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Miles Per Gallon

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Mulletwagon

Regular Contributor
In general, what conditions result in maximum MPGs ? Cruising at idle, trolling speed, just enough rpm to stay on the plane, or wide open/red line ?
 
Throttle up to plane,then backoff and keep on plane. What type of hull makes quite a difference also, displacement or planing
 
You measure fuel usage by gallons per hour GPH. Make sure it is propped correct for proper WOT rpms. Then find the point where the engine just reaches max timing advance. Put a timing light on and watch the progressive timing advance and slowly give it throttle note the point where the timing advance reaches its limit or all in spark advance. Put a mark on your throttle control and some tachs have a pointer you can just put the pointer at that engine rpm. An outboard you can just watch the trigger and put a mark on the controls when the spark advance stops moving.
 
You can get alot better GPH at lower rpms but if your bucking a strong current you can be going 0 MPH when using a GPS. Water speed is not the same as GPS speed.
 
You mean gallons per mile, not Miles per gallon.

Anyway, Trying to achieve the best fuel economy is usually bad for the health of your engine.

Lugging your engine with just enough throttle to stay on top of the water will produce elevated combustion temps.

The amount of money that you will save by doing this will not cover the cost of the repower.

Push the throttles up, You will feel the engines unload a little(their sound will change) that is where you want to be.
 
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Interesting comments – five opinions all opposed. Timing, manifold vacuum, single engine, planerpm, plane plus rpm – wow. If someonecan provide actual numbers from personal experience it would be instructive. Assuming calm seas, no wind/current, and twinengines; a simple calculation would be to divide the GPS MPH (or actual waterspeed if known) by the instrumented fuel burn rate in GPH under variousconditions to determine the resulting MPGs. Cruising at all the conditions mentioned (including the single engine option)and calculating the estimated MPGs would yield some interesting data points whichmay provide an authoritative answer. Howto achieve optimal MPGs would be good info. If offshore and fuel is low (something which should never happen) itwould be good to know how to maximize the chances of making it back to thedock.

 
And calibrate it!!!

comparing results across different hulls and engines may not apply to your combination....even two boats of the same configuration will yield different results if loaded differently....too many variables.
 
REAL LIFE and a BIT OF HUMOR

True story...

As a very wise man said to me....prior to me buying a "bigger" boat......

I was looking out the window of this fellows residence, (he lives on the south side of Lake Ontario) and about a mile or so a very large cruiser went by....looked like a lovely boat.

I said, "I wonder how many miles to the gallon it gets?"

My friend replied, "If you have to ask, you can't afford the boat ".:eek:
 
NB NB SB SB avg
prop pitch rpm gear ratio theor.travel in "/min SB speed in kts NB speed in kts Avg. speed in kts actual travel in "/min slip Port GPH STB GPH Port GPH STB GPH Total GPH nm/Gal

"Islander" 16 1000 1.52 10526 5.5 5.3 5.4 6562 38% 1.9 2 1.9 2 3.9 1.38
16 1500 1.52 15789 7.7 7.3 7.5 9114 42% 3.9 3.5 3.6 3.4 7.2 1.04
16 2000 1.52 21053 9.1 9.6 9.35 11362 46% 5.9 5.6 5.5 5.3 11.2 0.84
16 2500 1.52 26316 15.9 15.6 15.75 19139 27% 8 7.5 7.5 7.2 15.1 1.04
16 3000 1.52 31579 20.7 19.4 20.05 24365 23% 10.2 10.8 9.6 10.2 20.4 0.98
. 16 3250 1.52 34211 22.7 21.9 22.3 27099 21% 12.1 11 11.4 11 22.8 0.98
16 3500 1.52 36842 24.4 23.5 23.95 29104 21% 13.4 13 12.4 12.5 25.7 0.93
16 3750 1.52 39474 26.2 25.5 25.85 31413 20% 15 14.7 14.5 14.6 29.4 0.88
16 4000 1.52 42105 27.9 27 27.45 33357 21% 19.8 17.9 18.1 18.1 37.0 0.74
16 4250 1.52 44737 28.9 28.4 28.65 34815 22% 22.1 22.7 20.8 19.3 42.5 0.67
16 4500 1.52 47368 29.2 35484 25% 24 24 48.0 0.61
 
one engine operation port engine on stb engine on
16 1000 1.52 10526 4.9 4.9 5954 43% 2 2.45
16 1500 1.52 15789 6.4 6.4 7777 51% 3.9 1.64
16 2000 1.52 21053 7.9 7.9 9600 54% 6.2 1.27
16 1000 1.52 10526 4.7 4.7 5711 46% 2 2.35
16 1500 1.52 15789 6.5 6.5 7899 50% 3.9 1.67
16 2000 1.52 21053 7.8 7.8 9479 55% 5.7 1.37
 
copied from excel, so a bit of a mess in formatting.

NB= north bound; SB= south bound; 16 = prop pitch

the very last number on each line is nm/gal.
I couldn't beat 2.45 nm/gal, even with only one engine on.
I recall below 1000 rpm the mileage got a bit worse, no doubt due to what MM said about max vacuum condition.
 
Dave – Your numbers are very interesting since they bothsupport and refute conventional wisdom. For twin operation it sounds likeefficiency is good at no-wake speed, drops off during the transition to planewhen the bow is high and the boat mushes through the water, than maxes out whenup on plane (assuming plane rpm is between 2500 and 3000 rpm), and finallydrops off again as rpm is increased beyond plane speed. As a general rule the data suggests minimumspeed on plane is the MPG sweet spot. Thesingle engine data is something of an epiphany. Conventional wisdom generally suggests the elevated drag induced by thebow yawing heavily toward the operating engine (boat actually moving sideways),and the additional drag of the stalled (or even worst windmilling) prop expendsconsiderable energy which does not contribute to forward motion. Based on your data it would appear theincreased drag may be over estimated and does not dramatically impactefficiency. May be something to the suggestion that single engine operation (ona twin) might yield the most MPGs. Welcome more comments on single engine operation.
Interesting numbers – thanks for posting.
FL Panhandle

 
Yep. And best efficiency at 43% prop slip! This boat doesn't have a pronounced bow rise transition. It does rather plow a bit at the 11 to 14 kt range, making trolling for wahoo and tuna there a bit strained.

I redid all these tests with 4 bladed props; not a lot changed.
 
Try different arrangements on how you store gear and bodies when running. After a while you get to know what trim angle and weight distribution gives you the best results.
 
$(KGrHqUOKo4FJgvnYFIeBS,qw4oVf!~~60_57.jpgI thought this was an interesting concept. Owner was selling it on ebay. Said he used the outboard in low speed zones. Twin 454's were cranked up when he wanted to go.
 
Fwiw my 1986 Sea Ray 390 Express with 454 Quadraets will get 1.1 mpg below hull speed about 8 mph. It will get .6 mpg on plane above 2700 rpm and 17 mph. It really doesn't change above 2700 rpm all the way up to 3500 rpm and about 25 mph. I have twin Garmin GFS10's networked on my Garmin 4212. So I just run the boat at a comfortable speed based on load and conditions.
 
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