A 70 liter tank in a 41 foot boat
is rather small, so the amount of fuel returned by the engine
is proportionally greater than it would be with a larger tank, and that makes the fuel in the tank heat up faster than it otherwise would.
That said, the fact that the fuel heats up isn't really hurting anything as long as the engine has enough reserve power to push the boat in adverse conditions. When engine manufacturers rate the power of an engine, the rating conditions usually state the temperature and density of the fuel used, and that is usually something like 845 g/ liter density and 20*C temp.
If your fuel is heated up to perhaps 70*C, it will require a greater throttle setting to push the boat at the same speed as a lesser setting when the fuel is only 20* C. As stated earlier, this may not be a problem if you have adequate power reserve.
Should you install a fuel cooler? Why not, it won't hurt anything. Lots of high performance engines are equipped with coolers so that they don't suffer power and performance loss, and modern electronically controlled engines routinely measure fuel temp so that the ECU can compensate for power loss automatically.
Get yourself a small cooler and install it in the seawater circuit between the seawater pump
and the heat exchanger
, and make sure the seawater passages in the cooler are large enough that there is no seawater flow restriction. Run the return fuel line from the injection pump
to the cooler and then back to the tank. It's best to plumb the fuel flow so that it's flow within the cooler is in the opposite direction as the seawater is flowing. And that's about it. If nothing more it will give you piece of mind.