3988 prop size

[CBAB]

We have had our 3988 with Cummins 330's for nearly a year and have done all the checks, cleaning, and adjustments and now believe the engines are doing their best. But at wide open throttle we get 2700 RPM max and 25.2 knots with 22" Diameter x 22" Pitch x 4 blade Nakashima props. So, it looks like we need to re-pitch to get another 200+ RPM. But before we do that, I am wondering what size props other 3988's with Cummins 330's have and what is their max wide open throttle RPM and boat speed.

[Uncle_Bob]

You are rated for 2800 WOT rpm. Purists will say you want more than the rated RPM, but if you are already loaded with fuel/water and boat junk and achieve 2800, you're doing well. You will lose some top end speed as you go down in pitch. To be sure of your RPM you need to know if your tachs are accurate by using an external tach.

[The_Other_Gary]

I have 22x21 Nakashima props for salt water but my normal props in freshwater (5% less dense) are 22x20 Michigan Hi Skew. I am fully loaded as a liveaboard and reach 2875 at WOT.
Your engines will thank you for the reduction.

[guntar]

It is extremely important to properly prop your yacht. You should get max RPM or better with average loads. If you don't then you are really hurting your engines and life expectancy. I know all the pain from that when we bought our 3988 and found out the hard lesson as the prior owner ran the boat with wrong size prop and pitch and cracked the heads on both engines from loading. Of course this didn't show up on survey. I had engines rebuilt and new props installed. 22x20 now.

[CBAB]

Well, I had hoped to hear from a few more 3988 owners just to know that I am in the same range as other 3988s with 330 Cummins. But the above responses of 22D x 21P and 22D x 20P give me a target range.
In the interim I had a prop analysis by Michigan Wheel and they say that a 22D x 21P should be about right. My uneducated reasoning is that a 1" decrease in pitch (5%) should give me a 5% increase in RPM which would put me above the 2800 RPM rating and also falls in the same range. I would probably prefer a little higher RPM but Michigan Wheel also tells me that my current props have already been re-pitched with a 1" reduction and the advice seems to say that re-pitching more than 2" is not wise. So, the next time I haul out I'll re-pitch to a 22D x 21P. Thanks for the replies.

[noah2858]

I have a 2000 3988 330 Cummins I repitched from 22d x 22 to 22d x 21 wot went from 2750 to 2850 top speed went from 26kn to 24.5kn salt water. the boat is heavy loaded weighs about 30000lbs. I think you are on the right train of thought at 21 pitch.

[CBAB]

I decided to repitch the props to 20.5" (7% reduction) just to be conservative. So, now at WOT I see 2900 RPM and 25.4 kts. I also find that it accelerates to top speed much quicker than before.