Aluminum vs carbon steel for supporting chainplates / shroud load calculation

Hi there!

My 34' sailboat has three shrouds on either side whose chainplates bolts on to a double 80x80x3 mm beam structure that again is bolted to the bulkheads. I've removed both beams from the boat as they are corroded due to water intrusion (I've since re-cored the deck).

I would have to sand and paint these beams prior to installation, but I'm thinking that maybe I could have them fabricated in aluminum,. It would be a pretty easy weld job for a shop, bsically two SHS side by side with spacers and a flange in either end.

Will aluminum be structurally sound for this application? It would of course depend on which alloy, but I would guess what the shop has lying around is 6060 extruded with a E modulus of 70 GPa.

I've done a smiple beam calculation, and putting the boats weight (5000 kg) as a center load on said dual 80x3mm SHS would deflect the beam 35 mm, which would mean too much lifting on the chainplates.

Is setting the boats weight as shroud load sound design practice, or is it excessive? I've read this on this forum at some point I believe..

 

The material use you, aluminium in this case, is not the issue. You can easily calculate how much "material" is required.
The issue is how to transfer the load from the chain plate into the structure that is supporting it with confidence.....is it first via a glued joint..or..???

 

The objective is to shed weight so if I'd have to beef it up it defeats the purpose. I might have been imprecise describing the shroud connection, I have U bolts that run through deck, that are bolted to the plate, which is welded in between the beams. I've sketched it up.

 

An aluminum beam with the same dimensions (height, width and thickness) as your steel beam will deflect 3 times more under the same load. If you want the same deflection you have to increase it's moment of inertia. If you have enough room you can use a beam with more height to achieve the same deflection. One thing I would modify is the attachment of the plate to the 2 beams. The actual arrangement is good for steel but I would not do it this way with aluminum. I'd rather use a length of channel between the beams. The welds would be vertical between the channel flanges and the beams side.

 

Poor arrangement for guaranteeing good weld penetration.

You will not get a good weld connection on the plate to the RHS. The distance apart also means the under side weld will be poor penetration at best. Would be better to have the plate (and wider) on the underside of the RHS...and then you can get a full pen weld along the length of the plate to the RHS. The inner weld to the side face of the RHS will be, at best, just a sealing run. But the plate will not act in tension, and all you need to do is allow a good FoS for the weld of the plate to the RHS. It also doe snot totally rely on the weld. Since if the weld fails, the plate must be pulled through the RHS as it is wider than the gap between the 2 RHSs..which is next to impossible if all glued in anyway. So this way it adds a double FoS if you like.

 

Thank you for your input, I will have to check how much space I have above the headliner.