Approximate Side-Load Strength of One Leg

Important: This is only a rough first-pass estimate, not a structural certification. A real design like this needs a naval architect and a marine structural engineer. Wave slam, fatigue, weld details, buckling, stress concentrations at attachments, corrosion, and dynamic resonance can all reduce safe load a lot.

1) What I assumed from your description

For each main leg/float:

I interpreted your question as:

Big uncertainty: The actual strength depends hugely on the internal structure. A 1/2-inch aluminum skin alone is very different from a leg with internal webs, frames, bulkheads, and a strong root attachment. In practice, the connection to the main triangle structure is likely the most critical part.

2) Simplified structural model

To get an estimate, I modeled the leg as a hollow rectangular section approximately:

For sideways loading, the leg bends in the fore-aft direction, so the important section modulus is based mainly on the 10 ft chord depth.

Using a thin-wall rectangular approximation:

Typical marine aluminum yield strength varies by alloy and weld condition. A rough range:

Converting section modulus to cubic inches:

1.17 ft³ × 1728 = 2020 in³

Estimated yield bending moment:

Convert to ft-lb:

3) Distributed sideways load that causes that moment

If the submerged length is L = 9.5 ft and load is uniform, then for a cantilever:

M = wL²/2

So:

w = 2M / L²

With L² = 90.25:

Total distributed force on one leg:

F = wL

Rough answer for one leg:
If treated as a simple hollow aluminum box with 1/2-inch skin, the shell-only bending estimate says on the order of 0.7 to 1.1 million pounds of evenly distributed sideways load on the submerged 9.5 ft would be needed to drive the section toward yielding.

4) But real failure may happen much earlier

The numbers above are not a safe working load. Real failure can happen much sooner because of:

A practical first-pass allowable load might need to be only around 1/4 to 1/6 of that yield estimate unless the leg has substantial internal stiffening.

That would suggest a more cautious “don’t even get near this” range of roughly:

And even that is still a very rough conceptual estimate.

5) What wave height might cause that much side force?

This part is much more uncertain than the structural estimate.

For a side force on the submerged portion of one leg, a crude hydrodynamic estimate is:

F = 0.5 ρ Cd A V²

Where:

Because your geometry is unusual, I used a practical projected side area of about:

A ≈ 9.5 ft × 10 ft = 95 ft² = 8.8 m²

Then solving for water particle velocity needed for various loads:

Total side force on one leg Approx. water velocity needed Comment
100,000 lb ~10 to 14 m/s Very severe, extreme breaking-wave type flow
250,000 lb ~16 to 23 m/s Exceptionally violent slam / breaking crest event
700,000 lb ~27 to 39 m/s Implausibly high for ordinary wave orbital flow; more like impact/slam territory

Those velocities are enormous. That suggests:

Very rough wave-height interpretation

There is no single clean conversion from wave height to side force because it depends on:

But as a rough conceptual guide:

Wave condition Likely implication
3 to 6 ft beam waves Usually far below shell-yield loads, but may cause uncomfortable motion
8 to 12 ft steep beam seas Could produce significant cyclic loads, especially if periods match roll response
15 to 25+ ft breaking beam seas Now you may get severe impact/slam loads and dangerous dynamic events
Bottom line on wave height:
A simple static estimate suggests that ordinary waves are unlikely to directly bend a leg to shell yield.
The more realistic threat is large steep or breaking side waves, likely in the 15+ ft range, especially if they cause impact, rapid roll, or repeated fatigue loading.

6) Final answer

Using a very simplified cantilever model for one leg made from 1/2-inch marine aluminum with approximate section 10 ft × 3 ft:

But for design purposes, real-world allowable load may be much lower, possibly more like:

As for wave height:

7) Strong recommendation

Before building this, you should get:

If you want, I can next produce an improved engineering estimate using a more realistic model with:

Prepared as a conceptual estimate only.