Big speed, little speed....2 motors

Originally Posted by Hullaby

Apex1, To clarify: is the fixed prop that is optimised for high power(big generator) still fairly optimal at a lower rpm (small gen)?

The prop is´nt, of course. A fixed prop is only optimized (if so, most are just roughly calculated) to operate at one given rpm to provide best thrust, everything else is over- or underloading the engine. With a Diesel El. drivetrain you still have no optimal prop, but assuming your Hotel load is a high one, you have to operate a genset anyway at all time, so the D/E setup can be more efficient than a conventional Diesel / Gearbox / fixed prop installation. If the Hotel load is not constant, you are much better away with a CPP on single engine and a separate set of genny´s.
You must bear in mind that a pair (or more) of genny´s is running constant at the given rpm (usually 1800 for 60Hz), and consumes fuel at optimal efficiency only when the genset is under full nominal load. Say your gen. consumes 200g/KW/h at 1800 rpm laden, and has 160 KVA of which you use 60 for Hotel and 100 for propulsion you have 32kg of fuel consumed in one hr. Now you shut down your El load of 60KW, the gen. still turns at 1800rpm, but now it operates at underload condition and the diesel consumes 300g/KWH/h, which is translated in 30kg fuel/h. Means the engine is less efficient (and wear and tear increases), you pay more for less work done. (relatively).

A CPP allows you to provide a sufficient workload on the engine, no matter which rpm set, so the diesel runs almost always at, or near optimal specific consumption point. In our example 200g. And it does´nt coke, does´nt spoil engine oil by unburned fuel, and so on.

So, if your Hotel load is a big constant consumer, leave the prop/shaft as it is and go Diesel El. with different sized genny´s, if that is´nt the case, go CPP!

Regards
Richard

[WestVanHan]

My old boat had 2 Cummins.It's best cruise for temp/load and such was ~16 knots.

BUt I don't want to go that fast and burn all that fuel,with uncharted rocks everywhere and deadhead logs.

But if I turned off one engine and slowed down to 10 knots,my burn was the same as with both engines on,due to the one prop's drag.And unhealthy low load.

Now with CPP,I could turn one engine off,feather the prop,and have a nice economical cruise with one properly loaded engine.

Paid for itself before I sold it.

You would do better running on both engines at idle but with sufficient pitch on the props! There is no big difference (apart from some inner, frictional resistance) running on say 100 hp out of one or two engines, consumption is for 100 hp running. But there is additional drag of prop, even feathered, and hull,running not straight, and rudder, larger angle. Altogether higher resistance, and consumption, than the additional engine running.

[WestVanHan]

Well,I know what my FloScans told me-you don't.

Have you thought that running at idle with some pitch dialed in would lower the rpm, and make it run rough.

Originally Posted by WestVanHan

Well,I know what my FloScans told me-you don't.

Have you thought that running at idle with some pitch dialed in would lower the rpm, and make it run rough.

Of course I cannot know your special case, and when you say you´re fine with that kind of operational mode, you´re fine. And it is not sensible to run a engine really at minimal rated rpm (or worse, below), by applying pitch at idle without giving throttle accordingly.
What I was referring to, is the fact that when you run at a 100 hp setting it is the better solution to divide that by two and run on both engines at 50hp instead of one at 100. Naturally that is´nt true in every case.