I just bought a 1979 eliminator daycrusier with 460.  water plumbing looks like chit.  there is a ball valve by drivers seat to control temp. WTF. 

$10 gate valve from local hardware store here. HD didn't have any for 3/8 or 1/2.

Ball valve is more on/off. Gates are what you want for flow control. 

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The internal combustion automobile engine does use heat to some extent and which helps the engine make power and also the engine will last longer (reduced wear) if the operating temperature is higher than lake water temp.  With a wide open gate valve, most boats typically run at 120*-130* F.  If you start turning the gate toward the closed position you will eventually begin to see a water temperature rise in your engine (usually less than 1-1/2 turns open from fully closed).

You will get varying opinions on a good water temp for a jet boat; I like to run my engine at about 160" as controlled by the gate valve (no thermostat) while running hard. Then, upon entering a 5 mph zone the temp might spike to 190* (because water flow has decreased with decreassed rpm) and then it will settle back down to 170 or so. So I don't go any higher than 160* while running across the lake because I don't want to "steam" (boil the cooling water in the engine) when I enter the 5 mph zone.

LO