Fuel efficient engines

[shugie]

With the steady increases in the price of diesel it seems sensible when considering a new engine to try and find one which will go further on each expensive litre of the smelly stuff. But I see no sign that engine providers are trying to make their engines more economical, at least for narrowboats.

 

What's the best engine configuration to keep the diesel bill down? Smaller engine? Electric drive?

 

Although the Beta table suggests a 40hp minimum for a 60ft narrowboat, I suspect for much of the time on a shallow and narrow canal the actual power needed is much less.

[ChrisPy]

because the specific fuel consumption of a diesel engine is, in theory, fixed, then the only way to achieve fuel efficiency with a diesel engine is to fit a smaller one and to improve the hull design to minimise the power requirements. Narrowboats are not ideal - too deep, too heavy, displace too much water which is particulary power-consuming in a narrow shallow canal.

 

a slipper launch hull form drawing about 9" of water would be ideal, but then you don't have any headroom.

[Timleech]

With the steady increases in the price of diesel it seems sensible when considering a new engine to try and find one which will go further on each expensive litre of the smelly stuff. But I see no sign that engine providers are trying to make their engines more economical, at least for narrowboats.

 

What's the best engine configuration to keep the diesel bill down? Smaller engine? Electric drive?

 

Although the Beta table suggests a 40hp minimum for a 60ft narrowboat, I suspect for much of the time on a shallow and narrow canal the actual power needed is much less.

 

Actual power needed to propel the boat at ordinary canal speeds is around the 2 or 3 hp mark. Extra is needed for effective manoevring, especially stopping, also modern trends to lots of electrics means maybe another 2 or 3 hp, sometimes more, just for that. One way to increase efficiency is to use a larger, slower turning propellor, but this is limited by the draught and increased draught in a restricted channel means more power or less speed. Older boats swinging props of 24 to 28" dia have plenty of manoevring power, and perfectly adequate speed in open water, with 15 to 20 bhp in a 70' boat. I'm unconvinced of the need for 40hp, though to some degree the smaller the prop the more hp needed.

 

Tim

[Proper Job]

Its not just the efficiency of the 'engine'. It the overal efficiency of the engine/gearbox/propshaft speed (depends on the reduction ratio of the gearbox) and diameter/pitch of the prop.

 

As Tim say's - traditionally marine engines were slow reving, had low propshaft speeds and threw large diameter props. A 9hp Bolinder running at 450 rpm, 1:1 drive throwing a large diameter prop would move a fully laden 70' er. They upped it to 15hp for a motor & butty.

 

Unfortunately, the option to fit a slow speed diesel is not available to most of the boating public any more (at least not cheaply - ask RN for a quote on a new 2DM).

 

A lot of efficiency is lost in the prop with modern high rpm set ups. A small prop running at higher speed introduces a shear effect on the water that it is trying to move ('slip' as marine architects call it). A slower prop rpm, larger diameter prop is much more efficient.

 

 

Electric drive? - I'm not sure there's a viable solution out there at the moment (put's head above parapit and prepares to have it shot at ). The trouble is, every time you convert energy you introduce losses (reduce efficiency). Engine to generator to batteries to electric motor to prop = chemical energy (fuel) to mech' energy to elec' energy to chemical (batteries) to elec' (motor) to mech'. Efficient - no. Quiet - depends on the generator (you could of course have a very long extension lead running off the grid )

 

The best way to better fuel efficiency - slow down a bit!

[Timleech]

Its not just the efficiency of the 'engine'. It the overal efficiency of the engine/gearbox/propshaft speed (depends on the reduction ratio of the gearbox) and diameter/pitch of the prop.

 

As Tim say's - traditionally marine engines were slow reving, had low propshaft speeds and threw large diameter props. A 9hp Bolinder running at 450 rpm, 1:1 drive throwing a large diameter prop would move a fully laden 70' er. They upped it to 15hp for a motor & butty.

 

Unfortunately, the option to fit a slow speed diesel is not available to most of the boating public any more (at least not cheaply - ask RN for a quote on a new 2DM).

