Would anyone hazard a guess as to litres per hour my 28hp Volvo should be using at moderate cursing speed?
Select to expand quoteTrek said..
Would anyone hazard a guess as to litres per hour my 28hp Volvo should be using at moderate cursing speed?
More info needed. Length of boat, displacement (weight) what you consider an average causing speed, age of engine.
Length of boat will determine max hull speed and pushing a boat close to it may use more fuel
Newer engines are a lot more efficient than older ones.
Prop ratio will also pay a part as it will determine the revs required to maintaining "average causing speed"
Some boats have a better hull shape which is usually reflected in the length to displacement ratio.
Sea state (and wind direction) will also play a major part in the fuel consumption. A boat may sit on 5.5 knts in calm conditions but to maintain this same speed (increase the revs) pushing into a swell with a head wind may double the fuel consumption.
My 30hp Yanmar manual actually had a revs/litre drawing which was pretty dam accurate. I also had a variable pitch prop that I had adjusted to give me 5.4knts at 1500 rpm in good conditions.
There will be a point where your boat will be at its best for speed verses revs in different conditions and it will be unique to your boat. You need to find this out. Trying to maintain the same speed for different conditions will result in wasted fuel. I monitored my fuel usage against distance travelled in different conditions from just motoring, motor sailing, pushing into a swell etc. This gave me a better understanding of how far my fuel would last in different conditions.
So many variables.
The surveyor (who is a ships engineer) who surveyed my boat last year said that as a rule a diesel in a yacht will use approx 1 litre per 10 hp per hour
So working on his theory yours would use approx 2.8 litres per hour at cruising speed
It will be interesting to see if other forum users agree with this
Regards Don
Which 28 hp Volvo
A 2003?
If it is and depending how propped there the fuel curve has two big increases about 1650 and at about 2150.
Don is right, 1 litre per 10 horsepower used although smaller engine throw this out a bit.
So if 1650 is delivering 15 hp than start with 1.5 litres an hour but increase it bit due to size of engine.
At cruise at about 1650 a 100 litre tank gave about 60 hours steaming and 2150 gave about 40 hours in good conditions.
Sea state will through this out.
Thanks for answers. It is a Volvo 2003 and agree there's lots of variables. Can you tell me where to get that fuel curve Lydia? Would love to see it. Boat is Matzcraft 35 (Roberts 35) with 2 blade fueling prop. It has a motoring sweet spot about 2000 rpm where speed is about 5 knots. Cranking up engine past that rpm creates a lot more noise and not much more speed. I've marked "hours" on fuel tank sight gauge and want to see if I'm roughly right.
If you can a hold of a workshop manual, I am not sure if I have mine anymore as the boat was sold.
what you are looking for the is the torque curve and fuel curve that are in that.
I think you will find that about 2000 rpm is best torque for revs.
Sounds like you might be a bit under propped, what is the max revs you get to.
There is discussion on a YBW discussion page if you google 2003 torque curve.
Has the same observations listed here.
Select to expand quotelydia said..
If you can a hold of a workshop manual, I am not sure if I have mine anymore as the boat was sold.
what you are looking for the is the torque curve and fuel curve that are in that.
I think you will find that about 2000 rpm is best torque for revs.
Sounds like you might be a bit under propped, what is the max revs you get to.
There is discussion on a YBW discussion page if you google 2003 torque curve.
Has the same observations listed here.
Thanks Lydia I will look. I think boat is under propped previous owner liked racing so I think it's as light as possible.
Don't forget cleanliness of hull (significant effect on speed/consumption) and of prop (dramatic effect).
Cheers, Graeme
Our Volvo 2003T is rated at 43hp and our yacht is a Najad 34. When motoring at 1700rpm we consistently use less than 2 lph. There are certainly many variables - but this should help with a rough guesstimate.
5.6 ton boat 30 HP Perkins, 2000 revs, 5 - 5.5 knots, between 2.5 and 2.9 l/hr depending on sea state. That 1 liter per 10 Hp is not far off at something like cruising revs of 2/3 max.
Select to expand quoteBristolfashion said..
Trek, I like the idea of moderate CURSING speed - sums it all up!
Bristle
Ha ha ha I didnt see that!
I have a volvo 2003 28hp in cavalier 37 3 blade feathering prop around 2000 revs is about 5.5 knots and just under 3 litres an hour 
I have a 50 hp kubota based WM conversion in my 32 fisher and it uses approx 2.5 l/hr at 1800-2000 rpm, with a clean bum and calm sea state
If we are not talking about nm/h which was not your question, l/h is roughly as been stated, 1-1.25 l/10hp thou for older engines l would hazzard a guess of conservative 1.5 l/10hp.
Mine, a 5 ton (loaded) boat, a 28, 10.5hp engine, clean hull and twin folding prop at 3000 revs 6.2 knots, 1.25 l/h. At 2500 approx 5 knots and that is more than enough for 3 days motoring on my 72 liter tank.
My Sole 20hp on a 6.5 tonne S&S34 uses 1.6 litres/hour. I have kept accurate records over 11 years and it has always been between 1.5 and 1.6. Closer to 1.5 on long trips like Lord Howe.
A diesel engine fuel metering unit will put in the fuel needed based on the revs and load. If you are driving a inefficient hull, one carrying its own Barrier Reef, a dirty or wrong pitch/size prop or very close to hull speed, fuel consumption will rise lots. A clean and efficient hull with a good prop will use a lot less.
30 hp BMC Thornycroft 6 ton Dunc 37 cruising speed 1.5 litre per hour.
50hp Perkins Prima 12 ton Adams 42 cruising speed 2.5 litre per hour.
It depends on a lot of variations, check engine specs , hull weights , year of manufacturing , keep a running log.
Thanks Greg. I got graph. Tomorrows a boat day since I worked on Weekend. I'll give it a shot. - Southace - log is good idea. Im getting to that. Lately havent been boating enough to get good data.
My plan is that I can go downstairs and look at fuel tank sight gauge and read off "miles left" without having to mentally work it out.
Heading out of Broken Bay in search of dolphins with a friend on board who's busting to see some :-)
Not sure if your motor is in here but this tends to be fairly accurate
www.boat-fuel-economy.com/volvo-penta-diesel-d1-30-fuel-consumption-liters
Select to expand quoteSandS said..
let me know if I win the meat tray !
Its porridge for you Sands 
Get a Red one
