Preserving hull integrity in polyethylene hulls
Polyethylene is a remarkable material with positive buoyancy and a degree of flex and impact damage resistance not found in fibreglass hulls
Due to the roto-moulding process, where polyethylene beads or powder heated to around 150 degrees in a steel mould literally flow into the bull shape required, solid colour throughout the hull is possible.
My first experience of a polyethylene hull was a Nylex Pioneer 8 dinghy I borrowed in 1987 as a tender to my 6.7m plywood Bluebird yacht. The hull had sufficient flex not to damage the painted topsides and cleverly Nylex had moulded in a bottom channel section that not only gave the hull good directional control when rowing, but also provided a strengthening point for the internal liner.
But the main issue was the weight, which at around 50kg was quite heavy for a 2.4m dinghy.
The weight issue is common in smaller polyethylene hulls as the material must be thick enough not to flex over waves or at the transom when maximum recommended power is fitted. For example, the polyethylene 300 Tuffy (rated to 15HP) from Polycraft in Bundaberg weighs a hefty 108kg, compared to around 60kg for a three-metre aluminium punt. However, as hull sizes increase the weight get closer to comparable fibreglass hulls. For example, Polycraft's 599 Frontier weighs 940kg and is rated to 175HP, whereas a six metre Seafarer Victory weighs around 1100kg and is rated to 225HP. The clinker sides in Polycraft hulls add to localised topsides and longitudinal strength.
Polyethylene hulls have some longevity issues, the major one being hull support on a trailer. Using a multi-roller trailer suitable for a fibreglass hull is not recommended as the material has poor ability to handle localised stresses imposed by the rollers and can actually bow inwards, ruining bottom shape. Only full skid trailers that support the bilges from transom to forefoot can be used, with rollers for the hull centreline. The bilge skids should angle downwards aft of the transom to help guide the hull on to them when retrieving the hull and have a soft rubber topping to prevent scuffing the material.
Because of the thickness of the material used in polyethylene hulls it's possible to attach fittings with self-tapping screws but I suggest not doing this as the relatively soft nature of the material provides poor grip for the screw threads. If screwing on a fitting to the gunwale, such as a rod holder, I recommend fitting a plywood backing pad (preferably varnished to prevent end-grain de-lamination) and through bolting the fitting to the gunwale instead of screwing it into the pad, using a drill at low rpm to prevent melting the polyethylene through drill bit friction. And fit a washer top and bottom on the bolt to help spread the load.
It is important to preserve hull integrity
Another aspect of polyethylene hulls is that on 40–50-degree days in Oz tropical climates the material can go soft, and flex more than it would normally. This is particularly noticeable around the transoms of larger hulls when using maximum rated power which can actually lose strength. The suggestion from boaters who use polyethylene hulls in the tropics is to keep them in a carport or garage when not being used, away from direct sunlight. Australia has the highest amount of ultraviolet rays of any country in the world and this can affect longevity of polyethylene. The good thing is that when the material cools it returns to its original shape, unless it's been sitting on a multi roller trailer.
Scuff resistance is another issue and I recommend that, just like with a fibreglass hull, not leaving a polyethylene hull on a concrete launching ramp while retrieving the car and trailer. Fortunately, if the hull is gouged from hitting a jetty when coming alongside it's possible to repair the damage with a heat gun and a strip of polyethylene.
The gouge is sanded to smooth the sides then a heat gun applied to the polyethylene strip to literally fill the gouge then when cooled lightly sanded back to level with the surrounding material. However, there'll always be a surface gloss difference between the original moulding and the repair but really no different to fibreglass when repaired unless the area is gel coated.
Google Polycraft and check out its clip on how to repair a polyethylene hull.