Aluminum Boat/Aluminum Chainplates - What bolts/bushings to use?

[Ginkgo]

New to me boat needs new chainplates. The boat is aluminum and my new chainplates will be as well. They will be professionally welded on, and then bolted with 1/2" bolts. What type of bolts should I use? Galvanized? Stainless Steel? Some other kind? Do I need bushings to electrically isolate the bolts from the aluminum hull? Thanks!

[ccannan]

Use 316 stainless bolts and nuts. A bushing is probably not necessary but you could add one. Use nylon washers.

[kmacdonald]

Fatigue in aluminum is fairly unpredictable so you really need to over design them if made of aluminum. If they are welded on there is no need to bolt them also.

[a64pilot]

I wouldn’t bolt either, but if I did, maybe Monel?
I don’t like what happens with SS and aluminum over time myself, they seem to become one.

[Fuss]

Originally Posted by Ginkgo

New to me boat needs new chainplates. The boat is aluminum and my new chainplates will be as well. They will be professionally welded on, and then bolted with 1/2" bolts. What type of bolts should I use? Galvanized? Stainless Steel? Some other kind? Do I need bushings to electrically isolate the bolts from the aluminum hull? Thanks!

You don't say what size/type of boat.
Why does it need new chainplates, corrosion?

For aluminium boats with aluminium chain plates, you need stainless bushings and you need to use the right bolts as described by the rigger.
The bushings need durolac or tef gel to isolate them from the aluminium and they need to be a press fit.
Then contact between the bolt and the pressed in stainless fitting does not matter, what is important is the pressed fit stainless bushing into the chainplate with lots of durolac. This then will last years as it is permanent. Thats how all aluminium boats have the chainplate to rigging connection.

Its a professional job and needs to be done together with a good rigger as well.

Somehow , if you ask this question here then I think the guy welding the chain plates might not be experienced enough as he should know this.

Also on aluminium boats, all stainless must be isolated where it is in contact with aluminium.

[Ginkgo]

I was going to have them bolted and welded only because the current plates are bolted and welded. Thanks for the tip!

[Ginkgo]

Sorry, new to posting in forums. Boat is 34 foot Thomas Colvin Saugeen witch gaff schooner from 1974. Old chainplates are 44 years old, and look fine, but I will be taking the boat offshore and would like to play it safe. As for the rigger, there is no rigger yet. I am trying to educate myself so that I know the variety of possible viewpoints that exist on the subject and/or the right answer if there is a single right answer. I have found that it helps to at least know what questions to ask, and I am learning that right now. Thanks!

[foufou]

You might consider aluminum bolts. No corrosion issues then. The do exist. In another life I changed out all the bolts on a motorcycle to save weight

[Fuss]

Originally Posted by Ginkgo

I was going to have them bolted and welded only because the current plates are bolted and welded. Thanks for the tip!

Maybe better to check why this was done in the past....it does not really make sense to me to do both as once welded then the bolts do nothing. Bolts seem to me to be an added complication. Normally if bolted then the whole mounting design would be completely different to a welded chainplate concept.

I think a 2nd opinion would be good at this point.

[Benz]

Don't bother bolting. Make sure your welder is good. Make sure the surface you're welding onto is good. Make sure you don't damage the hull as you remove the old weldment.
PS. If you're worried about the old aluminum chainplates, aren't you equally worried about the old aluminum hull?

[estarzinger]

A couple things:

typically, properly done

(1) the aluminum chain plates are 'Hard anodized' (after shaping andd drilling but ofc before welding to hull) - like good blocks are - prevents corrosion and pitting - makes them look better and last longer and is not hard to do.

(2) the chainplates are just welded, not also bolted. The anodizing is ground off just for the area of the weld. The load from the chainplate needs to be spread, preferably into the hull structure (the frames) and not point loaded to one spot on the skin.

(3) Where the stays attach typically there is a stainless bushing hydraulically pressed into the hole.

[Panope]

Panope is a sister ship to the OP's boat, hull built in the same yard.

When she was schooner rigged (prior to my modifying to sloop), there where eight chainplates (four per side) spread along the gunwhale. This low aspect rig, combined with a somewhat tender hull, means the rig loads are far lighter than comparable sized modern boats. We used 1/4" galvanized wire. One wire to each plate.

The point of mentioning the above, is that it trivial to size chainplates large enough to eliminate concerns about strength, fatigue and longevity.

Also, these boats are built with a somewhat "industrial" flair. Therefore, oversized, chunky looking components will fit right in

As mentioned above, forget about the bolts. Just weld.

The stainless bushing (where the shroud attaches) is a nice touch. An alternate to the pressed "interference fit" that estarzinger mentioned, is to first chamfer the hole at both ends, then secure a loose fitting bushing by battering (or clamping) cone shaped mandrels, thus expanding the lips of the bushing into the chamfers.

That said, when I built new chainplates, I did not bother with a bushing. I use industrial, galvanized turnbuckles (the ones with crude, lumpy pins) in an oversize chainplate hole. My new chainplates (just one per side) are now 1 inch thick by 6 inches wide (where they are welded). This crazy big for a 7 ton boat. They will last forever.

Steve

[rpjn59]

One question and one statement. Does the interference fit of pressed in SST fitting prevent galvanic corrosion? I have never heard of that. A good welder does not necessarily have any thing to do with rigging as someone has suggested, although a little knowledge of aligning the plate angle would be very helpful.

[jpstevens]

Wow, if it were me, I'd surely begin a dialogue with Panope!