boats and fuel consumption

[gjgm]

dont know much about the efficiencies of diesels, but following an earlier thread, I looked up VP data on my boat/engine performance. As far as I can read, fairly obviously the faster you go, the more fuel you burn per hour.
Of course, most of us arent aimless driving in circles, but going from A-B, in which case L/NM is more meaningful. What surprised me, was that,roughly, between 2500-3500revs,which is approx 25-35knts, L/NM is constant. In other words, for a given journey, it doesnt matter what speed I drive at in terms of fuel consumption.
http://www.windy.no/downloads/28_GHIBLI_%20KAD300DP_Ggearratio_1_68_1.pdf
Thats not what I expected.

 

[Whitelighter]

Tht is what I found on the Bav when I got the MPG read out on the fuel computer. Between 2400 rpm and 3100 rpm, the MPG figure was always around 1.9. Give it beans over 3100 rpm and it started to drop off dramiticly. WOT was 3800 rpm and MPG was a mear 0.97 /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif

 

[[2068]]

It does make sense. Between 2400rpms and 3000rpm the increased losses due to air resistance / water friction etc. are being compensated for by the props "slipping" less and the outdrive working in its sweet spot ... until over 3000rpm, where the engine isn't as efficient.

dv.

 

[hlb]

It's about getting to max efficiency of the engine, it used to be reckoned about 200 revs less than WOT.

 

[[2068]]

If that were true, then I could put one outdrive in forwards, one in reverse, accelerate to 200rpm below WOT, and I would have my optimum mpg?

(point is, the drivetrain and the boat come into it too.)

dv.

 

[DAKA]

It can be more economic to go faster ! /forums/images/graemlins/cool.gif

My engines use the same fuel at 3200rpm as 3400rpm

Thats an extra 4 knots free or 4 nm extra per hour, in an average trip that is a fuel saving of 16 nm and 1/2 hour saved.
/forums/images/graemlins/smile.gif





Obviously if I went to the other extreme and reduced to 8 knts it would be more economic, I am talking about once you have used the power to lift out the water it does not need much extra power for extra speed.

 

[clivew05]

[ QUOTE ]
...Obviously if I went to the other extreme and reduced to 8 knts it would be more economic.....

[/ QUOTE ]

Ummm not too sure that that is correct - at 2800/3200 rpm I get the best economic speed for fuel consumtion (about 15gpm) - at 8 knots I get between 9/10mpg - so I try not to drop below 25/27knts as a rule (obviously depending on sea state) but rpm wise 2800/3200 is my best

 

[marine]

/forums/images/graemlins/shocked.gif[ QUOTE ]
at 2800/3200 rpm I get the best economic speed for fuel consumtion (about 15gpm) [ QUOTE ]


Crikey, think I would get off and tow the boat myself, /forums/images/graemlins/laugh.gif

 

[gjgm]

thats with the hook still down, I think! Uk is now slightly closer to France.

 

[clivew05]

[ QUOTE ]
/forums/images/graemlins/shocked.gif[ QUOTE ]
at 2800/3200 rpm I get the best economic speed for fuel consumtion (about 15gpm) [ QUOTE ]


Crikey, think I would get off and tow the boat myself, /forums/images/graemlins/laugh.gif

[/ QUOTE ]

Ah well thats because I wrote that with my other head - I meant 15 Gph ........... /forums/images/graemlins/laugh.gif

 

[qetoo]

I have a fuel flowmeter in my boat and have done the measured nautical mile and plotted the results:

The boat is a 31-foot hull, 16,000 lb deep-vee bottom, with a single gasoline engine. Note that the data is in US gallons.

 

[Mister_Smee]

Has anyone calculated the most efficient RPM's for a Volvo TAMD 63P?
I have twin 63P's in my Phantom 38, and I usually run at around 1900-2000 rpm gives a comfortable ride and I think its about the most effiecient fuel rate but would welcome anymore info on this
Just to save me doing it over the summer.
Cheers

Stuart

 

[jfm]

Haydn that's just not true, if you want to minimise mpg at least. Yes the engine is more efficient (in the sense it produces the most kinetic energy per dollop of fuel burned) but if the boat is going faster then there is lots more work (overcoming resistance) per mile travelled than if you travel slowly. So as you speed up, you gain a bit in efficiency, but lose lots in overcoming resistance. Kinda one step forward, 3 steps back. MPG will always be the highest when the engines are tickover ish

