Engine Revs question

[DeanS]

How can I phrase this....

 

If my boat has a slower engine...and I increase the throttle to make the engine run at 1500revs for example, is my engine using more effort/energy/fuel to produce those 1500revs, than a newer boat which does 1500revs at a lower throttle position.

 

and...at 1500revs, is my boat doing the same speed as a newer boat at the same revs.

 

I guess I'm trying to ask whether my boat is working harder to achieve a speed, than a newer boat does to achieve the same speed....

 

Im asking this because I always feel other boats are in a rush, and maybe I should be taking my throttle up more...but then I think...why should I make my engine work harder...I'm happy with being slower.

[Tim]

Don't worry about the others, you travel at a speed you're comfortable with.

[pistnbroke]

A diesel always adjusts its fuel input to match the load ...thats the governers job ....if you go faster you use more fuel simple as that ....go as slow as you like as long as you pull over to let me fly by .....

The 4 mph limit is a fairly recent idea ..designed to slow everything down so the railways could compete ..not in force in the commercial hay day of the canals ...

[Tony Brooks]

How can I phrase this....

 

If my boat has a slower engine...and I increase the throttle to make the engine run at 1500revs for example, is my engine using more effort/energy/fuel to produce those 1500revs, than a newer boat which does 1500revs at a lower throttle position.

 

and...at 1500revs, is my boat doing the same speed as a newer boat at the same revs.

 

I guess I'm trying to ask whether my boat is working harder to achieve a speed, than a newer boat does to achieve the same speed....

 

Im asking this because I always feel other boats are in a rush, and maybe I should be taking my throttle up more...but then I think...why should I make my engine work harder...I'm happy with being slower.

 

 

The speed an engine needs to run at to give a certain boat speed will depend upon:-

 

Gearbox reduction ratio

 

Prop size

 

An to a lesser extent

 

Waterway profile

 

How well the hull swims

 

so there is no direct correlation between engine speed and boat speed between boats.

 

However the engine will have a specific fuel consumption graph that looks like a very shallow dish so depending upon exactly where on that graph your present speed puts you speeding up might produce a very slightly lower specific fuel consumption. However the resultant shoving greater amounts of water around will probably cancel this out.

[Robin2]

I believe the best way to think about is that two otherwise identical boats but with different engines would probably use the same amount of fuel at the same boat speed - because the same amount of work is being done.

 

However this assumes that both engines are optimally matched to their propellers at that boat speed - which is unlikely. There will also be some differences in the efficiency with which each engine burns fuel. In general one would expect newer designs to be better - but 1500cc Isuzu diesel was not designed for the same work as an equivalently powered Gardiner so I'm not sure how their specific fuel consumption compares. The graph on this page illustrates the complex relationship between propeller power and engine power.

 

In general with a boat the fuel consumption rises disproportionately compared to the boat speed. I don't know the exact relationship but it won't be too far wrong to assume that doubling the boat speed would use four times as much fuel. If you want to save fuel, go slow.

[blodger]

If my boat has a slower engine...and I increase the throttle to make the engine run at 1500revs for example, is my engine using more effort/energy/fuel to produce those 1500revs, than a newer boat which does 1500revs at a lower throttle position.

 

AFAIK your engine was designed to propel a boat and was purpose built. Many modern and even some of the Vintage engines were not; they are just power units

[nb Innisfree]

How can I phrase this....

 

If my boat has a slower engine...and I increase the throttle to make the engine run at 1500revs for example, is my engine using more effort/energy/fuel to produce those 1500revs, than a newer boat which does 1500revs at a lower throttle position.

 

and...at 1500revs, is my boat doing the same speed as a newer boat at the same revs.

 

I guess I'm trying to ask whether my boat is working harder to achieve a speed, than a newer boat does to achieve the same speed....

 

Im asking this because I always feel other boats are in a rush, and maybe I should be taking my throttle up more...but then I think...why should I make my engine work harder...I'm happy with being slower.

 

Assuming identical boats both engines will use the same amount of fuel, it takes a certain amount of fuel to propel those two identical boats at a set speed despite one engine revving more.

 

The faster you go the more drag encountered so the higher the consumption, same as a car.

