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Old 09-10-2018, 03:23 PM   #57
DDW
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City: San Francisco
Join Date: Apr 2018
Posts: 3,093
Let's first define "overpropping". On this thread it seems to be a prop or pitch selected such that in calm water, at full throttle, the engine cannot reach it's rated rpm steady state.

All that means is the prop absorption curve meets the engine output curve at less than max rpm. At anything less than that rpm, the engine will be capable of more torque than the prop can absorb, so it is running at reduced load. It is worthy of note that with a higher displacement (more fuel or water or stores) or in a headwind, or in a seaway, a perfectly "propped" boat by the above definition is unlikely to be able to reach full rpm, and will therefore become "overpropped" by that definition.

If we pick a boat and an rpm and a displacement and a weather condition, the result will be a certain speed, and a certain slip ratio. Increasing the prop pitch but changing nothing else, several things will happen: the prop will absorb more torque. The engine will fuel more (the rack or the electronics) to achieve the same rpm. The boat speed will increase, but not in proportion, as hydrodynamic resistance is non-linear. Therefore the slip ratio will increase.

For a modest increase in pitch, the increase in torque required is modest, and the headroom the engine has in torque is usually large except near top rpm. As others have said, whether this will increase economy depends on the fuel map and the resistance curve of the hull. Very difficult to predict the result, even with those in hand.

Here is a engine and prop curve plot taken from this article on sbmar.com, which is a pretty good discussion of the subject.



Note that you do not need to run at much reduced rpm to unload the engine (relative to its capability) by 50% or more. In this example cutting from 2000 rpm to 1700 reduces its loading to 50%. Changing the prop pitch a little bit moves the prop curve up and to the left a little bit. It only has to move a little bit to change the intersect point (max rpm) a fair amount. But it changes the cruise power demand much less.

I have a typically overpowered SD trawler (AT34). Max rated rpm is 3000, but I cruise it at 1400 or so. Even a pretty substantial change in pitch will not overload the engine at 1400, if anything it might be considered underloaded. If I habitually ran it at full throttle and only got 2700 vs the 3000 spec, then yeah, it's overloaded. But I don't do that.

Another wild card in the discussion is mechanical vs common rail electronic control. The "throttle" on a mechanical diesel just adjusts the governor speed set point. The governor then adds fuel to achieve that rpm. If you overload the engine such that it cannot achieve the set rpm, then it will be constantly overfueled and eventually damaged. Electronic engines are considerably more efficient, the more modern ones will not allow this kind of damage to occur, they will de-rate to prevent it. These controls were developed for truck engines, and in light duty applications (pickups) they run a low rpm and low load for a very long time.
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