Twin v Single fuel costs. Theory and reality

[tom52]

I know that the arguments for and against twin or single engines have been discussed many times and I don't want to go over them all again. I am just interested in trying to quantify solely the additional diesel costs for a twin engine compared to a single in otherwise identical boats.

Apologies in advance because I am no engineer

So being no engineer I just looked at the Performance Curves on the Yanmar website of the 260hp (4LHA STP) engine and 125hp engine (4JH3 DTE), which is the nearest 2:1 comparison I could find.

At 3000 revs one 125hp engine produces half the power at the prop shaft that the 260hp engine does at about 2700 revs, but uses almost exactly half the fuel. So it appears that two 125hp engines produce approximately the same power as the 260hp engine for approximately the same amount of fuel, albeit the big engine revs more slowly.

(I know the manufactures performance figures may be optimistic but presumably they are consistently optimistic for both engines.)

So am I just misinterpreting the curves or in theory would two 125hp engines produce the same power for the same fuel as a single 250hp engine ?

Is the reality that the twin engines just use more fuel in the real world because of the extra weight, double the friction losses and also the extra drag of two props versus one.

In which case what is the real world size of the fuel difference . Do twins use 5% more, 10% more, 25% more, 50% more or what ?

Help !!

 

[Whitelighter]

That is true, but you need more than twice the power since you are dragging through the water an extra propellor and shaft/outdrives which adds a heap of drag as well as weight. To compare the performance of a single 260hp motor your twin set up would need more like 300 or 320hp, so 150-160 ho per side.

I don't know if a single big engine weighs less than two smaller ones but I suspect it does and that will increase the weight/drag penalty with the twin set up also

I guess twins probably use 25%-30% more fuel but I have no stats to back this up and there aren't many boats that come in either single it twin to really try it out

 

[jrudge]

The only comparison I know is light aviation. Add another engine, go about 10 knots faster ( maybe) double the fuel and more than double the maintenance. As a consequence twins have almost no value!

If they made decent motor boats with one engine I would buy one. If I was daft enough to fly round for 15 years in a single engine plane with hi consequences of failure, then I am more than happy with one ( far newer) marine engine and relatively low consequences of failure ( in most conditions).

Sadly anything of size has 2 engines ( probably because it is big!) so I am stuck with 2.

if the size of boat you are looking for can be supplied with one then I would buy it. Stick on a bow and stern thruster and you will be set ( and yes I can handle a boat, and yes I love thrusters and am not looking for a thruster debate!)

Jeremy

 

[Latestarter1]

I know that the arguments for and against twin or single engines have been discussed many times and I don't want to go over them all again. I am just interested in trying to quantify solely the additional diesel costs for a twin engine compared to a single in otherwise identical boats.

Apologies in advance because I am no engineer

So being no engineer I just looked at the Performance Curves on the Yanmar website of the 260hp (4LHA STP) engine and 125hp engine (4JH3 DTE), which is the nearest 2:1 comparison I could find.

At 3000 revs one 125hp engine produces half the power at the prop shaft that the 260hp engine does at about 2700 revs, but uses almost exactly half the fuel. So it appears that two 125hp engines produce approximately the same power as the 260hp engine for approximately the same amount of fuel, albeit the big engine revs more slowly.

(I know the manufactures performance figures may be optimistic but presumably they are consistently optimistic for both engines.)

So am I just misinterpreting the curves or in theory would two 125hp engines produce the same power for the same fuel as a single 250hp engine ?

Is the reality that the twin engines just use more fuel in the real world because of the extra weight, double the friction losses and also the extra drag of two props versus one.

In which case what is the real world size of the fuel difference . Do twins use 5% more, 10% more, 25% more, 50% more or what ?

Help !!

Tom,

You are forgetting the golden rule...'Propellers move boats engines merely turn them'.

Where are you reading power, from the full load curve or the propeller demand curve? Later Yanmar data sheets give us no number for the exponent, used to be 3 for some engines 2.7 or 2.5 for others, unless you actually know, no point in surmising. IF exponent IS 3 propeller demand is around 80 hp not 125, The consumption curves are shown as exponent and not full load, as you can see by the shape but once again with no number??

Also remember 4LHA engine ratings to ISO 3046 making it 237 real hp. In addition some Yanmar models use differing fuel density which makes a complete nonsense of spec sheet comparisons.

Unless you start comparing engines with data sheets from Cat, Cummins, Deere, Detroit MTU and Steyr and are using same exponent it is as much use as herding cats.

 

[Hugin]

Is the reality that the twin engines just use more fuel in the real world because of the extra weight, double the friction losses and also the extra drag of two props versus one.

Theoretically two small engines shoull not have double the internal friction of one bigger with equal power. The total friction will be higher, but most likely not double
Otherwise you are right, extra weight, engine friction and appendage/prop drag are the reasons for higher fuel consumption at any comparable power output.

 

[blackmagicthe1st]

Herding cats

Tom,

You are forgetting the golden rule...'Propellers move boats engines merely turn them'.

