I recently bought a used Mohawk Blazer 16. Just prior to the sale, the previous owner replaced the middle thwart with a piece of 1x1x1/16 aluminum tubing. It looked like he cut it too long, because the sides of the boat appeared to have an outward kink at the gunwales. I have ordered a carrying yoke, but in the meantime, I removed the piece of tubing, cut off 1 1/2", drilled a new hole, and replaced it, reducing the maximum beam at the rails from 36" down to 34 1/2". There still appears to be a slight unfairness in the curve of the rail, but I think another 1/2" reduction might just about fix it. My question is, what is the maximum beam at the rail SUPPOSED to be?
What is it when there is nothing there? How does the curve look then?
The 1997 Canoe Magazine buyers guide lists it at 36". Memory says the gunwale line took a bend at the center of the canoe. It was not an arch from bow to stern. The bottom is fairly flat and pulling in the gunwales too much might hog the bottom.
That makes sense. When I disconnected the thwart, it didn’t spring in or out, just stayed put. I had to pull the sides in. I checked the line of the bottom to make sure it wasn’t inducing negative rocker. I guess I’ll know for sure when I take it out tomorrow.
Why would pulling in the gunwales
tend to hog the bottom? I think the opposite would be the case.
I paddled one of these on Chattooga 2, many years ago. The gunwales were pretty much straight up and down.
By opposite, you mean make more rocker?
No, that can't happen. Pulling in the gunwales will increase the distance between the *top* position of each stem (The rails are the same length no matter what you do, but they are a lot longer than the boat itself and making them straighter converts more of their total length into "boat length"). If the distance between the tops of the stems gets longer, and the length along the keel line stays the same (nothing will change that dimension), you end up with less rocker, or even reverse rocker (or a boat that's "hogged"). I've heard that the degree of shape change is hard to predict though.
I’ve pulled gunwales in, and pushed
them out. Pushing them out is more likely, in real life, to require adding support between the center thwart and the hull bottom to prevent “oil canning” or hogging. That was the case on my OT Tripper and on my little MR Compatriot. In both cases, a foam pedestal crammed between the center thwart and the bottom kept the bottom from rising. In the Tripper’s case, it improved what was already a great whitewater hull, by producing a center pivot zone that allowed the boat to spin better.
In general, the more “tubular” hulls (Dagger Rival) are less prone to hogging, while square sectioned hulls (Dagger Ocoee) are more prone to hogging. Pulling gunwales in tends to increase tubularity, while pushing them out will decrease it. So both on this basis and on my practical experience, it’s hard to see how pulling gunwales in a bit could produce hogging.
I see your point, and …
... we are not talking about exactly the same thing. I think you are right that a boat with a more tubular cross section will have a more rigid hull bottom, and with flexible materials like Royalex, such a shape will be one that doesn't "pooch upward" as much due to water pressure as one that's more "rolled open" (like you'd get with longer thwarts). On the other hand, and this has been described several times by some of the old-time canoe gurus here, if you make the end-to-end dimension of a boat longer than specified by reducing the curvature of the rails, something's gotta bend, and that bend can show up as a change in shape along the keel line (less rocker or reverse rocker). Of course, you could cram something between a thwart and the hull bottom to counteract that (and induce bending to occur elsewhere), but that's a whole different issue.
Someone once used the example of an envelope to illustrate this. If you pull the two sides of the opening of an envelope apart, the bottom edge of the envelope will no longer be a straight line, but will become "rockered". It's by that principle that some people have increased or decreased rocker in canoes. Several people have posted on this in the past, and maybe they will chime in.
ww hulls vs flatwater hulls
The short deep rounded ww hulls you are so familiar with will react differently to a change in center thwart length. The side to side profile will flatten as you widen the gunwale spread. The end to end gunwale profile on a ww canoe is straighter due to the flared bows and pulled in centers. The stems will not be affected too much even if they are forced farther apart by the slightly straighter gunwales.
The subject Mohawk Blazer is a wide shallow tandem. It has a flat stiff bottom that is not going to curve much if you pull in the gunwales. More likely you will sharpen the curve at the chine. Pulling in and straightening the gunwales will try to move the stems farther apart. Where the fiberglass gives will depend on the reinforcement and I would not take a guess where that would happen on a Blazer.
Just best the guy leaves the width where it was when Mohawk trimmed the boat at manufacture.
Yeah, but the OT Tripper is not
a short ww boat, and the MR Compatriot is more like a tiny MR Explorer than a modern ww boat. My MR Guide Solo is not a modern ww boat, nor is my 17’ Bluewater. All of these behave exactly as I have described if the center thwart is pushed out or in. Widening the center of the boat will tend to flatten the bottom at the center, and make it more inclined to oil can. Narrowing the center of the boat will tend to make the bottom more convex, downward, and less inclined to oil can.
The ends of canoes do have constraints in how they respond to beam changes at the center. It’s tricky to compare canoes to paper envelopes. Usually, modest changes can be made at the center of the boat without distortion of the ends coming in to play.
As for the Blazer, my recollection is that it is a combination of FG cloth and chopper gun FG, and its bottom is fairly stiff. The sides are far less flexible than a Bluewater or a Millbrook, but in my opinion, pushing the sides in or out at the center thwart is not going to affect the bottom very much. This will be true as long as a beam change is confined to + or - an inch. Going beyond that is hiding stress in the hull which can bite you if your boat goes sideways and runs into a rock. My instinct on the Blazer would be to narrow the gunwales carefully for the sake of solo paddling.
THANKS for all the info, y’all!