Fully HDPE 57ft Narrowboat – Why We Built It (and Why It Matters)

Subject: Fully HDPE 57ft Narrowboat – Why We Built It (and Why It Matters)

Hi everyone,

Some of you may remember my last post when we put our first HDPE prototype in the water—a hybrid of steel and HDPE. That experience taught us a lot, especially about how different materials behave. The expansion rate between HDPE and steel simply isn’t compatible long-term, so we made a bold move:

We’ve now built a 100% HDPE 57ft narrowboat. No steel. No compromise.

And yes—I know this might not be for everyone. Canal boating is steeped in tradition, and I respect that deeply. But this boat isn’t trying to replace what exists. It’s about offering a viable, modern alternative for those who want something more resilient, low-maintenance, and environmentally conscious.

So, why HDPE?
Here’s the detail for those interested in the material science behind it:

• Material: High-Density Polyethylene (HDPE 300-grade marine sheet)
  
  
• Density: ~0.95 g/cm³ (much lighter than steel’s ~7.85 g/cm³)
  
  
• Flexural Modulus: 900 MPa – Flexible, but strong with the right engineering
  
  
• Brittle Point: Below -50°C – stays ductile even in harsh winters
  
  
• UV Resistant: Our HDPE is UV-stabilised for long outdoor lifespan
  
  
• Thermal Expansion: ~1.2mm/m/10°C – which is why we designed a fully integrated structural rib system
  
  
• Corrosion/Rust: None – zero metal means zero corrosion
  
  
• Insulation Properties: HDPE has low thermal conductivity (~0.46 W/m·K) compared to steel (~50 W/m·K), offering passive insulation benefits
  

Structural Design
Since HDPE is flexible by nature, we engineered a full internal skeleton structure to give the shell its rigidity. While traditional steel narrowboats use ribs that run partway up the hull, our HDPE version uses full-height ribs throughout the entire boat, from bow to stern. It behaves more like a monocoque structure—designed to flex together, not fight itself.

Development & Paint
We also built a 60% scale model to test everything, from the structure to buoyancy. It helped us develop a specialist HDPE-compatible paint system—so now you can customise finishes properly, with coatings that actually bond and last.

I’m not here to knock steel boats. I love the canal world in all its forms. I just believe there should be more choices. HDPE has served the workboat industry for years—tough, zero-maintenance, recyclable, and built to last. So why not bring that into the leisure world too?

This boat represents over three years of work, countless hours of testing, and a lot of heart. I genuinely believe HDPE can offer something valuable—not instead of tradition, but alongside it.

Thanks for reading. Whether you're curious, sceptical, or just watching from the towpath—I appreciate every bit of interest.

 

Hi @shaun Moir, I find your experience very interesting, and we'll have to find a niche within the broad small boat sector where HDPE is absolutely advantageous compared to other materials.
I've also tried my hand at using HDPE, although, to my knowledge, no project of mine has yet been built. Therefore, I don't have any proven practical experience, and I'd like to ask you what design stresses your engineers use when calculating the structure.
Thank you, and much success with your boats.

 

You may or may not be aware, but ISO certification specifically for HDPE canal boats doesn’t yet exist. That said, I’ve been asked to be part of the committee helping shape those standards, which is a huge honour. We want to make sure any HDPE canal boats going forward are built to the right levels of safety, strength, and performance.

Until formal ISO standards are developed, we’ve based our work on ISO scaling reports for 17m vessels, but we’ve gone further. Our HDPE canal boat uses a 40mm thick structural spine and 20mm HDPE throughout the rest of the build — compared to the standard 17mm typically used in HDPE workboats.

We made this decision primarily due to HDPE’s flexible nature. Unlike steel, HDPE naturally absorbs impact and stress by flexing. That’s a strength in many applications — but it also means we had to re-engineer how structural strength is distributed across a narrowboat. On a traditional steel narrowboat, the ribs go about halfway up the internal frame. But on ours, the ribs are spaced and extended throughout the entire boat — from bottom to roof — creating a continuous skeleton that locks everything together.

Our design loads (in kN/m²) are as follows:

• Bottom (longitudinal): 14.5
  
  
• Sides: 11.8
  
  
• Roof: 5.0
  
  
• Bottom (overall): 14.5
  

We also had to think differently because of the unique demands of canal boat layout. Most large HDPE workboats — like the 23m+ ones you might find in offshore or industrial use — have a central console for crew and controls. On a canal boat, the cabin spans the entire length of the hull, which introduces totally different stress points and load demands. Using a single HDPE console wouldn’t work at this scale without adapting the structure — so that’s exactly what we did.

