Raw water cooling - why run the engines so cold? Also, closed cooling conversion?

[loco]

I can't get my head around the necessity of running a raw-cooled engine so cold. It's something like 65C, which seems crazy low. It would reduce the efficiency of the engine, and of course reduce the amount of water suspended in the oil that gets evaporated.

Why can't we put in a ~75C (167F) thermostat, and run them hotter?
The only thing I can think of is that the coolant (sea) always stays cold, so whenever the thermostat would open, the temperature would drop too quickly, too fast, but then why not have reduced flow/a smaller impeller?
I'm sure there is a reason - just curious.

On that note, has anyone had experience with converting a raw-cooled engine to closed cooling? I have <150 hours on this engine, and wouldn't mind doing the swap if it was cost effective.

Thanks in advance, as always.

 

[Maclin]

Marine engine has a harsh duty cycle, almost always under a load, either planing or trying to maintain plane. The lower temp will keep internal components like the valvetrain from overheating (ex. tuliped valves) due to hot spots that automotive engines alleviate during no load cruises. Also keeps the potential for detonation in overheated cylinders low.

Switching an engine that has been in raw water mode for any length of time is problematic due to the scale build up in the water jackets. That breaks loose and has nowhere to go in closed cooling setup. In the raw water setup the junk just goes on out with the spent cooling water.

 

[Scott Danforth]

to further Maclin's post

the primary reason is salt water comes out of solution at about 150-160 degrees, so to prolong the block, raw water cooled motors get a 140 degree F thermostat, fresh water cooled get 160 degree F.

to convert your motor from raw water to fresh water, the motor will have had to been operated solely in fresh water, otherwise the scale build-up on the inside becomes a detriment.

and you are right, boats are horribly inefficient.