Regarding hose diameter, it’s best to go with the flow

Re ‘Hose diameter dilemma’ (Ask the experts, PBO Summer), I read this with great interest as we were faced with the same question when upgrading from our previous Volvo Penta MD3B to a Bukh DV36, both rated at 36hp. The MD3B had direct cooling through a 16mm hose, while the DV36 uses something much bigger as it features a heat exchanger receiving water from the through-hull. With engines of equal power, I suppose the right diameter for the hose depends upon the type of cooling system (direct or indirect cooling) and, potentially, other watercooled devices (gearbox, oil cooler). So, opting for the larger diameter should be the better option.

Lutz v. Meyerinck

Hamburg, Germany

Stu Davies replies:

Other ideas are always welcome: however, the bottom line is the thermal loading on the system. 25hp is roughly 19kW at peak power, and one has to get rid of the heat generated by that power production. Both engines generate roughly the same; and Westerly, we can assume, would have done the calculations.

The total peak power dissipated is roughly equal, including a heat exchanger and/or gearbox, so, IN TOTAL, the thermal loading is much the same. The old engine, if I recall correctly, was raw-water cooled, and VP designed them to run cooler than intercooled fresh water engines to avoid excess corrosion caused by really hot salt water to the cast iron engine parts. Surely Westerly and VP would, if anything, have designed the flow to be bigger to dissipate heat more quickly on the old engines?

My advice is to check the size of the new engine pipes – and that the less holes there are in the hull, the better.