Prop change on 2018 G21

[Az182]

Hi,
We’ve owned our 2018 G21 for a couple of years now. We surf 70%, wakeboarding 25% and cruise only 5%. We only run a light crew of 3 with stock ballast and our lake is at sea level. We’d like to change the prop in order to reduce the engine rpm at surf, wakeboard and cruise speeds. I’ve been in touch with ACME and they have suggested changing from the stock #2561--17.00X17.00VL4B 1.25.105 CUP to #2749--17.00X14.00VL4B1.25 .105 CUP. With the stock prop we’re surfing at 11.4mph turning 3250rpm, with the 2749 ACME have said we should see a reduction of 240-300 rpm which should hopefully make a noticeable reduction in sound levels on the boat, which our main concern.
Has anyone else changed props on a G21? I’m looking for recommendations before going ahead and ordering.

Thanks

Aaron

[charlesml3]

Before you go spend a lot of money on a prop:

- Get out on the lake and setup for surfing. Ballast, speed control, everything.

- Run up to your normal surf speed.

- Tap the speed control down until you shave off 300rpm (Ignore the speed of the boat).

Is there any real difference in the sound levels?

-Charles

[BasaltRMK]

We ran the 2749 on our 2018 G21 with the H6, at ACME’s suggestion, but our home lake elevation is 8,000’. Worked perfectly for us, not sure how it would be at sea level. Curious to hear what you decide and how it works out.

FYI - ACME has suggested the 3 3/4” shaft version of the 2749, the 3477, for our 2020 G21 with the ZZ-6.


2020 SAN G21
2018 SAN G21 (traded in)
2015 Tigé R20 (sold)
1972 MFG Gypsy-15 (sold)

[greggmck]

Hi,
We’ve owned our 2018 G21 for a couple of years now. We surf 70%, wakeboarding 25% and cruise only 5%. We only run a light crew of 3 with stock ballast and our lake is at sea level. We’d like to change the prop in order to reduce the engine rpm at surf, wakeboard and cruise speeds. I’ve been in touch with ACME and they have suggested changing from the stock #2561--17.00X17.00VL4B 1.25.105 CUP to #2749--17.00X14.00VL4B1.25 .105 CUP. With the stock prop we’re surfing at 11.4mph turning 3250rpm, with the 2749 ACME have said we should see a reduction of 240-300 rpm which should hopefully make a noticeable reduction in sound levels on the boat, which our main concern.
Has anyone else changed props on a G21? I’m looking for recommendations before going ahead and ordering.

Thanks

Aaron

Something is wrong. Going from a 17 x 17 to a 17 x 14 prop will INCREASE RPM as the lower pitch will provide less load on the engine. As a result it must turn faster to achieve a similar speed as the 17 x 17. I have never tested a prop on the G21 but I have tested several props on the G23. If you have the H6DI Engine something like a 17 1/2 x 17 1/2 would reduce rpm. It will reduce sound levels too as I measured this with a sound pressure meter and reported the result to ACME. The noise reduction at surf speeds were noticeable.

To reduce the RPM you will need to INCREASE prop diameter and / or pitch. The 17 1/2 x 17 1/2 was a bit too much prop for my heavily loaded G23 and resulted in greater fuel burn but I believe it would be ideal for a lighter G21.

You should see this thread: https://www.planetnautique.com/vb5/f...-ballast/page2https://www.planetnautique.com/vb5/f...-ballast/page2

[scottb7]

Before you go spend a lot of money on a prop:

- Get out on the lake and setup for surfing. Ballast, speed control, everything.

- Run up to your normal surf speed.

- Tap the speed control down until you shave off 300rpm (Ignore the speed of the boat).

Is there any real difference in the sound levels?

-Charles

Good advice!

[Az182]

Thanks for your replies.

Yes dropping 300 rpm does make a big difference in sound levels. With the ballast empty and 2 people in the boat the rpms are much lower, around 2800-2900 from memory, the engine sounds much more pleasant to me here. We’re actually 500 miles away from the boat and only use it on holidays so I’m unable to test exactly as you suggest until we’re up there in 3 weeks time. Also we’re in the U.K. so we’d have to import the prop from ACME which makes returning it if they send the wrong one a pain. Our engine is the 409

[greggmck]

As I said, I really noticed the noise reduction from the lower RPM when surfing behind the boat the most. The music was more clear and it was a bit more peaceful.

[Az182]

Yea the results look good. I’m going to get back in contact with ACME now and query the prop suggestion based on your pointe above. I asked them about the 3081 which seems a good option on the 23 but apparently it will not fit the 21. Do you find the boat cruises better too?