 

Thank you @shaun Moir for your explanations. You're very generous in sharing your experience and providing all this data. But I'm afraid you haven't answered my question. Let's see, since the ISO 12215-5 standard appeared around the beginning of the century, I've always used it to calculate my small boat projects. Since the 2019 revision, it can be applied not only to recreational boats but also to commercial vessels. I'm familiar with the ISO, and it provides design pressures for a multitude of materials, but not for HDPE.
The design pressures, or design loads, as you call them, are clearly established in the ISO and do not depend at all on the material being used. They are the same for all types of materials and are independent of the calculation procedure used (Simplified Method, Enhanced Method, FEA, etc.).
That's why I'm asking you. As an expert in the calculations of these structures, what values do you use for the design stresses?
The fact that you use frames all the way up doesn't tell me anything, since any structure can be almost any shape if calculated correctly. A thickness of 17 mm may be adequate, and without giving further details about the structure, it doesn't have to be worse than 40 mm.
I don't know if I've explained myself correctly. The design stresses you use are what I'd like to know. If it is a confidential matter, I will understand your silence.
Thank you so much for your patience and kindness.

Edited :
I wrote "... ISO, and it provides design pressures for a multitude of materials, but not for HDPE." , but what I wanted to say is "... ISO, and it provides design stresses for a multitude of materials, but not for HDPE".

 

Apologies, I didn’t answer that properly,

The honest answer is that this part of the process was handled directly by our naval architect, so I’m not 100% sure on the exact technical details behind those calculations. The design work was done very closely with a naval architect who specialises in HDPE – and that’s important, as while there are many fantastic naval architects out there, most are used to working primarily with steel or aluminium.

So unfortunately, I can’t give you a fully detailed answer on that one – but I’ll be happy to ask and come back to you if needed.

 

Yes, please and thanks a lot.

 

Important question, and straight to the point. And after they reply, we need to find out how the "welding" is regulated. From my observations, it seems to be very loosely defined.

 

Of course, not to mention that the two most important points for designing with this material (welding and design stresses) have been overlooked. The rest is just paraphernalia.
But I'm sure the OP will clarify these points for us.

 

I am very confused as to why you would build any boat out of HDPE……it recyclable…..OK…..but it’s heavy and not stiff…not exactly a great boatbuilding material.
You state it’s been used in workboats for years……..and so what….so has every other material used in boat building. You get one color and it scratches easily…..

 

I'm convinced, as I said in a previous post, that someone will find a sector of shipbuilding where this material is far superior to any other. For the moment that "someone" does not appear.
A thought: a material that requires a lot of thickness to be effective is not an effective material.
And another thing, we have two great topics to discuss:
- 3D-printed hulls
- HDPE hulls

 

You heard about whitewater kayaks ?

HDPE Plastic Boats - Legacy HDPE Boats https://legacyhdpe.com/hdpe-boats/

 

Oh you unbeliever, here's the required proof that the two topics are one, it swims.
Now before you ask why they didn't print the transom together with the hull, it's because of printer logistics, it's made from a glass fibre filled material while the hull is pure HDPE.

 

I have no difficulty seeing the advantages of using this material for narrowboats. Most of them potter about on calm water at low speeds and they need to be robust primarily to handle impacts from other boats driven by beginners. The elimination of rust issues would make them attractive because you wouldn't have continual worries about massive future expense to rescue them, and the better insulation is great because these boats are often just used as static floating housing, with only occasional trips being made, while almost all of them just stay in one place all winter while the occupants just struggle to stay warm. I would choose an all-plastic one without hesitation. There are places where people take narrowboats out onto the rougher water of rivers, and some even do sea crossings (UK to Ireland or across the Channel to France), but a boat that's aged and weakened could simply be kept in the canal network and never be stressed significantly again. Weight and hull thickness is not important for this kind of boat.

 

@montero , congrats, what a beautiful video!!!! But... just between you and me, now that no one's watching, what do you think this video proves?

@Rumars, I'm not a skeptic; perhaps I have a slight technical bias due to my academic training. That's why I like to know what's behind everything, and this material in particular interests me greatly. Since I have no practical experience with it, I ask these innovative designers to tell me, if it's not a secret, the technical bases they use to calculate the structures of their boats. I think that question, among honest technicians, is quite normal. I understand that marketing is very important, but I don't want to think that the quality of a design isn't.

 

I understand your problem , you saw this viedeo before and they have half milion views. Just answered @mudsailor scratching issue .Write to Legacy HDPE maybe they they count it somehow. Although I doubt it.