HDPE as a boat hull material

[JonathanCole]

I would be interested in hearing about people's experiences with HDPE as a boat hull material. I have seen 20 year old hulls from Hobies and other small boats that, other than some scratches and a bit of surface oxidation, are in excellent condition. Although this material is more flexible than GRP/FRP, recent molding techniques allowing for double walls with foam filling have increased the stiffness of such hulls considerably.

I am interested in exploring the use of this material for the hulls of the solar catamaran I am designing for mass manufacture and would appreciate any experiences of members.

By the way, I am already aware of the size limitations of rotomolding. I am planning multi-piece hulls that can be shipped in containers and assembled near the point of use.

The largest boat that I know about using this material is one designed by Morelli&Melvin for Triumph boats, the Winsor Craft 240 (http://www.morrellimelvin.com/powerb...4-Windsor.html)

[ted655]

It looks like a floating butter tub. Does the process have to be that "klunky" for all hulls? Are pontoons all that can be cast or can a monohull (with lines) be made? How do you envision your "sections" to fit & work?
The stuff is tuff, I;ll give you that, but a designers/stylers nightmare maybe?

[JonathanCole]

Originally Posted by ted655

It looks like a floating butter tub. Does the process have to be that "klunky" for all hulls? Are pontoons all that can be cast or can a monohull (with lines) be made? How do you envision your "sections" to fit & work?
The stuff is tuff, I;ll give you that, but a designers/stylers nightmare maybe?

AS far as the Winsor 240 goes, I agree that it does look rather bulbous, however it is not the design that interests me but rather the material. There is no reason that it has to be limited to use in pontoons. If you look at hobie cat hulls as well as many kayaks made from HDPE, they are certainly fair and designed hulls. As far as using it for monohulls, the limitations of the rotomolding technology is that the mold is made of aluminum (not cheap) and that the mold then must rotate in two axses while being heated. The HDPE powder/pellets are inside the mold and uniformly coat the inside of it. Things such as stainless steel fasteners (threaded inserts and/or studs) can be molded directly into the hull as it is being formed. Many products that require high abrasion and puncture resistance are made from this stuff. Road barriers, barrels, pallets and much more. It is also highly uv resistant.

I plan to have the sections (3 pieces) of the cat hulls to be joined with stainless fasteners. Each hull section will be a water-tight unit. A spar attached to the cabin will then run between the hull sections and be bolted to the hull sections for stiffness. This boat will have symetrical hulls with identical canoe ends, so only two molds will be required.

Anyone have any experiences to report on the durability and desirability of this material?

[alaskamokaiman]

Look up mokai.com they make a small jet boat with it.

[Pierre R]

HDPE is used extensively on kayaks, both sea and white water. It holds up exceptionally well to abuse as far as puncture but HDPE does has some disadvantages to building large craft.

HDPE is extremely hard to paint or have paint hold. HDPE is also embrittled by sunlight and these boats have a useful lifespan that is relatively short if stored out in the sun like a larger boat would be. HDPE also scratches easily and becomes hairy rather quickly and for this reason, a boat becomes ugly in a hurry.

Buyers what that gel coat look and want it to last more than a couple of years.

[RAWRF]

There is an aussie company that has been using HDPE for years on their airboat bottoms, but this material doesn't last long if exposed to the sun. Up here in AK everyone uses it for dog sleds and sled runners. I don't think the plastic on kayaks is the same thing I am talking about, because those kayaks last for a long time in the sun without cracking and looking terrible, I have a Prijon Expedition that is ten years old and has been in the sun almost constantly.

[RAWRF]

Never mind what I said, I was thinking of UHMW (ultra-high molecular weight) plastic.

[naturewaterboy]

I think that HPDE has a low strength to weight ratio - but foam sandwich construction will help with that. It does deteriorate with sun, and in some places the sun is more intense so could be a problem.

[JonathanCole]

This is from Hobie's web site
http://www.hobiecat.com/sailing/all_models.html

According to them rotomolded polyethylene (I'm not sure if it is HDPE) is a very durable material, but for some reason (perhaps weight) they say that fibreglas gives higher performance. They describe one of their sailing boats made of the material this way:

Standard Features:
>> Super Durable, Impact-Resistant Polyethylene Roto-Molded Hulls


Hobie's roto-mold technology will insure the long life of your new water toy.The rotomolded sailboat hulls are made of two layers of tough, durable, roto-molded polyethylene.

