IS USING JUST ONE ENGINE FALSE ECONOMY

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Now there all that seems simple enough - I read up a lot on the subject and spoke to prope designers, boat designers and those are my conclusions ...... I can see no reason why those conclusions can explode into another mega thread Richard

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The great Gludy cometh and hath spoken to the people. End of thread. Amen

(Actually I agree with your conclusions)
 
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My consulsions were:-

1. Dragging a moving prop is a lot more drag that dragging a fixed prop - a moving props actually sucks energy away in moving it.



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Is there any chance you can elaborate on this please Paul /forums/images/graemlins/smile.gif

I have herd mention of this before but I have never once herd a reason why ?

Surely it takes more negative power to hold a prop stationary than to allow it to rotate /forums/images/graemlins/confused.gif


Any chance you can also elaborate on which gear boxes are safe to rotate and which ones need to be locked ?

How I missed the first threads I dont know ?
Any links ?

cheers

Pete
 
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The boat is going to need the same amount of power

[/ QUOTE ]Tricky subject this one.
Indeed each boat needs X hps for a given speed, and that doesn't depend on the engines (leaving aside the drag for a moment).
But X is not necessarily what the engine(s) actually deliver. At the speeds we're talking about, and for a boat which is actually designed for higher speeds, the engine(s) will surely produce much more than X hps.
Now, the point is, two engines will generate Y hps at a given speed, whilst one engine will generate Z hps.
Both Y and Z are bound to exceed X, but Y will exceed Z as well.
A theorical figure might be estimated based on both the prop demand and the max power curves of the engines, with the associated consumption.

Just as an example, I gave a look at the Caterpillar curves of my engines.
Prop demand curve at 1200 vs. 1800 RPM:
27x2=54 vs. 69 hp
6.7x2=13.4 vs. 18.6 lph
Max power curve at 1200 vs. 1800 RPM:
80x2=160 vs. 190 hp
17.3x2=34.6 vs. 37.6 lph

See the huge differences, and how the single and twin figures are much more comparable for max power rather than for prop demand? Mind, rpm are exactly the same!

And in practice, each boat can behave in such way that the engines are anywhere in between prop demand and max power curves, so the answer to the original question "is it more economical..." is almost surely yes.
But if robyonfrome would have asked "by how much", I would have replied that he must check it himself on his boat.

...and all without even starting to consider drag, shaft cooling, etc.!
 
Good question. I have fuel flow metering connected to GPS so it computes instantaneous mpg and at slow speeds (say 6kts) I get about a 20% improvement in mpg if I use one engine instead of two. A few further things though

1. I dont have a shaft brake so that's with a free-rotating "off" engine. But at 6kts it doesn't turn much anyway, so the worries about loss of efficiency due to no shaft brake, and gearbox spinning are red herrings in practice inho, at 6knots for say 30minutes. They would become important issues at higher speed/longer trips of course. Remeber you have both lubrication and cooling issues with the gearbox. The engine needs to be spinning to circulate the oil and to drive the water pump that provides sea water cooling to the gearbox oil. So unless you're sure you're ok, best not take chances. do you have gearbox oil temp guage and does it work with the engine off?

2. On my boat at least, the power steering is fed by only one engine so that's the one that has to run

3. I think the bore glazing issue is a red herring so far as this question is concerned. For the low speed river passage you describe I also think the prop pitch thing is a bit of a red herring - the props are the "wrong" pitch anyway at 6 knots. Driving at 6 knots in a boat is like driving a 6mph in a car in 4th gear.
 
It's all down to efficiency.

Two engines barely working will consume a certain amount of power which is being wasted simply by running the engine and it's ancillieries.

One engine working a tiny bit harder is going to offer gains in efficiency, and a modest saving in fuel and, more importantly in wear and tear on the dumb engine!

Lets say the fuel consumption is only 20% better while in this state. Isn't that a good thing with fuel (soon) costing £8 a Gallon?

