DavidMcMullan
Well-Known Member
I'm in the process of making a boat speed/fuel usage chart. Am I correct in assuming that fuel usage is directly proportional to engine RPM? Does anything else influence the fuel usage at a given RPM?
RPM & Fuel usage
- Thread starter DavidMcMullan
- Start date
crewejd
Member
Surge-loading, possibly? head seas leading to climbing and surfing? Similar I guess for tail seas..
Also boat-loading - more weight, more work for the engines at a given RPM.
And planing speed vs displacement speed.
penfold
Well-Known Member
The manufacturer of your engines has already provided a graph of engine speed versus fuel consumption(unless it's incredibly old); it's easy enough to extrapolate and overlay the engine speed with boat speed.
Portofino
Well-Known Member
Along with fouled props + hull which increases the load and thus fuel burned , it might not even reach rated or anywhere near rpm , yet burn above ave or off your chart to the max fuel consumption .
fisherman
Well-Known Member
Displacement hull will be most economical at lower speeds, but as it approaches optimum speed, ie, as it starts to drag a stern wave, consumption rises dramatically. Mine would be fine at 1800 revs/7kts, but at 2300 revs, for only another two knots or so, fuel at least 50% more.
However, with the planing hull I had, just over the hump and throttle back a shade was best. 2100/14kts.
However, with the planing hull I had, just over the hump and throttle back a shade was best. 2100/14kts.
jrudge
Well-Known Member
Engine load drives consumption. If the hull and props are clean then your chart will be accurate. However when they get fouled you would be amazed at the difference. I cost me £500 difference in fuel to Ibiza then I cleaned the pops and had a netted journey back. Also rough weather when climbing waves dramatically increases consumption.
The engine management system keeps rpm constant. It does that by allowing more or less fuel based on load
The engine management system keeps rpm constant. It does that by allowing more or less fuel based on load
gordmac
Well-Known Member
Might be linear below hull speed, I don't know but after that it will be far from it. The fuel consumption will rise steeply until over the hump then be fairly level until close to flat out. I am talking litres per mile.
LittleSister
Well-Known Member
In short, no.
More load dependent.
More load dependent.
ChromeDome
Well-Known Member
The more power you demand from your engine, the more fuel it will burn. RPM alone will not tell you much although enginge max. output tend to follow rpm.
If you, on a bicycle, go uphill at a given speed you will need more (muscle-)power than going down the same hill at the same speed.
For combustion engines the key is 'Brake Specific Fuel Consumption'
If you, on a bicycle, go uphill at a given speed you will need more (muscle-)power than going down the same hill at the same speed.
For combustion engines the key is 'Brake Specific Fuel Consumption'
Soco2
Well-Known Member
The only real way to have an accurate graph is by having a fuel flow meter and then plot it vs speed.
but even then the graph will only be good for the exact conditions and state if hull/prop cleanness and also boat loading.
As others have said, best economy will be idle displacement speed.
but even then the graph will only be good for the exact conditions and state if hull/prop cleanness and also boat loading.
As others have said, best economy will be idle displacement speed.
Alicatt
Well-Known Member
Most diseasels have a feed and return so just measuring the fuel flow would not give you your consumption, you would have to subtract the fuel that is returned.
have a look at this video where they are testing the new Nordhavn 41
Ningaloo
Well-Known Member
Absolutely not linear. There will be a sweet spot, after which increased engine revs will punish your consumption.
On my 57hp (yacht) engine 2000 revs uses less than 4l/hr. 2500 revs uses 7l/hr. 3000 revs uses 12l/hour.
On my 57hp (yacht) engine 2000 revs uses less than 4l/hr. 2500 revs uses 7l/hr. 3000 revs uses 12l/hour.
gordmac
Well-Known Member
Sounds like 2000rpm is about max hull speed.
Ningaloo
Well-Known Member
14m (13.5 waterline) so hull speed might be just below 9kts. I'll be happy if 2000revs gives 6.5kts.
