"oil-canning" repair

My plastic Nordkapp has some hull concavity (aka oil-canning) from tie-down points, while racked, because the Thule cross-bars on my Subaru outback are not at the stronger builkhead points. I have purchased a J-rack for racking and started to store it upside down to reduce pressure while stored indoors, but I wonder if I can force out the rotomold plastic memory with application of hot towels or hot water or placing it on foam rests on the ground at the bulkhead points and using heat and some weight from inside the kayak.

I have read
to suspend the boat and pour boiling water into the hull. I tried it once and it seemed to help some but did not fuly correct the problem.

Leave it out in the sun upside down
for a while. This ususally helps with returning the hull to shape.

Concave compression spots:
can be helped a bit, but sometimes they are stubborn. I find placing it out in direct sunlight, upside down on sawhorses placed under the bulkheads works, but may take several attempts. Place a smoothed out black plastic bag over the affected area and give it several hours of direct sunlight. This may do it in itself, but often it will require some clever wedging with some scrap wood. Cool the area with running water, or wait until sundown, before removing any bracing. If you can’t get at the area to brace it, try pulling the dent with a plumbers friend. That works well for me. To prevent this from happening, you can make some wooden tee’s to support the hull during transport, like a temporary bulkhead. You may just be tieing the boat down a bit too tightly. Plastic Valley’s are tough. Ken…

Can’t be fixed, I’ve tried for 5 years.
Those dips will come out by themselves if you just leave the boat upside down in the sun. They will come right back again the first time you tie it down.

May take a little more than sun alone.
My plastic Necky Looksha Sport got a dent in the oil-can zone because I had to bring it home on an inadequate rack.

I cut a slab of Ethafoam (Minicell ok also) to fit between the deck of the boat and where the keel was supposed to be. I pushed it into place, forcing the bottom to where it was supposed to be.

Then I put the boat bottom up in the hot sun for several hours. By the next morning the boat was cool, I took out the foam slab, and the bottom was back to normal.

This may not work if the bottom of your boat is not only distorted, but substantially stretched in the process of being distorted. Stretched poly is more reluctant to return to normal.

Another thing
Most people tie their boats down way too tight on their cars. Snug is all you need with most good quality saddle systems. Bow and stern lines also. I’ve never had an issue in 15 years and many tens of thousands of miles of travel with a kayak on the roof.

I see people all the time torquing the straps down with all their strength, or in the case of my better half, with her whole body weight (I’ve been telling her for years it’s not necessary, but she’s a bit stubborn and hard to convince that she’s wrong).

For a poly boat, J cradles are the way to go - you’re carrying the boat on it’s strongest dimension - the gunwhales.

Turn it over
Decks don’t dent

try some weight
I have had oilcanning on new boats i’ve gotten from sitting on store racks, from putting it away for the winter the wrong way, and from tieng them down akwardly in a pinch. Sometimes the sun itself does take the dent out, but if not i always just sat it in the grass with 2 pfd’s or old couch cushions supporting the ends so that the middle still rests on the ground and putting 5 or 10 pounds of weights or sandbags over top of the dent, it uses the heat to make it soft, and slowly pushes it out to its original position. Just don’t use too much weight or it could dent out the opposite way or overstress another area. With such a small weight it can sometimes take a few days, but i have never not had one come out before.