prop performance question

[diamondlangus]

1) Performance issue you are trying to correct. Low RPM at WOT, it is currently running about 4400-4500 rpm

2) Current prop manufacturer, model, aluminum or stainless as a minimum. Solas rubex aluminum
3) Current prop diameter and pitch (required). 11" 15 pitch
4) Wide open throttle RPM and speed with an average load (very helpful) 4400-4500 rpm
5) Engine/drive make, model, year, and HP 1993 ish Force 70hp
6) Boat make model, year, length and weight 1995 spectrum 16' sport, boat weight with gear and me about 1450lbs

Don't know if it matters I live in WY and run at about 5200'. compression ranges from 100 to 108lbs. I am trying to decide if I should drop to a 13 or 14 pitch prop to get the RPM up. the boat is running 30 - 31 mph according to GPS. If I drop the pitch size but my RPM goes up will I maintain about the same speed or will it change much?

The Throttle is opening up all the way so I know that is working correctly. I am also going to put new spark plugs in they look a little old. This boat is new to me so I don't know how old they are.

 

[Starcraft5834]

Re: prop performance question

your numbers above sound in normal range for a stern drive... my 3.0 merc likes 4500 WOT,, dont know what outboard RPM suppose to be,, Im pushing a 2500 lb with a Solas 4 blade, 15p 14.75,, had a 3 blade in before hole shot was lousy and RPM at 4200,,,,,4 blade was proper move for me

 

[Maclin]

Re: prop performance question

The engine should be able to get to 5000rpm or more. Has this boat always been run at that elevation? If not and it was run at significantly lower elevation (like 1300' instead of 5200') in its' previous life then it was propped for lower elevation, that could be the main issue.

 

[SkiDad]

Re: prop performance question

I'd say 13 pitch but you will probably loose 2 mph. But your engine will be much happier - I went from 17 to 15 on my 125 and my rpm went from 4400/4500 to 5000 give or take a little.

 

[diamondlangus]

Re: prop performance question

I also discovered the boat is jetted for a lot lower elevation. i am going to replace the prop as well as the .096 jet with a .092 jet so hopefully this will help a little also.

 

[jestor68]

Re: prop performance question

Ideal rpm for that motor is about 5000 rpm.

If you're running 4500 now (the bottom of the rpm range), dropping to a 13 should raise it 350-400 rpm, and get you close to 5K.

Be careful rejetting the main jets. If you get it wrong, you could run too lean. Read the sparks plugs after a full throttle run to see if you need to change jets.