 

A lot of efficiency is lost in the prop with modern high rpm set ups. A small prop running at higher speed introduces a shear effect on the water that it is trying to move ('slip' as marine architects call it). A slower prop rpm, larger diameter prop is much more efficient.

Electric drive? - I'm not sure there's a viable solution out there at the moment (put's head above parapit and prepares to have it shot at ). The trouble is, every time you convert energy you introduce losses (reduce efficiency). Engine to generator to batteries to electric motor to prop = chemical energy (fuel) to mech' energy to elec' energy to chemical (batteries) to elec' (motor) to mech'. Efficient - no. Quiet - depends on the generator (you could of course have a very long extension lead running off the grid )

 

Some well-known gearboxes are available with a 2.5:1 ratio or thereabouts, IMO that should give a reasonable compromise between prop size/draught and efficiency with modern high-speed engines and maybe lead to the amazing revelation that you don't need 40bhp and more. It seems, though, that everyone wants to stick to the 2:1 option without questioning it.

 

Tim

 

The best way to better fuel efficiency - slow down a bit!

 

Exactly

[Yoda]

Forty horsepower on a canal boat is very unnessasary. The whole hull design is for no more than 5 mph otherwise the bows, swim and stern would be different.

 

The engine should be sized for the worst application of the boat and the prop should be sized to the sfc curve of that engine. This might mean that a five bladed prop is fitted rather than a 3 blade 24 inch one. Variable pitch props are more efficient.

 

There is no point in chasing the inefficiencies of the hull by fitting big engines.

[bottle]

The best way to better fuel efficiency - slow down a bit!

 

Exactly, for example according to Beta Marine their 43 uses 1.5 lph at 1600 rpm and 1 lph at 1400 rpm, I doubt the reduction of speed would be noticed but saving one third of fuel use/cost would.

[Kraken]

The engine should be sized for the worst application of the boat and the prop should be sized to the sfc curve of that engine. This might mean that a five bladed prop is fitted rather than a 3 blade 24 inch one. Variable pitch props are more efficient.

 

 

Can you tell me what "sized for the worst application of the boat" means? Does it mean, for example, on a river against the tide? That sort of thing?

 

And also, can you tell me how to size the prop to the sfc curve of the engine?

[steelaway]

Can you tell me what "sized for the worst application of the boat" means? Does it mean, for example, on a river against the tide? That sort of thing?

 

And also, can you tell me how to size the prop to the sfc curve of the engine?

 

Hi There

You had to open this can of worms again!

There seems to be two strongly held theories.

One is to tune your prop to give maximum HP available at peak revs - 17/18 x 10/12. I think this method is subject to cavitation and a bigger diameter prop is much more effective than increasing the pitch

The other one is to tune the prop to the maximum torque, this will enable you to criuse at lower revs and at maximum torque and achieve optimum MPG. Prop size maybe 19 x 10/12

You will not get maximum revs but when you ask for power the engine will be at the maximum torque and to be able to provide power quicker

Alex

[DHutch]

Yeah, get a proper prop on it and you half way there.

- If where building a boat to have a deisal engine in it, i would build roughtly propotions of emilyanne, with a pitch of propeller and a ratio of gearbox to match whatever engine i put in it. Nothing wrong with the lister HA2 unoriganal a thought tho it might indeed be.

 

Emilyanne swings a 26*32" prop.

 

 

Daniel

[Yoda]

Yeah, get a proper prop on it and you half way there.

- If where building a boat to have a deisal engine in it, i would build roughtly propotions of emilyanne, with a pitch of propeller and a ratio of gearbox to match whatever engine i put in it. Nothing wrong with the lister HA2 unoriganal a thought tho it might indeed be.

 

Emilyanne swings a 26*32" prop.

Daniel

 

Dan your on the beer again. You should be on your female and then the beer.

[Kraken]

Hi There

You had to open this can of worms again!

There seems to be two strongly held theories.

