Plastic joists for pontoon boat

Hi all

I'm building a 4.5 x 14 m (15 x 46 ft) pontoon boat for sheltered coastal use.
It'll have 3 HDPE hulls.

I'm looking for joists to run between the hulls to act as the main support.
Due to the moulding on the hulls they need to have a 50 x 150 mm cross section.

I'm planning on using joists made of recycled plastic (a mix of mostly LDPE, HDPE and styrenes). Alternatives would of course be treated timber or aluminium box-section. However, my preference is plastic as:

1) I'd rather not work with sawing/drilling/sanding/coating treated wood, but more importantly I'd be concerned about rot and regular maintenance of frequently-splashed but not fully-submerged timber for what is a hard to access but critical part of the boat.
2) I don't plan on learning to weld, particularly not aluminium safety-critical structures.

It's about twice the weight and cost of timber but on the overall boat cost including maintenance I can manage.

However - my main concern - is it strong enough?

Looking at two potential sources (I'm in the UK):

Kedal: Recycled Plastic Lumber - mixed plastic - Ultra 150 x 50mm https://www.kedel.co.uk/mixed-plastic-lumber/recycled-plastic-lumber-mixed-plastics-ultra-150-x-50mm.html
technical: https://www.kedel.co.uk/user/Ultra 2018.pdf

Manticore/Envirobuild: Heavy Duty Plastic Lumber Joist 50x150mm | EnviroBuild https://www.envirobuild.com/products/plastic-lumber-joist-50x150mm
technical: https://a.storyblok.com/f/162000/x/5c1311ec9c/manticore-lumber-technical-specification-v1-0.pdf

....there's quite a bit of variance but they both seem considerably less strong (but more flexible) than typical wood or aluminium. I have very limited knowledge of applying technical material specifications to real-world applications however (what's the worst that can happen! /s), so I'd be very grateful for any insight.

Have others built pontoons with plastic joists? I'm not sure yet of the support weight required but it'll be a cabin approximately 3.5 x 11 m made of SIP's. Some of the float modules (13 per hull) will be used for fresh and black water so that'll be below the supports.

The floats can support a load of 10,000 kg at 55% draft (after discounting modules used as balancing and water storage) so given that upper limit it's how much the joists can support, particularly given the dynamics of the sea (albeit in sheltered water at low speeds).

I've attached a CAD screenshot for visual reference. I've only included the main lateral joists, I think I'll need some longitudinal elements too but welcome suggestions. I also can't get lengths greater than 3-3.5 m off-the-shelf so will need to figure-out the best way of joining/extending them too.

Yes, ideally I'd find and pay for a naval architect, but I'd welcome any constructive thoughts beforehand. As you've probably guessed, this is a new thing for me. Though I have a doctorate in engineering it's... electronics.

Matt

 

Welcome to the Forum Matt.

Re your 'sheltered coastal use' in the UK, whereabouts are you, and how 'sheltered' is it - is it always flat calm, or can you get short waves?
What is your intended usage with the boat?

No matter what material you use for your cross beams, you should do a detailed weight estimate of all the different items that will go into your trimaran pontoon boat, including the weight of the hulls, and the cabin and all the outfit equipment - you might be surprised by how quickly it all adds up.
The plastic cross beams in the links above seem to be generally more dense (hence heavier) than wood (they might just float, submerged, in salt water).
And you ideally do not want to go past half the depth of the pontoons in the maximum expected load condition (and preferably a maximum draft of less than half the depth).

The structure of the cabin should also provide useful transverse strength re holding the hulls together if it is designed and built well - and if so, a boat with a cabin should have 'better' transverse strength (ie resistance to breaking in half transversely) than one without, all else being equal.
Will you use recycled plastic beams for building the structure of the cabin as well?
Do you have any sketches of your proposed superstructure for the pontoons?
And what sort of power are you intending to fit - maybe two outboard motors on brackets between the hulls?
.

 

Sawing, sanding, drilling treated lumber is very tame health wise by comparison to same actions with most plastics.
The weight will be prohibitive, and the spans very short.
You might be interested in a product called G10. It is available in many structural shapes, and is extremely strong.
https://www.alro.com/dataPDF/Plastics/PlasticsBrochures/Brochure_EXTREN.pdf

 

Read that very, very carefully.

Matt-
Why three pontoons?
Two would be better.

I'd go with full dimension 2x4's or 2x6's lumber incorporated into the cabin floor for strength.
Or even engineered beams (wood).
But so much depends on what you intend to build for a cabin and the boats purpose/use.

And what diameter are those pontoons, 24"?

 

Thanks very much for your reply!

It'll be coastal so definitely not always flat calm. Having said that it'll be moored in a reasonably sheltered parts (either a marina or up an estuary) for most of the time, only taking it out when the forecast is as reliably calm as it can be. It'll definitely need to handle short waves though.

Usage is a full-time touring house boat (always occupied but not long-term moored in any given place).

Yes, the weight will be added up including furniture/appliances (and crew!). The cabin will be constructed from Structured Insulated Panels (SIPs) which is foam sandwiched between two wood panels. It's meant to provide both insulation and structure as a single module, rather than need to use timber frames and add insulation separately. The modules/panels are self-supporting and slot together on all edges. So yes I hope they'll add a useful degree of additional strength, but it's difficult for me to quantify.