Now the wear and tear savings on the engine aren't to be sniffed at either, with a modest engine costing upwards of £20,000, a 2% increase in life expectancy is worth £400 per engine.

On that basis, I would run the boat on one engine unless circumstances dictate otherwise.... /forums/images/graemlins/smirk.gif
 
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Indeed each boat needs X hps for a given speed. But X is not necessarily what the engine(s) actually deliver. Now, the point is, two engines will generate Y hps at a given speed, whilst one engine will generate Z hps.

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Mapis I think that's a very incorrect analysis. Leaving aside friction and inefficiencies outside of the engines, X is what the engines deliver. It has to be, else where is the energy going? All energy has to be accounted for, you can't lose any, laws of physics

When you have two engines as opposed to one, and ignoring the extra inefficiencies of the second engine (from having to run a second cooling pump etc and general friction), X, Y and Z are all the same number. They absolutely MUST be, e;se you are re-writing the laws of physics and thermodynamics.

I know that if you take data off the engine manufacturer power/consumption curves you can get a different answer, but that is not correct use of those curves. As I say, X=Y=Z (ignoring frictional and prop efficency losses etc)

Thus the only reason why one engine might be more economical than two, or vice versa, is frictional and efficiency losses of running two, versus running one and dragging the other
 
Perhaps someone in MBY can dig out the previous thread on this subject ... Seem to recall it went into loads of details etc.

Now if we just had a good search engine in here...... /forums/images/graemlins/smile.gif
 
Daka
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Is there any chance you can elaborate on this please Paul

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The best way is to think about it is that you require a certain amount energy to turn a prop.

A fixed prop will cost the turbulence around it - that is the drag of it. However this is small compared to the amount of energy required to turn the prop and if left free the energy in the boat is what is now turning the prop.

If you use a child's play handheld windmill thing and wave it in the air - it will turn. You are inputting the energy to make it turn. This is just like the energy being removed from the boat to make the prop turn.

Now taking the same kids windmill, tape the windmill part so that it cannot turn and wave it through the air - it is much easier to wave - it requires less energy as you only have the drag of the windmill - you do not have to turn it.

It takes the same energy to turn drive a prop using an engine at say 500 rpm as it does to turn a loose prop at 500 rpm. So a dragged prop turning at X speed needs the same energy as driving one at X speed and that energy can only be subtracted from the energy being supplied by the other prop. If both turned at the same speed the boat would not move!

The gearbox has to be designed so that when the engine is not driving it, it is still lubricated- some are and some are not - you have to find out from the engine/gearbox people.

Hope that explains.
 
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else where is the energy going?

[/ QUOTE ]I'm surprised to be asked such question from you jfm.
Laws of physics - the answer is heat, as I'm sure you'll agree, if you rethink about it.
 
I'm still in a "not sure" over this. I have, perforce, run only one engine between Windsor and Sunbury, due to cooling issues with t'other, and was not conscious of reduced consumption. Bear in mind that IMO if you have two screws then use them for locks, and that diesels overfuel a bit on startup, you might, possibly, dump some of the saving as smoke (more smoke in my case) and a small slick (more slick in my case!) depending on how far apart are the locks.

Jesting aside, the single vs twin argument is probably less significant when either engine is more than sufficient to reach hull speed, probably not using more than 50HP or so anyway, than it would be in a lower powered craft where you need to give a single engine some beans to get the same result as running both. For those running in and out of harbours, I put it to you that any fears on glazing would be allayed by a bit of a handful when safe and prudent so to do when out at sea. Its those of us on rivers that may need to think more about that one. I have, a couple of times, gone up to the weir pool and neutral turned a few times under more than river cruise power to blow the engines out a bit. Don't know if it helps but it might do.

As for shafts and gearboxes, I've got two shafts and four boxes, so don't want to muck 'em up. Oh, and starboard smokes worst, but makes the hot water, port runs clean, and makes the best electricity:-)
 
Thank you Paul,

I now understand the theory.