Same applies in a car. MPG will ALWAYS be at its max at idle-ish speed (1000rpm or whatever minimum the engine will run at in gear without conking out) in top gear, 28mph ish depending on the car. Not at 56mph or some other random number. It's nothing to do with thermal efficiency of the engine (which i agree is not at all good at low RPM). Rather, it is about doing the least work (in the physics sense) per mile travelled

 

[jfm]

That looks good data QE2. Exactly what you would expect, and broadly similar shape curve as we get with twin diesels

This shows that

1. max rpm and range is ALWAYS at tickover. Same in a car - see my other post.
2. But once the boat is planing your mpg doesnt fall too much if you plane quite fast rahter than plane slowly (though, MPG does still ALWAYS fall, the faster you go)

 

[mjkinch1]

I found my old write up on the TAMD 63P Engines on the Phantom38 - it shows;

which shows about 1800-2100 as the optimum range? I generally get up on the plane at 2200, then ease back to around 2000 and it seems about right.

 

[hlb]

Well, I know all that. I think. /forums/images/graemlins/confused.gif But was just trying to point out why the graph was not just a straight line. There's puddles of other things come into it. Oh hell, I was trying to avoid writing war and peace. Bit like some pillock above suggested optimum revs forwards with one engine and optimum reverse int tuther, wont work. But I was to stupid to mention that.

Q E two, points out what I mean. In his Giraffe. /forums/images/graemlins/confused.gif

You can some times go twice as fast, with same fuel as slower.

 

[BrendanS]

Really depends on the boat.

This is profile for a boat a bit smaller than mine, but has similar characteristics, and with similar size engine. My sweet spot is 28-30 knots. Slower or faster means higher consumption

 

[jfm]

The red line is basically the same shape on all planing hulls. The precise shape of the curve and speed at which the kink occurs will vary depending on the boat, but not the basic zig zag shape of the line. ALL boats have max MPG at tickover rpm, and mpg falls as speed increases, except around the hump, which is the kink in the graph. At the hump there is a small bit where you increase MPG by opening the throttle, becuase just before the boat hops over the hump there is high fuel burn and low speed as you push a stack of water.

You/all of us know all this. I agree your sweet spot might be 28kts. Sweet spot isn't a scientific term, and i think you mean a speed when you are not wasting fuel by being caught before the hump, and you get a nice compromise of speed and MPG. But it aint your point of max MPG - that still occurs at tickover in-gear, 6 knots or whatever, and that always applies - it doesn't depend on the boat. And even if you make the sensible assumption that you want to plane not displace, your max MPG occurs just past the hump, so maybe 12knots or something (I dunno your boat), but not 28, for sure

 

[BrendanS]

I'm well on the plane at 14knts, and over the hump. The rise in the graph in this case is because of a stepped hull where you rise higher and higher out of the water, until a minimum is left in the water, at which point normal drag issues start to arise, causing decrease.

 

[jfm]

Martin your 11 knots is on the kinked part of Brendan's red line graph. That figures, becuase the max displacement speed of phantom 38 using the 1.5x sq root w/l length rule of thumb is about 9 knots. If you had a data point at 700rpm tickover and 6 knots you would get about 3MPG which is much higher MPG than any of your other data points in your table

Hey I'm not trying to suggest folks go everywhere at 6kts. I'm just dispelling the myth that there is best-MPG at some kind of 20knottish "cruising speed", rahter like the car's 56mph. There isn't, that's a myth. The max MPG and therefore max range is always at tickover speed, and it is always the case that MPG falls as you speed up, except on the kink in the graph when the boat is just the worng side of the hump. That's about 8-12 knots in the case of your boat.

If you make a working assumption that folks want to plane rahter than displace in their planing boats, then the sensible fuel efficient speed is just past the hump, praps 14kts in your p38. Just after the kink, 2650rpm on Brendan's graph. Brendan's boat might have a comfortable sweet spot at 28kts but it's nonsense to say its max fuel efficiency (in MPG terms, not thermal efficiency) occurs at 28kts. 12 knots would be more like, or whatever speed it is just over the hump

Same anaylsis applies to a car, except there's no hump. Every single car on the planet will give max mpg and hence max range at tickover speed in top gear, being something like 28mpg and 900rpm or thereabouts. It's utter pants frexample to say 56mph or whatever is the most fuel efficient speed. Not that I'm suggesting you drive everywhere at 28mph...