Lateral strength?

I will be building some kayak carts out of PVC, and I was wondering about side stress, using schedule 40, versus schedule 80? I noticed you mentioned that 80 is thicker than 40, and will handle higher pressure / higher temperature. Will this equate to higher sideways stress?

My cart will start as a basic "H" shape, with additional cross bars at the bottom, (where the axle will go through), and up the body, for bracing, with the top of the "H" left open. The top will get slipped into the scupper holes, and allows the kayak to be rolled along without the need for strapping the kayak to the cart. The last one I built worked well, but when I pulled my boat over a curb, the lateral stress caused the PVC to break. That cart was made out of schedule 40, and I was wondering if building it out of 80 would help?

On a related note, it there any material I can pour into the cart, to increase strength? My initial thought was to fill it with epoxy, though that would be expensive.

Thank you for this great class.


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jef400dread20 days ago

I hear ya with the idea to have the kayak sit in the scuppers, but I'd warn against it. You already know that the smaller size required to fit in said holes lacks the rigidity to withstand much lateral force. If you add too much strength to the cart (like if you built it from metal tubing instead) those lateral forces that previous caused the cart to fail, can break the plastic of the kayak scuppers - a much worse problem. Then you've gotta find color matching plastic from the kayak manufacturer, teach yourself how to weld plastic, then hope like hell it holds up the next time you're on the water.

I built one using this design(with the recommended 1" pipe):

It eventually broke and I rebuilt it with 1.5" pipe, and it works to transport a loaded Hobie Outback, with a rigged weight exceeding 100lbs.

criffster (author)  jef400dread19 days ago

Good points, regarding stronger material.

Thank you for including the design you built.

I've been thinking more about this...and I think Hobie does (or at least did) make a cart that is designed to fit inside the scuppers in at least one of their fishing model kayaks - perhaps the Pro Angler. I remember the cart was made of Aluminum. Unless you have a Hobie, or see something on your manufacturers website that shows pictures of a cart using those holes, I'd assume that they were not engineered to withstand lateral forces or possible impacts in those sections of plastic. I imagine that there's minimal thickness of plastic used within the walls of those scupper holes, and if one of those breaks, you're taking on water.
And you can drop the pool noodles from that design. A few years ago when I started kayak fishing, I used pool noodles everywhere - the cart, scupper plugs, on the roof all rips off eventually.

Hey there, honestly with the kind of kayak you are making, you're just better off increasing the diameter of pipe that you are using so that there is less flex (unless you want a lot of flex). Either schedule will work since you are not actually plumbing water through the pipes.

Where in the cart are you trying to pour material? Into the pipes? Epoxy is always a last resort.

criffster (author)  audreyobscura1 month ago

Hi Audrey, thank you for the answer.

The most limiting factor to pipe size, is the diameter of the scupper holes. They are about 1" across. 3/4" PVC fits easily enough, but 1" is a very tight fit. I agree that pouring epoxy into the PVC pipe should be a last resort. The weak point are the two vertical pipes, which run from the scupper holes down to the axle. Maybe I could slip a piece of 1/2" square tube aluminum in each upright?


Could you insert a smaller diameter metal conduit pipe? It sounds like the strut/tube to the axel is

not super long.

3/4" is still pretty rigid and strong. If you have relatively short segments between cemented joints, it becomes less flexible. If you need to bend a piece, don't include any connectors between the segments.

In general, I wouldn't want to put anything besides water or sand in the tubes.