I am certainly not going to argue against the principal.

I am lucky in that I do have a child's windmill at home and I intend to try it /forums/images/graemlins/smile.gif

I had previously thought the prop as a solid round saucer ( you can not see through it from ahead or astern) until it turns when the gaps open up, a bit like screwing a bolt through a nut.

I will try a child's windmill through water first locked and then unlocked.

Cheers

Pete



Any one thought they spotted a typo /forums/images/graemlins/wink.gif
 
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It takes the same energy to turn drive a prop using an engine at say 500 rpm as it does to turn a loose prop at 500 rpm. So a dragged prop turning at X speed needs the same energy as driving one at X speed and that energy can only be subtracted from the energy being supplied by the other prop. If both turned at the same speed the boat would not move!

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The fallacy there is the implicit assertion that the drag on the unbraked idle prop will make that prop turn at the same speed as the driven prop.
 
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I will not travel on one engine unless I have to as it costs more on one than 2.

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Sorry Pete. Got to disagree with you on this one. The more cylinders = more fuel.

I'm sure JezBanks checked this out some time back on his Bavaria, as he'd fitted fuel flow meters. I reckon it was more economical on one engine but not hugely - 20% ish.

I'm sure he'll be along soon....

R

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oh gosh, now there's and intro.

I did indeed try it out, but all the data is at home so i will dig it up and let you know.
 
is the revs / fuel consumption relationship fixed on any given engine ? What I mean is, if an engine uses 10lph at 1000 revs, can those numbers fluctuate, or is it just the speed that can alter due to loading? IE, can there be circumstances where the boat engine is using 15lph while running at 1000revs?
 
A lot of formulae can come into play with this question. Running on one engine has been the subject of much discussion in Passage Maker magazine (for Trawler Yachts), and tends to depend on the boat itself.

For example, on our Fleming, travelling at a displacement speed of 8kts (hull speed is 9.5kts), we burn more fuel on one than on two (to keep 8 kts).

The reason we burn more is mainly down to two factors. First, the rudder angle having to be deployed to keep the boat straight which creates more drag and requiring more power to overcome it, and second, because the power is assymetric on one engine (not along the centre-line) the boat crabs about 6 degrees which creates more drag. This means more fuel is burnt.

So, you maybe burn more fuel (as I do), but engine service intervals increase which is a good saving.
 
We do this quite often because we have 2 V8 petrol engines. At 6 knots with two engines (1200rpm) we use about 3gph. At 6 knots on one engine (1400rpm) we use about 2gph.
 
That makes sense. I would guess (and my own recent experience indicates) that at substantially-lower-than-hull-speed, one engine will burn less than two.

Do you know anything about the wear on gearbox issue of an undriven, unbraked prop? What would be the effect if the engine on the idle prop was placed in reverse (non-electronic gears/throttles)?
 
Interesting that you find less fuel when using one, only. Is this at the same speed? The reason for questioning is that to move the vessel at a particular speed takes a set HP, so whether produced in total by 2 or just 1, the fuel burn is not normally significantly different.

Have you got some figures?

Onto the question of a windmilling gearbox. It is said that a windmilling prop greates more drag than a stationary prop - more drag = more fuel burn.

But you should check with the manufacturer of the gearbox to ensure no damage is being done.

On my TwinDisc, I emailed the qn to their tech centre and received a tech reply. In essence, it's OK on mine provided I meet some reasonable criteria.
 
I am pleased I am not the only one who is not convinced that one engine uses less than two.

Certainly my personal experience over a lot of miles and several years was that I used less fuel using both engines. ( close to hull speed of 8 knots)

Electronic fuel sensors and GPS are useful to a point but you would need to cover a lot of miles in order ti discount current/windage, fuel, passengers load etc

For me it is certainly twin are better than one.
other considerations

In a harbour/river I want twin engines for safety and manoeuvrability.

Your crabbing inefficiency perhaps explains why the theoretical locking of the prop still does not work.
 
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