Consumption guesstimate perkins 4108?

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pcatterall

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Boat is new to us and we have not been able to get any idea of fuel consumption to date.
As we plan to do a fair bit of canal work it would be nice if we could get a better handle on it.
Boat is a a 33' Neptunian, her beam is 10 feet, I don't have the displacement figures to hand.
She is fairly happy at 2000rpm doing 6 knots.
Any figures please?
 
pcatterall said:
Boat is new to us and we have not been able to get any idea of fuel consumption to date.
As we plan to do a fair bit of canal work it would be nice if we could get a better handle on it.
Boat is a a 33' Neptunian, her beam is 10 feet, I don't have the displacement figures to hand.
She is fairly happy at 2000rpm doing 6 knots.
Any figures please?
Click to expand...

1 ltr per 10hp used
3000rpm would be around 3.5 lts assuming your engine is rated @ 3000rpm as mine was i.e approx 37hp
 
Key word there is "used". A 50hp engine using just 20hp will be a bit more inefficient than a 30hp using that much power but generally it's per 10hp used. At those sorts of revs you're surely using just over half power?
 
Yes, at 2000 rpm you are drawing around 20-25 hp - you can check if you have the power curves for your engine. The estimate of 1l per 10hp is pretty robust - you will also find the specific fuel consumption curve (fuel used per hp or kw) is flat from about 1700-2800, which is the normal operating range, then gets steeper as you get near max power.

Net result is that you are unlikely to exceed 2.5l hour and in the canals probably nearer 2l. How far/fast you go then depends on sea state, wind, tide etc.
 
The consumption figure for a Diesel is approx. 1 gallon/hour per 20hp developed, that would be about 2.25 litres for 10hp. I expect the consumption at 6 knots would be no more than quoted already (3 - 4 litres) as you are probably using no more than 50% engine power.

I think the 1 litre/10 HP figure is a value for typical cruising consumption based on Max engine power for a typical displacement installation?


Edit,
Just found this useful info:
http://www.ltsc.co.uk/yacht-articles/564-diesel-engine-fuel-consumption-quicl-calculation
 
Last edited:
ithet said:
The consumption figure for a Diesel is approx. 1 gallon/hour per 20hp developed, that would be about 2.25 litres for 10hp. I expect the consumption at 6 knots would be no more than quoted already (3 - 4 litres) as you are probably using no more than 50% engine power.

I think the 1 litre/10 HP figure is a value for typical cruising consumption based on Max engine power for a typical displacement installation?


Edit,
Just found this useful info:
http://www.ltsc.co.uk/yacht-articles/564-diesel-engine-fuel-consumption-quicl-calculation
Click to expand...

Don't think the figures in the link are correct. The 1l per 10hp used per hour is pretty robust - and about 50% of that suggested in the link.

This subject comes up regularly and many people who do lots of motoring and keep good records confirm it. You can also get confirmation by taking the specific fuel consumption (which varies little from engine to engine) and is expressed in terms of grammes per hp/kw/ per hour and convert it to litres using the SG of diesel and you will get the same result +/- 1l hour.

My own personal experience also confirms it with .8l hour on a Yanmar 1GM and 2.2l hour on a Volvo 2030 - the latter over about 180 hours almost continuous motoring at 2400 rpm on the last long trip.
 
ithet said:
The consumption figure for a Diesel is approx. 1 gallon/hour per 20hp developed, that would be about 2.25 litres for 10hp. I expect the consumption at 6 knots would be no more than quoted already (3 - 4 litres) as you are probably using no more than 50% engine power.

I think the 1 litre/10 HP figure is a value for typical cruising consumption based on Max engine power for a typical displacement installation?


Edit,
Just found this useful info:
http://www.ltsc.co.uk/yacht-articles/564-diesel-engine-fuel-consumption-quicl-calculation
Click to expand...

That's about twice my actual usage - but it's about the same as the static load figures quoted for my engine, someone else will explain but there's less load for a boat moving through the water, isn't the static load sort of assuming infinite inertia but once we've got moving you need much less power to maintain that movement? Actually, don't quote me on that!
 
The Lazy Kipper said:
That's about twice my actual usage - but it's about the same as the static load figures quoted for my engine, someone else will explain but there's less load for a boat moving through the water, isn't the static load sort of assuming infinite inertia but once we've got moving you need much less power to maintain that movement? Actually, don't quote me on that!
Click to expand...

So therefore you are using half the power! The "static load" for the engine is not to do whether its in a moving vehicle, its measured at a fixed RPM against a dynometer or brake. It is also the maximum power figure (or braking force against) which the engine can hold a certain RPM. In use the engine can produce (and probably will) less power than shown at all RPMs below the maximum achieved. Sometimes a typical propeller power curve is given in the spec.
 
Tranona said:
Don't think the figures in the link are correct. The 1l per 10hp used per hour is pretty robust - and about 50% of that suggested in the link.

This subject comes up regularly and many people who do lots of motoring and keep good records confirm it. You can also get confirmation by taking the specific fuel consumption (which varies little from engine to engine) and is expressed in terms of grammes per hp/kw/ per hour and convert it to litres using the SG of diesel and you will get the same result +/- 1l hour.

My own personal experience also confirms it with .8l hour on a Yanmar 1GM and 2.2l hour on a Volvo 2030 - the latter over about 180 hours almost continuous motoring at 2400 rpm on the last long trip.
Click to expand...