One is to tune your prop to give maximum HP available at peak revs - 17/18 x 10/12. I think this method is subject to cavitation and a bigger diameter prop is much more effective than increasing the pitch

The other one is to tune the prop to the maximum torque, this will enable you to criuse at lower revs and at maximum torque and achieve optimum MPG. Prop size maybe 19 x 10/12

You will not get maximum revs but when you ask for power the engine will be at the maximum torque and to be able to provide power quicker

Alex

 

 

But does anybody know how you actually do that?

 

Sorry about the can of worms, but its kind of an interesting can don't you think?

[DHutch]

Dan your on the beer again. You should be on your female and then the beer.

No beer today.

[Yoda]

No beer today.

 

Well get your female off the keyboard and get a beer!!

Edited January 26, 2008 by Yoda

[chris w]

Hi There

You had to open this can of worms again!

There seems to be two strongly held theories.

One is to tune your prop to give maximum HP available at peak revs - 17/18 x 10/12. I think this method is subject to cavitation and a bigger diameter prop is much more effective than increasing the pitch

The other one is to tune the prop to the maximum torque, this will enable you to criuse at lower revs and at maximum torque and achieve optimum MPG. Prop size maybe 19 x 10/12

You will not get maximum revs but when you ask for power the engine will be at the maximum torque and to be able to provide power quicker

Alex

 

 

Torque is the important thing on a NB because we don't have a selection of gears - just one forward gear.

 

With cars we aim to change gear at maximum HP in order to still give us a reasonable torque when the revs drop after changing up. With a boat we don't have to worry about that so peak torque is the all important figure.

 

Chris

[ChrisPy]

tune the prop to the maximum torque, this will enable you to criuse at lower revs and at maximum torque and achieve optimum MPG. Prop size maybe 19 x 10/12

You will not get maximum revs but when you ask for power the engine will be at the maximum torque and to be able to provide power quicker

how does that work then? if the engine is cruising and already producing maximum torque, and the prop is sized to match, then there is no possibility of increasing the engine/prop speed and therefore no way to increase the power when you ask for it.

[Proper Job]

how does that work then? if the engine is cruising and already producing maximum torque, and the prop is sized to match, then there is no possibility of increasing the engine/prop speed and therefore no way to increase the power when you ask for it.

Chris

 

What steeaway and Chris W are saying is that the all important thing for boat propulsion is Torque, not hp. Torque is by definition a 'turn moment'.

 

If you pair the prop to the point of max torque/rpm, then that is the most efficient prop/engine match. When people say, "I can't get the max rpm out of my engine - all it does is black smoke", often it max's out just below the red line, ie at the maximum torque. Perfect.

 

When cruising, you are never(?) at max engine rpm anyway. However, what Cris is saying is, when you need that extra 'umph', because the engine/prop is configured against the torque curve rather than the power curve, it is there quicker and more responsive.

[chris w]

Yes, torque is what you feel when you accelerate in your boat or your car. You don't feel HP. HP anyway is a "derived" figure being (Torque x RPM)/5252 where torque is in ft-lbs. Torque is what is actually measured.

 

Chris

Edited January 27, 2008 by chris w

[Boxman]

I am in the process of fitting a new gearbox with different gear ratios the will increase the engine speed by about 200 rpm for a given prop shaft speed. It will bring the engine speed closer to the peak torque at cruising speed. Will this be more or less fuel efficient?

[Yoda]

I am in the process of fitting a new gearbox with different gear ratios the will increase the engine speed by about 200 rpm for a given prop shaft speed. It will bring the engine speed closer to the peak torque at cruising speed. Will this be more or less fuel efficient?

 

You will use alot more fuel. You should match the prop to the sfc curve.

[Billypownall]

You will use alot more fuel. You should match the prop to the sfc curve.

