Prop cupping question.

[Gary H NC]

I have 2 different 19 pitch SS props.
One is the factory Johnson prop and one is the Stilleto AdvantageII
Both are very similar with the aftermarket prop being high polished and the stock prop is more of a brushed SS prop.
I noticed the Stilleto has a good bit more cupping in the blades.
Will this affect WOT rpms? more cupping less rpms?
Just wondering being i have not really run the original prop but the Stilleto will only let me reach 5100 rpms at WOT. I'm thinking of trying to trade one of them for a 17 pitch.

 

[nlain]

Re: Prop cupping question.

Cup will affect wot rpm, I recently change props, aluminum not stainless, Mercruiser 14 x 23 to Turning Point 14 1/4 x 23, Merc went slightly above wot rpm, TP would not let me get to wot rpm, I contacted TP and they swapped for 14 1/4 x 23, which I have not been able to run yet. The more cup the more bite or less slip the prop has, the TP prop gave about 4 mph increase at normal cruise rpm but killed the wot rpm.

 

[Dhadley]

Re: Prop cupping question.

Remember - going down in pitch to simply gain rpm is a last resort. That generally reduces efficiency.

Cup does several different things depending on where it is and to what degree. In the 2 props you are comparing the Stiletto will run at a higher X dimension that the Johnson prop. If you run them both at the same motor height the Stiletto should grab or bite harder and therefore allow less rpm.

The additional cup at the tip of the Stiletto not only lets you run a higher X dimension (less drag) but it should have more lift. More cup, in the right place, produces lift which reduces wetted hull surface (again less drag) which produces more rpm.

 

[Gary H NC]

Re: Prop cupping question.

Good info..
I can trim full out without any prop blowout with the Stilleto.
Maybe i should look at raising the motor one more hole and testing it again before i start changing pitch..

 

[Dhadley]

Re: Prop cupping question.

Very general rule of thumb (not an absolute) - if you can use 95% + of your trim range, that prop is running too deep on that rig.

 

[Gary H NC]

Re: Prop cupping question.

Thanks DHadley,Yea i can use all of the trim without blowout so its time to do some measuring..
I raised the motor one hole before.The previous owner had it as low as it would go.
Something else for the winter project list...

 

[Dhadley]

Re: Prop cupping question.

Time to consider adding "the button"!

Actually I know that's probably not in the budget but here's something to ponder - if you're looking for bow lift, and most of us are, some sort of setback will certainly help. Setback in a manual plate sure makes playing with props much easier. You'd be surprised how much difference 5" of setback changes the fulcrum point.

 

[Gary H NC]

Re: Prop cupping question.

Yes,I would love to add a setback plate....I have been looking at the manual operated jack plates....Not in the budget right now but by spring maybe.
If i go with a 5 or 6 inch thick jack plate i could get away with the same cables and rigging without changing anything.
It sure would help dial the motor height in...

 

[Dhadley]

Re: Prop cupping question.

I really like the Bob's convertable 4-n-1 but they've come out with a new deal. It's a hydraulic plate with no hydraulics. It has a manual adjustment and when (if) you decide to go with a "button" all you do is add the hydraulics. I thought it was a pretty good idea.

 

[Gary H NC]

Re: Prop cupping question.

The Bobs Machine are the ones i have been looking at..
Maybe Santa can give to the jack plate fund..

 

[bowman316]

Re: Prop cupping question.

If a prop is cupped, add one inch of pitch to it. Even if it says it is a 19, if it is cupped it will perform like a 20.

 

[Dhadley]

Re: Prop cupping question.

Not necessairly. Not all cupping is created equal just like not all 19" pitch props are created equal. It depends on how much cup, what kind of cup, location of cup and how it all ties in with blade design (rake, progressive pitch or not, etc)

 

[Benny1963]

Re: Prop cupping question.

hello guys gary hadley been watchin this one because im trying to tune my
mckee with 175,need something with stern lift that would mean some cupping right? wanted to run 21p cupped im currently runnin a 21 no cup
but want to come up on engine height ,is aggrsive 21 cupped capable of running high on boat and pull the same rpm as my old 21
man im confuzing my self know
gary and all happy holidays

 

[bowman316]

Re: Prop cupping question.

you will loose rpm's if you go to a cupped prop, everything else being equal.
I don't think it will nessecarly give you any more lift.

 

[Dhadley]

Re: Prop cupping question.

Actually you may gain rpm if you use the cup properly. Depending on where the cup is and what the boat requires, cupping can produce more lift which decreases drag. If the boat needs bow lift the cup goes in one area. If it needs overall lift the cup goes in another area.

We've been thru this more than once. A guy has a (for instance) a 19" prop and needs more rpm. He goes and buys a 17 simply to get more rpm but loses speed. At this point we put on a quality 19" prop and change the set up to take advantage of the technology built in the prop. In the end he can turn more rpm with a 21 and get more speed. At this point it's likely the holeshot improves too.

If the boat needs help with bow lift you can do that with set up and cupping in a specific area of the prop. If the boat has natural bow lift, cupping in another area will produce overall lift which equals more speed.