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Old 30-05-2017, 06:48   #1
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Join Date: Jun 2015
Boat: Sweden Yachts 34
Posts: 31
Aluminum vs Carbon Steel for Shroud Support / Shroud Design Load

Hi there!

My 34' sailboat has three shrouds on either side that are fixed with U-bolts on to a chainplate that is welded between two 80x3mm SHS beams that are again bolted to the bulkheads. I've removed both structures 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 for weight saving purposes. 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 off the U-bolts.

Is setting the boats weight as shroud load sound design practice, or is it excessive?

There is probably a reason why the yard chose carbon steel from the get go, but that reason doesn't necessarily have had to to with mechanical properties. Material cost, welding equipment.. boat is built 1978, so hard to know. What I do know is that I don't want to go lean on what is holding up my mast.. These members are also stiffening the hull as they connect the bulkheads and are thus experiencing compression/tension loads.

Please let me know if I'm off track here and should just sand and paint the old beams, and possibly reinforce the chainplates.
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Old 30-05-2017, 07:43   #2
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Join Date: Feb 2013
Location: Oregon
Boat: Seafarer36c
Posts: 5,563
Re: Aluminum vs Carbon Steel for Shroud Support / Shroud Design Load

There is a picture in Ferenc Mate's book that shows the chainplate support you are talking about, it doesn't look that heavy. Is yours different?
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