It does indeed sound like you’re overpropped but there’s also the fact some heat exchangers on 4-108’s are fairly incompetent. This is for a variety of reasons and the first is that by design its a straight through (single pass) tube stack. Other engines use a 2 or 3 pass tube stack to get the best out of the
core, imagine a car radiator with just the centre exposed to full airflow and the perimeter with no airflow… runs at thermostat temp till you meet a hill on a hot day…. then the temp climbs, same with the
perkins core and seawater. Second problem is the
coolant side of the core, I think it makes a difference if the round holes in the outer sleeve are mis aligned but more importantly the support casting baffle in the housing ( about mid tube) seems to corrode away leaving a gap that allows the
coolant to bypass the outside of the bundle rather than weave its way in and around the tubes ….defeating effective heat rejection/exchange. Finally,
raw water flow is a biggy, that
pump needs to be in good nick with all of its impeller blades intact, cover and end plates smooth and unworn , correct and unworn cam and no suction restriction. You can test the
pump flow by filling a bucket with a known volume of water, putting the seawater pump suction hose in it and running the
engine at idle and timing till empty then at say 2000rpm and timing again, then, if you want to
experiment further, re connect the seawater suction and do the same tests at the
exhaust outlet and time the fill.
These are just some of the weaknesses in the
cooling system, but parasitic loads ( dirty prop, large output or multiple alternators, overpropped etc) and an
engine thats
lost power ( loose or tight valves, bad timing etc) can all make it seem that the
boat is overpropped.
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