Undersize pins in chainplates

[kayakerChuck]

I messed up-

I ordered new standing rigging from a local shop. I brought them my old stays, which fit well, to be duplicated.

Mostly, they accurately reproduced what I brought them. What I brought them was WRONG. I never checked the hole sizes in my chainplates. I now know that they are all 7/16".

The rigging I brought them, and the rigging they made for me, has 3/8" pins for the uppers, and one set of the lowers. One set of lowers had 7/16" pins.

The old ones have been in service for some unknown time- probably more than 20 years. The chainplates have a bit of a boss around the hole, and are showing no wear- the holes are still round. It seems the 3/8" pins are more than strong enough, and O'day even used 3/8" U-bolts for chainplates some years.

How concerned should I be? I'm thinking of just making some bushings to adjust the fit and calling it good enough for a 40 year old cruiser.

[tkeithlu]

I think it just means that you need to do scheduled inspections, and not just "That looks OK." Pull the pins and check for deformation, egging of the holes, and cracks.

[kayakerChuck]

That works for me.

I'm on the Great Lakes, and store her with the mast down.

[Brioche]

It might be good to consider the overall limits on the shrouds in question. Looking at a random chart for stainless shackles, a ⅜ inch pin has a safe working load of 1700 pounds, compared to 2500 for a 7/16 inch pin. Your pins are probably shorter than in a shackle, and would probably have higher safe loads. But what are the limits on the cables and swages? If they are less than the limits for the pins, then your main issue is one of wear, and a bushing would help a lot. On the other hand, if the other parts of the shrouds are much stronger than the pins, then maybe you should be concerned about the strength of the pins. This doesn't give you a final answer, but it might help at least understanding the issues.

[kayakerChuck]

I did some poking around, and it seems that the O'Day 30, 32, and 34 share most of their rigging components. The 30 and the 34 both use 1/4" wire and 7/16" pins. In rough numbers, the 34 has about 25% more sail area, so let's say 25% more force.

Math, math, pie, math... the 3/8" pin has about 25% less cross section than the 7/16.

The 3/8" pin will be the weakest part of the system, but it's looking like all the other parts of the system are significantly oversized for the job, and the smaller pins look like they still give me about the same safety margin as OEM on the 34'.

[Jim Cate]

If they survived for >20 years I reckon they are quite adequate. A bushing to match the sizes up will be a good improvement, but I'd not be worrying about "improving" upon the proven.

Jim

[Locquatious]

Pins that are too small often lead to two types of weaknesses:

The pin acts as a point load on the chainplate hole and the load concentration may lead to stress cracking.

Often the "jaw" for rigging that uses larger pin holes is often wider than the chainplate is thick. This puts the pin more in a bending load rather than a shear load. As a general rule-of -thumb, a smaller diameter cross-section bends more based upon the radius cubed. So a 3/8" pin would bend ~40% more than a 7/16" pin. As a general rule-of-thumb bending also increases as the inside width of the jaw cubed. So the bending load on the smaller pin with a greater jaw width is significantly greater.

If possible, drill out the chainplate hole, assuming there is enough material surrounding the hole. Also squeeze in the jaw to better fit the chainplate width.

[Moontide]

My concern would be about the point loading on the holes in the tangs, not the strength of the pin. Design is to spread the load on the whole surface of the hole, not just the top point.


A possible alternative would be to get a metal bushing to reduce the hole from 7/16 to the 3/8 pin

[Bowdrie]

Short of using a lathe, getting a sleeve/bushing to go from 3/8ths to 7/16ths
is a tall order.
that's a wall thickness of only 1/32".
Places that do electric motor rebuilding might have sleeves like that, (used for building up pitted/damaged armature shafts,) but are not of corrosion resistant metal and are usually a press fit.
If you think it's worthwhile you can use a sleeve bushing like this:
https://www.mcmaster.com/6391k876/
Don't let the "Max radial load" scare you, that's for a rotating shaft.
At that price you could have a bucket full and change them each year.
Don't go drilling on rigging parts, and don't try to squeeze forged rigging toggles to a smaller throat size.

[kayakerChuck]

Bowdrie-

That's exactly what I did- the McMaster bushings are about perfect. I bought about twice as many as I need. It's cheap insurance against dropping them.

All-
Thanks for the help!!

[sailcrazy]

FWIW, we found a similar issue when we redid our standing rigging in Phuket some 10 years ago, ie, smaller pins than holes in the chain plates. When I removed the chainplates, to check for corrosion, I found elongated holes and stretched chainplates. We had new chainplates made, with the proper amount of metal over and around the holes, drilled the proper sized holes in them, and reinstalled the rig with the correct size pins. I think we were very close to losing our rig! And after seeing a couple of boats struggle back into Phuket with broken masts.....I'm glad we found it before we broke the rig. I would take no chances....the stresses when you're hard on the wind in a big seaway are significant. I'd redo the affected chainplates.....correctly! Bushings surely will help, but it's not the proper fix!