There have been a number of threads here and on cboats dot net regarding the effect, or lack thereof, of modifying gunwale beam on boat rocker.
The conventional wisdom is that widening a canoe might have the effect of increasing rocker and narrowing it the opposite effect.
Some folks have noticed a change in rocker and others have not. I have pulled in the gunwales on a couple of Royalex whitewater canoes up to about 2 inches and I have not seen any change in rocker that I could see, measure, or otherwise appreciate. These were older Royalex boats. It might be that a newer Royalex boat that is less "stiff" might demonstrate more of a change or maybe there is some time delay between narrowing a boat and a change in rocker.
What I have gleaned is that the general consensus is that relatively minor changes in beam (2 inches or less) have relatively little effect on rocker in Royalex boats but might have more effect on the cross sectional hull configuration. Widening a boat might flatten the hull a bit and narrowing it might put more arch in the hull bottom.
You can certainly try spreading the gunwales a couple of inches using either bar or pipe clamps that can function as spreaders, or maybe just some furring strips cut a couple inches wider than the yoke and thwarts to push out the gunwale a bit, and then put a straightedge back on the keel line and see if it is different. My guess is that you will get more mileage with a foam spacer between the hull bottom and yoke though.
I have paddled a number of Old Town Discovery canoe which tend to become a bit hog backed with age. Many will probably take exception with this but personally I haven't noticed any great hardship imposed by a relatively minor degree of hogging.
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