Squirrely Canoe Analysis

-- Last Updated: Aug-05-10 10:14 PM EST --

I think I already know the answer to this question, but would like opinions of others. I built a 17+ft. Micmac stripper canoe 35 years ago. Glass over fir, 80 lbs. weight. It has a standard canoe bow, but an extended ducktail stern, intended as a tracking voyaging canoe. It's been a trooper and always done great---until this last weekend. With myself, son, and two grandkids, we totaled 660 lbs. I don't remember there being capacity ratings for the original design, but other wood canoes of same size and basic design have capacities of 624 to almost 1,000 lbs. I had always figured on a middle point of about 800+ load weight. The canoe had low freeboard, was uncharacteristically tippy, and for a boat that usually tracks like a train, couldn't hold a course to save itself, or us. It was unresponsibe, and for safety sake, we quickly called it a day. I had never had that much weight in it before, and had never seen it behave that way before, so my guess is that we were overloaded. One Old Town 17 ft. woodie in fact has a capacity of 624. I have never experienced that kind of behavior from the boat before. Would this be a typical behavior of an overloaded canoe? My guess is yes. Would anyone have more critical analysis?

Cross-Sectional Shape

– Last Updated: Aug-05-10 9:32 PM EST –

Think about two floating objects, one a lightly loaded canoe, and the other a log. We all know that logs roll in the water very easily, because if they didn't, the old-time lumberjacks would not have devised various competitions which are based on the difficulty of standing on a floating log. Canoes on the other hand are very stable.

Now think about the cross-sectional shapes of both of these objects, but ONLY the cross-sectional shape that is in the water. The canoe's cross-sectional shape within the water, as viewed from one end, is a skinny, horizontal rectangle, since nearly all of the canoe is above the surface. The log's submerged cross-section shape, viewed from one end, is nearly round (only a small portion of the log is above the surface). Now, you could take that log and carve it into the shape of a canoe, but as long as it remained solid wood (not hollowed-out inside like a dugout canoe so it could float higher), it would STILL be mostly submerged and have roughly equal dimenions in all directions. And it would still be nearly impossible to keep upright.

When your canoe is lightly loaded, the horizontal dimension of the submerged cross-sectional shape is many times greater than the vertical dimension. Thus, "tipping" the boat greatly changes the shape of that cross-sectional area below the water's surface. When you overload your canoe, its overall cross-sectional shape within the water becomes more and more like that of a nearly-submerged log, with much less difference in horizontal dimension compared to vertical. Of course, the canoe never gets quite as unstable as a log, but the lower it sinks into the water, the less difference there is between the width and height of the submerged cross-sectional area, which means the overall shape of the submerged portion doesn't change much when the boat is tipped, and that is why the stability decreases.

Squirrely Canoe Analysis
To be more specific, I’d be looking at center of gravity being too high above center of buoyancy adversely affecting stability, while hull being too deeply depressed would affect steering and tracking characteristics. But, I’d still like to hear what others think. Thanks, Jim

Centers of gravity, bouancy
Nope. Relating this comment to my post above, with the floating log, the center of gravity and the center of bouyancy are very nearly in the same place, yet the log has almost no stability at all. With a lightly loaded canoe, the stability is awesome if the center of gravity is near the center of bouyancy (like if you were to lie down on the bottom), but kneeling, sitting, or even standing up do not make the stability become unpredictable. The shape of the part of the canoe which displaces water, and thus how that shape changes as the boat tilts, is what controls stability in the practical sense (the “practical sense” meaning that this is independent of where you place your load in the boat, which is pretty much the same whether the boat is lightly loaded or over-loaded).

The thing that makes a boat stable is the amount that the center of buoyancy moves in relation to the amount that the center of gravity moves when the boat rolls.

The key concept there is that they both move in a stable boat.

If we take Guideboatguy’s cylindrical log and roll it the center of buoyancy and the center of gravity don’t move at all. They are always directly in line, one above the other. So the log is perfectly happy to stay at rest with any point up.

Now consider a stable boat. When it rolls both the center of buoyancy and the center of gravity move. Typically (but not always) in the same direction. But the center of buoyancy moves farther than the center of gravity. This creates forces that want to move the boat back to the condition where the CB and CG are lined up again.

Guideboatguy is correct, when you overload a boat you make it more log like. Rolling the overloaded boat doesn’t make the CB move as much as it does in the same boat when it’s light. The boat has become less stable.

All good points. Just a practical note.
Our new tandem is 17’ long, 14.5" wide, and 16" deep. It has a flattish shallow arch bottom and feels quite stable. The manufacturer gives a maximum capacity for good handling at a little under 600 pounds. Now, they could have done a 6" freeboard capacity, and it probably would have been close to 1000 pounds. But the manufacturer wanted to be responsible and give a capacity figure that is consistent with quality boat handling.

I’ve been around this board, and some others, for a few years now, and I just haven’t heard of people using ordinary tandems to take long camping trips carrying much over 600 pounds.

If a canoe is starting to wallow with, say, 650 pounds aboard, about the only things one can do are put the heaviest stuff as low as possible, kneel, and perhaps pray.

GZ Curves
For anyone new to this, the basics are all covered with pretty pictures in many places on the web: e.g. http://www.marineengineering.org.uk/navarch/navstability.htm

FWIW, optimal loading for a tandem generally seems to start somewhere near the 3" waterline and to end somewhere near the 5" waterline… and the published specs suggest that even Bell’s cavernous 17’ Alaskan is only optimised for loads of up to 700lbs - but that’s presumably assuming sensible trim, and in my experience, being down at the bow or stern has also sorts of interesting consequences, especially when you start moving!

Squirrely Canoe Analysis
Thanks to all. Very interesting. Guideboatguy, I like your clear presentation. Very nice.

Interesting post
and likewise for the replies.

However, methinks your best option will be to build another boat. Grandkids are only going to get bigger and who knows, could be more on the way. Maybe they could help.