Non-cupped prop to cupped prop: What does that do to RPMs?

[SuperPickle1967]

In really have a fine running boat. She's a 502 Mercruiser MPI in a Formula (Bravo One 1.5) and pushes along at about 4500 RPMs on a 25 pitch prop at WOT. She is crisp and sharp and never lets me down. Tuned up. Runs like a top.

The boat was surveyed and the surveyor advised me to 'get a more modern prop'. He mentioned better raking and cupping found on modern props will do more to lift the bow and prevent blowout in turns.

I also need a smaller prop, because I've found other owners of this boat and they are on 23p props spinning from 4800 to 5100 depending on load. One guy runs a 24p 4-blade. Time and time again, the stock setup I find is the 23p prop. Since the boats are all mid 90's models, the stock was non-cupped 23p prop.

If I drop from a 25p non-cupped prop to a 23p cupped prop, what's the overall effect on RPM going to be? Doesn't adding cupping add bite (resistance) and knock down some RPM? I don't want to go from an old 25p non-cupped to a modern 23p cupped and find the drop in pitch (which should yield more RPM) was countered by the cupping.

(The reason she has a 25p prop now is that the original 23p prop was stolen and the previous owner got by with the 25p prop he already had. He sold the boat because it [replacing the prop] was just one more expense he couldn't manage).

 

[steelespike]

Re: Non-cupped prop to cupped prop: What does that do to RPMs?

I can't be really specific;hopefully someone with more expertise will be along. A cupped prop in the right pitch will out perform your present prop. What is the max rated rpm?What is the hp?We need the specifics on the boat. you may have to find a dealer that will let you exchange props till you find the right one.If your sure these others are running non cupped props then it would seem you would need to go lower in pitch I have also seen a little cup removed to increase prop performance. Another words if you got a 23 cupped and your rpm was a little low a prop shop could remove a little cup and it would increase both rpm and speed.In testing various props try to stick with the same brand and model.

 

[SuperPickle1967]

Re: Non-cupped prop to cupped prop: What does that do to RPMs?

4900 lb dry weight. 80 gallon tank. Figure two passengers @ 300 lbs total combined and I don't carry lots of junk.

About 27'. Deep, non-stepped V hull. 23* deadrise. With 2-3 people and 3/4 tank the top speed is 59/60 at WOT at 4500 RPMs (a tick below the max operating range of 4600-5100). Other guys are getting 63, 64 at WOT, 23p Mirage, 4900-5000 RPM.

I'd rather be near 5k RPMs at WOT and a light load.

 

[steelespike]

Re: Non-cupped prop to cupped prop: What does that do to RPMs?

Maybe I'm over simplifying but it sounds to me you need a 23' Mirage prop.
If you have the possibility of gaining 4-5 mph with a simple prop change thats a pretty safe gamble.

 

[SuperPickle1967]

Re: Non-cupped prop to cupped prop: What does that do to RPMs?

I think that is the answer. Yeah, keep it simple silly (KISS), right?

Basic (non-cupped) 23p Mirage is going to give me the extra few hundred RPM and slight speed boost at WOT. Engine won't be lugging and she'll be in upper part of operating RPM range lightly loaded, they way it should be set up.

Although, then I don't get to enjoy the cupping benefits unless I get it customized by a prop shop. Q: I could ask them to add cupping (to gain bite) and to maintain my RPM (in which they take off slight am't of pitch)? Do-able, correct?

I am guessing a 21p cupped prop is just gonna be too small.

 

[Philster]

Re: Non-cupped prop to cupped prop: What does that do to RPMs?

The drop in pitch will get you more RPM. The cupping on a Mirage Plus might rob some of that RPM gain, but not all. The cupping will return some bite and you might be able to trim up a touch more, getting you the top speed back the boat had stock and saving 100-200 RPM (but still be in the max operating range, so as to not lug the engine).