17P Trophy 4 Blade only turning 3000RPM@WOT on 75HP, is this right?

[mrfishaholic]

I am only turning 3000 RPM at WOT with a 4 blade SS Trophy Plus 17 Pitch Prop. I was told that my 1991 Merc 75HP was supposed to turn 5400-5600 RPM at WOT. Is my tach reading correctly? I was pushing my 1994 Tracker Pro Deep V 16DC 35MPH on GPS at 3000RPM/WOT with this prop. Any recommendations? Is it even possible I was only turning 3000 RPM? My hull is aluminum. Thanks in advance, I am new to the forum.

I just got the motor, and she is running nice and smooth. Not rough or anything to give me any indication there would be something else wrong. What pitch should I be using on this boat? by my calculations to get to the 5400RPM I would need like a 12pitch and I am not sure that they make those?

The prop has 4 vent holes in it near where the prop meets the lower unit. There are not any plugs in those, would that make a difference?

What is my next step in trouble shooting my motor? Other than taking it in to a dealer with a blank check?

 

[bigtwin]

Re: 17P Trophy 4 Blade only turning 3000RPM@WOT on 75HP, is this right?

IMHO either your tach or your speedo is off or you have a negative slip situation (you are lucky if the latter is true)
make sure your tach is set to the proper pole, and is accurate, also verify if your speed is correct (the easiest way is to use a handheld GPS)

 

[mrfishaholic]

Re: 17P Trophy 4 Blade only turning 3000RPM@WOT on 75HP, is this right?

was using my gps for the speed, so I am sure that is right I have Eagle Fishmark 642C for my FF/GPS

Just installed the tach, how do I check the pole thing?

What is a negative slip thing also?

Sorry, kinda new to this stuff

 

[mrfishaholic]

Re: 17P Trophy 4 Blade only turning 3000RPM@WOT on 75HP, is this right?

Ok, the stupid tach is for a 20 pole alternator. Anyone know the converson factor? I will get a new tach ASAP!

 

[Texasmark]

Re: 17P Trophy 4 Blade only turning 3000RPM@WOT on 75HP, is this right?

If your 75 is like my 90 you need the 12P position for accurate readings. I just did a quick BAM slip calculation and here are the results:

Your gearbox is 2.3; same as mine.
Your stated speed is 35
With the 4 blade prop I estimated your slip at 13%
That gives you your 35 indicated mph with a 40 mph no slip calculation. (Had to guesstimate this to be able to back into the rpm calculation.)

Rpm at that setup is 5715 which is a tad high but personally I run that frequently with my 3 cyl 90.

Your ear is your best instrument for your tach error.

3000 on that engine is nice and quiet and you can hear hear each other talk at a normal level and hear the exhaust tubes resonate.....nice, quiet, purring 3 cylinder sound (no other configuration sounds like it).

5715 is loud and the engine is screaming (singing, ringing), even with the sound deadening material. Have to scream and lip read to talk over it if close to it.

HTH

Mark