One point that hasn't been mentioned- where did you site the centre of gravity on your model? For most Nuke boats with hulls of revolution, you can calculate the best point by dividing the length by 2.5. Take the answer (in the case of the Vanguard it's about two feet) and measure back from the tip of the bow. On the Vanguard it comes out at just past the second missile hatch, so that should be where the middle of the ballast tank is situated. If your C.G is forward of this, the boat will be very stable but less responsive to pitch and yaw, if it's aft the boat will be more responsive, but may be a little twitchy or unstable- experimentation is the key here. If you've ever flown aeroplanes, these principles are the same as having an aeroplane that is nose or tail heavy.