the physics behind a canoe's pivot point

[canoedad89]

In the thread about moving and turning a canoe with side slips, it was mentioned that the canoe's pivot point will shift a little bit fore or aft when the velocity of the canoe changes. The term used to describe this phenomenon is the "peripatetic pivot point". A google search explains this term by stating that when a boat is dead in the water, it pivots near its center, and as it gains forward speed the pivot point dynamically shifts toward the bow.

My own experience doing side slips seems to back this up. When I am doing a side slip using a static draw, as the canoe slows down and loses its momentum, I need to move the paddle back, or toward the stern, in order to keep the canoe slipping sideways without rotating about the pivot point. This theoretically keeps the vector from the power face of the paddle pointed at the pivot point. So far, so good.

But are we able to delve deeper into the physics of why this occurs? My search of old posts seems to suggest two schools of thought on why a tandem canoe won't go strait when both the bow and stern paddlers execute perfect forward strokes. To briefly summarize, the first school suggests that since the pivot point shifts toward the bow, the lever arm between the stern paddler and the pivot point is greater, thus magnifying the effects of the stern strokes. That makes sense. The second school suggests that the water pressure forces on the hull are stronger at the bow than they are at the stern, due to the eddys that form behind the pivot point, which would tend to hold the bow in place and make the stern easier to pivot. That also makes sense. So I don't know. Maybe all of this can be explained by Bernoulli's equation, or maybe it's just beyond the scope of this forum.

What are your paddling experiences with the shifting pivot point?

 

[Dana]

Sideways motion also shifts the pivot point. In aerodynamics we'd say the center of pressure shifts aft with increasing "angle of attack" (defined as the angle between the airflow and the chord line of the wing).

 

[BWCA66]

My pivot point always seems to be changing!

 

[Steve in Idaho]

My search of old posts seems to suggest two schools of thought on why a tandem canoe won't go strait when both the bow and stern paddlers execute perfect forward strokes. To briefly summarize, the first school suggests that since the pivot point shifts toward the bow, the lever arm between the stern paddler and the pivot point is greater, thus magnifying the effects of the stern strokes. That makes sense. The second school suggests that the water pressure forces on the hull are stronger at the bow than they are at the stern, due to the eddys that form behind the pivot point, which would tend to hold the bow in place and make the stern easier to pivot. That also makes sense.

Can't it be both?