Prop Sizing - low WOT

[rocky388]

Currently I'm running a Solas Rubex 4 Blade Aluminum 14.25" dia. X 19" pitch. My WOT is 4150 as per the gauge, and GPS speed is 49mph. The boat is a 1988 Chaparral 2100sx with a mercruiser 5.7, I believe the boats weight is around 2800lbs from what I saw in a brochure I found online. The motors WOT range is 4200 - 4600rpms, so I'm looking to change props to get me where I need to be. After some reading, I'm thinking a 3 blade 14.25" x 19" pitch should add another 200rpms, bringing me to the lower spectrum of the recommended range. Like almost everyone else that owns a boat, I like a quick hole shot, but also I like top end speed. If I can get to 55mph with a quick hole shot that would be ideal.

Any recommendations? Should I drop 2" in pitch with a 3 blade to bring me closer to the top end of the WOT range? I'm not really considering any stainless options, as price is a factor.

Thanks

 

[steelespike]

I think your tach is wrong, reading low. at 4150 and 49 mph = about 1% slip basically not possible.
If we use a more normal 13%slip rpm is 4695.Your 4 blade prop should be giving you a good hole shot.
​ My info indicates 4800 as a max rpm, 4600 seems reasonable.
Hole shot and top speed really don't go together.
You need to varify your tach perhaps borrow a shop tach.
There is a switch on the back of the tach note its position and operate it a few times to clean the contacts.May help.
Don't make any changes until you varify your tach.

 

[KYsean]

If you find your RPM's are correct, a Mercury Spitfire 17Pitch would do you well with hole shot and top end.. Won't break the bank either.

 

[rocky388]

Thanks for the replies. I'll verify my tach before going any further. Is it possible for a tach to be correct at low rpms, but out at high? I verified the tach when I did a tune up (plugs, writes,dist cap, etc) awhile back. I will definitely verify again, but just wondering if such an instance is possible?

 

[Starcraft5834]

Rocky.. As Spike said.. best hole shot and maximum top end speed dont go together....ie- the 4 blade will give you best hole shot and many other advantages over a 3 blade. The only advantage a 3 blade has over the 4 is top end speed. IMHO... gaining another 1-3 mph by going to a 3 blade is not worth tossing all the advantages a 4 blade offers. If you have a heavy boat...and you at near 3000lb may suffer going to a 3 blade.. you've got plenty of engine there for sure,, also the 4 blade will be more gas efficient too...quicker on plane, maintain plane at slower speed. all help in gas consumption.. as to why your top end Tach is not where it should be.. assuming your tach works right, drop down in pitch. 1 .enjoy!