Polyethylene boats

[magneticsailor]

I see hobby has embraced rotomoulded polyethylene hulls. Why is it that other brands have not? They are much more resilient and handle the sun better than fiberglass. I realize that they may be too heavy for racing. But they seem to be popular with leisure sailing. What thoughts do you have on the matter? I am curious.

[magneticsailor]

I see hobby has embraced rotomoulded polyethylene hulls. Why is it that other brands have not? They are much more resilient and handle the sun better than fiberglass. I realize that they may be too heavy for racing. But they seem to be popular with leisure sailing. What thoughts do you have on the matter? I am curious.

I meant hobie. Stupid auto correct.

[tonyquoll]

Recently browsed new Kayaks, where the vac-moulded hulls were dominant in the shop. They were cheaper, and the salesman boasted "you can watch a YouTube clip where a guy hits it with a sledgehammer - and the hammer just bounces off!"

For a manufacturer selling beginner boats with warranties there are a lot of advantages; cheap to produce, very durable & impact resistant.

However an ideal hull would be light and stiff, rather than heavy and flexible. Weight adds interia, making the boat / kayak slower to accelerate, and they sit lower in the water producing more drag. The flexible bows deform every time they slap a wave, which absorbs energy. A light, stiff hull will accelerate faster, have a higher top speed, and with stiff bows will maintain speed through waves & chop better.

The plastic kayaks weighed around 20kg, while Kevlar hulls are about 11kg. Kevlar also costs about twice as much. Not sure about home-made ply, as some make 20kg+ very strong ones, others very light racers.

In conclusion I see the plastic moulded boats & kayaks as designed for beginners, who dont know how to look after a boat and dont know how to make them go fast anyway. Curiously people develop a bond with whatever craft they have, and will see it as the best. As a Paper Tiger sailor, I love my stiff, light ply hulls which make it a fast racing boat, and extremely responsive and fun to sail. I do take extreme care to avoid damage, for example always using a trolley to get in and out of the water. Unlike, say, the Hobie 14 sailor I saw in Canberra sail his boat straight onto the shore, WHAM, and drag it over the concrete to the rigging area. Sheesh, if you want to treat a boat like that, and hit it with a hammer, them yes, a plastic moulded boat is for you.

[magneticsailor]

Thanks for that. Would foam sandwiching the polyethylene help stiffen them? I heard that somewhere.

[tonyquoll]

Dont think that would work, unless you're talking polyethylene mat as a replacement for fibreglass mat.

The Kayaks I saw saw were vacuum formed: a sheet is heated up to make it flexible, then sucked into a female mould to form the shape, then cooled to set it. It's a very quick process, but there's no opportunity to get foam sheet in there. The hull & deck moulds are later glued together. Vac-forming can be done fairly cheaply, as moulds can be CNC machined out of custom-wood, and the plastic heated with simple bar radiators.

You mentioned roto-moulding, where a liquid plastic is poured into a hot mould, which is rotated to form even spread of liquid over the surface. Once cooled, the mold is split and product is formed. The advantage is a hull could be formed in a single piece, with no joins. The massive machine required to spin a heated boat-sized mould would be very expensive to set up and run.

Foam-core hulls are made into a female mould, itself usually fibreglass made moulded off a male plug. The process involves spraying mould release, gel coat, laying cloth, spraying resin, laying the foam, adding more cloth, and a final spray of resin. A huge plastic bag covers the job and vacuum helps suck it down and hold it all tightly into the mould. As the foam is coated with lqiu8id resin on each side, there is good bonding and a super-strong, stiff composite is formed. Each hull and deck moulding takes a few hours to lay, and a few more to let set.

Same process can be done with carbon, kevlar or mix replacing the fibreglass cloth, in which case vinylester or epoxy would be used instead of polyester resin. This makes the craft much stronger and stiffer, so typically less material is applied to make it also lighted. The materials add substantially the price; eg:$6000 foam-core fibreglass hulls, $9000 for carbon hulls.

Back in the 70's, before foam-core was invented, people sprayed resin and chopped strand fibreglass into a female mould to make a hull. These were quicker to build than ply boats, but heavy and flexible - similar to the today's plastic moulded boats. In most fleets this technique went out of favour as soon as foam-core boats hit the market; light, stiffer & faster. I had a foam-core boat that was over 30 years old & still within 2kg of minimum weight (50kg).

Plywood craft are usually cheapest in materials cost, but very labour intensive, taking weeks or months to build, depending if wood drying and seasoning gets involved.

(Disclaimer: I've studied Industrial Design, Mechanical Engineering, worked briefly in a boat-builders making foam-core boats, in factories manufacturing plastic products and for a motorhome manufacturing making a plug & mould. I've never built a boat and someone with recent building experience may be able to improve this info.)

[magneticsailor]

Thankyou very much , I shall take all that on board.