Sponsons below the waterline

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 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

Let me be Chief Brody to your Quint. "You're gonna need a bigger boat."

I don't mean need--I'm sure you can make it work with the boat you have. My question is why mess with it? Just get a more right-size boat.

My $0.02, and over-priced at that.

I'm not an engineer ! I believe, Sportspal/Radisson meant for those Sponsons, to add flotation, to an aluminum hull, in the case of a capsize, not to add capacity.

They may have also wanted more stability, by widening their hull.

An aluminum hull. without flotation, would sink.

As Chip suggests, I'd get a bigger boat to fill the bill.

@dhdallas, welcome to the forum.

I'm certainly not an engineer but, yes, I would think that the addition of sponsons below the waterline would increase your ability to load (or overload) your boat. That said, I'm not sure how you could properly estimate the amount of floatation gained.

Given the current displacement of your hull, I doubt that the addition will result in an almost 30% increase in capacity.

Unfortunately, I suspect that Chip & Jim are correct: you'll probably need bigger sponsons or a larger boat in order to displace that additional 100# of water.

One risk I see is that the boat's secondary stability may not be enough to keep it from swamping easily. Secondary stability is the force that tries to keep the boat upright as you lean it. Your boat's hull gets wider near the top so when you lean it over the wider part resists leaning and keeps you upright. Having submerged sponsons will totally change the shape of the sides and make them more vertical and vertical sides do nothing to keep the boat upright when leaned. So if you mess with the secondary stability of a boat rated for 350 pounds and then put 450 in it the boat may roll over sideways too easily when you move around in it or experience waves. With sponsons above waterline just one goes underwater as you lean so it provides buoyancy where you need it; with both underwater the buoyancy forces cancel (for leaning the boat) since one will resist leaning while the other one will push to make the boat lean more.

I'd also be curious to see what happens when the heavier person gets into the front of the boat. It may be quite wiggly since the ends of the boat don't have the volume to support the weight properly especially if the sponsons don't run the full length of the boat. Maybe you'd have to both get in at the same time.

As a former lifeguard I have to say the idea makes me uncomfortable since you're adding risk.

i totally agree with Gumpus above. It makes no sense to me to attempt that configuration.
Plus, how would you securely attach the sponsons below waterline? if your motor gives you any advantage in any speed all, the force of water drag will llikely rip the sponsons off. If only one is lost, someone is going to get wet or worse.