Edited August 21, 2011 by nb Innisfree

[bizzard]

How can I phrase this....

 

If my boat has a slower engine...and I increase the throttle to make the engine run at 1500revs for example, is my engine using more effort/energy/fuel to produce those 1500revs, than a newer boat which does 1500revs at a lower throttle position.

 

and...at 1500revs, is my boat doing the same speed as a newer boat at the same revs.

 

I guess I'm trying to ask whether my boat is working harder to achieve a speed, than a newer boat does to achieve the same speed....

 

Im asking this because I always feel other boats are in a rush, and maybe I should be taking my throttle up more...but then I think...why should I make my engine work harder...I'm happy with being slower.

Providing that the propellers the correct one for your boat,engine ect, and at 3-4 mph should return about 4hours to the gallon of fuel. bizzard.

[Nickhlx]

Providing that the propellers the correct one for your boat,engine ect, and at 3-4 mph should return about 4hours to the gallon of fuel. bizzard.

 

Hmmm - that's about 14 mpg

 

I think that's not bad, considering it weighs about 15-20 times that of a car and is equivalent to moving "your whole house" for many people.

 

 

Nick

[Higgs]

If you're creating a bow wave that pounds the bank, your going too fast. Too much froth coming from stern, your wasting power or you have a collection of leaves around the prop, in which case you'll not be making much headway. Put drive in reverse for short burst then back into forward until it happens again; that'll be quite frequently in the Autumn.

 

In shallows you can't really do much speed or really gain much extra by opening up the throttle.

 

All the other things apply too.... like prop size, good boat design, torque, engine condition, etc..

Edited August 21, 2011 by Higgs

[jonk]

I am not sure that some of this is true. Surely the efficiency of the engine comes into the equation? If one engine is more efficient than the other then it will use less fuel for a given power won't it? All other factors being equal. This is mainly because the fuel is burnt less efficiently and some goes through without being burnt (or the power produced is not working towards turning the engine) - modern engines having pistons, chambers, exhaust etc refined for better fuel efficiency and should therefore propel a boat further for the same fuel. Where am I going wrong?

 

John

[Robin2]

I am not sure that some of this is true. Surely the efficiency of the engine comes into the equation? If one engine is more efficient than the other then it will use less fuel for a given power won't it? All other factors being equal. This is mainly because the fuel is burnt less efficiently and some goes through without being burnt (or the power produced is not working towards turning the engine) - modern engines having pistons, chambers, exhaust etc refined for better fuel efficiency and should therefore propel a boat further for the same fuel. Where am I going wrong?

 

John

I did mention this in post#5

[nb Innisfree]

I am not sure that some of this is true. Surely the efficiency of the engine comes into the equation? If one engine is more efficient than the other then it will use less fuel for a given power won't it? All other factors being equal. This is mainly because the fuel is burnt less efficiently and some goes through without being burnt (or the power produced is not working towards turning the engine) - modern engines having pistons, chambers, exhaust etc refined for better fuel efficiency and should therefore propel a boat further for the same fuel. Where am I going wrong?

 

John

 

The vast majority of engines are equally efficient, more or less, hardly anything to choose between them, even vintage engines are efficient. Latest common rail engines are better though having very precise injection throughout the rev range.

 

ETA: Of course all this is assuming the engine isn't under or overpowered and is correctly tuned.

Edited August 21, 2011 by nb Innisfree

[jonk]

The vast majority of engines are equally efficient, more or less, hardly anything to choose between them, even vintage engines are efficient. Latest common rail engines are better though having very precise injection throughout the rev range.

 

ETA: Of course all this is assuming the engine isn't under or overpowered and is correctly tuned.

 

Efficiency ranging from 35% old engines to 50% new engines? I am no Diesel expert but from reading about engine efficiency I would be surprised if new diesels were not more efficient than older models. There is probably a trade-off though since older models were often 'over-engineered' to provide a long and reliable life. I came across this bit of info http://en.wikipedia.org/wiki/Diesel_engine#Power_and_fuel_economy

which also indicates that turbo and intercooling can make an engine more efficient - for a trade-off against servicing and reliability. I don't say that we should turbocharge though!

