Perkins 4.108 no coolant flow through hot water heater

[Ruso]

Gentlemen,
Any advise appreciated.
New boat to me. PO said it was working before.
I suspect air block.
Left hose (see image) - exit hose to air heater(floor lever) then it is going to heater tank and return to engine (right hose)
Heater tank coolant hoses entries 1/4 feet below hoses enters into engine.

Where to start and how troubleshoot?
Many thanks!

[Lepke]

I doubt it worked. It appears the hoses both come from almost the same area of the head. There can be no flow. Usually the supply comes from near but below the thermostat. The return comes back near the suction side of the coolant pump.
The normal flow has the coolant enter the block from the coolant pump, enter the head from the block, and leave via the thermostat to the exchanger.
The hottest coolant is near the thermostat, so that's heater supply. It can return either at the coolant pump suction, where it leaves the exchanger on it's trip back to the pump, or somewhere in between.
While you're plumbing, add valves so you can shut off the heater in case of a clamp or hose failure. So you can still run the engine. Also with the valves off, the engine will heat faster in cold weather because the heater circuit bypasses the thermostat. You can buy 12v solenoid valves so you can control it from some convenient location. Solenoid valve are reasonable on ebay.

[Ruso]

Thank you, Lepke.
I am sure it worked - PO was cruising with his wife for 7 years... He is still getting me up to speed with the boat and I am sure he doesn't have reasons to lie.
The exit coolant hose sits on top of water pump together with thermostat housing. Therefore extra pressure built by water pump applies to the thermostat housing to heat exchanger and and coolant exit hose.
Good point about shutoff valves. I would keep it simple by adding mechanical valves next to the engine entry/exit points. I am cruising warm waters - slow warming up is not an issue.
I strongly suspect because (right hose) kind of looping up above heat exchanger level might creates air plug..
May be adding purge valve in addition to shutoff valve can fix the problem.
Could you suggest mechanical shutoff valve with purging capability?

[Lepke]

Usually a coolant pump has enough force to push any air thru the system to where it can escape. Sometimes thermostats allow zero flow when cold and air will collect under the thermostat. As the engine heats the thermostat doesn't open because the air doesn't transfer heat like liquid. It usually leads to the engine overheating until enough heat finally opens the thermostat. This can happen on a new coolant refill. The solution is drill a small hole in the thermostat plate.