Pontoon Prop - pitch and cup change vs RPM; unexpected results

[jmoser]

I have a 21 foot pontoon with 60 hp Mercury 4 stroke BigFoot. Factory prop was 14" x 11 pitch; I usually got ~5400 RPM vs 6000 redline. Since we use this a lot for low speed trolling speed sunset cruising I changed to a BlackMax 14" x 10 pitch extra cup. I expected better low speed performance due to less 'lugging' of the engine but also expected to hit redline RPM at or near 6000.

The boat runs great at lower speeds just as I hoped but now I cap out at 5000 - 5200 RPM, appx 10% DECREASE not 10% increase. Does the extra cup really load the engine that much more so that a reduction in pitch from 11 to 10 still results in 10% RPM reduction ??

I have not checked top speed yet, have to use GPS as my boat does not have a speedo.

Overall I am happy with the change but I am a little surprised by the top end behavior.

 

[ufm82]

Re: Pontoon Prop - pitch and cup change vs RPM; unexpected results

Sounds about right- you should have gained 200-250 rpm with the pitch drop but the cup took that and more away. Cup can cost you a lot of RPM depending on how aggressive the cup is. My closest comparison is on a 18' bowrider we played with years ago. We took the 14.5 X 19 prop and had it cupped because we wanted a better bite and it seemed the prop slipped easily. (We were so stewpit back then...) and we lost 500 rpm in the process. The bite was phenomenal though. LOL Top end stayed about the same but we gained some bow lift.

UFM82

 

[Flhx]

Re: Pontoon Prop - pitch and cup change vs RPM; unexpected results

Is this the Black Diamond prop #QA2828X?
Thanks