Hulls made of HDPE

This looks like a new high tech meterial for hull construction...
HDPE - high density polyethylene

Here are some links for more information:
http://www.designinsite.dk/htmsider/m0002.htm

Material suppliers:
http://www.gipo-rpi.com/eco_panels/extruded.html
http://www.sdplastics.com/hdpesheet.html
http://www.tapplastics.com/shop/product.php?pid=344&

Welding of HDPE:
http://www.wegenerwelding.com/usa/flash/index.html
http://www.sdplastics.com/plasticwelders.html
http://www.columbineint.com/

One of the only boat manufacturers using HDPE:
http://www.boulderboatworks.com/bbw_home.html

 

These guys have been using it for a while. http://www.triumphboats.com/about/

 

I think WaveWalk.com (Yoav is on the forums) is using it too.

 

Hdpe

I'm having my boats molded from HDPE and I like this material very much.
However, I think there are limitations in using it:
1. Size: HDPE is molded in a process called rotational molding in which the entire hull is molded inside a mold that's being rotated in an oven.
In order to rotate a small boat like my 10' WaveWalk W1 (see: http://www.wavewalk.com) you need a very big machine and an even bigger oven. The bigger the machine the more expensive it is, and therefore there aren't many molders who have large-capacity machines.
2. Cost per unit: Producing a good quality (cast aluminum) mold becomes very expensive when you increase the boat's size, which makes the technology cost effective only for medium and large size production series. This is the main reason why many canoe, kayak and other small boat models are still hand-made from composite materials.

As for welding HDPE I have some doubts since HDPE is mechanically stronger when it solidifies uniformly as it cools during a controlled process.


Yoav

 

To take the polypropylene one step further you can use Twintex which is fibreglass reinforced polypropylene. Extremely tough and can be vacuum moulded but at 200 degrees C. Just need a big oven and a mould that can take this heat. Otherwise it is as simple as moulding pre pregs.

Twintex

 

Never underestimate the cost of a boat-sized autoclave.... these things are Very Pricey.

Polymer hulls like Yoav describes have their place. They're great for small boats, especially boats that see cottage or kids, since they can be banged around like crazy and still run happily for years. The trouble is the size limitations that you get from having expensive molds and expensive cooking setups. But I wouldn't be surprised to see the technology used for consoles, seats, and other parts of larger hulls that could be made cheaply and essentially maintenance-free.

 

You need just a regular oven. Autoclaves are pressurised ovens $$.

 

I have experimented a little with welding 1/2 sheets using a extrusion type welder when building some cleaning tanks for use in the bicycle industry. I welded a few scrap pieces together and found that the welded joints were almost indestructible. I would even say the welded joint were as strong or stronger than the parent material. I tested the pieces using a hydraulic press reversed to pull apart the welded pieces. Although this was not a scientific test it gave me an idea of how strong the welded material really is.

If you take a close look at the photos of the Boulder HDPE drift boats you can see that they are welded sheet.

I think my next boat building project will be a combination of wood and HDPE.

 

It's a good material. Nice and strong.

 

Yes rotomoulded HDPE is very strong but this is partly because it has so little stiffness that you have to use loads of it to match the stiffness of other materials like GRP, this makes it very heavy. Anything is strong when you use enough of it, the trick is to get high strength/stiffness and low weight. As John says Twintex goes along way toward acheiving that.

It also has the additional advantages:-

1. Impervious to water, unlike GRP which absorbes water through osmosis
2. Excellent resistance to abrasion, and impact resistance, better than GRP or HDPE.
4. High stiffness for its weight (same as GRP)
5. Easily recycled.
6. No Toxicity or emmisions.

The British Army use it for their landing craft and RIB's bacause of its toughness.

We have been using it for our multihull and we are very pleased with it.




Gareth
Warob Marine
www.fourhulls.com

 

The average wall thickness in polyethylene kayaks is 0.150", and those are big parts with uniform surfaces, where in many cases they don't use HDPE but softer grades I.E. MDPE (Medium Density PE).
HDPE is pretty stiff: In closed, small parts such as the 4 hulls of your boat you'd probably be OK with about 0.100" wall thickness if you aim at getting a low weight.


Yoav

 

Yoav,

I am not an expert with HDPE and everything is relative but if you compare the respective Modulus of HDPE, Twintex and GRP both GRP and Twintex are between 15-30 times stiffer than Rotomoulded HDPE depending upon the formulation and weave.

i.e Twintex and GRP 15-30GPa whereas HDPE is 1GPa. So while it may be pretty stiff it is not as stiff as GRP or Twintex by a long margin.

All the best

Gareth

 

Gareth,

I'm not an expert either and HDPE is not the stiffest material -It's less stiff than CF for example, but it's stiff enough for smal boats while being highly resistant and low-cost as well.
How thick is the wall in your boat's hulls and how much do the 4 hulls weigh together? -My point is that sometimes it's worth to sacrifice money and/or strength to get a boat that's lighter, and sometime it's not.
Using HDPE can also make the price per boat unit too high if you run small production series.

Yoav

 

I didn't know about water absorption. Good to know.

 

Yoav,

I agree with what you say, rotomoulded is the way to go if you are looking for a high volume boat where weight is not too much of an issue. With us weight is a big issue.

Our hulls are a sandwich construction and the thickness varies throughout, they weigh 10.5kg or 42kg for 4. When we were getting quotes for the hull, we were finding that rotomoulded came out about twice the weight to get equivalent stiffness.

Gareth