Going from alum to S.S. prop

[Daddy O]

If I go from a alum. prop to a S.S prop with the same numbers. Will the rpm's go up or down.

1991 Rinker 181
4.3L alpha 1 gen II
both 19 pitch prop.

 

[walleyehed]

Re: Going from alum to S.S. prop

If the SS prop has hgiher tech features, it may run higher RPM due to more lifting capabilities, which will equate to more speed.
If the hull does not react in a positive manner to the extra lift, it will likely be just a tad faster at mid-range, but lack about 100RPM from the alum prop.
Speed will likely be about the same even with the 100RPM drop due to a more efficient design at the same RPM..
Alot depends on the SS prop choice.

 

[Daddy O]

Re: Going from alum to S.S. prop

Thanks for the info.

Here's what seems to be happening to me. With what I believe is a Michigan Prop Alum. 78120-a40-19p. The boat would go between 45 and 50 mph @ around 4800rpm. This is by the boat tach, and speedo. This was last summer. I didn't tune the boat up this year.I did replace the u-joints and gimbal bearing. At the end of last summer I came across a SS Prop.48-73980a-4-19p.
This summer with the S.S prop my rpm's go over 5000 rpm and only about 40 mph. I backed off since rpm is maxed out at this point. I go back to the Alum and can get up around 45 mph and appox 4800 rpm. Is there something wrong with the S.S prop or is what you suggested applying at this pointed.

Thanks again for you input.

 

[walleyehed]

Re: Going from alum to S.S. prop

I would venture a guess and say the SS has either a different blade design, or has been worked to increase RPM which is one of the no-no's of working a prop to a lower pitch.
The number you give for the alum prop is a Merc number.

 

[Daddy O]

Re: Going from alum to S.S. prop

Thanks for the input. I think I figured out what is happening. I put the S,S. prop back on for Labor Day weekend, because I liked the hole shot better. Just didn't go to WOT.
Anyway the prop might have been slipping. When I installed the prop this last time. This time I tightened it as tight I could .
At the end of the day I did open it up while watching the rpm. The boat ran around 45 mph at 4800 rpm. Which is around the same as the Alum. prop I believe after a tune up, it will run even better.oo

 

[walleyehed]

Re: Going from alum to S.S. prop

shouldn't have any hub slip, even with the nut loose...
Could be as simple as temp change too.

 

[Daddy O]

Re: Going from alum to S.S. prop

Thanks for all your help Walleyeshed.