[karmann]

Hello All.
Application 95 barchetta z272 454 MPI 385 HP
Current prop that seems to work fine is a michigan wheel 15 1/4" x 21" - 3 blade SS
I want a spare, also would like to change to a 4 blade.
The props that are available are 14 1/4" dia, X 21 or 20.
My current prop seems to have plenty of power ( could use a little more dig) and has a max rpm of 46 to 4800 rpm. @ 53 MPH

I read where you should drop down a number on pitch when adding the 4 blade.
So my question is 20 or 21" pitch?
thanks for any help.

[mikeyt]

A 4 blade 20" will add about 200-300 rpm at WOT and knock about 2-3mph off your top end but...you will get the added low - mid range punch you are looking for. A 21" 4 blade will have less of the same effects.

[indykoch]

I'd have to question the drop of 1" in diameter though. My thoughts are the same as Mikey's, but wouldn't the smaller diameter allow it to spin up faster getting out of the hole and use the engine's power band?

I've been toying with the idea of dropping my diameter also. It's a 14.5" X 21, but it doesn't seem to get me out of the hole as fast as 300 HP should. I also can't get it over about 4600 RPM. I don't want to drop to 19 or 20 pitch since it would take off top end, and I'm thinking a drop in the diameter would give me a little better hole shot with possibly a better top end since it could allow me to get in the upper 4000 RPM range. Basically, it seems my 21 pitch is about right, but the higher diameter is bogging the engine down a little at take off and on top end.

What are your thoughts Mikey?

[mikeyt]

hi Ryan

I think you are in a similar situation to the original poster on this thread. Staying with the same size but going to a 4 blade should add 1-200 rpm and the extra blade should more than offset the smaller diameter. You should get out of the hole a little faster as well as have just a little more acceleration in low - mid range speeds. It will likely though reduce your top speed by 1-2 mph because of the extra drag (due to more surface area) at higher speeds. Its kinda hard to be exact about what will occur though because of the different blade shapes/curvatures/cupping etc but this is the most likely effect.

[al levy]

generally 4 blades are even number. keep in mind not all. my recommendation is going up. try a 22pitch progressive prop if you can. you might be able to go to 23 ( some mfgs.) . you will have to learn to drive the boat differently as the 4 blades will push the nose down but wait til you make turns. the progressive pitch wont let you lose any r's will make some difference coming out of the water but generally speaking you will love it