I built this as well and had the same problem (pictures on this site). Being a single shaft vessel all my electrics motor, battery, receiver, Rx battery, ESC are on the centreline. this gave it very poor transverse stability. My solution was to epoxy some torpedo fishing weights either side four in all spread longitudinally so that she sank evenly on the waterline, but spaced as wide apart transversally as possible right in the turn of the bilge. She sails a bit lower on the waterline but not enough to repaint it. It solved the problem and she is very stable. Another solution would be to have to two battery packs, eg 7.2v packs side by side.