How to cup prop and will it lower wot rpm

[harrisg]

I have a volvo penta 270 outdrive/aq125a engine with a 14x19 aluminum prop pushing a 16ft Invader fish/ski boat. I'm currently running at 4400 rpm and get up on plane really quickly. <br /><br />Would cupping my prop do anything for me<br /><br />How does a person cup their prop and where would I find info about it <br /><br />and, does cupping change the wot rpm

 

[Silvertip]

Re: How to cup prop and will it lower wot rpm

Check the prop to make sure its not cupped already. If not, you can't do it yourself. It can be cupped by a prop shop. As for "would it do anything for you" it depends on what you want to accomplish. It may allow better high speed turns with less ventilation (hangs onto the water better). It might slow you down some or it might speed you up some. It might increase hole shot some. My suggestion would be to talk with a prop shop or someone with the same vessel as yours and see what they are running. If you are happy with current performance, don't mess with success as it seems the setup is about right at this point. Any improvement from cupping would likely be relatively minor. The down side is it could upset performance to the point where you need to replace the prop.

 

[cm5408]

Re: How to cup prop and will it lower wot rpm

i own two of the exact same props for my 285 outdrive (volvo) that are made by volvo and are not cupped. i was constantly complaining about how the boat came out of the hole (seemed like the prop was slipping) and that the low pitch was creating WOT to be too high.

So, one day i took the boat out to the lake and ran her around for a bit. Took the prop off (while in water) and cupped it myself with a mini sledge and a block of wood on the back to control the curve that i was putting in. Put the prop back on the boat and loved the difference. I instantly noticed the hole shot difference, WOT came down about 300 rpms and actually speed went up 3 - 4 mph. I know this may seem like a joke, but i am telling the truth. I ran this self cupped prop for about 3 weeks and then took it to a prop shop to have it balanced (was not vibrating, i just wanted to check). When i got it back from balanceing, the guy corrected the differences in the cup that i put in each blade. For the rest of the summer the avg speed was 4 - 5 higher and the hole shot was better (not perfect, this wasnt a miricale worker just a better fix). I also noticed that in turns while above 40 mph, the boat felt like it was driving better in direction as apposed to "sliding".

Prop cupping has been around for a long time and was actually discovered by the aero industry in airplane propellers. It is a shame that with the older volvo props, that they were never cupped.