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

Practical Boat Owner - - Letters -

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.

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