Greetings...

I have a 1995 Universal M50 in my Polaris 43 sailboat (26,000 lbs) that has about 1000 hours on it. I've had the boat since 2002 and it has always steamed and will overheat if I exceed about 1900 RPM. The original owner said that it steamed since the engine was installed new in 1995 but I have no way to verify this. I have a Seahawk, 18 inch, three bladed feathering prop (similar to Max Prop). The boat will make about 6 to 6.5 knots at 1900 rpm which is way less than hull speed.

I have done the following:

Checked the raw water output / through put at the output of the raw water pump, at the output of the heat exchanger and at the output of the exhaust. The output is the same at all locations - about 4 - 4.5 gallons per minute. The specs for the Oberdorfer pump say that it should be about 6.5 gallons at 1750 rpm.

Checked the through hull - water comes in freely and with nothing attached, the water column is about 18 inches.
Replaced all hoses - raw and fresh water cooling systems
Replaced the OE Sherwood raw water pump with an Oberdorfer
Replaced the impeller - multiple times
Boiled out the heat exchanger (it didn't look bad in the first place)
Replaced the raw water strainer with one twice as big as the one that was there
Pressure tested the fresh water coolant system - it's fine
Replaced the thermostat in the fresh water system
Replaced the cap on the fresh water system

Re-pitched the prop - gave me more rpm and the overheating went away (the steaming DID NOT go away) but I lost a full knot of boat speed - max boat speed was 5.5 knots at max rpm which I couldn't live with.

Put the prop pitch back where it was originally, overheat problem returned.

I have talked to several local Universal experts and they are at a loss on the cause or solution to my problem. I am pretty frustrated that I have a relatively new engine which is more than big enough to drive the boat at hull speed yet it is so problematic and I can't find a solution.

So I turn to you... gentle reader as my last, best hope. I anxiously await your sage wisdom.