mercruiser 4.3 most economical rpm??

[1216bandit]

I dont have a fuel flow meter and I realize there are variables to consider but generally speaking, at what rpm is the 4.3 most economical? The boat is a 1993 22' aluminum starcraft islander, engine is carbed, prop is 14.5X19 3 blade.

Also wondering what would be considered max sustained operating rpm? I know there are those that would say its ok to run all day at wot but I am not in that camp. In an emergency situation hammer down and go! But other than that all I can do is guess...and I am thinking 1000 rpm below max would be ok to run for extended periods???

Take er easy...Dan

 

[H20Rat]

Looking over at boattest, for the MPI 4.3 its 2500 to 3000 rpm. And below that, the curve is pretty steep, but going up to 3500 doesn't drop much for most boats. As far as max rpm's, I also like to back off max rpm's, but I have no problem driving around in the low 4000's.

 

[matth121]

i might not be the best one to answer this but i'll give it a shot. The faster you go- the more gas you burn. unlike a car that has a transmission that allows the engine to run at a slower speed for economy, a boat can be thought of as only having one gear, well, one forward gear anyway since it does have reverse. The higher your engine revs, the more gas its going to burn. the most economical setting would be the minimum speed you can get on plane. anything faster is going to burn more fuel.
As far as max revs- i personally would agree that its probably not a great idea to go WOT all day, but how big is your tank cause i'm guessing that at WOT you'd probably run through a tank in around 2 or 3 hours, thats pricey.
again, i might be talkin out my ass here- just my opinion

 

[H20Rat]

i might not be the best one to answer this but i'll give it a shot. The faster you go- the more gas you burn. unlike a car that has a transmission that allows the engine to run at a slower speed for economy, a boat can be thought of as only having one gear, well, one forward gear anyway since it does have reverse. The higher your engine revs, the more gas its going to burn. the most economical setting would be the minimum speed you can get on plane. anything faster is going to burn more fuel.

Well, yes, faster usually equals more gas, but... Lets talk efficiency/mpg's.

Hulls become more efficient at speed as you get more hull out of the water, and engines also are in theory more efficient at burning fuel. (Technically the most efficient engine is one without ANY restrictions, no throttle plate, nothing.. Just a venturi sucking very carefully metered fuel. In reality that doesn't hold true usually.)

Take this 4.3l boat for example, it gets its best, and identical mpg's running at 25mph (3000 rpm) or 31mph (3500). And going to 38 mph at 4000 rpm only costs .5 gallon/hour more.

http://www.boattest.com/boats/boat_video.aspx?ID=581

 

[jack black]

I'm surprised no one gave the correct answer.


The correct answer is:


Exactly Zero RPM (really no brainer here).

 

[achris]

Interesting subject... I was out in my boat (4.3MPI) today with my mother (fishing) and on the way home, running at WOT, 4950 rpm and 39 knots, I commented on the fuel consumption, more than a litre a minute running down the hole, 63.5 litres per hour to be exact. Once we reached the marina I had to slow down, to 8 knots. Fuel was being burnt at 13.8lph. Integrating those numbers generates the same fuel consumption (near as) for both speeds. I have actually run a full revs vs speed vs lph graph, and for my engine in my boat, between 3,000 and 4,000 revs is the best economy... Interestingly I ran the numbers with both a 17" prop and a 19" prop, and guess what... For the same boat speed, the fuel consumed it EXACTLY the same. The revs are lower with the 19", but that just means a higher load than the 17" at those revs...

 

[achris]

I'm surprised no one gave the correct answer.


The correct answer is:


Exactly Zero RPM (really no brainer here).

Well, no... Fuel economy is distance travelled divided by fuel consumed travelling that distance... At zero revs distance travelled will be zero, and zero divided by zero is infinity... Sorry, your logic not so good :rofl:

 

[jack black]

Well, no... Fuel economy is distance travelled divided by fuel consumed travelling that distance... At zero revs distance travelled will be zero, and zero divided by zero is infinity... Sorry, your logic not so good :rofl:

Sorry, you didn't read the question correctly. The question was: [h=1]most economical rpm[/h]

 

[matth121]

dang, i knew i wasn't the right one to answer, lol

 

[achris]

Sorry, you didn't read the question correctly. The question was: [h=1]most economical rpm[/h]

I'm reading the OPs intent, not a bus-stop lawyers' interpretation...

 

[stonyloam]

IMHO the answer is that depends. Depends on the load and how it is distributed and how the boat is trimmed. Your best mileage at any given RPM Will be when the boat is properly trimmed. Try this: when on plain trim up and down and watch the tach. Maximum RPM at any given setting should be your most efficient throttle setting. I change my trim all the time to try to get the most efficient and comfortable ride, a holdover from sailing where you are constantly trimming to maximize the power you are getting from the wind. Once I am up on plain I will trim up till the boat just starts to porpoise then back down a touch for the best ride and RPM. My best mileage is at about 3/4 throttle, in the mid to high 3000's but every boat is different. Good luck.

 

[bruceb58]

I have a fuel flow meter on my boat and the most economical RPM is way past where it gets up on plane. Close to 3500 RPM. It is integrated with the GPS so reads out MPG.

Every engine/boat combo is going to be different. Same exact engine on a different boat will have a different point.

 

[achris]

Here's the chart I did....

 

[dubs283]

my whole take on fuel economy and the most efficient rpm in a boat is this...

if you're that concerned about fuel in your boat, maybe boating isn't for you

 

[achris]

my whole take on fuel economy and the most efficient rpm in a boat is this...

if you're that concerned about fuel in your boat, maybe boating isn't for you

I totally agree... I did my chart for a couple of reasons. 1. It' is something a lot of people ask about, and as a shop owner it is also something I should be able to inform them about. 2. Curiosity.... Did I change my driving habits because of it? Not in the less....