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
My first thought about this is that you need a bigger boat. That canoe seems very small to me because I have a 12’ Jon boat which is quite a bit wider than your canoe, and it’s a bit of a stretch to put two adults on board and then power it with a motor. It can be done, but only in very calm conditions. Of course, in recent years, the fact that the vast majority of fishermen who use boats have convinced themselves that gigantic boats powered by motors with hundreds of horsepower are the only viable option has resulted in a shortage of small boats. Boats of the size that you really need are no longer being built, and that has nearly dried-up the used market for them as well (since most people that have such boats are not parting with them). It’s a real shame that finding small boats nowadays has become so difficult.
So here goes, addressing your actual question. I looked at pictures online of Sportspal canoes, and I made a very rough guess that their sponsons are 6’ long, 6" high, and 4" thick. If that’s the case, each sponson has a volume of close to 1.0 cubic foot, so a pair of them would be close to 2.0 cubic feet. The density of water is 62.4 pounds per cubic foot, so if the volume of the sponsons matches my rough estimation, the pair of them would provide roughly 125 pounds of flotation when they are fully submerged. So it seems you might have a viable plan. Again, be aware that my estimate of dimensions of the sponsons (and thus volume) is extremely rough, so the flotation value might be a bit higher or lower than what I came up with.
One thing to consider is whether the sponsons are durable enough to provide such flotation continuously without over-stressing the attachment points. Is there some kind of reinforced “backing strip” which bolts will be clamping against the hull, or is the entire sponson made of fragile foam which might let the bolts “pull through” after a while?
Finally, no discussion of canoe load capacity is complete without talking about how load capacities are defined and what they mean. Usually, the stated load capacity corresponds to the boat being weighted down into the water such that a certain amount of freeboard remains (freeboard is the height of the hull that is above the water). I think a freeboard of 6" is commonly used to define maximum load on cheap canoes (some makers of good canoes make more reasonable estimates of capacity, defining it with a freeboard that’s greater than 6 inches). In most cases, a freeboard of only 6 inches is getting to be a pretty delicate situation as far as safety or the boat’s capabilities go. A canoe loaded down that far is much less stable than when it is less deeply seated in the water, and of course waves and leaning of the boat by moving passengers become problematic. Related to that is the difference between a cargo load and a live load, and some boat makers specify different ratings for each kind of load. A boat loaded with people is far more likely to have stability problems than a boat loaded to the same degree with cargo, because people are “higher” than cargo, and because people move around. Depending on how the load capacity of your boat was calculated, you may have kind of a dicey situation when it’s fully loaded, and of course that would apply for the target load capacity of the modified boat too since the added amount of flotation is roughly the same as the degree to which the boat would be overloaded right now in its bare condition. If you have already used the boat when loaded to either 350 or 450 pounds, you might have a better idea than I do about whether it will be reasonably capable of carrying that target weight after the sponsons are installed.
Finally, here’s hoping you are using a motor of reasonable power (that is, one with not much power).
Your reply is exactly the well thought out response I was looking for. The sponsons are 9’ long and 4"x3" thick. You point out a probable problem as I had not thought of the foam pulling against the bolts. If I do it, I will glue and bolt them on. Also I will take into consideration the load balancing situation. I plan on filling the canoe with 450 pounds of water and then marking the waterline so I know exactly where to align the top edge of the sponsons but like you said, a static load is not like a top heavy dynamic load. My motor is the smallest electric trolling motor made with 18 pounds of thrust so that won’t put any real added stress on the foam. This canoe will only be used on very calm lakes & streams. I am a tinkerer at heart and sometimes my crazy ideas work perfectly and other times…not so much. I should wear some protective padded clothing because if the canoe tips and dumps my wife in the water, she might whack me with a paddle. If I do it, I will post the results with pictures. Thanks so much for your reply. It really gives food for thought.
These sponsons appear to be available with stainless hardware and large fender washers to hold the sponsons in place. While you could additionally glue the sponsons to the hull it might not be necessary. You could always do this later if need be.
If you contact the manufacturers they may have suggestions as to optimal placement.
If adding a motor be sure to check local regulations as far as registration. Many states require motorized boats be registered which may also depend on size of the boat and engine.
Okay, here’s a followup to your additional comments. First, the pictures I looked at online showed sponsons on very short canoes and I know the ones I saw were not as long as the ones you are getting. It’s good that you will have this extra length, especially since they are not as bulky as I guessed. You probably figured out the workings of my calculations, but if not, your proposed sponsons are a bit smaller in volume than my first estimate, being about 0.75 cubic feet each. That gives you a total volume of 1.5 cubic feet and a total flotation value of about 95 pounds, which is still in the ballpark for your desired increase of 100 pounds.
You may have figured this out since you wrote about it, but I’ll mention it. If you load the bare boat with 450 pounds and mark the waterline, intending for that to be the location of the top edge of your sponsons, the sponsons will actually make the boat sit a little higher than that once they are attached. That may be fine, in that it might be close enough, but it will miss your ideal objective by some amount (but see the next paragraph too for a different viewpoint on this). For a boat that has non-uniform volume distribution throughout its height, I can’t imagine a good way to locate where the “450-pound waterline” will be after the sponsons are installed, unless it’s determined WITH the sponsons installed. Therefore, if it’s convenient for you to play around by the water for a while, you might try clamping them in place temporarily while you figure this out.
