Plastic canoes and kayaks are relatively inexpensive and highly impact resistant. However, plastic is heavier than fibre composites, and less rigid. The resulting flex can also make plastic boats less efficient to paddle. Plastic is also susceptible to denting or deforming and can develop drag-producing scuffs with use.
Almost all plastic boats are made of some kind of polyethylene. More expensive boats, and those intended for rougher use (such as whitewater, surf, or high-end touring boats) are made with better grades of polyethylene that are stiffer and/or more resilient. Manufacturers often give their particular polyethylene blends a proprietary name.
The cost, durability, and functionality of plastic boats is generally determined by type of plastic, number of layers, and how the plastic is formed into a boat.
Roto-moulded boats are made by placing plastic powder into a metal mould. The entire mould is then rotated and twisted while it is heated to melt the plastic. Boats made of a single layer of roto-moulded plastic are very inexpensive, they're tough, and adequate for very short, slim boats, such as whitewater kayaks. But for longer, wider boats, single layer plastic deforms too easily. Adding different materials during the moulding process creates multi-layer boats that are stiffer, more durable, and have some built-in buoyancy. Similar to laminates, adding layers improves performance, but increases the weight and cost.
Blow-moulded boats are made by closing the mould around a tube of molten plastic. Gas is blown into the mould to compress the plastic. These kayaks tend to be more UV and abrasion-resistant than roto-moulded models. They are stiffer, tend to deform less, and require fewer structural reinforcements, which creates more usable inside room.
The newest technology for moulding plastic boats involves laying a sheet of plastic over a mould, then sucking the air out from inside the mould, so that atmospheric pressure pushes the plastic into shape – an inside-out version of the vacuum bagging used to make some fibre composite boats.
The cost of thermo-moulded boats is somewhere between high-end plastic and entry-level fibre composite. While they're lighter than other plastics, they're heavier than many fibre composites. Repair is also trickier: you can't re-coat or patch plastic the way you can fibre composites. But thermo-moulded boats offer a pleasing, high-gloss finish that may also decrease drag in the water. They're also fast and tough.
Single-layer plastic is too floppy for the open structure of canoes, so multi-layer plastic sheets, such as Royalex® or Polylink3™, are required. The inner layers of laminates are tougher plastics, such as ABS, or sheets of foamed plastic. These layers add stiffness, performance, and buoyancy without significantly increasing weight.
Laminates are rigid enough to make very sturdy (though somewhat heavy) canoes. Some are so resilient they can spring back after being folded in half across river rocks. Royalex also has a "memory", even severe dents smooth themselves out when the material is exposed to sunlight.
The surface of scuffed, fuzzed plastic produces considerable drag in the water. Brave boaters have been known to gently apply a blowtorch to melt away this fuzz. If you do so, it's at your own risk: no warranty covers burnt boats.
Plastic is less rigid than fibre composites. Don't strap your plastic boat too firmly to a roof rack in warm weather – it can be deformed. Gentle heating in the sun or with a hair-dryer will sometimes cause a plastic boat to spontaneously pop back to its original shape.
Long-term exposure to ultraviolet radiation can degrade plastic boats, so store them indoors or under UV-proof wraps. Periodically rubbing your boat down with an UV-blocking wax or spray, such as 303 Protectant, will also help extend its life.
Read Fibreglass Kayaks for information about fibreglass and Kevlar® boats.