When shopping for used plastic boats, what is the best way to check for oil canning?
Oil canning happens when the bottom
flexes up and down while paddling, so is best identified while paddling.
Dents, aka wows, are visible just by looking at the bottom of the boat while upside down.
visual of hull
will reveal issues. Just look for significant deformation (usually into kayak) typically resulting from too much force against roof or storage racks combined with heat from sun. Some can be reversed with gentle and judicious application of heat. Poly is pretty tough and can with stand much abuse both from paddling and handling. Also look for symmetry left and right to center line as that can induce tracking issues.
If your definition of oil-canning is warped, or dented areas, it’s just a matter of looking. In this regard, l’m assuming you are generally referring to polyethylene boats. You will need to sight down the keel from bow to stern and visa versa to see that it is straight (boat upside down).
My definition of oil-canning would be areas of the hull, or deck that are soft, or squishy. On most boats, you will find a few spots that are a bit flexible, but it’s a matter of degree and exactly where the flexy area is. You have to distinguish between normal flex and flex that might be due to some type of damage. In which case, there should be other clues.
Some people define "oil canning" as a permanent, wavy deformation of the hull bottom that occurs most often in triple-layer polyethylene boats.
The more widely accepted definition of oil canning is a hull bottom that flexes up and back down in response to wave action or shallows, somewhat akin to the thin metal bottom of an old fashioned oil can that one popped in and out with the thumb to dispense oil.
If you are referring to the later, in my experience many canoes demonstrate this, especially thinner Royalex boats or single-layer polyethylene boats. Boats with a shallow V bottom or a lot of continuous rocker tend to have a bit more rigidity in the hull bottom as a consequence of their structure, and are less prone to oil canning.
If you turn the boat over and press on the hull bottom and it is relatively easy to push it in, it is almost certainly going to oil can. The only way to know for certain though, is to take one out and paddle it.
There are variations in the thickness and quality of Royalex also. In the early days of Royalex, manufacturers were more likely to specify relatively thick sheet that was more rigid and durable, but heavy. As years went on, makers speced thinner and thinner sheet more prone to wear and oil canning.
In the opinion of a majority of whitewater paddlers, the quality of Royalex took a nose dive in the early 2000s when OSHA requirements mandated a change in formulation. Also, during the peak years of Royalex canoe construction, the larger manufacturers stocked sheet in inventory and the Royalex tended to "cure" before the canoes made it to the market.
Also I have read that some manufacturers rejected up to 25% of sheet produced. In the latter years of Royalex makers tended to order sheet only at the start of the production year and the material was still relatively "green" when the boats reached market. I suspect that as the supply of Royalex started to dry up, less and less sheet got rejected.
Objectively written reviews
will make mention of oil-canning, if there is any.