prop swap, different diameter problem?

JGus81

Recruit
Joined
Jun 8, 2010
Messages
3
I have a 78 seray 18' with mercruiser 140 the boat weighs around 2500 lbs. Just bought the boat it had a 15x17p prop on it but it was in bad shape. The guy gave me a 15x19p prop that he said he put on but it never ran as good so he put the other back on. I changed the prop then took it out on the lake the boat took a long time to get on plane, and when it did rpms at wot were only 3700 @ 28 mph (speedo not gps). Would like to put on a 17 pitch to try, my dad has a 13.25x17p he would give me but what would the smaller diameter do? Also the outdrive has a "doel-fin" on it should i take that off? thanks guys
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: prop swap, different diameter problem?

Most likely the 13 1/4 is for a smaller gearcase.Was the 13 1/4 On an I/O?
I feel you have more problems than a prop.Your setup with a 19" prop should run
in the low 40s plane easily and run near or above the max rpm rating.
Check that the throttle plates are opening all the way.Be sure its running on all cylinders.The bottom is clean and smooth.Do a compression check,note the position and condition of the plugs.
 

JGus81

Recruit
Joined
Jun 8, 2010
Messages
3
Re: prop swap, different diameter problem?

Finally got my new prop and was able to take it out. I bought a turning point legacy 14.5x17 alum. I had to buy another trim tab because the rake on the prop was hitting the tab. But alas, the gains were exactly what I hoped for the rpms are now up to 4200 top speed 38 with out even touching the trim. Well worth the 110 dollars spent+25 for the tab.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: prop swap, different diameter problem?

Is the trim broken?It would be interesting to see the results with trim.
 
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