Where are you reading power, from the full load curve or the propeller demand curve? Later Yanmar data sheets give us no number for the exponent, used to be 3 for some engines 2.7 or 2.5 for others, unless you actually know, no point in surmising. IF exponent IS 3 propeller demand is around 80 hp not 125, The consumption curves are shown as exponent and not full load, as you can see by the shape but once again with no number??

Also remember 4LHA engine ratings to ISO 3046 making it 237 real hp. In addition some Yanmar models use differing fuel density which makes a complete nonsense of spec sheet comparisons.

Unless you start comparing engines with data sheets from Cat, Cummins, Deere, Detroit MTU and Steyr and are using same exponent it is as much use as herding cats.

Sorry latestarter1

I saw "as much use as herding cats" and could help but think of this advert...

http://youtu.be/pz1iNSqqixc

Makes me chuckle.

 

[Latestarter1]

Tom,

May I suggest that you copy the example of Jegs http://www.ybw.com/forums/showthread.php?400025-Prop-pitch and purchase a copy of Dave Gerr's 'Propeller Handbook' as you obviously have inquiring mind. Gerr is also excellent book if you suffer from insomnia.

Just consider this........If we take a theoretical semi displacement hull of nominal 30 feet waterline length and 4 tonnes displacement, SD factor not relevant at this time, it will take a TOTAL of 114 shaft hp to drive it at 13.7 knots. However if one wanted to cruise the same vessel at 10 knots hull would demand around 45 shaft hp in total.

At 2,000 rpm our Yanmar 4JH3 DTE's have the potential to each develop 48 hp, however at 10 knots our hull is demanding barely half of each engines potential power, so where is the other half going? Simple answer is engine is not producing it. Governor is holding 2,000 rpm with only sufficient fuel to develop roughly half the engines potential power.

 

[MapisM]

If they made decent motor boats with one engine I would buy one.

Define decent.
I can't think of any twin engines mobo which I would trust for (very) long distance cruising.
Not up to to 65' or so, at least.

PS, in reply to OP question: according to all I read, heard and experienced, real life difference can be anywhere between 10 and 20%. BUT, I'm not aware of one single case where the single/twin choice was driven by fuel burn.

 

[Nick_H]

I'm not aware of one single case where the single/twin choice was driven by fuel burn.

I think you mean by fuel burn alone? Certainly fuel burn is one factor in many people's decision, mine and the OP for a start. Don't forget (you haven't of course) that fuel burn is not just a cost issue, but also range.

wrt to the original question, doesn't it depend on boat speed. I've only really looked at the comparison for fast outboard boats, where the rule of thumb is 25%+, ie. twin 140's gives approx same performance as 1 x 225.

 

[MapisM]

I think you mean by fuel burn alone?

Yes and no.
I mean, of course fuel burn can be one of the considerations in any boat selection.
But whenever there is a sensible choice between single and twin, for any given vessel (in many boats you don't actually have the choice, namely planing boats above 30 feet or so) I never heard of anybody whose choice to go for a single screw was mostly driven by fuel saving. Actually, not even slightly, coming to think of it.

PS: incidentally, I agree that 2x140 vs. 1x225 is a comparison that makes much more sense than 2x125 vs. 1x260.

 

[Nick_H]

Yes and no.
I mean, of course fuel burn can be one of the considerations in any boat selection.
But whenever there is a sensible choice between single and twin, for any given vessel (in many boats you don't actually have the choice, namely planing boats above 30 feet or so) I never heard of anybody whose choice to go for a single screw was mostly driven by fuel saving. Actually, not even slightly, coming to think of it.

Out of interest, what is the reason that most long distance cruisers have single engines, or single + wing. I always assumed it was to maximise range and/or reduce running costs (ie, fuel burn is the driving force), although that was only an assumption and i'm happy to be corrected.

 

[baylabayla3288]

I recently saw a comparison of single versus twin outboards on a relatively heavy planning hull. Surpricingly the twin engined boat was more fuel efficient up to about 22-24 knots. At higher speeds the single engined boat had the edge. The reason being that outboards typically have too small prop diameters to efficiently push a big boat under heavy loads. Long range slow cruisers have shaft drive where the prop can be made large enough, optimized for a narrow speed range.

 

[MapisM]

Out of interest, what is the reason that most long distance cruisers have single engines, or single + wing. I always assumed it was to maximise range and/or reduce running costs (ie, fuel burn is the driving force), although that was only an assumption and i'm happy to be corrected.

LOL, funny that the OP in his first sentence said "I know that the arguments for and against twin or single engines have been discussed many times and I don't want to go over them all again". He must have seen the debate coming....!

Yep, eventually the overall economics are indeed a component.
But even some small(ish) Nordhavn were built with twins, in the (rare) cases when the buyer was obsessed by the idea that two engines are safer than one.
And if properly built/optimised, they can still have ocean crossing range, and a fuel burn not much higher than the single equivalents.
So, as well as these few owners went for twins mostly for reasons that had nothing to see with the fuel burn, the same is true for all others who went for single.
Well, in my experience, at least.

