Originally Posted by jwalker
What surprised me was how unstable it became with such a small amount of water on board.
Your stability results partially from your waterplane area, which is if you take a slice through the boat on the waterline, it's the area of the hull that is contributing to stability that counts. The further away from the centreline, the more effect it has.
In this instance the immersion of the aft end meant that the main hull wasn't adding to the buoyancy at all and the stability was being achieved by the thickness of the hull sides plus a bit at the bow, which is on the centreline and doesn't contribute that much. As a result it had very little in the way of stability.
If the inside of the hull hadn't become free flood and was just full of water, it still causes issues because you have a large body of water that naturally flows to the low side. It's called the free surface effect and is due to the size, particularly the width, of the free surface of the water.
As someone else has said, that is why tubes are so effective. They not only provide buoyancy but they also provide it some distance from the centreline which maximises the effectiveness.