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Old 10-22-2010, 07:43 AM   #17
timjet
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Operating a twin on one engine and fuel economy

There is no doubt the drag of the rudder corrected for the asymmetrical thrust and the wind milling propeller will slow the boat down and create inefficiencies.* This amount of drag will depend on hull design, the distance each prop is from the centerline of the boat and other characteristics unique to each boat. However any engine must produce a certain amount of power that is used to run the pumps, alternator (commonly referred to the accessories) and in the case of a diesel, the heavy fly wheel and crankshaft, power that is not delivered to the propeller, but uses fuel nonetheless. Another point brought up by Jeff is the fact that fuel flow will increase on an engine under load compared to an engine unloaded at the same rpm. The question of course is how much more of a load is produced by the asymmetrical thrust and the effort needed to counteract it.
My contention at least on my boat is the fuel consumed by an engine to run the accessories exceeds the greater fuel flow needed by the remaining engine to counteract asymmetrical thrust drag.

Another issue is the power to weight ratio of the boat. My boat unlike some trawlers has a higher power to weight ratio, allowing it to plane. When I shut an engine down and maintain displacement speed, the remaining engine is producing probably less than 50% of its available power. Perhaps as the power to weight ratio lowers, the argument shifts in favor of running on 2 engines.

The argument here is not 2 vs 1 engine as this is a personal preference issue that has no definitive answer that can be based on fact. There is a factual answer to the question of fuel performance at displacement hull speeds on a planning hull with one or two engines. Perhaps this question can only be answered by installing fuel flow indicators and further testing.* But my guess is this testing has already been done, I just need to find where the answer is.


-- Edited by timjet on Friday 22nd of October 2010 07:44:57 AM
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