I knew it had a name.
As to effect on initial stability, if the kayak heels the leeboard has to move water sideways, so the more board in water the more water has to be displaced and the more resistance there will be to the heeling motion, slowing it down. The only problem I can see is that when you want to return from any heel that has developed the same resistance will slow that down too, but I think it would certainly improve initial stability.
John

: In sail craft, they're called leeboards - they help prevent you from sagging
: off to leeward. They actually, like a centreboard, act as a foil,
: providing lift to windward - I'm uncertain what affect, if any, they would
: have on initial stability.

: You could try forming two bouyant topside additions, shaped like a torpedo
: bisected along it's lenght, then glueing that to your topsides so that it
: sits barely above the water when the kayak is properly loaded and trimmed.
: When the boat heels, it would displace water and gain bouyancy - how much
: you'd need to notice a difference is another question! I'd be tempted to
: make a prototype using that blue foam sheet insulation used in house
: construction (cheap, easy to work), then duct-tape it to the topsides and
: try it; if it works, then do a proper job of it in mini-cell, sheath the
: foam with FG cloth, and epoxy it in place. (Note - unsure how foam and
: epoxy react...)
: If you do make adjustments, let us know how it goes...