The figures are correct, and I have seen them published a number of times - 20hp/gall/hr diesel, 16/hp/gall/hr petrol. There seems to be confusion about how much (or rather how little) power is actually used to propel a typical yacht, compared to the rated power of the engine. If people are measuring half the given value then they are using half the power they think they are! Your figures show you are averaging 3.5hp for the Yanmar and 9.8hp for the Volvo, which seem reasonable figures.

As for specific fuel figures, for example, in the product brouchure for the latest VP D1-30 (nearest to your 2030) they give a consumption of 6.7 litres at max power or 27hp (at crankshaft), that would be 4.96ltrs per 20hp = 1.09/20hp gph.
The same brouchure gives a propeller curve and at 2400 rpm the power output/consumption is shown at 11hp/3ltrs hr, equating to 5.45ltrs per 20hp = 1.2 gph/20hp, so some loss of efficiency at the lower rpm.

It is also interesting exercise to put figures into the Vicprop calculator to find out what hp is needed for a certain speed (use your cruising revs and speed):
http://www.vicprop.com/calculator.htm

Regards
 
Last edited:
ithet said:
The figures are correct, and I have seen them published a number of times - 20hp/gall/hr diesel, 16/hp/gall/hr petrol. There seems to be confusion about how much (or rather how little) power is actually used to propel a typical yacht, compared to the rated power of the engine. If people are measuring half the given value then they are using half the power they think they are! Your figures show you are averaging 3.5hp for the Yanmar and 9.8hp for the Volvo, which seem reasonable figures.

As for specific fuel figures, for example, in the product brouchure for the latest VP D1-30 (nearest to your 2030) they give a consumption of 6.7 litres at max power or 27hp (at crankshaft), that would be 4.96ltrs per 20hp = 1.09 gph.
The same brouchure gives a propeller curve and at 2400 rpm the power output/consumption is shown at 11hp/3ltrs hr, equating to 5.45ltrs per 20hp = 1.2 gph, so some loss of efficiency at the lower rpm.

It is also interesting exercise to put figures into the Vicprop calculator to find out what hp is needed for a certain speed (use your cruising revs and speed):
http://www.vicprop.com/calculator.htm

Regards
Click to expand...

Have difficulty with both your figures and your reasoning.

My Yanmar is running at 3000 RPM at which it produces just under 7hp - not 3.5 as you suggest and is pushing 3 tons of boat at 5 knots (which is not possible with 3.5hp). My Volvo 2030 is running at 2400rpm producing approx 21hp and pushing 6.5 tons of boat at 5.4 knots. If your figures are correct and I am using around 5l hour with the Volvo then my 150l tank gives a range of 30 hours, or about 160nm at 5.4 knots. However, I comfortably managed Sardinia to Majorca - well over 250 miles non stop motoring comfortably on less than one tank of fuel.

Specific fuel consumption is measured in grammes per hp(kw) per hour and hp is a function of RPM - unconnected with boat speed or propeller efficiency. As I suggested earlier you can convert specific fuel consumption into litres per hour per hp by using the SG of diesel - and you will come out close to the estimate of 1l per 10hp used inb the normal operating range - typically 1800-3000 of a small diesel maxing at 3600.

As to making use of that power and converting into boat speed, propellers are sized to allow the engine to propel the boat (displacement) at maximum speed at maximimum rated revs. This allows a comfortable cruising speed at somewhere around 65-70% max power. So my boat is propped to achieve 7.2 knots at 3500 and 5.4knots at 2400 (70% max power).

There are already several posters here confirming consumption figures similar to those based on the estimate, and if you do a search on the subject (which comes up regularly on this forum) you will find many more.
 
pcatterall said:
many thanks guys, a little lost in the theory but I will work on something just under 1 gallon per hour and see how that goes. L'escargot quotes 3lt per hour but then he is probably travelling slow!!
Click to expand...

That would seem to agree with experience and theory (whether the correct or incorrect one! ;)).
 
There seems to be a lot of variety in this post (and a lot of professional theory - Very impressive) but, are the rules that finite?
We run a 6.5ton 30 x 10 foot motorsailer with a 4108 normally running at 1500 revs doing 5.5 - 6kn depending on (sea) conditions. This costs us about 1.8lit/hr. The prop/gear ratio is such that we get 2200 revs max in gear before smoke, so will never achieve "tugboat power" but it does seem that the economy calculations have been done (by a previous owner) and the engine seems to be very happy not much above tickover. It has been doing this since 1978 without complaint, so I am happy to remain an anomaly!
 
Billjratt said:
There seems to be a lot of variety in this post (and a lot of professional theory - Very impressive) but, are the rules that finite?
We run a 6.5ton 30 x 10 foot motorsailer with a 4108 normally running at 1500 revs doing 5.5 - 6kn depending on (sea) conditions. This costs us about 1.8lit/hr. The prop/gear ratio is such that we get 2200 revs max in gear before smoke, so will never achieve "tugboat power" but it does seem that the economy calculations have been done (by a previous owner) and the engine seems to be very happy not much above tickover. It has been doing this since 1978 without complaint, so I am happy to remain an anomaly!
Click to expand...

i would guess @ 1500 you are using around 18 > 20 of the 38hp that the 4108 will produce @ 3000rpm
 
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