fuel consumption

[takefive]

i am considering the purchase of a 30ft sea going launch fitted with two volvo penta aqd40 diesel engines and can find no indication of the likely fuel consumption.can any body assist.

 

[pvb]

If you assumed an average of about 50 litres an hour (2 x 25 litres/hr) when cruising, you wouldn't be far wrong.

 

[sailorman]

Rule of thumb is 1lt per 10 hp per hr

 

[pvb]

Rule of thumb is 1lt per 10 hp per hr

A little low for an AQD40, I reckon. This 22 footer admitted to 20 litres/hr on one engine.

 

[Searush]

Hi, is that the Take5 who was part of the North Wales gang a few years back? If so, welcome back & good luck with your purchase.

 

[FishyInverness]

Directly copied from the RNLI website :

Diesel
• A diesel engine burns about 1 gallon per hour for every 20hp. So a 90hp diesel would use about 90/20 = 4.5 gallons of fuel per hour. For those who prefer to work in litres then simply multiply the horsepower by 2 and then divide by 9. So a 90hp has an estimated consumption of 2 x 90/9 = 20l/hour.


Petrol
• Four-stroke petrol engines use approximately 1 gallon per hour for every 15hp. So a 90hp petrol engine would use 90/15 = 6 gallons. For a figure in litres multiply hp by 3 and divide by 10. For a 90hp engine 3 x 90/10 = 27l/hour
• For two-stroke petrol engines up to 50hp, multiply hp by two and divide by five. So a 10hp two-stroke would be 2 x10/5 = 4l/hour. For two-stroke engines over 50hp – simply divide hp by three. Therefore, our 90hp two-stroke would use 90/3 = 30l/hour.

 

[neale]

Directly copied from the RNLI website :

Diesel
• A diesel engine burns about 1 gallon per hour for every 20hp. So a 90hp diesel would use about 90/20 = 4.5 gallons of fuel per hour. For those who prefer to work in litres then simply multiply the horsepower by 2 and then divide by 9. So a 90hp has an estimated consumption of 2 x 90/9 = 20l/hour.


Petrol
• Four-stroke petrol engines use approximately 1 gallon per hour for every 15hp. So a 90hp petrol engine would use 90/15 = 6 gallons. For a figure in litres multiply hp by 3 and divide by 10. For a 90hp engine 3 x 90/10 = 27l/hour
• For two-stroke petrol engines up to 50hp, multiply hp by two and divide by five. So a 10hp two-stroke would be 2 x10/5 = 4l/hour. For two-stroke engines over 50hp – simply divide hp by three. Therefore, our 90hp two-stroke would use 90/3 = 30l/hour.

These kind of calculations only work if you are going flat out or you know how many of your engines horses you are using. A boat fitted with a 90hp engine may cruise using only 45 of those so the fuel consumption will be half. You can't work it out by rpm's or throttle setting either because the fuel flow is usually a curve rather than a straight line.

To give an example, my boat is fitted with 300hp so on the figures above it should use 15gph, and very possibly does at full throttle (3600rpm, 30 knots). I however cruise at around 3200rpm and 22 knots and the figures are showing I am using around 9 gph, which calculated back the other ways shows that I must be using around 180 of the 300hp available

 

[FishyInverness]

Thanks Neale,
I had assumed that the RNLI figures were a "rule of thumb" type estimate, but I just dropped them in there because I had them to hand and gave some indications....

 

[neale]

Thanks Neale,
I had assumed that the RNLI figures were a "rule of thumb" type estimate, but I just dropped them in there because I had them to hand and gave some indications....

They are very useful figures and def worth keeping as a guideline.

 

[Davy_S]

I would not trust the petrol 2 stroke figures, I think they are to low.

 

[BoyBlue49]

Litres per hour.

Volvo 87 bhp turbo at 2200 rpm, 50' sailing tupperware boat = 3.75 lts ph at 6 kn.

Volvo 85 bhp turbo at 2600 rpm, 50' sailing tupperware boat = 6.5 lts ph at 8 kn.

Both same hull design, 85 bhp boat 2 years younger but with Volvo folding prop.

Actual experience over long no wind passage. ( Troon to Penzance ).

 

[takefive]

thank you all very much for your most helpfull replies which are appreciated