Re: Stern thruster ?
Start at the intake through hull and work your way back.
1. Visual inspection of exhaust water flow. You should be getting a LOT of water at 60%-80% RPMs. But it can be tough to tell how much is a lot.
2. Pop the top off your sea strainer and clean-out basket. Now, turn-on the thru-hull valve. There should be a really good flow coming through. If not, you have some sort of obstruction in the intake - seaweed, mussels, plastic bag, etc.
3. Impellor. Change it. Have some silicon grease to lubricate it as it goes in. If you are missing any vanes, you'll find them in the first end of the heat exchanger (next stop)
4. Heat exchanger. Pop off each end (assuming it has two ends) and visually inspect the tubes. Here's a decent video showing the HE on a 4.108. https://www.youtube.com/watch?v=g18-xIyR9jM I've never heard of "Barnacle Buster" but it sure looks like it did the trick. Make sure it's safe for copper.
One of the above will fix your problem. If not, you have a problem with your raw water pump - on rare ocassions, the body or cover gets worn and lets water bypass the impellor.
1970 Willard 36 Sedan Hull #40