Jeremy2006,
If there are no hole in the hull then I'd start looking at the things going through the hull. As in the stuffing box/drivehaft and rudder tubes, etc. If one of those is the problem then run the boat in the test tank (bathtub) to be sure. Be prepared for you-wouldn't-believe-how-much-trouble from 'she who is to be obeyed'...
- 'Doc