RPM too High with a 15 pitch

[oavila88]

After installing a tiny tac, my bayliner capri 19' with a force 125 and 15 pitch prop was reading a max of 6600 rpm WOT and going about 33 MPG (GPS). Does that seem right? I had a couple of people on the boat (450 lbs). Idle it was reading 800 rpm.

 

[roscoe]

Re: RPM too High with a 15 pitch

Sounds about right. A 19pitch prop should be about right.

Better and more specific prop info would be in the prop forum.

 

[Jiggz]

Re: RPM too High with a 15 pitch

After installing a tiny tac, my bayliner capri 19' with a force 125 and 15 pitch prop was reading a max of 6600 rpm WOT and going about 33 MPG (GPS). Does that seem right? I had a couple of people on the boat (450 lbs). Idle it was reading 800 rpm.

I believe a 15P prop is an underload for 125HP. The standard issue prop for this engine is a 17P with a rating of 5000-5200 RPM at WOT. I have a 13 1/4-19P and will do 40 MPH with three average size adults (approx 380 lbs) onboard with a WOT RPM of 5000 RPM in calm waters. Of course hull design and trim settings will make a difference but a 15P is I believe an undersized or underload. Of course that is just my own POV.

 

[jerryjerry05]

Re: RPM too High with a 15 pitch

I believe that boat came with a 19p(if my memory's right)
Jiggz,you tell him you have a 19p and how it works on your boat,then don't tell him the size or the kind etc.
Give him something to compare to.
And it sounds like your underpropped too if you can only get 5000rpm?
(3 average adults 380#) you guys midgets?

 

[steelespike]

Re: RPM too High with a 15 pitch

What year is the motor?
I think your tach is getting too many pulses. at 6600 you would be doing about 43 mph at 10-% slip.
If you calculate from the speed end 33 gets about 5000 at 10% slip.
You need to use the bottom plug wire and get the wire outside the motor in as short a distance as possible.
Multi cylinder motors can interfere with induction tachs. Usually suggested to get a special antenna wire.
Be sure to follow instructions carefully.

 

[Jiggz]

Re: RPM too High with a 15 pitch

I believe that boat came with a 19p(if my memory's right)
Jiggz,you tell him you have a 19p and how it works on your boat,then don't tell him the size or the kind etc.
Give him something to compare to.
And it sounds like your underpropped too if you can only get 5000rpm?
(3 average adults 380#) you guys midgets?

Actually we are, two ladies standing at 5'4" each weighing in at less than 115lbs each and a 5'10' guy weighing in at 150#! But maybe more of the malnourished type rather than midgets, LOL. If I only get 5000RPM wouldn't that mean I am over propped? Since if I go for a 15P it means my RPM will increase at WOT? But the rated RPM at WOT is 5000~5200 RPM so I think I'm within specs.

 

[steelespike]

Re: RPM too High with a 15 pitch

We seem to have 2 threads going Jiggz you should start your own thread.
A 15" pitch would be a lighter prop allowing the motor to rev easier. Under propped increases rpm.Over propped lowers rpm.
Depending on the boat a 19 seems like a lot of prop. What size is the boat.What year is the motor?

 

[jestor68]

Re: RPM too High with a 15 pitch

That boat has an empty weight of about 1655 lbs, so it's not that heavy.

The Mercury prop selector comes up with a 19" for that size boat with a 125. A friend has a 19 ft Bayliner cuddy with a 17" on it, which works good.

 

[oavila88]

Re: RPM too High with a 15 pitch

Thank you all for your replies. I will try to get readings again after I make sure the tach is properly installed.