# HDPE pontoons

Discussion in 'Boat Design' started by Chris Chadwick, Jul 19, 2022.

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I want to build a work barge of 14m x 6m with a carrying capacity of about 12 tons. What dimeter pontoons in HDPE do I require

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The barge will be catamaran configuration, steel deck, aluminum superstructure. powered by 2x300hp outboards

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### bajansailorMarine Surveyor

Welcome to the Forum Chris.

You mention a carrying capacity of about 12 tonnes - but first you need to do a weight estimate of all the other weights that will be in this barge.
You need to try to calculate the weight of the HDPE pontoons (a rough initial calculation will do for now), the steel deck, the aluminium superstructure, the two 300 hp outboard engines, the fuel tanks (full of fuel), the weight of the crew (how many?), and all the other various bits and pieces that make up a boat, like anchors and cables, tools, maybe a water tank, etc.

Lets say that you calculate the weight of the hull and outfit at 10 tonnes, and you want to be able to carry 12 tonnes - that will give you a loaded displacement of 22 tonnes.
If you have two cylindrical pontoons 14 metres long, then for weight purposes you can consider them as one pontoon 28 metres long.
You do not want to go past the half immersion point of the pontoons, and ideally the maximum load on the barge should be less than half the total buoyancy available - lets say 40% rather than 50%.
The volume of a pontoon is the cross-section area x length, where the cross section area is (3.142 x D x D) / 4
And D is the diameter.
I presume that you will be using the barge in salt water, so the displacement will be the volume x 1.025 (where 1.025 is the density of salt water).
And we don't want to go past 40% immersion, so the maximum allowable displacement will then be 0.4 x volume x 1.025.

Hence we have 22 = 28 x 0.4 x 1.025 x (3.142 x D x D)/4

And this gives diameter D as 1.56 metres (or approx 5').
Or in other words you are going to need to have some pretty massive pontoons!
And if the overall beam is 6 metres, then you will have 2.88 metres between them.
Note however that this is using an assumed hull & outfit weight of 10 tonnes - it is possible that you might be able to reduce this, depending on how your weight estimates for everything 'work out'.

Edit - I made a mistake previously re the above calculation - I only used one 14 m. pontoon instead of two. The required pontoon diameter is now 5', rather than 7' as calculated previously - it is still quite large though!

Last edited: Jul 20, 2022
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Catamaran barge not the best choice for structural simplicity or economy. If built of HDPE it will need more engineering to determine the thickness of the pontoon skins and the method of attaching them to the platform deck.. Without doing the tedious calculations I can imagine that HDPE will be heavier that an equivalent steel pontoon and might even cost more. To the credit of the HDPE, it ain't gonna rust and it might be less attractive to all those sea creatures that like to attach themselves. Teridos would probably gag on the plastic, as well. Little concern with electrolysis and so on.

The OP used the term "diameter" which implies a round cylinder. It would be easier and safer to use a square section pontoon. In that case the load bearing per unit of draft would increase in a linear fashion. Whereas the round section would work well enough until the draft exceeded half diameter. When draft becomes more than half Diameter the buoyancy or loading capacity decreases exponentially with further immersion. A fully loaded barge with half cylinder immersion will not be as stable as a rectangular pontoon system when in rough water or when encountering large wakes from other barges.

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Many thanks for your replies. Makes total sense!

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### waikikinSenior Member

Hi Chris,
Some great advice in the replies above.
Nearly always with large deck loads a flat top barge seems the way to go, these can be sectional for transport. It will give you the highest load capacity within the size envelope and as mentioned by others above construction of this may be simpler than building of the connective system for separate floats that will need to deal with twisting/racking on the water. They are simply a box potentially with a swim end forward and a cutaway aft to aid manoeuvrability, bulkheads and framing within the depth of the barge.
Although it relates to Australia the link below quite handy- I'm sure the area your at has some form of regulation but plenty of handy info to enhance understanding- very often work platforms are operated by those with heavy machinery backgrounds and there's some stuff to be careful of.
All the best from Jeff.
Construction barge safety https://www.amsa.gov.au/construction-barge-safety

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### baeckmoHydrodynamics

Now wait a minute, don't restrict your thinking unnecessarily....... Pipes are attractive due to cost, availability and inherent strength, they are produced to industrial operating pressure standards. Building a rectangular "block" might be more of a welding nightmare if built in HDPE. Bajansailor exemplified with a "catamaran", but within the 6 m width limit you can fill it up with f.i. 6 x one meter dia pipes, or 2 layers (12+11) of 0.5 m dia, whatever gives the required volume. Then you check against stability requirements and structural strength of the connecting elements.

BTW, good and clear info from waikikin's link!

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### rnlockSenior Member

Wondering what the extra 550 hp is for? A round bottomed, cylindrical hulled cat is not exactly a planing hull. I'm sure 50 hp would be more than enough if it was swinging a sufficiently large prop. If you go too fast with these HDPE floats, they will probably be inadequate for the loads, since HDPE's stiffness and strength are only a couple of steps up from dried bubble gum.

I think a barge like hull, as suggested by Jeff, built in more usual materials*, may be the practical way to go. With approximately the right shape, it might even plane with those huge outboards. It could be shaped like a gigantic jonboat.
http://www.jimsboats.com/jonlns.gif

For a work platform, maybe make the sides straight up and down, rather than flared. One advantage of such a shape is that it would have much less draft.

