Another prop question

[bigtahuna]

I have a '98 SF23 that has a 225 Optimax with a 17P Mirage Plus prop on it and I'm not able to get the WOT RPMs up to where I think they should be. My question is, has anyone run a 15P Mirage Plus on a similar setup or should I be looking at engine height or some other limiting factor? The cav. plate is just under the water when trimmed up. Thanks for any help.
Dave

[bj]

What RPM's are you getting with that prop? I would have thought that would be the correct pitch. I have a 2003 21' with 200 Merc that turns around 5600 rpms at WOT with that exact prop. It is very sensitive to trim as I can only reach around 5200 with engine trimmed all the way down and around 5600+ trimmed up. Have you tried trimming it up to see what your WOT is?

[bigtahuna]

The highest I've been able to get the RPM's is around 5200. I'd like to be able to reach 5600-5700 with a light load, then loaded it would be fine to only turn 5200 or 5300. I've experimented with the engine trim and the tabs, I have a digital Floscan and it's easy to tell when you've trimmed the motor correctly, the RPM's and boat speed go up and the fuel burn goes down, to a point, then it starts going the other way. Cool instrument, well worth the money in my opinion. Others on this site have said that the 17P is probably the correct pitch for this boat but I was wondering if anyone had tried a 15.

Dave

[Bushwacker]

. . . The cav. plate is just under the water when trimmed up.

Sounds like motor might be mounted a little too low. Where is cav. plate relative to bottom of hull? Can be about 1" ABOVE bottom of hull with a cupped prop. Most dealers mount motors with cav plate flush with the bottom, so you might be able to raise it 1 hole and then see how it handles regarding ventilation, etc. Need to get that sorted out before you go too far with changing props.

[bigtahuna]

The cav. plate is a little more than inch above the keel now.

[joshmon71]

Dont swap props till the engine height is good. When running on plane the cav plate should be just skimming above the water. NO tabs involved, WOT, trimmed down, start trimming up slow, when cavitates then back down. get a gander over the stern and see if cav plate is above water. You said it was below but might be misunderstanding. if you turn on plane, if you have a little bit of cavitation, but not when straight, that is about right for height as well. I have a mirage plus 17P not on it right now and the boat gets wot 6300 RPM, 1977 SC 23. Prior owner had some prop work done so it is not stock. I put on a merc rev 4 17P and get about 5850RPM. I may swap this out for a rev 4 15P though.

For fishing I want a 4 blade for low speed plane and better hole shot, but for going fast and efficient a 3 blade. No prop does everything its a compromise.

Sounds like you can bring the engine up 1 notch....having the cav plate under the water when running is a big drag. Not sure who rigged it, but mfrs seem to err on caution side of life and rig lower.

A 4 blade may allow you to go a bit higher, but in my case I swap props around so no dicey.

I might be selling my new this summer 4 blade Merc Rev 4 17P as well....prob not right prop for you though-you lose about 400 RPM I think from a 3 blade. Unreal hole shot and low speed planing, and better handling near the dock, def a big fan of the Rev4.

[pl99]

Hey how much ya selling prop for.. Chris

[eggsuckindog]

I would also move it up a bit to at least an inch above to 1 1/2 but that alone won't get you to 5600 - I would start shopping for a 15 to do what you want. That should give you right at 400 RPM and it works pretty darn good for a rule of thumb.
I would be interested in a 17 by the way

[bigtahuna]

If I pull a string line down the keel and adjust the motor so the cavitation plate is parallel with the string the plate is 1 1/8 inches above the keel line. If I raise the motor one hole, that will put the plate at 1 7/8 to 2 inches up from the keel, will that be too high? I don't know, I've never done this before. If I decide to sell the 17 Mirage Plus I'll let you know. Josh, I hear what you're saying about the four blade props, this boat had a four blade Merc. Offshore on it when I bought it. I liked the way it handled, especially in turns, but it was a 19P, way too much. I could probably sell that prop too. I'll have to do some more testing next time I get out to see if I can trim the plate out of the water flow.
Dave

[captbone]

I would go with the Enertia over the Mirage Plus. I switched from a 15pitch Mirage Plus to a 15pitch Enertia and picked up 2mph and 200rpm. You could most likely drop to a 16pitch Enertia (they make it) and get 400 rpm more from it over the 17 Mirage.

The Enertia is designed for heavy singles.

Just my 2 cents.