The outer layer is composed of the latest polyethylene development for roto-molded boats. It is stiffer, tougher and more heat resistant than earlier molded boats. The second layer is polyethylene foam, which significantly complicates the manufacturing process, but provides a host of benefits.
>>Increased stiffness which means less weight.
>>Decreased chance of developing leaks.
>>Reserve buoyancy in the unlikely event of the hull filling with water.

This is the process and materials used in the production of the Hobie Wave, which has been in use at resorts and rental facilities throughout the world for nine years-the most taxing conditions for any boat!

[e39dream]

I finally realized where I saw HDPE before this thread at work tonight- on the lids of 5 gallon buckets.

I personally think the material would be too soft, it's not hard to put a deep scratch in the bucket lid.

[PI Design]

Topper International's range of sail boats (apart from the actual Topper) are HDPE - in two layers with a foam core. They are much derided by 'serious' sailors as ugly and slow, but it serves a purpose. Some of the newer designs are not so ugly, but the material is uninpiring to look at. It is slow because it is heavy and (even with the foam sandwich) fairly flexible - you can't get much rig tension on without bending the boat. Roughly speaking Topper's boats are 50% heavier than standard GRPm construction and twice as heavy as a 'lightweight' racing boat. The main advantage that Topper state is that it is virtually maintenance free - but then so is GRP in my opinion. You are less likely to put a hole in a HDPE boat (the hull flexes and absorbs the impact), but it not often you hole a GRP boat either. And at least you can fix a GRP hull. HDPE boats are not so easy to patch. The real benefit of HDPE is that it is cheap. Rotomoulded boats need less labour and are suitable for batch production. I'm not sure if there is a cost saving for a one-off boat due to tooling costs.
So, in short, HDPE is suitable for low performance, mass produced boats.
How weight critical is your design?

[jrl5678]

A lot of the sit on top cheeper Kayaks are HDPE. It is almost impossible to repair. You can paint it but it does not hold well because it is not porus and it is not reactive to almost any solvent. It makes good small boats light and flexable cheep. It is a thermo plastic, meaning it gets soft when hot pressed in to a mold cooled poped out. It does break down in UV light, we have some on the Hudson which are very old 10 years or so now we store them under cover but they spend long days in the sun.

[JonathanCole]

Originally Posted by PI Design

It is slow because it is heavy and (even with the foam sandwich) fairly flexible - you can't get much rig tension on without bending the boat. Roughly speaking Topper's boats are 50% heavier than standard GRPm construction and twice as heavy as a 'lightweight' racing boat. The main advantage that Topper state is that it is virtually maintenance free - but then so is GRP in my opinion. You are less likely to put a hole in a HDPE boat (the hull flexes and absorbs the impact), but it not often you hole a GRP boat either. And at least you can fix a GRP hull. HDPE boats are not so easy to patch. The real benefit of HDPE is that it is cheap. Rotomoulded boats need less labour and are suitable for batch production. I'm not sure if there is a cost saving for a one-off boat due to tooling costs.
So, in short, HDPE is suitable for low performance, mass produced boats.
How weight critical is your design?

My solar live-aboard catamarans are to be mass produced. I am trying to open the market for a large (16m x 7.5m) boat to a new group of buyers. Not your standard sailing crowd. It must be priced well and it must be bullet-proof. Since the portion of the boat I am talking about is the demi-hull section which is below the water line, I don't think that weight is that critical. This is not a speedboat, but instead is boat designed to enjoy living on the water with very low operational costs. It must be high performance in terms of efficiency, user-friendliness and comfort. As far as stiffness goes it will have a longitudinal spar/box that stiffens the overall hull. It is fine if it is locally flexible if that prevents punctures, cuts and tears.

[JonathanCole]

Originally Posted by jrl5678

It does break down in UV light, we have some on the Hudson which are very old 10 years or so now we store them under cover but they spend long days in the sun.

This is a response I got from Mokai.com who makes a very hi-tech small power boat utilizing Super Linear High Density Polyethylene resin, treated for UV protection. According to them: "
In the 8 years of MOKAI we've never had a hull deteriorate due to UV exposure. We have boats in a variety of climates all over the world, though I'm not sure if any are stored outside, unprotected etc. The UV protection is actually a specification in our resin. Since it is part of the resin, it is not something you'd re-treat. I will check to see if there is something on the market to "re-treat" polyethylene type products. "

[PI Design]

Sounds like it could be okay for your boats. I don't know how mass produced it needs to be, to be cost effective versus fibre glass. Presumably the heavier structure will require more solar panels to power it.