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Old 12-20-2018, 10:24 AM   #43
Riverguy
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City: MN and FL
Vessel Name: Serendipitous
Vessel Model: Mainship 390, Bayliner 3258, Bayliner 4788
Join Date: Feb 2013
Posts: 285
Ok…let’s take a step back here.



The statement “overpropped is overloaded across the entire powerband” needs to be re-written as “overpropped at W.O.T. means the engine is potentially overloaded at any RPM.”


To explain, we first need to dispel a huge myth about the “Propeller Demand Curve”.



Expert analysis by Don MacPherson of Hydrocomp is here:
https://www.boatdesign.net/threads/p...er-curve.9726/

MacPherson’s supporting white paper is here:


http://hydrocompinc.com/wp-content/u...hECEngines.pdf


Summary: The “Propeller Demand Curve” you see in engine manufacturer’s docs is a fiction. It is completely useless when discussing propeller selection. PDC a very rough, theoretical approximation of horsepower required to maintain steady-state RPMs in perfect conditions (calm seas, no headwind, clean bottom, etc.), or more accurately, in a laboratory dyno-tank. As Mr. McPherson states, PDC’s are >>only<< useful for estimating GPH at RPM. They serve no other useful purpose because this curve does not exist anywhere in the real world.



The large gap (noted by twistedtree) between the max power curve and the theoretical, steady-state demand curve that exists at lower RPMs is absolutely necessary! The size of this gap >>must<< be maintained. This is because the instant you try to accelerate, you exit ‘steady state’ mode and propeller demand will quickly approach max power. Without that extra horsepower, a semi-displacement hull (like the Shannon 36 in this example) will never climb out of the hole it is digging as it tries to get up on its ‘semi-plane’. This will be exacerbated in rough seas, headwinds and heavily loaded conditions, or with a fouled hull or prop. If you have ever tried to out-run a following sea in a boat with flo-scans and watched GPH increase wildly as you climb the waves and drop quickly going down the waves, you have seen the fallacy of the ‘propeller demand curve’.


So, any over-propping will reduce the amount of extra HP available to deal with the above dynamic and real world situations. Even a modest amount of overpropping virtually guarantees that (in the real world) you will be frequently overloading your engine at low or mid-range RPMs. How often you will be overloading your engine depends on the typical conditions you are running in.


Finally…hull design does matter. A true full-displacement hull, one that never needs to climb out of a hole because it is impossible to do so anyway, will present a load that looks more like (but will never match) the theoretical propeller demand curve. As Mr. MacPherson notes, PDC’s are somewhat useful for “…slow speed displacement hulls with conventional propellers” but are “completely unsuitable” for anything else. So, one could argue (as I do) that a full-displacement hull can tolerate some overpropping because it’s real-world demand curve looks more like steady-state. That is, until you are running headlong into gale force winds...


In the end, I don’t understand the widely held view that ‘some overpropping is ok’ or worse, even desirable. Overpropping never meaningfully reduces fuel consumption. Overpropping always increases the load on the engine, which in turn increases wear, decreases oil life, increases exhaust temperature, maintenance costs and the chances of early-life engine failure. Overpropping always robs you of additional mid-range power you might need in an emergency. Over-propping is also the most common cause of ‘sooting’.


So, if one accepts that real-world 'propeller demand' is a dynamic number, and that the additional 'headroom' needed above the PDC is critical, especially at low RPMs, then an overpropped engine can be overloaded at any RPM, and murphy's law suggests it will happen at the worst possible time.
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