[Destroyer]

Originally Posted by THEFERMANATOR

The cooling analogy isn't a good one as it has been proven time and time again that a 90-95% duty cycle of an A/C unit is MORE efficient than an oversized unit that doesn't run as much. If an A/C is oversized, the unit doesn't run enough to remove the humidity, yes it runs less, but it uses more electric while running, so you end up using just as much electricity, but don't remove as much himidity leaving your home feeling cold and clammy inside. Going oversized is a common misconception though.

I did specify a small window unit. While I agree with you on a whole house unit using more electricity that's not really the case with small window units.
In many cases the difference between a 4500btu or a 5000btu is the size of the evap coil with the compressor being the same size for both of them. The electric usage is less per btu with the larger unit while achieving the same RH decrease and cooling. And of course, a case could also be made that using several window units to cool a home will cost more than running just one large central unit to do the same thing, because the central unit will be running one compressor on 220v while the window units will be running multiple compressors at 115V. (But that's no longer comparing apples to oranges, so it's really not a good comparison).

Additionally, the compressors duty cycle is a good thing to know while sizing a unit for a particular job.. Some need to have a rest period of a certain percentage of their operating time, to give the compressor time to cool down. You want this to happen to prevent breakdowns and to prolong the life of the compressor in your A/C unit. Usually, the duty cycle is expressed as a percentage of a certain time frame, often a 10 minute segment. So a duty cycle of 50% for a particular brand and model of A/C would mean that this particular unit could run steadily for 10 minutes, and then it must have a 10 minute rest period before it kicks in to cool again. Often it's the home owner that gets caught in the duty cycle trap, not knowing what the duty cycle of their unit is, and burning it out prematurely through too prolonged a usage. The point being that any money you might save in electricity by running a higher duty cycle unit might be negatively impacted if your unit fails in X years instead of Y.

I think the same could be said for a 200hp vs a 150hp. If all you're doing is cruising along at say 30 knots wouldn't the 200 be just loafing along at a lower rpm while the 150 would need to be running at a higher rpm just to maintain the same speed? I'd think that the 200 would be more fuel efficient under those conditions. (And really, when you're out in the ocean, you hardly ever get the nice flat seas that allow you to really let it all hang out. Most times it's 30-35 if you're lucky).