Engine loading & prop pitch


As I’m in learning mode here I wonder if I could ask a couple questions. First a comment, while I understand your articles on hybrid power, and you make excellent points, the one thing I’ve read that is not addressed is that at the speeds diesels are run at most of the time, they are significantly underloaded. Some power cats and very large monos use a VPP to keep their engines optimally loaded. In theory, that is one benefit of the Diesel-Electric System, no? Is it sufficient to offset everything else, I have no idea, I would assume not from your article. Just wanted to point out there is room for improvement. Some monohull friends overpitch their boats since they can’t get beyond hull speed anyway, they figure why not put a better load on their engine and get better economy in the cruising range.


Your observations are all good. Working vessels such as tugboats that have to operate over a wide range of loading justify the installation of variable pitch props primarily as a way of improving brute thrust. Recovering the relatively minor loss of efficiency would not justify the complications and liabilities attendant with such an installation in this size of vessel. As you suspect, the same applies to the diesel electric application; the engine loading efficiency gains would never compensate for the overall losses. The boat ghosting along at minimal power in a calm backwater is using so little fuel that any percentage saved would be miniscule.

Overpitching the props on monohulls with limited speed potential is quiet common but you sacrifice the full power available when trying to overcome head winds and seas. Unless the engine can come close to the designed revs, it will be operating somewhat below its potential power. For this reason, we pitch the prop to allow the engine to attain its full rpm in “normal” conditions, that way when it encounters some heavy going it won’t be dragged down close to a stalling point. A simple analogy would be selecting a particular gear for your car to run in on the highway that covers most occasions. The highest gear may be most efficient, until you come to a hill that pulls the engine rpm down excessively.

I believe the “under loaded diesel” problem to be highly overstated for today’s engines. Virtually every vehicle engine spends the majority of its working life “under loaded”. Some prominent engines such as the two cycle Detroit Diesels from several decades back may have had problems with clearing the products of poor fuel and oil combustion due to low running temperatures and pressures but these problems just don’t bear on the engines we are considering. The efficiency issue remains but the law of diminishing returns applies. With very narrow hulled catamarans the hull speed barrier is virtually nullified so you may as well pitch the props to go as fast as you can.