One must also consider the thickness of the outwales which typically adds an inch or more to the width of each side of a flared or straight-sided molded hull.
I think that actually makes a lot of sense. The outwales on my tandem canoe stick out almost as far as the widest part of the boat, so compared to a straight side hull, the tumblehome does compensate for the gunwale width and allows for the paddle to be close to the hull, while at the same time providing a channel for the shaft hand to travel.
As Erik pointed out, even a pretty mild degree of heel will get the outwale of a flared or straight-sided hull in your way very quickly.
I see that, although I try to not heel during forward travel in the tandem.
Narrowing the gunwale width of the hull by even an inch makes cross strokes much easier. It also makes the switches much easier for those who paddle sit and switch.
Do you often see cross strokes and tumblehome together?
The David Yost designed “Fire” boats (Flashfire, Wildfire, Starfire) have shouldered tumblehome and have been among the most popular canoes for freestyle paddlers. Freestyle uses cross strokes all the time.
Whether a boat turns off-side or on-side in response to a heel to one side depends a lot on hull design. While it is true that a longer, relatively straight-keeled hull with a sharp water entry will usually turn away from the side it is heeled to, on on-side heel can often be used in solos to reduce the number of correction strokes required to maintain a straight course, or reduce the frequency of switches required for those who paddle hit and switch.
This technique has been known for a long time and has been called “paddling an inside circle” by Tom Foster and Charlie Wilson. This method has been rechristened the “2x4” method by Andrew Westwood and has virtually replaced traditional technique for whitewater open boaters paddling the new, short boats. The “cab forward” technique requires keeping body weight balanced forward, keeping strokes short and well forward, utilizing the tendency of the unweighted stern of the canoe to skid and the tendency of a bow wave that builds up on the off-side of a moving boat to resist a turn to the off-side. Instead of going absolutely straight, the boat is paddled in arcs of a circle of various radii and it is possible to go very nearly straight with no correction whatsoever using only forward and cross-forward strokes, or using only forward strokes with occasional switches. This short, somewhat cute video demonstrates the idea. The actual on-water demonstration begins around the 4:20 minute mark:
The paddler here is mostly carving fairly tight on-side and off-side circles and figure of eights but it is possible to open up the radius of the circle to very nearly approximate a straight-line path.
While this technique is especially applicable to short, highly-rockered hulls with a sharp chine, it will work even with much longer, less rockered hulls with a shallow arch cross-sectional contour.