# Aluminum vs plywood bending

Discussion in 'Boat Design' started by Deering, Aug 25, 2019.

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### DeeringSenior Member

There have been a number of postings here recently from folks looking to design/build small aluminum boats. That got me wondering how bending aluminum plate compares to plywood. Assuming plates of comparable stiffness (say 3/8” ply and 1/8” AL as a random guess), would they both behave similarly when used for developing hulls?

I’m wondering how feasible it would be to use some of the thousands of plywood boat designs out there and adapt them for aluminum? Obviously there would be differences in assembly and framing, etc, but could the plate replace the ply in most cases?

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### Mr EfficiencySenior Member

Can't see why not.

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### jehardimanSenior Member

You could use the same shape, but you would need to recalculate the loads to ensure you have enough sectional modulus. You would need to cover the primary, secondary, and tertiary loading. A beachable plywood boat may not be beachable in aluminum due to sectional modulus or point load properties.

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The bend radius is proportional to the Young;s Modulus and the yield strength.
To achieve a permanent plastic deformation when rolling/bending plate for a hull, the maximum bend radius is given by the following relationship :

R = t.E/2σy

Where: t = thickness, E is the Youngs modulus and σy is the yield stress.

Plate rolling and bend radii was discussed in Proboat Magazine Feb 2014, issue 147.

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### trip the light fandangoSenior Member

Yes, cut your lines carefully out of mdf sheets and produce a male and a female mold, strip plank them and use them as a press. Those huge aluminium road signs that get knocked down by absent minded drivers look like such a waste just lying there..and that aluminium glue they used on PT boats..hmm... ha . 3mm.. 1/8 inch in 6061 I've read. Cut the sheet lengthways into sizes that will bend in the male female mold perhaps..then glue with a strap, like a thin stringer behind of ally or just weld if your good at it .

Last edited: Aug 27, 2019
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### DeeringSenior Member

I was thinking more along the lines of not exceeding the tensile strength of the metal, maintaining form through framing and stringers. Most folks looking to build a small boat likely don’t have access to plate rollers. So fabrication would proceed like stitch-and-glue.

Unfortunately I don’t have a subscription to ProBoat. But I did buy a bunch of their back issues. I’ll have to see if I have it. Thanks for the reference.

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### DeeringSenior Member

In the size range I’m talking about, say less than 20’, I wouldn’t expect point loading to be a major issue. A one-off builder is likely using welding to fabricate, and that limits how thin the material can be before heat distortion becomes unmanageable. Commercial aluminum boats, such as Lund skiffs, have far thinner bottoms and seem to be quite beachable.

Without having the tools or expertise to conduct a rigorous load analysis, I’d probably just copy the framing and scantlings from a similar proven hull.

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### Mr EfficiencySenior Member

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### JSLSenior Member

Keep in mind that welding can distort aluminum (stress relief) and probably more so when stressed due to curvature.
For the structure make sure you copy a 'good' boat. You don't want a boat that goes snap. crackle, pop, as it runs over choppy water.

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