 

 

John

 

I did mention this in post#5

 

Yes - and I agree, but most posts seem not to agree? Perhaps I am mis-reading them?

 

John

[Tony Brooks]

I believe the best way to think about is that two otherwise identical boats but with different engines would probably use the same amount of fuel at the same boat speed - because the same amount of work is being done.

 

However this assumes that both engines are optimally matched to their propellers at that boat speed - which is unlikely. There will also be some differences in the efficiency with which each engine burns fuel. In general one would expect newer designs to be better - but 1500cc Isuzu diesel was not designed for the same work as an equivalently powered Gardiner so I'm not sure how their specific fuel consumption compares. The graph on this page illustrates the complex relationship between propeller power and engine power.

 

In general with a boat the fuel consumption rises disproportionately compared to the boat speed. I don't know the exact relationship but it won't be too far wrong to assume that doubling the boat speed would use four times as much fuel. If you want to save fuel, go slow.

 

 

I think you are spot on there because vehicle wind resistance increases to the square of speed and water is just a more viscose fluid. However a boat gets into wave making energy and hull attitude that probably affects things more at higher speeds in a displacement hull.

[Robin2]

Efficiency ranging from 35% old engines to 50% new engines? I am no Diesel expert but from reading about engine efficiency I would be surprised if new diesels were not more efficient than older models. There is probably a trade-off though since older models were often 'over-engineered' to provide a long and reliable life. I came across this bit of info http://en.wikipedia.org/wiki/Diesel_engine#Power_and_fuel_economy

which also indicates that turbo and intercooling can make an engine more efficient - for a trade-off against servicing and reliability. I don't say that we should turbocharge though!

 

 

Efficiencies above 35% will be difficult to realize in practice. From reading the reference in the wiki article the higher efficiencies were in bigger engines. I think the low fuel consumption figure at the top of the article refers to a slow speed diesel in a large ship or power station.

 

Also bear in mind that engines are most efficient at a particular RPM and in boats and road vehicles it is unlikely that the engine will be working at exactly the right speed. For the Isuzu 3LB engine the specific fuel consumption varies between about 250 and 285 grams/kW-hr. That is an efficiency range of about 33% to 29%.

 

In a boat the greatest inefficiency is the propeller - even the best propeller - because it must slip in the water to create thrust - unlike a wheel which is in firm contact with the ground.

[Nickhlx]

Efficiencies above 35% will be difficult to realize in practice. From reading the reference in the wiki article the higher efficiencies were in bigger engines. I think the low fuel consumption figure at the top of the article refers to a slow speed diesel in a large ship or power station.

 

Also bear in mind that engines are most efficient at a particular RPM and in boats and road vehicles it is unlikely that the engine will be working at exactly the right speed. For the Isuzu 3LB engine the specific fuel consumption varies between about 250 and 285 grams/kW-hr. That is an efficiency range of about 33% to 29%.

 

In a boat the greatest inefficiency is the propeller - even the best propeller - because it must slip in the water to create thrust - unlike a wheel which is in firm contact with the ground.

 

If efficiency in car engines is measured by mpg, then surely cutting edge engines must be the most efficient ? Take a look at e.g. BMW diesels - astonishing mpg and power compared with even 10 years ago. I think their 2 litre is some 163 bhp now whereas the Beta is around a 1/4 of that - I know that is a loose comparison and of power outputs not efficiency, but I imagine that the BMW engine would be more efficiently converting diesel to kWhs if each were put on a test jig.....

 

Nick

Edited August 21, 2011 by Nickhlx

[nb Innisfree]

If efficiency in car engines is measured by mpg, then surely cutting edge engines must be the most efficient ? Take a look at e.g. BMW diesels - astonishing mpg and power compared with even 10 years ago. I think their 2 litre is some 163 bhp now whereas the Beta is around a 1/4 of that - I know that is a loose comparison and of power outputs not efficiency, but I imagine that the BMW engine would be more efficiently converting diesel to kWhs if each were put on a test jig.....

 

Nick

 

Try putting it in a canal boat and see how efficient it is then.

