JimNut

Looking for some recommendations on which prop I should try next. The boat is an 87 24' Checkmate Enforcer with a fresh 350 that has some decent go fast parts in front of an Alpha 1 (1.47:1 gears). Realistically its probably in the 325-350 HP range (heads, cam, intake and higher compression). I still have a bit of tuning to do on the carb so I'm expecting to pick up a little in the upper RPM once I get it dialed in.

I have tried (2) different props with this combo so far and these are the results. I did both of these tests back to back in the same conditions on the same evening so they should be very comparable.

21p Mercury MiragePlus
- 4900 RPM
- 51mph (GPS) (half a tank of gas)
- Slip = 23%

23p Quicksilver Mirage (not a plus)
- 4550 rpm
- 53mph (GPS) (might have seen 54 once) (half a tank of gas)
- Slip = 24%

Both props get the boat on plane very easily, the 21 is almost effortless even without using trim tabs and the 23 benefits slightly with the tabs down a bit but still is very easy to get on plane. The 23" does seems to cruise a bit better but unfortunately I didn't record my speed and RPM for that.

I'm just not sure what to try next. I wouldn't think my boat would require a 4 blade as it is not that big and heavy of a boat but I'm just not sure what else to try. All thoughts and recommendations are appreciated! Thanks!!

Tinkerer

The one time that I drove one I found that the drive was mounted VERY deep. I believe that is a pad bottom boat. How far below the bottom of the boat is your propshaft centerline? I think that most of your problem is that your prop is way too deep to go fast.
The one I drove had a BBC in it. It was a pig - LOOKED real fast but wouldn't go.

sabo

iTrader: (2)

Is the ratio verified? (take drive off and spin shafts and count rotation?) If it was 1.62 ratio slip would be within range.

Unless you have an extremely high x-dimension you shouldn't need a 4 blade

Other causes could be , damage to drive, or drive shower tube causing turbulence, skeg guard, hydrofoil?

JimNut

I have not verified the gear ratio but I am pretty certain it is 1.47. What makes this whole thing even more confusing is I was running the same prop last year with the old motor and had a MUCH lower slip number. With the tired motor I could barely get the motor to spin to 4100 rpm with the 21 MiragePlus and seen a best of 49mph (gps). That equates to a rough slip of 12% which is what I would expect for the boat. Now this year with the new motor and same prop I am over 20% slip. The only thing that has changed was the motor. Same drive, same everything else. So essentially I gained over 800 RPM running the same prop and picked up 2 mph.. Makes sense doesn't it? haha

sabo

iTrader: (2)

that is odd, is it the same motor just rebuilt?

Maybe your rpm tach is off somehow?

JimNut

It is physically a different engine as I didn't want to wait for the rebuild time so I built it over the winter and swapped it over in the spring when I pulled the boat out of storage. Still just a run of the mill 350 sbc with better heads, cam, intake & higher compression.

I have verified the Tach (factory analog) in the past and it was within +/- 50 rpm or so of my timing light's digital tach but I have yet to do that this year. Sounds like that will be my next step. Something has to be off because this just doesn't make any sense at all to me. I really hope the tach isn't off though since that would mean the motor isn't making any more power than the old tired engine was haha

sabo

iTrader: (2)

only other thing i can think of is trim height, if you over trim you could loose speed and increase rpm and slip, try and get a cruise speed @ 3,000 rpm and see where you are.

JimNut

I know this is a long shot but what are the chances that my engine coupler is bad/going bad? I don't have any signs of a burning rubber smell or any debris around the drive shaft that you usually see when they fail. Also the boat never really feels like its obviously slipping like most people mention. I'm pretty sure I already know the answer to this but is there any way to check this in the boat without having to pull the engine?

I highly doubt it is the coupler but I'm running out of ideas...

Tinkerer

I would still like to know what your prop shaft distance is below the pad. It could be that the prop is so low that he is having to overtrim and that is causing the high slip #'s. I was not impressed with the other boat - It just wouldn't go no mater what I did. The prop was way low on that boat. If the drive is too deep throwing HP at it wouldn't make much of a change. Any chance the boat has developed a hook in the bottom over the winter?

JimNut

Just did a quick measurement and the prop shaft is roughly 7" below the vee. The hull doesn't have a flat pad on the bottom. I can upload a picture in a little bit.

I checked the bottom of the boat with a straight edge and it's straight as an arrow. No hook whatsoever.

If it because of my drive height would a prop with more stern lift potentially help?