Are smaller motors really much better in fuel consumption?

[ShoestringMariner]

If a boat moves slower with the smaller motor, does it not consume the same amount of fuel that a larger motor would travelling the same distance in less time?

say comparing a 50hp horse versus a 65hp versus 80hp, all versions with the boat planing at top speed. If each traveled the same distance at different max top speeds, wouldn’t fuel consumption not be pretty much all equal or very close? ( test would be on the same boat, same weight, same wave, wind and weather conditions.) Motors would be also be same era (not new vs 20 years old etc) and all either 2 or 4 stroke

 

[Chinewalker]

Doesn't work quite like that. Every motor and hull combo has a sweet spot for efficiency. That goes for gallons per hour and miles per,gallon. A smaller motor running at full boogie is working harder than a larger motor at cruising speed, so the smaller motor is likely using more fuel to accomplish the same thing. The larger motor on the same hull wouldn't have to work as hard, even at full tilt. However,,the larger motor will also use more fuel the faster it goes. Really depends on the motor, the boat, and load.

 

[jimmbo]

Speed cost money, so a bigger motor at WOT will consume more fuel. It will get to destination sooner, so run time is less but total fuel consumption is greater. If all engines can plane the boat, the larger engines when throttled back to the slowest boats top speed will get better economy than the smaller engine at WOT.
I acquired some of Mercury’s engineering data regarding the development of their V6 outboard in the early 70s. The development engine, which eventually was released as a 175, was compared to the 1972 140hp. When run at the top speed of the 140, it was going 27% further than the 140 using the same amount of fuel

 

[roscoe]

A better comparison would be with all the motors running at say 3600 rpm, hull comfortable on plane, but using far less fuel than at 5500 rpm. If all the motors are propped correctly, fuel consumption would be very similar. Have read articles/tests on this, but no, I can't reference them right now.

 

[jimmbo]

All engines at 3500 rpm is an apples to oranges comparision, as they may have different gear ratios, props and will yield different speeds
Of course if you want a GPH comparision, that would work

 

[dingbat]

Rpm, LU ratios are eliminated when you simplify to Work.

1 hp = 2,544.43 Btu/h.
1 gallon of E-10 = 111,836 btu

same boat. Same distance, same speed , large or small HP motor, same fuel usage

 

[ShoestringMariner]

Rpm, LU ratios are eliminated when you simplify to Work.

1 hp = 2,544.43 Btu/h.
1 gallon of E-10 = 111,836 btu

same boat. Same distance, same speed , large or small HP motor, same fuel usage

Very cool. Thanks

 

[JimS123]

Rpm, LU ratios are eliminated when you simplify to Work.

1 hp = 2,544.43 Btu/h.
1 gallon of E-10 = 111,836 btu

same boat. Same distance, same speed , large or small HP motor, same fuel usage

Kinda simplistic approach. Theoretically maybe. But different era motors have different efficiencies. Carbed, fuel injected, 2-stroke, 4-stroke, etc.

My little tinny with a 25 2-stroke, compared to my Whaler with a 60 4-stroke, compared to my bowrider with a I/O all use different amounts of fuel on the same course.

 

[Doh]

Kinda simplistic approach. Theoretically maybe. But different era motors have different efficiencies. Carbed, fuel injected, 2-stroke, 4-stroke, etc.

My little tinny with a 25 2-stroke, compared to my Whaler with a 60 4-stroke, compared to my bowrider with a I/O all use different amounts of fuel on the same course.

Your Tinny, 500#s, your Whaler 1000#s, your I/O 2000#s

Your Tinny, 20mph, your Whaler 30mph, your I/O 40mph

 

[444]

It takes a certain amount of HP to maintain speed on plain. It takes a certain amount of fuel at a minimum to product that HP level. Beyond that there is a difference in efficiency. They don't put 500 cubic inch engines in toyota corollas for better mpg because they're not working hard just as they don't put 4cyl corolla engines in 1/2 ton pickups and wring them out to the edge.

Also, keep in mind in terms of speed, resistance increases at something like a rate of the square of the speed difference.

 

[tomhath]

Rpm, LU ratios are eliminated when you simplify to Work.

1 hp = 2,544.43 Btu/h.
1 gallon of E-10 = 111,836 btu

same boat. Same distance, same speed , large or small HP motor, same fuel usage

Sorry to reopen an old thread, but it's not that simple.

The increase in horsepower doesn't go up linearly with the amount of fuel burned, running wide open is usually much less efficient.

Another important factor is how the power of the engine is applied to the water by the propeller. Water is forced backward in order to propel the boat forward. I'm not an expert in fluid dynamics but I can tell you that, in general, the more the water is accelerated the less efficient the propeller will be. In other words, a larger volume of water pushed at a slower speed will be more efficient that a smaller volume at a higher speed (think a big slow turning prop versus a tiny very high speed jet).

Put the everything together and you'll find a certain motor size and prop size/pitch will give the best economy for a given boat at a given speed.

 

[thatone123]

I know that a OMC 20 of the long stroke old school (circa 1967) burns a lot more gas than a more modern OMC 15 short stroke (circa 1985) at any speed. If the 20 is run at about 10-15 hp throttle is still burns way more gas than the 15.

 

[boobie]

A simple old rule of thumb to remember. A motor will consume 10 % of it's horse power at WOT in gallons.

 

[racerone]

I recall a Cadillac with a 429 V-8------About 23 miles / gallon on the super highway.------------Now picture a Volkswagen with a 1200 CC engine and getting about 40 Miles / gallon.------Fuel consumption is not easy to understand for some.-----The power required from the fuel to move a vehicle along is what determines things.-----Vehicle drag / amount of air consumed , used by engine / engine internal drag / bearing drag / wind resistance all come into the picture.