If you needed to pull things up through the middle, you could have an open well in the center. If that wasn't large enough, you could have two skinny, joined jonboat hulls.

*steel, aluminum, plywood, fiberglass....

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### shaun MoirJunior Member

hello everyone,

I work for a fabrication company that has recently built a 40ft barge all out of HDPE

using the manufacturing process, we have done makes it even more unique as this is the same way that most shipbuilding especially larger boats are made, (by welding process)

Having a sturdy, durable build is crucial to the success of a boat against the elements. Most would not think of plastic as a viable material for this purpose,

Let’s explore some of the benefits that polyethylene and other forms of boat plastic can offer.

HIGH LEVELS OF RESISTANCE TO THE ELEMENTS
It’s difficult to prevent your boat from sustaining damage caused by the elements. Salt water not only causes abrasions to a boat’s hull over time but also corrosion and deterioration to the bolts and bearings holding your craft together. UV rays from sun exposure cause discoloration, warping, and delamination. Plus, having constant exposure to the weather can leave your boat with unpleasant Odors.

plastics, particularly polyethylene and polycarbonate, are resistant to all of the above conditions. Plastic for boats doesn’t require paint that can chip or bubble from long-term sun exposure. When used as bearings or joints, it protects the different sections of the boat from pulling apart, as opposed to metal staples that corrode over time. also helps reduce the wear and tear on your boat from the waters you sail in.

Overall, if you want a material that can endure the elements better than any other, plastic is your top choice.

VIRTUALLY INDESTRUCTIBLE
Plastic boat hulls are increasingly known for their near indestructibility. Compared to other boat materials, a plastic hull can more easily bounce off any debris or rocky obstacles you may encounter. You might wind up with a few scuffs, but your hull may still remain uncompromised, removing the stress of having to make a speedy patch job.

DESIGNED FOR CLARITY
One of the most popular applications of plastic for boats is as a replacement for glass windows or windshields. Polycarbonate plastics are designed for strength and clarity in even the harshest weather conditions. This boat plastic is resistant to yellowing, sturdier than other plastics, and abrasion-resistant, meaning you’ll have a clear, shielded view of the sea no matter the weather conditions.

SAFE EVEN WHEN WET
As you traverse the waves, your boat is going to get wet. Many materials can become especially slippery when wet, making nasty falls a concern if you use these materials for flooring.

Luckily, AntiSkid HDPE is the perfect solution. This high-density polyethylene is designed to prevent slippage, even when wet. Using it for decking, gangways and handrails will make your vessel much safer for you and your guests or crew.

ENVIRONMENTALLY FRIENDLY
In our increasingly eco-conscious world, it is important to choose materials that are sustainable and built to last. plastic is easily fabricated and repaired, producing less waste than other traditional boat materials. Additionally, plastic is readily recyclable, meaning your old parts won’t have to wind up in a landfill.

TO SUM UP THE BENEFITS OF PLASTIC:
• plastic is resistant to common forms of weather-related damage, like abrasions, corrosion, and discoloration
• The flexibility of plastic boat hulls makes them nearly indestructible
• Boat plastics, especially polycarbonate blends, are perfect for windshields because they are stronger than glass and just as clear
• Some plastic, like HDPE, is skid-resistant, making it perfect for decking
• Plastic is environmentally friendly because it’s easy to recycle

I would love to speak to anyone who is considering buying a "steel etc " barge and explain different options to you

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### bajansailorMarine Surveyor

@shaun Moir does your company have a website or a Facebook page for reference?

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### shaun MoirJunior Member

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### shaun MoirJunior Member

this one was done as a POC hence the colour etc when we will build the next one it will be in any colour that the buyer would like

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### CacciatoreNaval Architect and Marine Engineer

Simply hire a Naval Architect . Not easy to reply in a forum without misunderstanding.

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### shaun MoirJunior Member

HDPE is a brilliant way for barge boats as bumps and scrapes would not matter there is no paint to scratch which then would mean no exposed metals etc to the elements,

of course, there is a lot to consider however I believe by doing these in 'plastic' will be better for the environment

below is the video of the process hope this helps everyone to understand any questions let me know

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### BarrySenior Member

Shaun Moirs list of advantages appear to be tantalizing but some of the negatives of plastic were conveniently omitted

You will note in the video that they have had to lay in a deck support structure of steel to stabilize and or stiffen the hull. The manufacturer recognizes the lack of stiffness and strength that HDPE possesses.

So some cons
1) A quick check of the modulus of elasticity of plastic against steel or aluminum will show the difference between the three materials. To increase stiffness of the material, the thickness has to be increased dramatically which will add
weight. Weight that you will have to push throughout the life of the barge. So while appearing "environmentally friendly" for the BUILD, extra fuel must be burned over the life of the hull

2) HDPE as most plastic do, have a tendency to creep under heat and load.

3) HDPE has a coefficient of thermal linear expansion 10 times more than steel and 5 times more than aluminum. What this means is that a 14m barge will increase in length by 6 inches through a temp range of 50F to 100F. ( steel will increase only 1/2 inch) This will create a fastener challenge between the steel deck and the HDPE pontoons.

4) A comparison between the rest of the properties of the three materials will reveal serious deficiencies in considering HDPE for a high loading application

While HDPE has merits for boat building, and Tideman boats from the Netherlands is a world leader, an HDPE barge could be built, a complete workup by a N/A or engineer should be undertaken before considering this material