Basics on Plastics for small boats
.
As 'plastic' is also a recognized and quite popular small boat material, I thought it should at least be included in the list, even though it’s the one boatbuilding method listed here that is not available to the homebuilder.
Because of that and also that I’ve no direct experience building boats with these materials, I will just outline some basics.
Although Polypropylene (PP) is also used for a few boats, Polyethylene (PE) is by far the most commonly used plastic but even aside from any pollution it causes, it has three major physical faults. Its flexible, its heavy & its surface is problematic, as it gets rough with use and does not hold paint well.
Almost anything can be molded using Polyethylene once the expensive cavity molds* are made, but as it’s SO flexible, the shapes used can not be flat but MUST be rounded with lots of ‘knuckles’ and added edges, to give the boat at least ‘some’ stiffness. This flexibility limits the size of the boat for which PE can be used to generally about 18ft and even then, only for a slim boat, so I would say up to a maximum limit of Length (ft) x Beam (ft) of about 45.
*Polyethylene plastic in small beads or powder foam is feed into the mold that will control the eventual hull thickness. The mold is then heated and rotated (hence the expression 'roto-mold'), to distribute the plastic and then it/s allowed to cool before removing the part. As one can imagine, making the inner and outer molds perfectly match each other with a precise gap is nigh impossible, so the final hulls extracted will also have a slight variation of hull thickness over the whole surface. But once the molds are made and fine tuned, this process can produce boat hulls 'fast'. like making muffins in a bake pan ;)
The material is very tough though, so works well for small boats that will see rough treatment.
Small ‘knock-about’ dinghy tenders, kayak rentals and kayaks for running rapids (hitting rocks) are the main ones. But as the material is relatively heavy, it’s seldom used for competitive use where other options are permitted because boats of ABS, fiberglass, Kevlar and carbon fiber are each progressively fairer, smoother, stiffer and lighter … making each boat material progressively faster with rising cost.
IF the boat is very small and all boats made nominally the same, then such boats can be raced competitively even if no two roto-molded PE boats are ever exactly the same. The BIC O’pen Skiff is one good example though … and it works as it’s only 9ft long x 3’9” wide, Even then, the bare hull still weighs 100 lbs. By comparison, a hi-tech 11ft Moth hull in CF only weighs 30 lbs. The O’pen Skiff 
shape is (and needs to be) very rounded in all directions in order to have enough stiffness. Boats of polyethylene do not have 1 sqft foot of flat area … it would ’tin-can’ (flip in-and-out) if it did, so such plastics that are not stiff can never replace plywood for flat surface shapes, unless used in some form of sandwich.
(Side Note: Hard to not notice the large bow wave from the 'bowl' bow. You will never see that with fine vertical sides, but its the drag effect from forcing lift via a Vee'd bow)
Typical panels to replace plywood are compressed composite panels with a light, weaker core and hard, strong but thin exterior veneers. Coosa (a polyurethane panel with a small amount of fiberglass) is one exception but it’s still about twice as flexible as plywood of the same thickness, so needs to either be thicker or have internal framing to support it, and even at the same thickness, its more expensive. Hard to handle too with fine FG fibers often rather loose on the surface.
Until carbon-fiber came along, the most competitive boats were ALL made of wood .. mostly laminated in some way, or of strip-cedar with a thin fiberglass skin. Quality marine plywood was also used.
Take a 16ft sailboat such as the Wayfarer that is 6ft wide. Long made of painted plywood, it was finally copied in fiberglass. They now race together but even if 30+ years old, the plywood boats are often still faster, as for the same weight they are stiffer. For this and many other restricted racing classes, carbon–fiber is not permitted due to cost.
A Wayfarer of a plastic such as polyethylene would be like a wet spaghetti noodle … totally distortable and VERY slow. Polyethylene surface finish is also matt and not very smooth. It can be waxed and looks ok when new, but soon gets scratched up, so such boats are for knock-about only. I own two polyethylene kayaks … but they are not fast and as already mentioned, they are heavy … but the bonus is that they require almost no maintenance and they are just about unbreakable. A typical single kayak weighs about 60lbs though. In plywood it could be built for 45 lbs and still be stiffer, and in carbon fiber, more like 35lbs.
So with plastic, the only way PE might match plywood stiffness is either to be ribbed all over or find ways to create a composite panel.
The Laser Pico does that, but it’s not cheap! The Laser Pico, uses a foam-core, molded in place with an inner layer of thin polyethylene.
Perhaps there is a way to create a foam core from old plastic (something looking like the core of an Aero chocolate bar) and then bonding skins to that under pressure ….perhaps skins of thin bamboo could be in our future too? Who knows.
Such a sheet might then replace plywood and even be used with the Stitch & Glue or the ABC-system for boat designs that suited such systems. A sheet with a 3mm cork core and skins of something sustainable or biodegradable, would certainly be better for the environment than using ANY plastic.
Here are a couple of other articles on the subject if this interests you.
https://www.boats.com/reviews/plastic-fantastic-2510/
originally written - Mike 2020
Posted Feb 2022
Your comments are welcome and can only help to make better boats in the future, either with plywood or other competitive panel materials that occasionally come on the market.
"See the Copyright Information & Legal Disclaimer page for copyright info and use of ANY part of this text or article"