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I'm now convinced I know how the engine cooling system functions. Here's the concept:

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Essentially, the lake water is picked up and pumped to the pressure-valve/thermostat housing. If the 'stat is closed, much of the water leaves the housing via a spring-loaded release valve, and is fed into the top-front of the exhaust manifold. Picking up some manifold heat, the water exits the back of the manifold and returns to the lake.

Some of the water is routed through the thermostat half of the housing (the housing, not the thermostat) and on to the engine's pump. After passing through the block, heads and intake manifold the water travels to the front of the intake manifold, picks up more heat and either 1) exits the manifold into the exhaust, or 2) travels via a hose back up to the 'stat housing. I call this path the "lower loop," because water travels in a loop back up to the thermostat housing. The volume of water exiting this "loop" (at the back of the exhaust manifold, into the exhaust) and being replaced by cool lake water is determining engine temperature.

Here's another diagram:

Click above for larger image.
The two red "p's" in drawing shows location of open
petcocks used to simulate running with "lower loop"
partially restricted, i.e., the side slots were closed w/gaskets.

Today, at the temp gauge sensor, with boat running from idle to 42 mph, the temperature was about 110F, and won't go much higher unless we obstruct water flow of the "lower loop" out the exhaust. This has been clearly demonstrated with various tests, including

• obstructing the flow completely, LIKE THIS
• partially, LIKE THIS or
• not at all LIKE THIS.

I'm now convinced the thermostat does not open at all during "normal" running, and won't open unless we obstruct flow at aft end of manifold -- i.e., restrict flow through the SIDE SLOTS of the lower loops.

Yes, the GPS stayed connected (strategic use of rubber band.) At 42MPH, the burgee did not.

On Apr 25, 2016, at 3:27 AM, Duane Parsons wrote:
Kinda looks like either you live with 110 degrees or construct some sort of "orifice" or valve to restrict the flow. I just checked my thermostat and it opens at 130 degrees in a pan of water. Last fall I could only achieve temps similar to yours. But the P.O. had not installed a thermostat at all so I may be OK. You sure have a nice outfit!!
Duane

On 4/25/2016 Grant MacLaren wrote:
Duane, Thanks for the compliments.
I've pretty much decided the same about the temperatures.
Do you think "living with it" (110F) will harm the engine? (My 160 thermostat opened at exactly 160 when I tested it in a pan of water. Also, THREE OF US here CAN'T REMEMBER seeing a 'stat when we dis-assembeled the engine. I'm guessing there wasn't one.) I've now decided to NOT NOW build a new front seat, nor engine box; maybe next winter.
Now, priorities are to get the boat ready for getting a few (5 for sure) kids up on skis this summer. Don't tell Pete, but that may entail drilling one mounting hole in the pylon.
Again:
Do you think "living with it" will harm the engine?
And, have you posted more photos of your boat?
-=Grant=-

Duane's response -- I think I would swap out the 160 for a 130. The 130 will probably NOT open either but comes closer to being correct. I am not real fond of 110 but I don't feel that it will kill anything either. I think there are probably more boats out there with no thermostats than ones with them.
Duane

Here's a better way:

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