Motor Slipping? That is what I am keying in on. Sounds like the prop is either caviting or slipping. If the prop won't turn with the shaft it's slipping. You can check the prop and make sure that it's tight on the prop shaft and that the nut is locked into postion with a key pin. Also check the blades of the prop for wear and tear. Worn or broken blades can cavitate. Cavitation occurs when low pressure on the back side of the blade while it's under the water creates air bubbles from the pressure drop. Water normally holds the air is solution but when the pressure on the dissolved air is decreased the air can form bubbles. Since props are designed to work in water not in air you can loose thrust when your prop is cavitating. Also it will make a lot of noise, well lots if you are a nuclear submarine trying to remain unheard. By going deeper the subs can get rid of the cavition as the water pressure increases as you go deeper.
I would bet that its the nut on your prop that needs to be tightened a bit. Unless your motor still is using shear pins but most modern motors don't use them anymore from what I understand.
Make sure that you have water running out fo the engines so that you know that the water pump is working. If you have not had the water pumps rubber impeller replaced in a few years you may want to have that checked as part of your routine maintainence. Also have the thermostat checked to make sure it's still in good shape ever few years.
Originally Posted by slabbuster