I use water ballast in the form of four or five 3-gallon plastic
bottles when paddling my empty canoe. It is a high-capacity solo canoe
intended for wilderness trips and has a shallow-arch bottom. If I
paddle it empty, the water-line beam gets very narrow (it's narrow
enough to frighten the novice paddler when loaded). The result is a
VERY tender craft. An extra hundred pounds of weight forces the hull
down and broadens the beam to acceptable limits. It has the added
advantage that if I swamp the canoe, there is no effective weight added
to the canoe, such as would happen if I was using 100 pounds of sand
bags.
I suspect that a marine architect would tell you that a water-ballasted
boat derives the majority of its stability from the hull shape, and that
the purpose of the water ballast is primarily to lower the hull to
achieve that shape, while still allowing a light enough boat for
trailering.
Take care,
Mark
P-14 #202 "Apres Vous"
Sunnyvale, CA