# HDPE hulls

Discussion in 'Materials' started by Silverbreeze, May 10, 2013.

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### SilverbreezeBruno Tideman

Hello to you all!
We seek advice on construction of HDPE hulls from sheet material.
Anyone has done this before and knows what thickness to use as equivalent of 4 and 5mm aluminium plates?

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### daiquiriEngineering and Design

The mechanical properties of HDPE are very different from those of Aluminum. Here are some numbers:

HDPE:
yield stress: sigma,y = 22 MPa
elastic modulus: E = 860 MPa

Aluminum 5086-O:
yield stress: sigma,y = 170 MPa
elastic modulus: E = 71000 MPa

These numbers can give you a base for comparison of structures made with these two material. Take a case of two simple flat plates subject to bending moments (due to hydrostatic pressure acting on its surface, for example). Let's assume that the external pressure is the same for both plates, and that the only thing in which the two plates differ is their thickness.
Let's call them plate A and plate B. The plate A is made of HDPE, the plate B is made of aluminum.

I will save you from the math, which can be checked in any text book about structural engineering, and will jump directly to the results.

If you want to make the plates A and B work inside the same factor of safety for a given external load, the plate A (HDPE) will have to be 2.5 times thicker than the plate B (aluminum).

However, since the rigidity of HDPE (expressed throught the quantity E) is 83 times lower than aluminum's, the plate A will flex much more than the plate B, for the same thickness. In other words, if the aluminum plate B flexes .1 mm under a given load, the HDPE plate A will flex around 4 mm - which is a very big number.

Hence, in order to keep the hull flexing in a reasonable range, the HDPE boat will require much denser internal framing than the original aluminum boat.

You can also keep the same frames spacing, but then you will have to increase the thickness of the HDPE plate. Precisely, in order to keep the same deflection of the two plates under a given external load, the HDPE plate needs to be 4.5 times thicker than an aluminum plate.
So, if the original project calls for 5 mm thick aluminum plate, the HDPE replacement will have to be roughly 22 mm thick. If the idea behind the project was a weight saving by replacing the aluminum with HDPE, these numbers tell you that the end result might be very disappointing.

Of course, these numbers serve just to give you a ballpark figure. As you can understand by looking at these figures, you are embarking into something that goes well beyond a mere replacement of the hull material. The corresponding change in thickness of all structural members will require a thorough re-thinking of the whole design.

Advice: get a help from a professional boat designer or from a structural engineer.

Cheers

Last edited: May 13, 2013
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### tomasSenior Member

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### jonrSenior Member

> Hence, in order to keep the hull flexing in a reasonable range, the HDPE boat will require much denser internal framing than the original aluminum boat.

Good points. You might want to look at using two layers of HDPE with a foam core. Or foam filled cavities.

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### upchurchmrSenior Member

HDPE sucks as a hull material if you care about weight.
If not let us know how it goes.

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### gonzoSenior Member

I think that a scow type hull with corrugated skin could help with the rigidity issue.

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### tomasSenior Member

Be mindful of the considerable heat-induced expansion of this material.

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### SilverbreezeBruno Tideman

Sample

We have succesfully made these Boats in hdpe and tested it for more than a year!
Many thanks for you advice and input.
See some results on www.dutchworkboats.com

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9. Joined: Sep 2008
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### jonrSenior Member

I see that you make HDPE and aluminum boats. What are the advantages and disadvantages?

How do you make the HDPE boats - roto molded, sheets, with foam filling?

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### SilverbreezeBruno Tideman

We make our HDPE boats from welded sheets.

It is light, tough, no corrosion, no paint, no electrolyte, limited growt, uv resistant, no radar signature and easy to produce.

But there are many disadvantages as well

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### daiquiriEngineering and Design

With all due respect for your work, calling HDPE boats "tough" strongly depends on how you define "tough".

The boat in your picture apparently shows some pretty visible deformation of the hull sides in the bow area, imo confirming the previously mentioned need for denser internal framing and/or thicker hull shell.

Also, a vessel with no radar signature can be an advantage only in military applications. For all other uses, a good radar signature is an important safety feature.

Cheers

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### SilverbreezeBruno Tideman

Correct on both issue's

We say though and mean difficult to damage, the flexibility absorbs a lot of the energy in case of a collision. The deformation does not matter in case of a work boat

Radar signature can be an advantage for smugglers as well, we do not have such a customer

Thanks for the feedback?

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### jonrSenior Member

I expect that roto moulding can produce a similar boat with more ability to vary hull thickness and provide many stiffening ribs. It can even do foam core (like Hobie does with small sailboats). Of course you have to make a metal mold.

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