on the head of the bolt. It seems to me that with a flatter inclined plane the mechanical advantage would be even more.
Granted they do look VERY nice all polished up and aligned, and in the instance of a bow plate, where you have sail all around and bolts in at a number of angles, I'm sure the would work quite well.