It would be good to make sure your sponsons are not mounted any lower than necessary, as the boat will have better stability if the sponson on one side starts breaking the surface as soon as the boat leans a little bit (if both sponsons are well below the waterline, the boat will have to lean farther before this unequal flotation kicks in and helps keep the boat from leaning farther still). You will get an even stronger “corrrective force” that opposes any leaning of the boat if both sponsons are at least slightly breaking the surface when the boat is level, since in that case, when the boat leans, one sponson immediately starts providing less flotation and the other starts providing more. Of course your boat will be sitting slightly deeper in the water in that case too, but the slightly greater ability of the sponsons to contribute to stability might be a good thing. And of course, that’s the situation you will end up with if you determine the installation height of the sponsons by the method you first described.
There are lots of non-answers there but it’s probably more food for thought.
My comment about the motor was more about safety and boat behavior. Very strange things happen when you try to push a small, heavily-loaded boat at any kind of speed (I could tell you how I know, but I write long enough posts as it is!). You won’t have such issues with that little electric motor.
You could temporarily clamp the sponsons on the hull and then add the weight of water to determine optimal placement. It would take approximately 54 gallons or water to equal 450 lbs.
If you know the floatation of the sponsons, the easier method would be to subtract this from the 450 lbs of water. This should give you the waterline with the boat at capacity with the sponsons properly installed.
Carrying capacity is rated for a live load and includes a safety margin. In theory, recreational monohull boats up to 20 feet in length are required to have capacity labels indicating what’s safe to carry in terms of engine horsepower, cargo (including gear and engines), and passengers, although this does not to apply to kayaks, canoes, or other vessels not designed to have a motor. If your boat does have this label, again in theory, if you ran across a really cranky government official you could possibly be cited for being over capacity as add-on sponsons probably would not count.
If a boat was sold with attached sponsons, this would probably be figured in on the capacity label.
Thanks for the helpful comments. I will have to figure out how to clamp them on temporarily to see if the idea is workable.
I will definitely have to clamp them on so I can find an optimal position. Thanks.
I had already mentioned the idea of temporarily clamping the sponsons to experiment with installation location, but yes, that would work.
But the other comment deserves a response.
“If you know the floatation of the sponsons, the easier method would be to subtract this from the 450 lbs of water. This should give you the waterline with the boat at capacity with the sponsons properly installed.”
This would in fact be easier, but unfortunately the result will not be correct. This is precisely the kind of false logic that I was addressing when I made the comment earlier about canoes having their volume unevenly distributed across their height profile, though I suppose I could have spelled it out more specifically and explained what that really means. There are no canoes which sink incrementally deeper into the water in a linear fashion in response to incrementally increased loads, and again, the reason is that the volume is non-uniformly distributed along the height of the boat. Volume increases with draft at a rate that is faster than a linear progression, and also which is not consistent in its rate of increase at various draft values. The method described in the quoted passage might turn out to be “close enough” when applied to a canoe that is already well beyond its realistic load capacity, so that any change in waterline occurs within a height on the hull where flare is minimal, but even the lesser degree of flare which still is present at that height (and don’t neglect to consider the contribution of the sloped stem and especially the transom) can easily add up to enough displacement to make a big difference. Think about how just two inches of water in the lowest part of a canoe - not even covering the floor - can make it far too heavy for two people to lift, as a way of picturing how much additional volume might be accommodated within those zones of the hull which have even minimal flare. I hope this makes it easier to visualize how erroneous it is to expect the waterline to change in a linear fashion.
On the other hand, 100 pounds doesn’t change the draft of a canoe by very much once it’s loaded beyond the zone of most-rapid flare, and it might be that trying to nail-down the details this accurately is pointless. “Close enough” might be close enough. But I did want to explain why I had that earlier comment since the meaning of that remark apparently didn’t register.
Getting away from the issue with a very heavily-loaded boat, for any canoe that’s carrying a load that’s within a “reasonable” capacity rating (usually this would be much less than a rating that’s based on 6 inches of freeboard), the change in draft with changes in weight is extremely non-linear, because in this case the waterline is located with that zone where the most pronounced rounding occurs in the transition from the sides to the bottom. Bell Canoe used to have a website with data illustrating this. I don’t know if “the new Bell” provides similar information.
If an unloaded canoe is weighed down with water the weight distribution should be close to determining the degree to which freeboard can be determined. Obviously how the boat is actually loaded may vary from this determination.
This whole discussion seems to be a risky attempt to make a boat suitable to perform safely outside of its design parameters.
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Thank you everyone for all the info and advice. I can’t figure out how to delete the post so I just muted all future notifications. If I go ahead with the sponsons next month, I will post the results.
Indeed, the 6" freeboard capacity is a volume capacity that doesn’t tell you much about the load with wich a canoe performs well for its intended use, often called ‘performance’ capacity. With deep canoes their `performance capacity’ is often far less than their 6" freeboard capacity – often not even half of that…
For other kind of uses than the canoe is designed and intended for, the usefull capacity could be more or less its ‘performance’ capacity.