I have a square stern 11’ 2-person fiberglass canoe with a weight capacity of 350 lbs. I need to increase that to 450 lbs. If I buy the foam sponsons made for a Sportsal/Radisson canoe and mount them outside just BELOW the waterline, that should make a noticeable difference, wouldn’t it? Yes, I know it will add drag but I am using a motor so that is not an issue
Increasing weight capacity
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Re: Increasing weight capacity
Adding extra volume to an object will increase it's displacement of water, ie it will sink into the water less (obviously, whether this helps depends on how much lighter than water this extra volume is - foam would work well, whereas lead wouldn't...) But safe load carrying also depends on other factors, eg stability/resistance to capsize. Unless someone has already done the legwork with your proposed modification, you'd need to be doing a load of trials to find out how it behaves in different conditions. Until you've done that, you really need to assume you're going to end up swimming alongside a boat which can't be righted, and limit your trips accordingly. (I've done stuff like this with boats, and it's great fun, but you have to treat it as a mad experiment, i.e. keep within safe swimming distance of shore, taking into account what you're wearing and what the sea conditions are like etc)
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Re: Increasing weight capacity
The nominal load capacity for kayaks defined by the manufacturer will be set on the basis that it's safe for all reasonable types of load and water conditions. A boat with two tall heavy people on an open lake with waves is going to be a bigger risk than the same weight made up from two smaller people and some luggage in the bottom of the boat, on a canal. Some boats become more stable as you load them, others can get less stable if overloaded. Your boat has boxed in seats which mean the people have to sit on them rather than kneeling with their feet under the seat, which is the usual way to to get weight low and improve stability in a canoe.
Fixing sponsons below the surface won't be easy; each one is going to have 50lb pushing up. Assuming you're not going to make holes through the boat to fix them, you'll either need beams under the boat or across the top and somehow held down.
An extra 100lb will push that boat down about an inch into the water. You could try it with 450lb on calm water, shallow enough to stand up or near the bank, and see how it feels. Give it a bit of rocking about and if it feels safe, build up to more adventurous stuff - if it doesn't feel safe, I'd sell it and buy a bigger one.
Fixing sponsons below the surface won't be easy; each one is going to have 50lb pushing up. Assuming you're not going to make holes through the boat to fix them, you'll either need beams under the boat or across the top and somehow held down.
An extra 100lb will push that boat down about an inch into the water. You could try it with 450lb on calm water, shallow enough to stand up or near the bank, and see how it feels. Give it a bit of rocking about and if it feels safe, build up to more adventurous stuff - if it doesn't feel safe, I'd sell it and buy a bigger one.