96' 150 HP Johnson low RPM's! Prop issues??

[sct_b]

I just bought a 96' stratos fish/ski 18ft. with a 96' 150HP Faststrike Johnson. Motor runs great and jumps out of the hole. Haven't tried pulling a skiier yet. It tops out around 50mph with just me on the boat. At WOT I am only getting about 4300 RPM's, which I know is way to low. The prop is a 14.5 x 24 Raker. No jack plate. Are the low RPM's a result of too high pitched prop? What is the recomended prop for this set up? What should max RPM's be for my motor? Is there anything else that could be causing this? Any suggestions will help. Thanks

 

[jestor68]

Re: 96' 150 HP Johnson low RPM's! Prop issues??

I would verify the accuracy of the tachometer before doing anything else. "Jumping out of the hole" and low WOT rpm don't normally go hand in hand.

 

[steelespike]

Re: 96' 150 HP Johnson low RPM's! Prop issues??

Agreed, terrific hole shot and low wot rpm usually don't go together.
Jumps out of the hole must be less than 10 seconds?
Preliminary calculations indicate 4% slip at 4300, not likely. Is the speed by gps?
We need an accurate speed precisely stated.
Do you have trim tabs or a hydrofoil?
If we use 10% slip your rpm is about 4500. If 50 turns out accurate your at the the bottom of your rpm range.4500-5500
And the tach may not be too far off.4500 is ok if you don't normally run extra heavy.But if you can't get comfortably into your rpm range with a typical load perhaps a 23" would be better.
Further investigation indicates the Stratos is about 1200 dry. I think your speed should be faster perhaps the 24 is too much Maybe a 22 would be better.But confirm the tach and the speed.

 

[sct_b]

Re: 96' 150 HP Johnson low RPM's! Prop issues??

Ok, after running it some more and from yalls advice, it is very possible that the tach could be off. Is that a common issue? How do I go about testing that? The speed reading is just on the speedometer, not on a GPS. I will verify the speed next time out. I don't have a hydrofoil or trim tabs. I do know this. After running it this weekend, I figured out it will plane out with 2 ppl within 3 sec, and within 5 sec with 5 ppl. Which seems good to me. The slip calculations are a little over my head, so not sure what that means. Thanks for the help.

 

[steelespike]

Re: 96' 150 HP Johnson low RPM's! Prop issues??

The tach may have a switch on the back note its position and operate it a few times to clean the contacts.
You may be able to borrow a shop tachor buy an induction tach.Works on any motor.We will need the speed by gps.
With that hole shot it appears you have plenty of power.
Slip is the measurement of the efficiency of the prop in the water.The lower the number the better the prop performs.
4% is possible but unlikely 10% is pretty typical.
You need to use a prop calculator to find slip.