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Old 17-12-2012, 11:42   #1
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Aluminum floor beams in a steel hull?

Aside from the galvanic corrosion potential if a joint were to get repeatedly wet, can anyone see a problem with this? I'm trying to carve as much weight out of Sabre Dance as I can, so this one area I'm looking at.

The current beams are made of half rotted spruce 2x4s, which I was going to replace with 1.5x1.5 or 1x1 steel angle, 1/8 thick. Now I'm thinking perhaps larger sections of aluminum might be better for support.

I plan on epoxy covering the steel and the aluminum ends before joining them with bolts, and they are not likely to get wet very much.

Comments? Concerns?
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Old 17-12-2012, 11:49   #2
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The general rule for selecting aluminum or steel in a structure is:

Strength with light weight: aluminum.
Stiffness with low cost: steel.

I would think the floors in a steel boat would be steel unless weight is a serious issue. Wood is a good choice too.
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Old 17-12-2012, 11:50   #3
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Re: Aluminum floor beams in a steel hull?

There is just too much galvanic to risk it. It could turn to powder pretty fast.... especially in that damp environment. I dont know the design, but aluminum has roughly 1/3 the ability to resist flexing than steel also. The weight is low in the boat... find other places to loose weight!
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Old 17-12-2012, 11:51   #4
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Re: Aluminum floor beams in a steel hull?

I would not do it. The galvanic problems will be very difficult to eliminate. There will be condensation and dampness with the salt air and this enough moisture to create a galvanic cell.
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Old 17-12-2012, 11:51   #5
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I meant to add that floors should be stiff. Strength is rarely an issue.
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Old 17-12-2012, 11:59   #6
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Re: Aluminum floor beams in a steel hull?

They don't have to be wet to have galvanic corrosions - just being in contact with each other will do it.

Easiest way to stop the problem is to buy some sheets of very thin phenolic - around 0.0625" thick (1/16") and cut out pieces so it total covers the joint area. This will keep the aluminum/steel from being in direct contact with each other.

You can use stainless bolts to joint them together, but be sure to put a phenolic or some type of fabric washer under the SS washer where it rests of the aluminum - again to prevent corrosion.

1.5 x 1.5 x 3/16" aluminum should be plenty strong. Go to 1/4" if you're really worried about strength.
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Old 17-12-2012, 12:06   #7
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Re: Aluminum floor beams in a steel hull?

Quote:
Originally Posted by SabreKai View Post
Aside from the galvanic corrosion potential if a joint were to get repeatedly wet, can anyone see a problem with this? I'm trying to carve as much weight out of Sabre Dance as I can, so this one area I'm looking at.

The current beams are made of half rotted spruce 2x4s, which I was going to replace with 1.5x1.5 or 1x1 steel angle, 1/8 thick. Now I'm thinking perhaps larger sections of aluminum might be better for support.

I plan on epoxy covering the steel and the aluminum ends before joining them with bolts, and they are not likely to get wet very much.

Comments? Concerns?
You can buy structural shapes made of glass that will do the job you are trying to accomplish.
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Old 17-12-2012, 12:07   #8
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Re: Aluminum floor beams in a steel hull?

The floor "joists" on our Selene trawler appear to be fabricated from fiberglass. Also you might consider laminated wood beams used in residential construction.
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Old 17-12-2012, 12:47   #9
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Re: Aluminum floor beams in a steel hull?

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They don't have to be wet to have galvanic corrosions - just being in contact with each other will do it.
.
Galvanic corrosion requires an electrolyte, but its very hard to prevent metal on a boat developing some moisture.

Your suggestions on isolation are good, but with structural members like this its very difficult to completely isolate to the two metals. There is a very large steel area and a smaller aluminium area.
Steel beams would be a much better choice.
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Old 17-12-2012, 12:49   #10
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Re: Aluminum floor beams in a steel hull?

How about using 1" x 1.5" (W x D) steel angle? Small where it counts and strong enough not to flex.

When refitting Boracay I seriously underestimated just how much load (and flexing) the sole gets. If you can frame up to not much more than 12" x 12" panels with steel angle then weight, money and headroom can be saved with thinner wood.

As mentioned, the steel is also going to be superior in terms of underfloor space and maintenance.
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Old 17-12-2012, 12:58   #11
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Re: Aluminum floor beams in a steel hull?

Forget all the bad advice, if you isolate the beams from steel and timber you will not have an issue at all.

This means where the beams bolt to the steel use an insulator ie plastic sheet, where the timber bolts or screws down use epoxy or poly sealant.

If it's going to be flooded or wet then don't but bilges should be dusty/dry, ours are and there is not a problem with dissimilar materials...
Cheers....
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Old 17-12-2012, 13:00   #12
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Re: Aluminum floor beams in a steel hull?

Quote:
Originally Posted by Doug Brown View Post
They don't have to be wet to have galvanic corrosions - just being in contact with each other will do it.

Easiest way to stop the problem is to buy some sheets of very thin phenolic - around 0.0625" thick (1/16") and cut out pieces so it total covers the joint area. This will keep the aluminum/steel from being in direct contact with each other.

You can use stainless bolts to joint them together, but be sure to put a phenolic or some type of fabric washer under the SS washer where it rests of the aluminum - again to prevent corrosion.

1.5 x 1.5 x 3/16" aluminum should be plenty strong. Go to 1/4" if you're really worried about strength.
Correct..
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Old 17-12-2012, 13:15   #13
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Re: Aluminum floor beams in a steel hull?

I lowered the salon floor in my boat 2 inches for more hear room. I used steel angle and bolted wood to that. If (when ) the wood need replaced, its just a few bolts and 10 minutes with a table saw. presto bango done.
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Old 17-12-2012, 13:58   #14
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Re: Aluminum floor beams in a steel hull?

Have you done an actual calc of weight saved? Why not just replace the spruce with an appropriate timber impregnated with an anti-rot resin?
Consider weight down low as ballast.

Regards,
Richard.
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Old 17-12-2012, 14:18   #15
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Re: Aluminum floor beams in a steel hull?

does anyone consider using the "composite" wood used for outdoor decking found at the home supply stores? I've used it to make some things in my backyard that don't rot.
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