Ok, This is my first performance prop boat I have owned. In the past I have owned 18-21’ High hp Jet boats. My last was 1050hp Pickle fork capable of triple digits. With my jet as the Impeller to wear ring clearance got excessive it would allow the RPM’s to increase and speed decrease. The reason I stepped up to a bigger boat was to move up to do some boating on Lake Michigan. I also do a lot of boating here on the chain o lakes (very shallow).

So I get this boat 1990 251 GTX Convinsor Checkmate a 330 hp bravo 1. It had a cam change, exhaust, intake and carb. So maybe bump up to 400 hp. I never got a base line test done. But it would run 4900- 5000 RPM’s (50 MPH on GPS) with a nicked up Mirage 23p. Ran it a years like this.

This year we install a 502 with Aluminum heads. Motor will spin this prop to about 5000-5200rpms (depending on trim setting) with a GPS speed of only 53 MPH.I am sure this is like 20% slip. I would have though my RPM’s would have increased even more with this motor and poor prop condition and low speed? I would think this boat should be in the upper 60’s lower 70’s with this motor. I have no ill handling as is.

Bottom line is I need new prop. What prop would you recommend? 3 blade or a 4? Pitch?

I am also sure the bottom paint is stealing a few MPH., that is next on my things to do to this boat.
Thanks
Mike

If you have a 1.50 gears you about 28% slip using 5100 RPM. This is very high as you should be from 11%-15%. If you hae a 1.65 gear ratio then your slip is approx 21% which is still to high. You should try to determine why your slip is so high with your set-up. Can you measure the prop shaft in relation to the lowest point of the hull for us and we can go from there. If you are running a shorty let us know, this could contribute to your slip.

Julie

No shorty, stock Bravo housings, and 1.50 gears. My prop is in very (very) bad shape.

Then with 1.5 gears at 5100 RPM, running a 23P you have 28% slip. Can you take the measurement of your prop shaft in relation to the bottom of the hull for me, please. I want to better understand your set up.

Julie

Maybe if you could post a photo of your prop this may help.

Julie

If i can get home before the sun goes down I will get ya one and of the out drive. Am I wrong to think a better bitting (IE no nicks and dings) will bring the R's down?
Mike

No, you are not wrong. There are several ways to control RPM but I am concerned about your slip being so very high. I suspect there is more to it than prop.

Julie

These vintage Checkmates are gluttons for bow lift!

If your propellers is a standard Mirage and has been reworked at one time or another, it could be loosing what bow help it had. Yes, dings and nicks can cause cavitation on the face of the propeller blades and raise the slip percentage.

It will be interesting to see the prop picture. The normal depth of the drive from that time frame at Checkmate should put your cav plate about an inch above the bottom. This means propping and weight distribution may be your only avenues to improve efficiency.

Ok Got pic today. bbladesprops was right cav plate about an inch and a half above the bottom. Is this prob salvageable. I hear the 4 blades work better for this hull?

It's hard to tell in the picture how much cup there is in the prop, if any. The prop is definitely salvageable, it's really not that bad and can be restored to like new condition. The only unknown is how much, if any, of the blade area has been removed.

Matt