alloy chainplates

[gramos]

i have had leaks from my cats chainplates , which are 8mm steel ,bolted through the aft bulkheads .on removal of the panelling i found they are positively agricultural ! big ,heavy and slight rusting . i want to remake these in 10mm aluminium , welded and braced. 38 ft cat, alloy 13m mast B&R rig , any problems i should look out for ?

[lazeyjack]

I use alloy chainplates on my alloy boats
i usually bolt theem to a structure, the reason being, I do not want to weld round the plate where it exits deck
You are best to anodise them,, and insert an ss eye into the plate
Need your wire size and then can give you thickness and distance from top of plate to hole centre
for example,
3/8 wire, width of plate 55mm, thickness 19mm, pin 5/8in, distance centre pin to top of plate 38mm

[northerncat]

can you just use normal flat bar for the chain plates or is there a marine flat bar available?
sean

[lazeyjack]

Originally Posted by northerncat

can you just use normal flat bar for the chain plates or is there a marine flat bar available?
sean

no (normal flat ) is usually 6063, you need 5083 cut from plate, or 6061 extruded t6

[alan white]

I would not worry about slight rusting. Remove them and polish them bright (like they weren't to begin with). Polished stainless doesn't rust as readily as raw unfinished stainless.
Yes, aluminum is lighter---- and weaker! All things being equal, they must now be larger sectioned to equal the stainless' strength. You can calculate how much by knowing the tensile capacity of both metals and increase the sectional area by the same ratio as the difference.
Then too, you will bolt them with stainless--- correct? Mixing metals in hidden places is not a great idea.
My advice is to accept that any and all efforts involving lightening relatively small items such as chainplates will result in throwing good money away.
Building new, you aren't throwing away perfectly good parts. I liken this to throwing out your engine to save 50 lbs.
A cat 38 ft long might weigh 6000-7000 lbs. The change in metals might amount to 12 lbs total. That's about 1/500 decrease in weight, or .2%.
There's great security in keeping what has not come close to failing in previous use. Polish and reinstall is the practical approach.

Alan

[masalai]

If you must, make them out of glass and epoxy to your hull on the inside through the deck and reseal the glass to the deck too.

alternately
Scrubbing (angle-grinder) off the outside gelcoat could make an "unattractive patch but less intrusive on the integrity of the hull/deck join/seal... Get a reputed fibreglass worker to advise or do the job...