19 Alum vs. 21 SS

[thejetman]

Heres the skinny.

93 Wellcraft Excel. 4.3 Volvo Outdrive.

It currently has a Solas 14.5x19p Aluminum prop on her. Got a good deal on a Solas 15x 21p New Saturn prop.

I am trying to bring my rpms down about 600-800 rpms. You guys think this should dial me in pretty good? Its wicked outta the hole but overrevs. Confirmed that the onboard tach was accurate last weekend, and that you could push the motor to 6000 rpms without watching. I can hit 44-46 witgh just me on board, half tank, calm seas right now, but thats at 5600 rpms.......So assuming everything is correct, the new prop should give me a higher cruise speed while bringing the rpms down?

If all else fails, I will have one tube/ski prop and one "haul ***" prop, right?

Any thoughts?

 

[Robbabob]

Re: 19 Alum vs. 21 SS

Depending on the cup of the SS, I see up to 500-600 rpm drop; 400 for the 2" change in pitch, another 50+ due to diameter increase, then the added grip/cup of the SS should lower yet again. Don't think you'll get the 1,000 reduction you're looking for. Can you be trimmed too far?

Just a thought or two.

Good Luck!

 

[thejetman]

Re: 19 Alum vs. 21 SS

Well it will bring me closer to where I need to be, And I'm ok with that. She runs great I just want to get everything out of her. I am just hoping to get a higher cruise speed. Outta curiosity are you part of the "VOL NAVY"?

 

[Robbabob]

Re: 19 Alum vs. 21 SS

Well it will bring me closer to where I need to be, And I'm ok with that. She runs great I just want to get everything out of her. I am just hoping to get a higher cruise speed. Outta curiosity are you part of the "VOL NAVY"?

Not heard of the VOL NAVY, so I guess I'm not

Enjoy the ride!

 

[thejetman]

Re: 19 Alum vs. 21 SS

I live over here in Knoxville, and when the Vols play, everyone brings there boats to the Calhouns on the river by the stadium. Its pretty cool really. Its like tailgating, but better!

 

[bob johnson]

Re: 19 Alum vs. 21 SS

well I have this notion that the ss 21 will act almost just like the 19 aluminum...

I had a 17 pitch aluminum prop and was running 5400 rpms, and I installed a SS 15 pitch prop and the motor turned 6100 rpms!!!!! telling me that SS props of the same pitch will turn higher rpms than aluminum.... I do confess i dont know how the blade number would effect the situation...I cant beleive the .5 dia difference means anything

bob

 

[Robbabob]

Re: 19 Alum vs. 21 SS

Jetman,
I see what you mean... we boat in the Chattanooga area, but can certain understand how that "boat-gate" would be so much fun!

Bob,

The 700 RPM change you saw is quite high for a 2" pitch change. However, depending on the model and quality of the AL design v the SS design, it's possible. Going up in Pitch would have reduced the rpm.

Happy boating to all!

 

[thejetman]

Re: 19 Alum vs. 21 SS

Lol, boat gate! Well, hopefully my new prop should be on my doorstep in the morning! I plan on trying it out this weekend. I will report back and tell ya how it is. I did pick up the missing line cutter/spacer for it today too, should get rid of the 1/8 inch of play in it too.

 

[thejetman]

Re: 19 Alum vs. 21 SS

Update.
The boat ran great!!!!
With 7 people on board she would do 40 at 5100 rpms. It could still pull a tube with a full tank too! Later on when it was just my lady and I on smooth water I was able to hit 48 at 5100 rpms via GPS. It brought the cruise speed up to 33-35mph at 3500 rpms. So it was a success. I went from a 14.5x17p Alum to a 15x21p SS prop, no loss on low end, got higher cruise speed and an overall smoother ride.

Congratulations men, Mission accomplished.

 

[Robbabob]

Re: 19 Alum vs. 21 SS

Update.
The boat ran great!!!!
With 7 people on board she would do 40 at 5100 rpms. It could still pull a tube with a full tank too! Later on when it was just my lady and I on smooth water I was able to hit 48 at 5100 rpms via GPS. It brought the cruise speed up to 33-35mph at 3500 rpms. So it was a success. I went from a 14.5x17p Alum to a 15x21p SS prop, no loss on low end, got higher cruise speed and an overall smoother ride.

Congratulations men, Mission accomplished.

Jetman,

Great that you made an adjustment and came back with the update! Also, happy to see the performance where you want it. Still a little afraid of you hitting that 5100; think you should be a tad lower, but that is the deal you had in front of you. Be nice to that engine and it will give you plenty of years of performance.