Mercury 115HP EFI RPM question

[dradulescu]

Hello,

I have a question, please help.
I have a 115 EFI and a 17.1 fiberglass, and with my usual boat load the max RPM I can reach is 5000.

My question is, considering this, if it is ok to run at 5000 RPM or this will stress the outboard and reduce the life time ?



Thank you,
Dan

 

[sschefer]

Re: Mercury 115HP EFI RPM question

Dan, to answer your question correctly please provide the max RPM that is shown on the motor id plate on the transom bracket.

 

[dradulescu]

Re: Mercury 115HP EFI RPM question

Hello,

Here we have the specs
http://mercurymarine.com/engines/outboards/fourstrokes/enginesetone_specs.php?ID=75

Thank you,
Dan

 

[ufm82]

Re: Mercury 115HP EFI RPM question

That would be a classic description of lugging an engine. Since your WOT range is 5800-6400 and you are a minimum of 800 RPM below that you must be overpropped quite a bit. By typical standards you should drop 2 full pitch levels I.E. from a 19 pitch to a 15 pitch. (I pulled a number out of the air.)
Now, is the thing running right? No other issues?

UFM82

 

[dradulescu]

Re: Mercury 115HP EFI RPM question

Hello,

I was aware that the engine works hard since it can't get over 5000 RPM with full throttle, my question is would that be bad for the outboard during time ?

Because my guess is that if I lower the pitch from 21 ( currently ) to 17 then in order to get the same speed I need to go maybe with 6000 RPM as the propeller would push less water per rotation.

Basically the dilemma is the following: in order to keep the speed the same and the outboard healthy is it better to go with 5000 rpm at 21 pitch or 6000 rpm at 17 pitch ( full throttle ) ?

Dan

 

[sschefer]

Re: Mercury 115HP EFI RPM question

Hello,

I was aware that the engine works hard since it can't get over 5000 RPM with full throttle, my question is would that be bad for the outboard during time ?

Because my guess is that if I lower the pitch from 21 ( currently ) to 17 then in order to get the same speed I need to go maybe with 6000 RPM as the propeller would push less water per rotation.

Basically the dilemma is the following: in order to keep the speed the same and the outboard healthy is it better to go with 5000 rpm at 21 pitch or 6000 rpm at 17 pitch ( full throttle ) ?

Dan

The answer is yes. You wll damage the engine if you run it under more than a light load over a long period of time. It the engine is over proped and cannot reach maximum RPM it will be under more than normal load throughout the RPM range.

 

[dradulescu]

Re: Mercury 115HP EFI RPM question

thank you !