Back to your Q on the pros of single screw, there are quite a few.
In fact, it's not accidental that it's a very common choice in commercial boats, and also in many ships - most (if not all) of them with no wing engine at all.
Not only a single prop can be larger, as has been said, but also much better protected, and it sits deeper in the water.
The shaft can be more horizontal than with twin engines, and doesn't require any bracket.
Also the engine can be installed lower in the bilge, leaving with more space around it, and also more headroom, allowing in turn also an easier access/maintenance.
With singles, it's also easy to build a much stronger and better protected rudder.
With twin props/shafts/rudders, a comparably strong setup requires very substantial skegs, but in turn they add a lot of drag.
And I'm sure I'm now forgetting some other technical pros of singles....

 

[bumpy_the_dog]

Define decent.
according to all I read, heard and experienced, real life difference can be anywhere between 10 and 20%. BUT, I'm not aware of one single case where the single/twin choice was driven by fuel burn.

The step change in diesel costs is still relatively recent, and people take time to change their behaviour. Maybe we're in the middle of a real shift in UK mobo habits because of more expensive fuel, but we need a few more years for it to become clear what those changes actually are?

 

[MapisM]

Well, there are many other forumites much more qualified than myself to guess whether any significant UK habits shift will take place or not.
Time will tell, I suppose.

One thing is for sure, there's one and only way known to mankind to burn less fuel when boating (sailboats aside, obviously): slow down.
The difference between 1 or 2 engines, a heavier or lighter boat, plastic or steel or whatever: the relevance of all these things is ridiculous, when compared to the difference between cruising at 30 or at 8 knots, for any given boat size.

 

[paul salliss]

Hi Mapism

Well said I agree with you, I was moored in Swanwick for 4 years, most went either no where at all or stayed in the Solent therefore my argument was what difference does it make it you travel across the Solent at 8 knots or 20 knots the trips are so short why not just slow down and enjoy it, someone in this thread made the comment that he would be interested in single screw if a decent British manufacturer made one, well there are several excellent Brit single screw manufacturers who in my opinion make a better built product than Fairprinseeker ever will, buts that's just my opinion

 

[Dill et Bound]

I must first of all admit to being no expert on this,but having owned and been involved with boats for many years, when it came to me buying my last boat, and it probably will be. I chose a single..
My boat is 25 years old, and designed for twin petrols, and most left the factory with that option.
With a single Diesel it is a bit under powered at 200 hp, but because of the tankage, has a huge range.
Because it is a planing hull, slowing down is not a good option, as it comes off the plane, and so burns more fuel.
Therefore it is for me either 10knts or 21 to 24.
The cost of running is not so much a problem, or factor, but the cost of maintaining is..
Maintaining 2 engines to a good standard would cost me twice as much as a single.
This was another factor as again I found that as engines have become more complex in the way they produce the power economically.
They have become more expensive to maintain, and more expensive to repair if they do brake down..

So is there a shift towards slowing down, and a change in behaviour, who knows.
It will probably be driven by the manufacturers anyway.
We seem to buy into whats on offer, Branding seems to rule the purse strings.

 

[Hugin]

I recently saw a comparison of single versus twin outboards on a relatively heavy planning hull. Surpricingly the twin engined boat was more fuel efficient up to about 22-24 knots. At higher speeds the single engined boat had the edge. The reason being that outboards typically have too small prop diameters to efficiently push a big boat under heavy loads. Long range slow cruisers have shaft drive where the prop can be made large enough, optimized for a narrow speed range.

You just added another layer of complexity to this topic. The issue of prop efficiency. A small fast spinning prop with little blade area is not as efficient as the bigger slower prop with larger blade area for the same hp.

That can explain why the two smaller outboards had lower fuel consumption than a single bigger engine. The two smaller engines probably had almost double the total blade area compared to the single bigger engine. The disadvantage of double prop/appendage drag is not so damaging at modest speeds, but becomes a significant hp-eater at higher speed (double the speed and you have 4 times more drag). At the same time the smaller engines' props also become more inefficient as speed increase, while the bigger engine's prop doesn't go through the same decrease in efficiency as speed go up - it is relatively speaking inefficient at all speeds (bar the very low).

But interesting there's a tipping point

 

[Hugin]

well there are several excellent Brit single screw manufacturers who in my opinion make a better built product than Fairprinseeker ever will, buts that's just my opinion

Maybe the comment about single screw was to be understood that he would be interested in a single screw boat that had the appearance of a Fairprinseeker product. Something that has the looks but not quite all the muscles.... and a lower price, of course

 

[rustybarge]

Maybe the comment about single screw was to be understood that he would be interested in a single screw boat that had the appearance of a Fairprinseeker product. Something that has the looks but not quite all the muscles.... and a lower price, of course

....Nimbus make some nice single screw boats.