Do plastic boats deform in water?

I think I already know the answer to this question. But a friend disagrees so I’m asking here.

Friend thinks that plastic sea kayak hulls deform in the water and this makes them slower than advanced material boats.

I do agree with him that this is true. Because the entire kayak is supported by the water evenly.

Thoughts ?

My understanding is that it could happen with single layer plastic boats, especially those with thinner plastics. But I am not sure the drag would be noticeable if you had 2 exact same boats - 1 rotomolded plastic and one of a stiffer material.

The gouges and little strips of plastic that have a habit of forming in plastic boats when they hit things are more likely to provide drag, but once again not clear if it would be enough to be noticeable with 2 exact copy boats of different materials.

The weight difference (composite and thrmoformed boats are generally lighter than rotomolded plastic) would also make for a (very) slight speed advantage.

Yeah in this instance we both have quality boats. He’s in a venture and I’m in a boreal designs. I could accept his point in pelican world.

Plastic hulls deform more.

A lot depends on design and layup/materials. I have two solo canoes that are similar in length (15’ 6" - 15’ 9") and materials (fiberglass mix). Also similar in age - ~1991. The Independence will flex it’s bottom as a wave goes under while the Rendezvous with the center rib construction & maybe 5 - 8 pounds heavier does not as best as I can notice.

A friend with 40+ years experience in OC-1 OC-2 marathon and wildwater racing mentioned the other day that he doesn’t like Carbon hulls because they are TOO stiff.

A hull that deforms will not perform as well speed wise.

I paddled a Coleman canoe once and noticed the bottom flexed with every wave that passed under. it was a barge to begin with and I imagine this did not help with it’s speed. Kind of reminded me of the bubble in the middle of a “Sorry” game.

That’s not necessarily true, except perhaps on flat water. Some Aleut baidarkas have hulls that are designed to flex along the keel, which helps them ride the waves more efficiently by maintaining a longer waterline.

Skin-on-frame kayaks are generally pretty efficient, yet their skins flex constantly.

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It largely depends on how the boat is used, and what boat.

Plastic boats do not have as much of a turn of speed as composite, thermoformed, or even wood strip boats.

Plastic boats have a tendency to oilcan when you try to push them through the water harder than they were built for. That oilcanning, bottom flex, takes some of the energy from the forward stroke. That happens in flat or big water, but big water has more energy of its own that can counter act that.

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Flex is good in a HDPE kayak when you hit something!

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All boats flex it is more a matter is the flexing perceivable and is it detrimental in any meaningful way.

To make a boat stiffer may require adding more weight and the weight penalty may be worse than the flex penalty.

From my limited experience it seems kayaks even the cheap single ply molded plastics are reasonable in not having excessive flex and my guess is having the enclosed decks and a rounder hull provide the structure that resists flex.

Canoes IMO are more likely to oilcan or hog as there is a nearly flat area on the bottom of the hull and that slight curve can be forced pretty easy to the reverse direction it wants to go. My Three-layer OT canoe had such a problem and I felt it did interfere with maneuvering the canoe as it messed with the slight rocker it should have had. It was easily fixed with a foam block added in compression between the hull and the base of the center seat. I believe I corrected between a half and a full inch more than OT original design and I feel that was a good thing. What I liked about the foam is per the above comment it still allows some flex when impacting a rock or such.

The rest of any flexing that goes on I don’t think is enough to worry about and I would much rather have flex than the alternative (breaking).

The larger the boat, the more likely it will flex. Larger rm boats flex alot more because thickening the poly walls would make the boat too heavy.

Rotomolded boats were not available when I bought my first kayaks. We called them tupperware boats when they first appeared, and they were prized because they would bounce off rocks that would shatter glass boats. But they do weigh twice as much as fiberglass or kevlar, and IMO their flexibility has a negligible effect on speed compared to their weight.

Every hull will flex and de-form. Even battleships will. It is all about by how much.

Just imagine the empty boat on the water, and then the heavy person in the middle. The center will bend down a bit, but likely not enough to make a difference for speed unless we talk really flimsy.

This is more a problem with inflatables that don’t use drop-stitch or cheaper iSUP.

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They blueprint bottoms of race boats for a reason.

Efficient because of weight.

Depends on the speed of the water and the amount of obstructions.
Flat water should not deform anything.

I don’t think Archimedes would support that statement.

Thanks, that one gave me a chuckle…