Does lower rpm mean better fuel economy ?

[jabfd]

hello all
i just changed my prop to a 4 blade 18 p ( mercruiser 470 engine alpha 1 ) my question is does lower rpm meann better gas economy i now can cruise at 3000 rpm and be at 20 mph . if i go up to 3300 rpm i get 24-25 mph is it what would give me more fuel economy thanks jeff

 

[UncleWillie]

Your best economy will be at No Wake Idle Speed.
As your RPMs go up your fuel usage in Gallons per Hour (GPH) will increase,and your Miler per Gallons (MPG) will Decrease.
As you get on plane the GPH will continue to increase but the MPG will also increase.
Most boats have a sweet spot right near 3500 RPM where the MPGs peak.
Any faster or slower will reduce range.

Look at the Test Results for Boats similar to yours at BoatTest.Com for comparison.

 

[Silvertip]

Engine rpm, prop rpm, hull type, load, etc., all factor into what economy any combination offers. Any boat must be optimized to achieve best fuel economy. If you put a high pitch prop on a low power engine you obviously have very low engine rpm but very high throttle setting which results in very poor economy because the engine is being severely "lugged". So to answer your question, NO! Lower rpm does not automatically result in better economy. The only way you can determine what combination is best is by carefully measuring fuel consumption at various speeds, always with the same (or what you would consider normal load). That is best done with a fuel flow monitor. Conversely from the above example, if you put a low pitch prop on a high power engine, the engine will likely over rev, have terrific hole shot, but again, poor economy because it running very fast to achieve very low speeds which results high fuel consumption for the distance traveled. There is a happy medium. Whatever prop allows the engine to operate at or very near the manufacturers recommended wide open throttle rpm with what YOU consider an average load is an optimum setup. Fuel economy will be whatever that combination provides and attempting to better that economy will mean you compromise somewhere else. You either over rev or lug the engine. Neither is an acceptable solution. And an engine that is not in a good state of tune or is simply "tired" is often the cause of poor fuel economy but folks begin messing with props, tabs, and other gadgets to compensate. That is simply a waste of money.

 

[thumpar]

Usually somewhere between 3k and 3.5k RPM depending. You will have to figure out what works best for you setup. Some people say the slowest you can go but still be on plane but that is usually not where you get the best economy.

 

[midcarolina]

Do a search in I-boats for vacuum gauge......... that is your most simple best way to maximize your mileage........way too many variables for anybody to be able to say run it at X rpm.

 

[HT32BSX115]

hello all
i just changed my prop to a 4 blade 18 p ( mercruiser 470 engine alpha 1 ) my question is does lower rpm meann better gas economy i now can cruise at 3000 rpm and be at 20 mph . if i go up to 3300 rpm i get 24-25 mph is it what would give me more fuel economy thanks jeff

Only if it's lower than maximum RPM at WOT. (REALLY!)

Assuming your boat is not over-loaded, doesn't have a dirty bottom, not water-logged, and/or is in otherwise "ok" running condition.........

There's no one RPM that is always going to be better than another. It's going to vary based on how much weight you have in the boat and where it is positioned inside.

You should "prop" your boat (regardless of how many blades you have in the water) for the "top" of the recommended RPM range when the boat is loaded at the weight you're typically going to operate it.


Then, depending on the hull shape and other characteristics the most fuel efficient cruise speed is going to be someplace below maximum speed and someplace above minimum planing speed. Then you'll need to adjust the center of gravity AND drive angle (thrust angle) and maybe move people and gear around to minimize the "whetted" surface on the water to reduce drag to the lowest point. At the MIN drag point you will find your max fuel economy.

There's also a MAX number of prop blades there too. 2 blade props seem to be the most efficient (actually a 1 blade prop is theoretically the most efficient in aircraft engines but no one liked the idea very much. (the one blade is always operating in undisturbed air and the theory follows in a liquid too but I wouldn't want one!)

3 blade props in liquid produce more thrust at lower RPM (less slip) but each additional blade produces more drag (on the engine) at higher speeds.

Mercruiser has 4x4 Bravo III. They produce incredible holeshot thrust and acceleration but at somewhat less top speed than a single prop drive.

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You can determine your fuel mileage (and efficiency) experimentally by first installing a fuel flow gage and an accurate speedometer (GPS)

Adjust drive trim and determine where the max MPG occurs at various speeds. The trim will be different for different speeds and even where you position people and other gear in the boat.

If you have TABS, adjust them (up) OUT of the water (not touching at planing speeds. They just add additional drag) You would use drive angle to adjust thrust angle with little or no drag penalty.

Most gasoline marine engines exhibit approx the same Brake Specific Fuel Consumption (BSFC) at medium power settings (there's not even a HUGE difference between EFI and carbureted engines at medium power settings)...........notice I said power settings and NOT RPM....... because at a particular power output you could have a different throttle and RPM settings ...... There are *some* lower RPM settings that might be more efficient than higher RPM settings but you would need an adjustable prop to find them.

This factor works well in an airplane with an (cockpit) adjustable pitch prop not so well in a boat. (unless you have an "on-the-fly" operator-adjustable prop!!)

UncleWillie above mentioned BoatTest. They do extensive testing at many different speeds and RPM with just about all the boats they test. They have even tested particular boats with different engines installed but otherwise loaded and operated the same (speed)

The fuel flow at similar speeds and boat loading produced similar MPG regardless of the engine (size) installed.

So, Prop for max RPM (@WOT) at typical loading and experiment with a fuel flow gage if you really want to find that "sweet spot" .............otherwise do like the rest of us who still prop for max RPM (@WOT) & are tuned correctly, and then run it at around 3000-3500 RPM and just go boating!



Cheers,

Rick