RPM questions

[Nitro2014]

I have a 1985 nitro mx17 fish/ski boat.
It has a 1985 2 stroke 115 hp Johnson outboard motor.

Question 1: What is the rpm operating range at WOT?
Question 2: How many rpms should there be right before break over on the hole shot?

Thanks!

 

[Sea Rider]

Welcome to Iboats,

1-Someone will know, or ask Google
2-Running a motor as loaded towards its max wot rpm range achieves a faster hole shot with slight top end speed decrease.

Happy Boating

 

[flyingscott]

First question Is 5500 rpm. Second question can't be answered because to many things affect holeshot.
Prop pitch
Prop style
Weight of the boat
Weight distribution in the boat
Health of the motor
Condition of hull (any rockers or hooks in it)
Is the hull waterlogged
How much weight are you carrying in the boat
Are you always carrying the exact same weight.
As the fuel tank empties hole shot will change.
Just a few things that will affect holeshot.

 

[Scott Danforth]

in addition to above

wind direction and current are another two factors

so what RPM are you turning at WOT flat-out and trimmed right?

 

[Nitro2014]

Thanks for the replies and great insights. My WOT throttle RPMs are 5200. Was thinking the operating range was either 4000-5000 or 4500-5500. If it’s 5500 as stated above, it sounds like I could go down a couple prop pitches. My current set up now is a four blade SS with 20 pitch.

 

[flyingscott]

4500-5500 is your range. You are propped pretty well right now. But I would double check that with another tach because a 20" pitch 4 blade is a lot of prop for that motor.

 

[Nitro2014]

Thanks! What do you mean by 20 inch pitch is a lot for that motor? Do I need to go down in pitch?

 

[flyingscott]

Thanks! What do you mean by 20 inch pitch is a lot for that motor? Do I need to go down in pitch?

You need to double check your RPMs with another tach. Most.of.those motors I have worked on have 17-19" pitch 3 blade props on them. When you go to a 4 blade you usually drop in pitch. I may very well be wrong it is something I would double check.

 

[Nitro2014]

Great, thanks!

 

[Sea Rider]

If max wot rpm for said motor is 5500 and motor achieved just 5200 the motor is + 300 wot revs off from its max wot rpm range. Better go for one less pitch and check if near max wot rpm range as factory stated. Going 2 less in pitch from current tached 20" /4 blade prop will be too much. The RoT (rule of thumb) that states +,- 200 wot per one less/more pitch most times doesn't apply, expect to achieve way more than 200 wot revs. For the record, post whichever wot rpm was achieved with a 4 blade 19 pitch same prop brand as currently loaded.

Happy Boating