The documents I have for a similar pontoon boat designed by the people I'm getting the pontoons from ( https://www.rotoplius.com/hb-pontoons , but document in question was emailed) state a maximum of 55% draft as mentioned but yes - I've read that half depth of the pontoons should not be exceeded not least due to the diminishing returns of its curved cross-section. I'll make sure it won't following as comprehensive as I can weight calculations (float buoyancy calculations are provided by the manufacturer).

I've got some 2D drawings of the cabin layout but nothing 3D yet. I've attached a general SIP construction image (mine will of course only be single story with a fairly flat roof).

For added challenge I'm going all electric. So it'll have a single 40kw outboard ( X40 Electric Outboard Motor https://www.epropulsion.com/outboards/x-series/ ) and a load of LiFePO4 batteries ( G102-100 https://www.epropulsion.com/g102-100-battery/ ) that'll be both propulsion and house power. Solar panels on the roof (though mostly charging off shore power as our trips between moorings are unlikely to be long in both time nor distance). It'll be slow and not super-easy to manoeuvre but I think that's ok for its use case.

In terms of redundancy however, I'm actually building two pontoon boats(!) One will be the day boat (living room, kitchen, bathroom/utility) and the other the night boat (4 bedrooms, bathroom). While the night boat is being built (hopefully with lessons learnt from the day boat) we'll sleep on sofa beds in the living room. It's a bit odd but it'll work. I think. Anyway, two boats will then give us a degree of redundancy, particularly regarding engine/power but each will have a tender too that could help if need be.

[edit: spelling]

 

Thanks for this. Fibreglass structural shapes was not something I knew existed. I think similarly to treated timber I'd be weary of working on fibreglass too, but again it's just a personal view and I appreciate with proper PPE it's not going to be an issue. A quick search didn't find Extren available in the UK but it was only quick.

I agree plastic lumber isn't great from a strength/weight side. The pretty much maintenance-free aspect though is a big plus though, if I could get it to work.

 

I originally designed it with two. I then thought three would add welcome extra buoyancy, particularly as I'd be using the inner one for water storage too so relieving the outer two of this. It also means my span length between pontoons is smaller so better for strength? I'm sure it'll add more resistance, but the boat's not designed for performance so I thought it's a fair compromise?

Regarding cabin floor, on top of the beams/joists I'm planning on installing composite decking, pretty much like a garden deck. Being composite it should be low maintenance/high lifespan and keep the saltwater splashes off the likely timer base of the SIP cabin. It'll retain some gaps for thermal expansion but also allow water to drain back through it (on the deck outside the cabin). Not sure it'll add much in the way of extra strength though, but will help spread the load.

Pontoons are 1m diameter, here: https://www.rotoplius.com/product-page/pontoon-hb-mid

 

You don't need to use treated lumber, nature provides rot resistant species galore.
You will get very intimate with PPE anyway on account of all the epoxy you will end up using to protect your SIPs from moisture.

I would build myself a nice foam cored deck, stiffend with top hat stringers and molded to the top of the pontoons. If that's to high tech for you, the decking lumber vendor can provide proper rainforest hardwood in 50×150mm that won't rot in your lifetime. If the rainforest stuff is to ethically questionable the EU provides plantation black locust instead. On top you screw sheets of filmed plywood with epoxified and painted edges. If you don't trust plywood you can use something made entirely from plastic (several options available).

The problem with HDPE in this application is stiffness, you would need to learn to weld it to be able to construct something viable.

 

Thanks for the ideas! Seems black locust is hard to get hold of in the UK, but could be done I expect - I'll look into it.
I was thinking of garden-style composite decking for the next layer above the joists, either covering the whole area or just using large squares of HDPE sheet for the hidden parts under the cabin.

 

Is drilling and bolting good enough, rather than welding?

 

And it will be a bit less than this at 45% draft (which would be a preferable maximum draft, or even 40%).
Before you start getting too involved in detailed design, please do carry out an initial weight estimate of EVERYTHING that has to go into / onto the boat.
If you do it on a spreadsheet, and note the vertical and longitudinal centre of gravity of each component, you can then also tally up the overall vertical (VCG) and longitudinal (LCG) centres of gravity of the boat at the end.
You ideally want the boat to trim level, or very slightly down by the stern, so the overall LCG should be approx amidships, assuming that this is where the centre of buoyancy of the hulls is.

Very few boats end up being 'too light' when first launched - rather, I have seen many that were too heavy, because nobody had kept tabs on all the little bits and pieces that were added.
Including a couple of home designed and built power cats here which when launched, floated with the deck almost awash - and this was before any passengers were put on board (they were supposed to be coastal party boats).

 

The problem with HDPE besides weight (too much) is stiffness (too little). Extruded aluminum or fiberglass box sections would be better suited for joists. There's no need to weld aluminum, properly designed bolted connections are used all the time.

As a retired Architect, I'm a real fan of SIP structures but as a boating nut I'm very leery of OSB panels in a marine environment. There are foam covered panels that use aluminum sheeting primarily for cold storage buildings but there would be a lot of devilish details in connections and the like.

That being said I just remembered I did just that sort of thing on a very small scale years ago. Contest 2002 - Solus https://www.duckworksmagazine.com/02/contest2002/06/skip.htm