Increasing prop pitch

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THOMMO59

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Wollongong, NSW, Australia
I have the chance to grab a 24 ss tempest at a good price. Currently running a 22 ss laser2 on my z6, 115hp pro xs and getting 5750 - 5800 wot one person in the boat, 53mph GPS.

So my question for those who like to calculate, will it push it ok? What revs and top speed would I expect??
 
I don't know what numbers you will get, but I can tell you that I am running a Tempest 24p on my Z7 with a 150, and I am only able to turn 5250-5350 RPMs at WOT. That is right at the low end of Mercury's recommended RPM range. My guess is that 24 will be too much pitch for a 115 and your RPMs will be low.

Mike
 
It's a big jump I know, about 10% pitch increase. The actually model change has me more concerned as I'm just not familiar enough with the difference in the 2 models.
 
Thommo59,

Do you know if there is an rpm limiter on that motor? I have a 2001 175 Opti and they max out at 5500rpm. Based on your first message, I would guess that you are close to the maximum.

The final piece of information that is needed to do the calculations is the gear ratio of the lower unit. Typically they are 1.87:1

With that you can calculate the top speed of the boat with the following calculation:

(((pitch (inches) x max rpm x 60min/hr) divided by 63,360in/mile) divided by gear ratio) x (1-slip factor), where slip factor is the amount of slip on the prop (3% slip = 0.03 slip factor).

Adding more pitch without being able to turn the rpms will just bog down the motor and burn more gas while not improving performance (hole shot and speed).

Good luck...

Mike
 
Thanks mike,
I'm pretty sure it does have a limiter, and that's what's in the back if my mind, is the current pitch enough or am I hitting the limiter. Most limiters I have been involved with start to misfire when they ate hit and this doesn't happen, some limiters cut fuel, others cut ignition advance etc. my theory was maybe I'm on the limiter and without it I could turn more revs, therefore adding pitch would be beneficial
 
Thanks mike,
I'm pretty sure it does have a limiter, and that's what's in the back if my mind, is the current pitch enough or am I hitting the limiter. Most limiters I have been involved with start to misfire when they ate hit and this doesn't happen, some limiters cut fuel, others cut ignition advance etc. my theory was maybe I'm on the limiter and without it I could turn more revs, therefore adding pitch would be beneficial

Thommo59,

you might want to get the manual for that engine. It will tell you the max rpm. The opti's don't sputter, they just don't go higher.

There are places that let you try a prop. A good prop dealer will know your boat and motor combo and give you the right prop. This board has proved invaluable on such things as well.

Good luck.

Mike
 
We have no dealers locally with test props unfortunately, I tried that a little while ago. I'm pretty sure this engine has 5750 as the limit, that's why I want to find out if I actually have a little to go and by putting a little more pitch on, maybe I'll still get close to the top rpm.
 

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