prop size

[rockyman]

i have a 19.5 1981 galaxy 165 inline 6cyl mercruiser motor with 1978 165 mercruiser outdrive.my question is this, i changed my 19 pitch prop to a 21 pitch prop but the 21 pitch prop is smaller in diameter than 19 pitch by about half inch, what will that change and how will it change anything .the reason for the change was with the 19 my wot was about 5200 rpms which is to much i know, with the 21 it should put me around 4300 rpms which is where it be . is there a potencial problem with this change. thanks

 

[hwsiii]

Re: prop size

Rocky you will find out that changing from a 19 to 21 pitch will normally only bring your RPM's down by about 400 RPM's, not 900 as you are assuming. 1" change in diameter changes about 400 RPM's and 1" change in pitch changes RPM's about 200. Hope this helps you.


H

 

[steelespike]

Re: prop size

Is the 19 in excellent shape?
If the 21 is listed for your setup the diameter difference is simply a product of the design.You need to check that the tach is accurate.It may be set for a 4 cylinder.

 

[rockyman]

Re: prop size

with the same size and diameter 21 pitch prop ,what would change if i went to a stainless steel prop

 

[Randybeall]

Re: prop size

Change to Stainless is a whole different ball of wax. Since stainless is so much stronger it can be thinner and in more complex shapes. That means that the average Stainless prop will move more water in the same size prop than an alumnium. So.....the stainless prop will drop the engine rpm a little more than the alumnium, but there is no rule of thumb as to how much. You just have to try it on your application and find out. Many suppliers have an exchange program available so you can return the prop if it is not quite right and get the correct pitch in exchange for freight charges only. Check with your supplier, I know Iboat does this. The 21 SS will likely be the prop your looking for.