Daytank excess heat issiue

Hey,

I have 40ft 1994 steel boat. I got two keel tanks and one 80L stainless steel daytank.

The daytank is in the engine room, hanging above the engine. This is the original setup it's been running with for years.

I changed out the engine when I got it, because the last one was dead and apparently unfixable (2004 Yanmar ~75hp)

Current one is 2025 a Yanmar 4JH-80.

Thing is, under prolonged engine stints 8h +, the day tank gets very hot. Can't hold my hand on it, guessing maybe 80*C. Eventually gives and alarm and the engine goes into safety mode and limits rpms.

I've been managing this by pumping in cool diesel form the keel tanks every few hours or so. But looking for a permanent fix. Need a reliable engine with hopefully a long working life ahead.

I think the main culprit for the heat accumulation is the return diesel from the engine. Room temp is never crazy, but it's snug in there, maybe gets to around 40*C at the most. I've tried making a heat shield between the engine and the tank, but can't notice any difference from that.

Looking into a heat exchanger on the existing seawater cooling from the engine.

Any one have experience with this?

Thinking about for example connecting the heat exchanger on the sea water line between the gearbox outlet and the impeller housing. Diesel after the injectors of course and straight to the day tank.

But, sure it's not necessary as simple as that, with water, fuel, pressures and flow to consider.

Hope to see any ideas or suggestions from you guys.

Thanks in advance