Prop Change Caused HUGE drop in RPMS

[HoosierDaddyo]

Good morning!

Here are the specs prior to the SS prop purchase:

OEM Aluminum 14.25x21P
WOT 4700 RPM with gear and four people weighting approx. 550 total
Mercruiser 43.L Thunderbolt Ignition (180HP?)
2002 Larson 210SEI approx. 4500lbs dry.

Here is what I have with the SS prop on the boat:

Solas Titan 3 14.5x21p
WOT 4000-4100 RPM with gear and four people weighting approx. 550 total
Mercruiser 43.L Thunderbolt Ignition (180HP?)
2002 Larson 210SEI approx. 4500lbs dry.


Should there be that big of a drop in RPM's moving up a 1/4" in prop size but staying the same pitch?

I'm baffled and I can't seem to find a 14.25x21 SS anywhere!

Thanks

 

[tpenfield]

The diameter change is only going to account for about a 4% reduction in RPM, whereas you saw about a 13% drop in RPM. So, the material change (from AL to SS) accounts for most of the change.

Stainless steel props have thinner blades and cupping, so they take a bigger 'bite' of the water vs. an aluminum prop.

You did not mention the speed change . . . :noidea:

Rule of thumb is to drop a notch in pitch when going to stainless, or going up in the number of blades, or increasing diameter. Right now your thumb must be hurting . . .

 

[Texasmark]

Adding to that SS usually incorporates Rake, Progressive Pitch, and Cupping, all described if you go to the top of this view and get in the prop section and look at definitions. For what you are doing, those attributes aren't necessary...including the thin blades, but for a guy/gal and their pal in a Bass Boat running 65 MPH they are necessary and very desirable. That's why folks caution....when you change pitch, stay in the same brand/type of prop or reconsider your numbers. I realize you didn't change pitch, just wanted a SS prop.

I suggest you remove the SS now for safe keeping.....keeping it in pristine order. Find yourself a prop shop that will work with you in finding what will work for you. Work an exchange for your current SS on his new one....save you just putting 500 bucks up on a shelf and collecting dust.