I think I have read your comments about higher revs using more fuel Yoda some time ago and you may be right. However the increase is very small and hardly worth considering. When our boat was delivered with a 42 h.p. engine and a 17x12 prop. we were using 1200 r.p.m. for about 3.6 m.p.h.and were going too fast past moored boats on tickover. Furthermore we had no end of flat battery trouble. We fitted a 17x10 prop and found real benefits. No more battery trouble at all. At our 1400-1500 rev.for cruising our two alternators really bang it out. The engine feels much smoother and definately quieter. Some of this is due to a faint but annoying transmission whine at about 1200 r.p.m.which disappears at about 1400. As to fuel cunsumption I have noticed no difference. In any case this forum started by querying fuel consumption from various engines. I think modern small diesels are very economical and our fuel costs are a tiny percentage of our year's running costs. I am not very technical but I think prop size should be matched to a cruising speed of about 1400-1500 rpm,say just under half the potential maximum r.p.m. of a modern diesel engine. Surely that cannot be far wrong.

[Timleech]

I think I have read your comments about higher revs using more fuel Yoda some time ago and you may be right. However the increase is very small and hardly worth considering. When our boat was delivered with a 42 h.p. engine and a 17x12 prop. we were using 1200 r.p.m. for about 3.6 m.p.h.and were going too fast past moored boats on tickover. Furthermore we had no end of flat battery trouble. We fitted a 17x10 prop and found real benefits. No more battery trouble at all. At our 1400-1500 rev.for cruising our two alternators really bang it out. The engine feels much smoother and definately quieter. Some of this is due to a faint but annoying transmission whine at about 1200 r.p.m.which disappears at about 1400. As to fuel cunsumption I have noticed no difference. In any case this forum started by querying fuel consumption from various engines. I think modern small diesels are very economical and our fuel costs are a tiny percentage of our year's running costs. I am not very technical but I think prop size should be matched to a cruising speed of about 1400-1500 rpm,say just under half the potential maximum r.p.m. of a modern diesel engine. Surely that cannot be far wrong.

 

That sounds like a fairly common sense approach, having a prop chosen to give a 'comfortable ride'. I think it's probably a mistake to try to be too precisely analytical.

'Matching the prop to the SFC curve' is fairly meaningless without more detail. Which sfc curve, for a start? One derived from dynamometer readings will bear no relation to what happens when the engine is driving a propellor, unless the dyno has been set up to match the prop load (not usually done). Even then, it will only relate to that one size of propellor, changing the prop will alter the whole thing.

Having said that, changing the prop to get the right charging regime would seem a bit ar*e about face to me

 

Tim

[Yoda]

That sounds like a fairly common sense approach, having a prop chosen to give a 'comfortable ride'. I think it's probably a mistake to try to be too precisely analytical.

'Matching the prop to the SFC curve' is fairly meaningless without more detail. Which sfc curve, for a start? One derived from dynamometer readings will bear no relation to what happens when the engine is driving a propellor, unless the dyno has been set up to match the prop load (not usually done). Even then, it will only relate to that one size of propellor, changing the prop will alter the whole thing.

Having said that, changing the prop to get the right charging regime would seem a bit ar*e about face to me

 

Tim

 

Wrong. It bears a direct relationship to the load. Specific fuel consumption is lbs per hour per horse power and the curve shows where the engine is most efficient.

[Timleech]

Wrong. It bears a direct relationship to the load. Specific fuel consumption is lbs per hour per horse power and the curve shows where the engine is most efficient.

 

Usually based on full load at each point on the curve. The prop load follows a cube law, so the only point the engine is at full load, or anywhere near it, is at full rpm. You will rarely be running for long periods at full load with your boat (hopefully!), so the sfc at that point is pretty much irrelevant.

 

Tim

[pwl]

Forty horsepower on a canal boat is very unnessasary. The whole hull design is for no more than 5 mph otherwise the bows, swim and stern would be different.

 

The engine should be sized for the worst application of the boat and the prop should be sized to the sfc curve of that engine. This might mean that a five bladed prop is fitted rather than a 3 blade 24 inch one. Variable pitch props are more efficient.

 

There is no point in chasing the inefficiencies of the hull by fitting big engines.

 

Where does one fine 'sfc curve' information for a given engine? Ours is a BMC 1.8. Just curious.

 

P

 

Edited for typo.

Edited January 28, 2008 by pwl