Propeller change

SPC028

Recruit
Joined
Oct 16, 2017
Messages
3
I have a 1974 sea ray 20' with a pre alpha drive originally it has a 302 and I put a 351w in it went to break it in and went WOT only hit 3500 rpms at 36 mph I can't find on the prop or hub the pitch or diameter I don't know if alpha one props will work I want to up the pitch and keep the same diameter How do I find out what pitch it is
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,092
and went WOT only hit 3500 rpms at 36 mph

putting a bigger pitch wont make you go faster or increase your WOT rpm. For most V-8`s the 14 x 19 is a starting point
 

wahlejim

Master Chief Petty Officer
Joined
Jul 23, 2015
Messages
884
This is one of those instances where finding a reputable place of business is worth it. You will have better luck dealing with a local prop shop dialing this in. They may even let you try a few rather than stabbing in the dark and buying online to save a couple bucks.


First step would be to get good data. RPMs from a shop tach, not your boats dash. Laser tachs work well too. You will also need your gear ratio and GPS speed, not boats speedometer.

The shop will be able to figure out your pitch and prop slip, recommend different props to try, etc.
 

harringtondav

Commander
Joined
May 26, 2018
Messages
2,438
This is one of those instances where finding a reputable place of business is worth it. You will have better luck dealing with a local prop shop dialing this in. They may even let you try a few rather than stabbing in the dark and buying online to save a couple bucks.


First step would be to get good data. RPMs from a shop tach, not your boats dash. Laser tachs work well too. You will also need your gear ratio and GPS speed, not boats speedometer.

The shop will be able to figure out your pitch and prop slip, recommend different props to try, etc.




Very good advice. My short answer is to try a lower pitched prop until you get close to 4800 rpm @WOT. This will help determine if you have a power problem. I guessed your prop's pitch @ 20" using a web calculater.
 

SPC028

Recruit
Joined
Oct 16, 2017
Messages
3
I will try a local prop shop I'll see what they recommend the 302 that was in it had a cracked block so I swapped with a new 351 I know its not a fuel issue the 4 barrels are open I just thought I could get more speed it had 188 hp I put in 260 HP and only 36mph
 
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