[Robin2]

If efficiency in car engines is measured by mpg, then surely cutting edge engines must be the most efficient ? Take a look at e.g. BMW diesels - astonishing mpg and power compared with even 10 years ago. I think their 2 litre is some 163 bhp now whereas the Beta is around a 1/4 of that - I know that is a loose comparison and of power outputs not efficiency, but I imagine that the BMW engine would be more efficiently converting diesel to kWhs if each were put on a test jig.....

 

Nick

MPG measures the efficiency of the complete package taking account of power losses in the transmission, tyres etc and the effect of air resistance. The huge improvements that have been achieved are partly because they started from a very low base.

 

The relationship between 2 litres and 163 bhp says nothing about efficiency. Car engines are not designed to run continuously at their maximum power whereas the Beta and Isuzu etc diesel engines are - which is why their rated power outputs are much lower.

 

I would not expect much difference between the BMW and Beta engines in terms of specific fuel consumption - grams/kW-hr. If BMW have figured out a significantly better way the others will quickly copy them. Most of them use Bosch injection equipment anyway. I previously had a workshop manual for a Yanmar engine and I was amazed at the fine tolerances to which it was manufactured. I'm sure the Beta (Kubota?) and Isuzu engines are as good.

 

ETA why is it only after posting that you spot the typos?

Edited August 21, 2011 by Robin2

[JDR]

All engines have sweet spots where they are happy to sit all day.I find that as a rule of thumb this is just below the point where secondary vibration and noise first begin to become irritating.You have probably sussed out where this is for your boat so stick with it.If you looked at a torque curve for your engine I would be very surprised if maximum relaxed cruising revs didn't bear a strong correlation to peak torque (and probably peak fuel economy).

[Robin2]

All engines have sweet spots where they are happy to sit all day.I find that as a rule of thumb this is just below the point where secondary vibration and noise first begin to become irritating.You have probably sussed out where this is for your boat so stick with it.If you looked at a torque curve for your engine I would be very surprised if maximum relaxed cruising revs didn't bear a strong correlation to peak torque (and probably peak fuel economy).

 

Vibration has little to do with efficiency. It depends on how well the designers balanced the engine. With anything less than 6 cylinders there is going to be a lot of vibration and you are quite right to say that it is worst at certain engine speeds and smoothest at others. If the designers bothered I would expect them to try to minimize vibration at the speed the engine would usually operate - probably closer to full power than maximum torque. However they may in practice be more concerned to minimize vibration at maximum rpm so the engine does not break in the warranty period.

 

None of this is relevant for a boat unless the boat builder also carefully (or accidentally) matched the gearbox and propeller to obtain a suitable cruising speed at the smoothest engine rpm. What cruising speed should he choose? In practice it could well be that when the engine is at its most efficient point the boat goes too fast and wastes power, even though the engine is creating it efficiently. Alternatively if the gearbox and propeller are carefully matched for cruising the engine may not be able to attain full power and the boat would be underpowered on rivers.

 

The short answer is that the slower a boat goes the more mpg you get. You need to balance fuel against frustration.

Edited August 21, 2011 by Robin2

[JDR]

Agree with above just my experience that peak torque often coincides with smoothness.Main point I think is that OP shouldnt worry about other boats.Odds are some of the boats he sees rushing by are overengined, some are being caned by people with no mechanical sympathy, some people will be on a tight schedule and some may have better designed hulls and/or better matched props.

 

As long as your boat is ok doesnt use much more than a litre an hour and doesnt smoke like a Dot Cotton then dont worry about it!

[mickeyduff]

I suspect that most of these replies to Deans topic have confussed him even more

[nb Innisfree]

I suspect that most of these replies to Deans topic have confussed him even more

 

Or did you mean concussed?

[JDR]

I suspect that most of these replies to Deans topic have confussed him even more

 

I think Tim nailed it in second post! Would still like to test this peak torque theory though.

 

Ive got a Beta BV1505 I would say the "sweet spot" is between 1500 to 1700 revs ie: engine will sit there all day without being stressed, up to 1900 is ok against a flow but the engine sounds like its working and anything much over 2000 is just not pleasant for sustained periods.I cant find my manual and dont have full web access-how do these figures correspond to torque curve for this engine?