Help, need the prop experts again

[chucksorce]

This is the 2nd post about my performance problem with my boat so ill try to make it short, my boat is a 04 22'crownline
CCR cuddy with a 5.0mpi alpa 1 21p alum prop, 39mph per gps @ 4000rpm WOT. My boat wont get up on plain to save a life so I took the information I got from my last post and purchased a new 17p 4 blade mercury spitfire prop, with the new prop my boat wont go fast enough to plain off, it just over revs the engine so should I try a 19p spitfire?

 

[Chris1956]

Something is wrong. A 17P prop should easily plane that boat. You should plane at about 20MPH or maybe even less, and that prop should reach that speed easily. Your max RPM should be about 4800RPM, and top speed should be in the low 30s, I would think. Maybe the prop is spun? Maybe there is some ventilation issue?

 

[steelespike]

First I would put the 21 back on and see if it does the same. I've seen two independent tests of the Spitfire and they are close to amazing.
I'm thinking the Spitfire mary be defective. Normally if your 21 was at the correct rpm you would swap the same pitch in the spitfire.
But needing 800 rpm a 17 seems reasonable. The typical 5.0 4 blade is at least 14" diameter some approaching 15"
I wonder if your prop may be intended for a outboard using a smaller diameter prop.
Does it meet up with the gear case correctly?
​

 

[KYsean]

I would make sure the prop and hub are on correctly. If you can, drive by your local Marine service and ask a tech to check the prop/hub assemb. I wouldn't imagine they would have any problem checking for you. It's to important not to get a second pair of eyes on the problem.

 

[chucksorce]

the prop im using is 13" in diameter, could this be my problem? And when I went to the prop selector on the mercury website the spitfire was not one of the recommended props for my boat, I choose it because I wanted to keep as much top end as I could.

 

[MikDee]

First I would put the 21 back on and see if it does the same. I've seen two independent tests of the Spitfire and they are close to amazing.
I'm thinking the Spitfire mary be defective. Normally if your 21 was at the correct rpm you would swap the same pitch in the spitfire.
But needing 800 rpm a 17 seems reasonable. The typical 5.0 4 blade is at least 14" diameter some approaching 15"
I wonder if your prop may be intended for a outboard using a smaller diameter prop.
Does it meet up with the gear case correctly?
​

I second this opinion, I think 13" Diam. is a bit small, more like an outboard prop, not one for a 22' boat? 14" to 15" Diam. is the norm for this. If your hub is too small for the gearcase, then exhaust gasses are escaping, & flowing over the prop, defeating the normal performance.

 

[KYsean]

the prop im using is 13" in diameter, could this be my problem? And when I went to the prop selector on the mercury website the spitfire was not one of the recommended props for my boat, I choose it because I wanted to keep as much top end as I could.

Sounds like it is a problem but still sounds like there is more to the story. Prop should be around 14.25 dia. I still think there is something not right with the hub setup or shaft. I would highly recommend taking it in. When all is said and done, what ever 3 blade prop works with your boat to get proper RPM's at WOT, you would stay with the same pitch in the Spitfire.

 

[MikDee]

the prop im using is 13" in diameter, could this be my problem? And when I went to the prop selector on the mercury website the spitfire was not one of the recommended props for my boat, I choose it because I wanted to keep as much top end as I could.

Yes, That's what I'm saying, maybe it's an outboard prop, and that's why it's not recommended for your boat.?

 

[chucksorce]

OK, thanks for the feedback everyone