SOP Kayak - Paddler's Centre of Gravity

I wonder if someone might want to illuminate me on the stability curves of a sit-on-top kayak?

At first blush, it seems that on many of those boats a paddler's COG is moving pretty close to a boat's metacentric height. I get that high initial stability is part of the design brief and that mitigates its somewhat. But but but ...

My question is this: how close can we put a paddler's COG to the transverse metacentre before the boat becomes useless for practical purposes?

 

To achieve maximum stability, the CoG should be as low as possible and the metacenter as high as possible. Trying to approximate both points is, therefore, a wild idea.
If you define the minimum of stability you want to get we could advise what to do, assuming, of course, that you provide us the body lines plan and the list of weights and CoG of your boat.

 

Several thoughts:

The paddler's Center of Gravity will depend on the size and shape of the paddler. I probably have a significantly higher CG when seated with my legs in front of me than most paddlers, and find some kayaks and canoes which others claim to be stable to be close to neutral stability for me. In one case I got into a kayak which others enjoyed paddling and found the intial stability was negative.

Paddler's do not sit rigidly, fixed in position, fixed in position when a kayak heels. If the kayak lists one way the kayaker tends to instinctively lean the other way to counteract the list. I've watched paddlers who's torso stay almost vertical while the kayak heels back and forth due to waves. The paddler's weight will usually be greater than the kayak's weight sometimes by a factor or five or more. That means the CG position of the combined kayak and paddler will be very dependent on the position of the paddler. A standard naval architecture stability analysis which usually has the CG position fixed relative to the vessel (for a given loading condition, excluding free surface effects, etc) may be pessimistic for a kayak with a paddler who reacts to the kayak heeling.

The stability characteristics when the vessel lists may be as or more important to how a paddler perceives stability than the upright slope of the stability curve. A kayak with lower initial stability but which "stiffens" quickly when heeling may be perceived as more stable than one with higher metacenter and higher lower initial stability but which does not "stiffen" as quickly.