Minnesota Canoe Association

Boat Building How to... 

2000

Altering Existing Plans (h27) by Al Gustaveson

So you found your "dream canoe," the one you’ve always wanted to build, and now you’re scheming on how to save enough money from household expenses to pay for the materials. The only problem is that it’s drawn as eighteen-footer and you really want a smaller hull. What to do?

This is a dilemma we encounter at least once a week here at Northwest Canoe. First of all there is no "perfect" canoe that will meet all your requirements. Canoes, like hammers, are designed for specific chores. There are framing hammers, trimming hammers and sixteen-pound mauls, and there are white water canoes, tripping canoes and racing canoes, so your choice is going to be a compromise right from the start, but that doesn’t mean you can alter one plan to re-engineer an all-new design. The most common error is to think that by reducing the station spacing (the distance between forms) you will end up with a smaller version of a well-loved hull... Nope! What you get is a shorter canoe that doesn’t paddle anything like the original. The same thing happens if you expand the distance between forms. To expand or reduce any hull it must de done in every dimension, width, length and breadth. For instance, let’s say you want to make a 16-foot canoe as an 18-foot model that the water will "view" in the same way as the original. That is, with the same general characteristics, only bigger.

Well, 16 feet is 88% of 18 feet; that means you should expand all the measurements 12%. If the original canoe was 16 feet over all you stretch the length to 18 feet. If the sixteen-footer had a maximim beam of 30 inches, the new expanded beam would be 33 5/8 inches. If the height at the bow were 13 inches it would become 14 1/2 inches and so forth. This entails re-drawing all the forms—a lot of work.
If you just reduce the station spacing you are going to end up with a pumpkin seed hull that won’t paddle any thing like your original. If you expand the spacing the result is a skinny hull, with a flat spot in the center of the sheer, that doesn’t have the righting moment of the original and may seem like a "plank on edge" when it’s in the water.
I guess the short answer is, don’t do it. Find a plan that more closely represents what your requirements are and build that, and if you must alter an existing plan consider all the ramifications of your action. Most builders put in 150 hours building any model, so before you chop, shop—or else you may end up with a canoe nothing like what you wanted to start with.