Fuel Consumption

[boatmike]

OK folks I know this has been discussed before but can't find it. Aquastar 33 has 2 TMD 41a 150HP engines. at WOT she achieves 16 knots (semi -displacement) at about 3600 RPM. What fuel consumption should I expect ?

 

[burgundyben]

150hp is going to be about 7.5 gallons an hour. So the pair will be about 15 gallons/hr.

I reckon 2/3rds top speed will be 1/2 fuel.

 

[Momac]

I get 60 litres per hour at 23 knots , 2x170hp
33ft planing hull boat .
Semi displacement hull uses more fuel I guess.

 

[boatmike]

Thanks both, Yes Martyn I think that's right. The difference is that you will probably achieve 23 knots for your 60 litres while I will get 15/16. I would rather be in mine doing 8 knots in a F7/8 than yours though! Horses for courses I guess. The reason I have asked is that I am over propped at the moment and when that's corrected I expect to achieve about 16. Interestingly at 8/10 knots currently I burn about 28/30 litres/hour but that's little more than hull speed so requires very little HP

 

[stelican]

OK folks I know this has been discussed before but can't find it. Aquastar 33 has 2 TMD 41a 150HP engines. at WOT she achieves 16 knots (semi -displacement) at about 3600 RPM. What fuel consumption should I expect ?

72 litres per hour from tests I did many years ago.
Sorry just noticed 41 series engines I quoted 31 150hp

 

[Momac]

Yes Martyn I think that's right. The difference is that you will probably achieve 23 knots for your 60 litres while I will get 15/16. I would rather be in mine doing 8 knots in a F7/8 than yours though! Horses for courses I guess.

I admit to aiming for relatively calm sea conditions.

 

[BruceK]

I cant see speed having any affect on fuel consumption at WOT. Is that simply not a function of how much the fuel pump can throw in in these analogue engines?

 

[burgundyben]

I cant see speed having any affect on fuel consumption at WOT. Is that simply not a function of how much the fuel pump can throw in in these analogue engines?

Speed of the boat is a function of the propeller curve, the fuel burn is a function of prop curve, so the two things are linked.

 

[BruceK]

I'll take your word on it, but I struggle to see it. I've burnt the same amount of fuel per hour going slow at WOT in rough head seas as I do when fully on the plane and skipping along in calm seas.

 

[boatmike]

Bruce, I think you are over thinking a simple statement. Of course it is throttle opening that determines fuel consumption but a planing hull in good conditions will go faster than a semi-displacement in good conditions using the same power. No contest. I think Bens talk of prop curve (I think he means pitch) is a red herring because overpitched as I am now I don't get max RPM and underpitched I would get the RPM but not use the power. I think we can only assume optimal prop dia and pitch and compare fuel consumed at WOT which should be pretty similar for similar HP. All we were saying is that not all boats will achieve the same speed for HP consumed so you are right that it is HP at WOT that determines consumption not speed. The choice of prop determines how efficiently we convert power available to vessel speed.

 

[scottie]

Bruce, I think you are over thinking a simple statement. Of course it is throttle opening that determines fuel consumption but a planing hull in good conditions will go faster than a semi-displacement in good conditions using the same power. No contest. I think Bens talk of prop curve (I think he means pitch) is a red herring because overpitched as I am now I don't get max RPM and underpitched I would get the RPM but not use the power. I think we can only assume optimal prop dia and pitch and compare fuel consumed at WOT which should be pretty similar for similar HP. All we were saying is that not all boats will achieve the same speed for HP consumed so you are right that it is HP at WOT that determines consumption not speed. The choice of prop determines how efficiently we convert power available to vessel speed.

Prop curve refers to a graph that gives an indication of a notional power against speed through water and does not have any relation
The prop curve assumes correct pitch and diameter
A propeller absorbes the same power when turned at the same speed regardless of the power available and cannot Turn any faster than the torque available will allow similar to going up a hill in high gear restricts your speed

 

[BruceK]

Bruce, I think you are over thinking a simple statement. Of course it is throttle opening that determines fuel consumption but a planing hull in good conditions will go faster than a semi-displacement in good conditions using the same power. No contest. I think Bens talk of prop curve (I think he means pitch) is a red herring because overpitched as I am now I don't get max RPM and underpitched I would get the RPM but not use the power. I think we can only assume optimal prop dia and pitch and compare fuel consumed at WOT which should be pretty similar for similar HP. All we were saying is that not all boats will achieve the same speed for HP consumed so you are right that it is HP at WOT that determines consumption not speed. The choice of prop determines how efficiently we convert power available to vessel speed.

OK. What I was alluding to is a TMD 41a 150HP will have the same fuel consumption at WOT and whether on a planing hull or SD hull will make not one jot of difference as both will be developing 150 ponies and for that engine to deliver it it needs fuel. The same amount of fuel. (props being good for the boat and engines just attaining max rpm).

 

[BruceK]

edit, we are talking fuel consumption rather than fuel efficiency right?

 

[Restoration man]

OK. What I was alluding to is a TMD 41a 150HP will have the same fuel consumption at WOT and whether on a planing hull or SD hull will make not one jot of difference as both will be developing 150 ponies and for that engine to deliver it it needs fuel. The same amount of fuel. (props being good for the boat and engines just attaining max rpm).
[/QUOTE

I agree with Bruce they will both use the same amount fuel per hour ,but the planning boat will get to the destination faster so it will be running for less time Therefore using less fuel ,?

 

[gordmac]

A diesel doesn't have a throttle, you set desired engine speed with the lever and the governor alters the fuel supply to maintain the selected speed. That means the fuel consumption can vary significantly for the same rpm. For a given power demand though the source of the load doesn't matter, the fuel consumption will be basically the same.

 

[boatmike]

This all makes I smile. I asked a very simple question that was answered (I think quite correctly) in the first reply. All the other posts have been written by Fred Drift...... Good 'ere innit????

 

[BruceK]

Well actually your question was ambiguous because you added a whole lot of irrelevancies that clouded the question.

 

[boatmike]

Well actually your question was ambiguous because you added a whole lot of irrelevancies that clouded the question.

OOOh! Sorry!!! I will try to keep it simple next time..................

 

[BruceK]

Apology accepted, 5 minutes off your time with the dunce cap in the stupid corner

 

[Assassin]

Bruce, I think you are over thinking a simple statement. Of course it is throttle opening that determines fuel consumption but a planing hull in good conditions will go faster than a semi-displacement in good conditions using the same power. No contest. I think Bens talk of prop curve (I think he means pitch) is a red herring because overpitched as I am now I don't get max RPM and underpitched I would get the RPM but not use the power. I think we can only assume optimal prop dia and pitch and compare fuel consumed at WOT which should be pretty similar for similar HP. All we were saying is that not all boats will achieve the same speed for HP consumed so you are right that it is HP at WOT that determines consumption not speed. The choice of prop determines how efficiently we convert power available to vessel speed.

Bruce overthinking??? I know someone who wont forget that one.