Big speed, little speed....2 motors

[Hullaby]

Mathdieselhead help requested:
Pushing my 60 foot hypothetical "canoe" at somewhere around 10knts requires something around 100hp(diesel).
Same thing at 15knts requires something like 400hp.
Forgetting weight, and drag penalties(extra engine, shaft and props..) for the moment and just looking at engine efficiency, would I save significant fuel by running the 100hp in it's "zone" (say 75% of the travel time) vs. running the 400hp at lower rpm...ie out of it's ideal "zone"?
I burn something like 50-75,000gals a year, so the answer could be important :D

[apex1]

Mathdieselhead help requested:
Pushing my 60 foot hypothetical "canoe" at somewhere around 10knts requires something around 100hp(diesel).
Same thing at 15knts requires something like 400hp.
Forgetting weight, and drag penalties(extra engine, shaft and props..) for the moment and just looking at engine efficiency, would I save significant fuel by running the 100hp in it's "zone" (say 75% of the travel time) vs. running the 400hp at lower rpm...ie out of it's ideal "zone"?
I burn something like 50-75,000gals a year, so the answer could be important :D

Yes in theory you would. When you run a diesel at optimum rpm setting, the specific consumption is the lowest. Running it outside of that "zone" as you name it makes specific consumption worse. At 50.000 gallons you should think about a Controllable Pitch Propeller CPP installation, before you think about a engine replacement or additional complication.
The CPP allows you to "load" the engine always to achieve a good specific consumption point, that keeps the engine happy and scrooge. The cost, I guess will be about the same as a second engine and cheap gear.

Regards
Richard

[Hullaby]

Thanks Apex. I have no experience with ccp, but lots of experience with adjustable pitch feathering props. They are not up to the day after day abuse of my world thus far. Max props are fine on cruising sailboats, hate Autoprops, but suspect that CCP might be a little prone to wear and tear like Max props....Any experience in harsh commercial applications?

[apex1]

Thanks Apex. I have no experience with ccp, but lots of experience with adjustable pitch feathering props. They are not up to the day after day abuse of my world thus far. Max props are fine on cruising sailboats, hate Autoprops, but suspect that CCP might be a little prone to wear and tear like Max props....Any experience in harsh commercial applications?

If you call Scandinavian waters, Iceland, Greenland during winter season harsh, then yes. All the Scandinavian fishing fleet uses CPP systems only!

Even if the engine drives a winch or a shaft generator, the rpm can be kept constant – and the speed of the vessel can then be regulated by means of the propeller pitch.
At lower than the maximum speed, the engine’s fuel consumption can be reduced considerably by increasing the pitch and lowering the rpm´s while maintaining the required speed. In this way the loading of the engine, and the total efficiency of the unit is increased. Using fully reversible propellers, only a reduction gearbox is needed.

The feathering props for sailing boats, or Max props are a completely different world. Toys

Search for "Hundested", "Helseth", "Westmekan" to name just the best proven and most common here.

Regards
Richard

[FAST FRED]

The simplest , and cheapest solution might be to purchase a transmission as found in US Navy landing craft.

Same design since WWII , so used , rebuilt is readily avilable.

These units usually have a pair of Detroit Diesels so have SAE 1 bellhousing.

A lever can disconnect either engine , even underway , tho the engines must be stopped to reconnect to tranny.

With a CCP , you would have the ability to cruise on either , or both , and fine tune the pitch to optimize engine loads.

Last I saw was adverts in Boats and Harbors for US NAVY fully rebuilt drive packages on pallet , with (2) 6-71 and 2-1? tranny for about $6,000 US.

FF

[Hullaby]

Ok, time for me to cough up a little more detail....
My specific application is passenger carrying. Without filling you up with too many details this vessel will cover more than 50,000 miles a year of stop and go, accelerate rapidly, stop NOW, and most important, always deliver on time, everyday. Excuses not understood, nor listened to.
Against the above, the vessel must be very light, fuel efficient, and of course, pretty fast. Current example: Catamaran that has about 500,000 miles on it, cruises at 23knt and gets about 1.1-1.2 MPG carrying 40-60 humans at that speed.
The new project will be tasked with a higher efficiency, but lower speed(15knt cruise) demand. The initial reasoning in asking about the big and little motors is economy at high and low speeds plus redudancy. However, the redundancy and associated maint is only really worthwhile if the economy keeps pace.
So, what is more efficient; using cpp to lug the engine at low speed or having a smaller engine that matches that load? And how big is the difference?

[apex1]

If economy is the target, the CPP always wins! Only high speed operation at almost always same speed is not worth a CPP, due to the fact that one can have a fixed prop optimized to that conditions. And doubling engine lifespan and TBO is another advantage for a CPP system.
I had a Harbour Police boat in my collection of old workboats, that came out of 24/7 service after some 35 years, there was no sign of wear and tear in the drivetrain, and the engine had one major overhaul since new.

