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.
Warob Marine www.fourhulls.com