Best prop for highest speed at 3000rpm 21p3blade or 18p4blade

[timothy2222]

I have a 19ft Larson with a 4.3 mercruiser.
The current prop is a 18p aluminum 4 blade.
It revs out to 4700 at about 39 mph.

I very rarely hold it pinned at 4700. Most of my work is done around 3000rpm. About 25mph.

I would like to get more speed at 3000rpm.

I am considering getting a 21p stainless would this prop give me more speed at 3000rpm or should I just stick to my 18p 4 bladed.

 

[steelespike]

Re: Best prop for highest speed at 3000rpm 21p3blade or 18p4blade

Don't keep the year of your rig a secret. assuming a 1.80 gear ratio your figures are right on.
Going to a 21 ss will lower your rpm about 400 to 600 rpm easily putting you below the minimum. The usual result would be slower hole shot,the boat would struggle to stay on plane at reduced throttle.While a ss prop will usually out perform an al prop of the same demensions. When you step up 3" it will lower rpm.
The 21 might produce more speed but at what throttle setting.It will be harder to push the boat thus you will likely end up with a higher throttle setting.
I think your better off with the 4 blade 18. You might consider a 4 bld 18" ss prop.
Keep in mind your al 4 bld is doing nicely and when you change material the performance becomes more unpredictable and while you may get a slight improvement will it be worth the investment.