And I doubt a Lofot Fisher has anything to give away.

>>>>And how big is the difference?<<<< That is a bit too broad a question. How long is a string?

Regards
Richard

[FAST FRED]

using cpp to lug the engine at low speed or having a smaller engine that matches that load?

DIAMETER is the biggest absorber of HP so a prop sized for the "big" engine would be pretty efficient when de pitched , and spun by the smaller engine .

What you really need is called a FUEL MAP , that allows you to optomize the engine choice by comparing the efficiency at different loads.

Unfortunatly some engines are so poor outside their 80-90% rating area , engine Mfg wont give the info out.

Some will burn 200% to 300% more fuel per hp at very low loading.

FF

[apex1]

[QUOTE]so a prop sized for the "big" engine would be pretty efficient when de pitched , and spun by the smaller engine .


Fully concur Fred, but for what do I need a second engine then?
When I reduce rpm and load the big engine by giving pitch until the choosen rpm does´nt change, I operate in a fuel efficient "window". Probably not the absolute optimum possible for the given engine, but pretty close.
And 100hp, for example, delivered by a small engine at perfect load is about the same consumption (say 190g/kw/h), as 100hp delivered by the big engine under optimized load.

What you really need is called a FUEL MAP , that allows you to optomize the engine choice by comparing the efficiency at different loads.

That would help choosing a second engine for a fixed prop setup (if you can get a fuel map), but you do´nt need when using a CPP.

Unfortunatly some engines are so poor outside their 80-90% rating area , engine Mfg wont give the info out.
Some will burn 200% to 300% more fuel per hp at very low loading.

Unfortunately thats quite true, therefore: I guess you know already;)

Richard

[Hullaby]

Thanks for the input lads,
While we're on this subject....and just to add a wrench in the mix....
I know diesel electric has been discussed mucho in many threads, but I don't recall mention of coupling an electric motor to CCp. Would there be any kind of advantage? I understand the torque properties of electric. I just wonder if there would be a large gain with the bigger prop being tailored to the current low speed......This probably has Occam frowning already:rolleyes:

[apex1]

No,
with a fixed prop you would have a advantage going diesel electric, because you could use two (or more) different sized generators for high/low speed and would run the genny always at optimal load setting. If you have a substantial Hotel load on the vessel (El.) then a DE setup can be a very sensible way of propulsion. You could choose a say 160 kva Genny to provide 60kva Hotel load and 100kva for slow speed, and a 240kva for full steam. In that case a CPP does´nt make much sense.
Richard

[baeckmo]

Just handed over a 8 hp SABB one-cylinder diesel to a customer after routine service. It has a cpp that matches the old stomper beautifully, making precision manoeuvering a delight! Which urges me to verify what Richard is saying about the performance of the CPP (not VPP....) in Scandinavian fishing vessels. The operation of these boats covers all speeds from high speed-low load steaming (first catch to sell gives best money!) to low speed-high thrust trawling. Some 20 years ago we saw fuel meters being installed, and they immediately made skippers change their engine loading routine.

Fuel savings resulting from procedure change were up to ~25 %, so the instruments were payed off within days for those who had run their engines with high revs/low pitch.

For Hullaby's application, I would contact the Servogear AS in Norway, they have cpp's well suited for this purpose. The reason why the cpp has lost market shares to conventional reverse gears, is probably the increased speeds of modern diesel engines; there has to be a reduction gearing anyway! Most "marine engines" are just road vehicle engines modified for "wet jobs". And, btw, a propos the thread on variable transmisssions; the cpp is a "continuously variable transmission", hydrodynamic style!

[Hullaby]

Baeckmo, With the engine speed thing in mind: Is there a more suitable low speed motor to work with? We currently run twin Cummins C series (430hp).
We are mostly happy with them because of easy rebuild (getting 15000hrs and pulling out while still in good running...not from failure)
cheaper parts than yanmars etc and very important to the question, power to weight.
For the new project I would like to run something like half or less that hp. This seems quite possible based on tests with our current vessel. Running on one engine with the rudder oversteering we get the speeds I need for the new project....even into 20kt headwind and seas.
Also, I am not yet 'un-sold' on diesel-electric.

[Hullaby]

No,
with a fixed prop you would have a advantage going diesel electric, because you could use two (or more) different sized generators for high/low speed and would run the genny always at optimal load setting. If you have a substantial Hotel load on the vessel (El.) then a DE setup can be a very sensible way of propulsion. You could choose a say 160 kva Genny to provide 60kva Hotel load and 100kva for slow speed, and a 240kva for full steam. In that case a CPP does´nt make much sense.
Richard

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

[Hullaby]

Apex 1, Sorry further clarification: SHAFT RPM

[apex1]

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.

[apex1]

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.

[apex1]

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.