flow in the MD3b
runs from the sea water
pump to a "T" fitting mounted on the underside of the exhaust
manifold. This fitting acts to split the water flow into two paths.
First path goes into the lower side of the manifold, then into the heads and cylinders, then back into the upper side of the exhaust
manifold and to the thermostat which sits at the front of the manifold. When the thermostat is open the water flows thru the thermostat and into the thermostat housing from the manifold side, then out the top of the housing to the exhaust elbow
Second path flow continues thru the "T" fitting and then into a hose which runs to the front lower side of the thermostat housing, thru the front of the thermostat housing and then out the top of the housing to the exhaust elbow
When the engine is cold most all of the flow goes across the "T" fitting, into and out of the thermostat housing and out the exhaust.
As the engine gets hot the thermostat begins to open and allows water flow thru the engine and at the same time the thermostat blocks flow coming from the T fitting. So the thermostat acts as a selector allowing flow either thru the engine or around the engine.
If the thermostat is removed the flow direction will mostly depend on which circuit has the most resistance to flow. If the engine is full of corrosion
or mud, most of the flow will go around the engine and it might run hot. If the engine internals are fairly clean there might be enough flow in the engine circuit that the engine doesn't overheat.
Running cool for a few miles won't hurt anything, but it goes without saying that running hot for too long could be detrimental to the engines health
If you find things heating
up, you can apply a small "C" clamp or something similar onto the rubber hose between the T fitting and the thermostat housing below the manifold. This will increase flow resistance on that circuit and force more water thru the engine circuit, resulting in the engine running cooler.