Checkmate Enforcer - Lowering Slip (20+%)
 

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!!

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.

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?

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

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

Maybe your rpm tach is off somehow?

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

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.

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...

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?

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?

If you want to drive to Grand Rapids I will let you try a -2 Shorty. I don't know if a -3 would work on that boat. OH shoot I forgot You have an alpha drive. Find someone with an Alpha SS - they are 2 1/2 inches shorter than an Alpha. I used to have one on an 18 ft Glastron Carlson. Made a huge improvement in speed and handling.

This part makes no sense.
The same 21 Mirage Plus spinning 4100 rpms should turn the exact same mph if its trimmed the same.
It does no matter what the max hp/rpm of the engine is. It should still run the same speed at 4100rpms.

I finally had a chance to get to the lake and do some more testing. I was able to borrow a Holley 750 marine carb (4010 series) from a buddy of mine and that seemed to make a huge difference. I also re-checked my timing and was only getting around 28* total. Since I always run premium gas I bumped my timing to 34 degrees.

Throttle response and mid range is much, much better now. Definitely a very noticeable difference.

Here are the results of the latest test:

21p MiragePlus Prop

3600 RPM - 44 mph - 10% slip
4200 RPM - 50 mph - 12% slip
4800 RPM - 58 mph - 11% slip

23p Mirage (non Plus)

3650 RPM - 49 mph - 9% slip
4300 RPM - 57 mph - 11% slip


My high slip numbers I was getting before appears to be from me over trimming the drive to get the RPM's to what I felt the motor should be able to pull. With the motor running better now I didn't need to trim any where near as much which shows in the good slip numbers.

I still don't feel that the motor is running as well as it should and I'm not sure where to go from here. I know a 750 is a bit big for my 350 so I might try and find a 650 or so and test that. I haven't made any adjustments to the 750 that is on there. It was a brand new carb that has been laying around for years that I pulled out of the box and slapped it on. Only adjustment I made was for the idle.

One thing I have not done yet is verify the factory tach is accurate. My good digital timing light that shows RPM had the signal wire cut by the alternator pulley last time it was used and I haven't had a chance to repair it yet. Hopefully I will get to that in the next day or so.

Atleast I'm going in the right direction...

I'd say you are about where a 350 will get you, without crazy exhaust. My stock 97 Vortec TB V 350 is probably 275 or 280 horse and only pushes my fat load of a Formula 45 mph. Course I bought the heavy boat for the ride it gives in chop so it's my own doing.
58 is darn good in a 24 foot boat with a small block and a deep drive. I bet your boat is under 3200 lbs.
My old boat has the pre vortec swirl port heads and it would get to 59 with a carefully rebuilt qjet and a Comp Cams flat tappet XM262H, 112 LSA, 218/224 @.050., .462/.477" lift. It also has a deep Alpha 1 drive. Speeds were about the same with a 23 pitch aluminum Michigan Wheel 3 blade and a 21" Mirage. The SS Mirage would give the same fuel economy and better hole shot so that's what I ran. That 19" 6" boat only weighs around 2600 lbs with chubby me in it.
I think I had too much timing in it on my last run, I was going as I said 59 and it kind of stopped accelerating at 90% throttle. I don't remember the RPM, sorry. This was also a 1.47 ratio Alpha. I may have been getting a bit of detonation so I backed off. Should have tried it with 91 octane.
It had good compression before I cammed it. It might have got some more compression due to the cam timing. I never did a comparison.

Try a 21" Laser II and if that doesn't work add a little more cup to it and see what happens. Your are close with the 21" Mirage +. Another option, which wouldn't be for top speed is a 21" High Five. All depends on what your looking for.

I very much appreciate all of the suggestions. I am starting to accept (but not without a fight) that I am wanting big block speeds out of my small block and it's just not going to happen. Last weekend I took the boat out after making a few carb and timing changes and I very briefly saw 60mph on the GPS. This was with only myself in the boat and very low on fuel with a 1' chop or so on the lake. Not sure if the changes I made helped or if it was more of the conditions and being low on fuel. Its more likely the conditions but either way I'll take it. It's time to enjoy the boat for what it is rather than being constantly unhappy with the performance.I allowed my expectations to be to high with the new motor. I just didn't realize/understand how much horsepower it takes to increase the speed only a little bit in a boat.

This is the first performance boat I have owned and it has been a great learning experience. I'll keep the boat around for another year or so and then I'll start to look to upgrade to a better platform. Something along the lines of a Checkmate 253/259 Convincor would work very well for me I think.

Many mid 300hp small block boats have a hard time spinning a 23" mirage prop, but not much of a problem with 23" smaller diameter props. Most of these, are however, smaller lighter boats.

That said, a 23" anything will be 'too large' a pitch for your boat.

9 out of 10 (or closer) sbc boats that stay a 350cid (when modded) will end up using the same pitch prop they used to use (if it was the best prop for the boat), but just spin it higher. Seriously.

Concur. Same prop spun faster.

Could also be the drive is to high at 7"below the bottom. Sounds like the prop is slippin and not hooking up, put a 1/2" spacer in and see what it does.