Another thing to bear in mind is turbos. My engine (57hp) also comes in a 80hp turbo version, but recommended minimum engine speed for the turbo is 2200revs, I know of several people with this engine that would prefer the lower power non-turbo for this reason.
LittleSister
Well-Known Member
With a marine diesel the (misnamed) 'throttle' sets the revs by setting the engine governor. The governor will deliver whatever fuel needed to reach and maintain the selected revs (up to the engine's power capacity at those revs).
This might be not much fuel (e.g. lightly loaded, clean hull, wind up the chuff, flat seas or surfing down the front of waves) or at lot of fuel (heavily laden, fouled hull, into strong winds and large waves).
The fuel/revs graph in the manual will show what is the fuel consumption for the maximum power available at any particular revs. Much of the time, in practice, you won't actually need the full power available from the engine at those revs to maintain that engine speed.
I assume (but don't know), that marine petrol engines are controlled like car engines. The throttle sets the amount of fuel, and the engine reaches whatever revs that produces, dependent on the load at the moment. As in a car, you will need more throttle (and hence fuel) to maintain the same revs going 'uphill'.
The situation is also further complicated, of course, in that engine and prop revs do not translate directly to boat speed (unlike in a car): more load = more prop slip.
This might be not much fuel (e.g. lightly loaded, clean hull, wind up the chuff, flat seas or surfing down the front of waves) or at lot of fuel (heavily laden, fouled hull, into strong winds and large waves).
The fuel/revs graph in the manual will show what is the fuel consumption for the maximum power available at any particular revs. Much of the time, in practice, you won't actually need the full power available from the engine at those revs to maintain that engine speed.
I assume (but don't know), that marine petrol engines are controlled like car engines. The throttle sets the amount of fuel, and the engine reaches whatever revs that produces, dependent on the load at the moment. As in a car, you will need more throttle (and hence fuel) to maintain the same revs going 'uphill'.
The situation is also further complicated, of course, in that engine and prop revs do not translate directly to boat speed (unlike in a car): more load = more prop slip.
Alicatt
Well-Known Member
And why at very light loads in a marine diesel, eg. a following wind, sea, and tide, with minimal diesel being injected you get a very lean burn that can lead to high internal temperatures and glazing of the cylinders.
kashurst
Well-Known Member
Are you sure you really want to know?
Previous boat had fuel burn readouts, electronic charts etc - it didn't use any less for knowing it more accurately.
Now after a few long trips I have a pretty good idea what fuel burn is with a clean hull and what it is with some fouling.
Best not to dwell on it, just make sure everything is working as it should, then fuel burn will be the best it can be.
What I have found more useful is accurate fuel tank guages so you know how much is really there.
Previous boat had fuel burn readouts, electronic charts etc - it didn't use any less for knowing it more accurately.
Now after a few long trips I have a pretty good idea what fuel burn is with a clean hull and what it is with some fouling.
Best not to dwell on it, just make sure everything is working as it should, then fuel burn will be the best it can be.
What I have found more useful is accurate fuel tank guages so you know how much is really there.
ChromeDome
Well-Known Member
Indeed. And this is incorporated in flowmeters designed for that type of measurement: One flow metering device on each line, out and in.
TBH I'd be interested in stretching the fuel, but until a magic wand becomes available I just keep the tank full.
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Montemar
Well-Known Member
Which would use less fuel: an overpropped engine pushing a boat along at 10 knots and working hard or an underpropped engine pushing the boat at the same speed but revving higher and easier?
No difference?
No difference?
kashurst
Well-Known Member
Assuming the engine wasn't actually being overloaded, then no it wouldn't make much difference. The engine doesn't know what's on the end of the propshaft. All the fuel injection system knows is that a required speed has been requested (throttle position) and it will inject sufficient fuel to achieve that - if it can.
It requires a finite amount of energy to make a boat move through the water for a given speed. The prop size doesn't materially change how much energy that is, it only changes how fast or slow the prop needs to rotate to apply that amount of energy.
What does seem to change fuel burn is the design of the propellor. Sharrow props claim to be more efficient with less slip.