Heys guys,

Some quick background on the boat, it's a 1993 Checkmate Convincor 251 with a Merc carbed 502, bravo 1 drive, and 5 blade 21p prop.

Some background on me, I'm still just as green behind the ears as I was last year lol, but trying my best to learn my way around the performance boating scene.

So last year when I first purchased the boat towards the end of season I had a lot of issues with porpoising and wasn't able to get much top end performance out of the boat because once I was up on plane the second I started to trim up the drive the boat would start porpoising even worse (at this time the boat had a 24p 4 blade). The prop was then changed out to the five blade 21p and I was able to achieve slightly more performance due to attaining a higher rpm, but the porpoising was still an issue. Fastest speed attained last year was 53mph according to a gps speedometer on a cellphone.

Fast forward to this year. Got the boat out for the first time this past weekend, and while out on the lake the performance was severely lacking. I was only able to see a best of 45mph no matter what I tried. For the record, the boat did have some remaining fuel, with stabil, from last winter, but I also added in 30 gallons of new fuel. I did adjust the idle on the carb down a little as it was running pretty quick. Other than that nothing was changed on the boat.

I do have trim tabs on the boat, and I have tried running with them all the way down and all the way up, and everything in between but they don't really do anything about the porpoising. I am 99% sure I need to fix the gimbal ring on the drive but aside from that I do not know of any other necessary repairs at this time. Also, my altitude is 5400ft above sea level where I run the boat in Colorado.

And just incase it is driver error when I'm trying to run the boat WOT, I get up on plane using the tabs, accelerate to about 20ish, back tabs off, accel up to 35ish, trim engine slightly, then accel all the way out and try and trim up slightly. HOwever, any trim up on the drive causes crazy porposing so I usually have to back the drive all the way down and that is when I hit 43-45mph.

Any help or advice would be much appreciated.

Start by troubleshooting the engine. Once you figure out whats wrong with it and repair it, then start with prop selection. That 502 motor is severely ill or way out of tune if it will only push the boat 45 mph.

I have a friend coming over to check timing and help check the carb...Any other advice with respect to the porpoising?

What rpm's? Does the engine stumble? What type of prop?

45 with the drive down and nose burried does sound a little slow but there should be no porpoise.

First Id throw that 5 blade in the can or sell it. Its a torque prop. The Bravo one 24P will be the prop that boat likes. I havent seen a convincor that didnt like the bravo one 24P. Same as powerplay said, what RPM are you running WOT? My boat will porpoise (Spelling?) at 40mph or so until I speed up and start trimming the drive up. I always leave my tabs up and only use them to correct a list to whatever side, usually the port side.

I would check the bottom to see if any work has been done in the back, maybe the work is shotty or the glass is coming apart?

When I had my check, my check friends and I were all running 23P 3 blade props and never had a prob. In fact we all barely ever used the tabs unless in really choppy water. My friends with a 330hp doesn't even have tabs. He runs 57-58 on a 23P prop GPS. It does list to one side however.

The bottom design of that boat is damn near perfect. Something is way off. Have you checked the stringers and transom for water? Has anything been added to the back....like a custom swim platform?

With the 21p five blade I'm spinning right at 5000rpm without trimming up the drive at WOT. I have a 4 blade 24p but was told that due to the altitude out here that was too much prop for my boat. I'm currently trying to find a 4 blade 23p to try out figuring that would be right in the middle.

Engine does not stumble, however, it did stumble on the first acceleration of the day, after that it ran smooth. The bottom of the boat looks good, minor scratches from beaching but that is it. As far as checking the stringers or transom for water I have not done that. And there is a plastic swim platform on the back, but it doesn't look to be aftermarket, however I cannot confirm nor deny that fact given I bought the boat used.

Seth

Take a long straight board to the bottom and see how much hook if any is there. If I remember correctly it shouldn't have any. Try a 23-24p Bravo or Mirage. 5000 is right about where you want to be when your trimmed out. It sounds like a prop or set up issue to me and not the motor. Also grab the drive and see how much play there is up and down.

I don't think they make a 23P Bravo do they? Only 22, 24, 26 and so on. I agree with powerplay, try that 24P out again, set your tabs up, trim the drive and see what happens. All else fails, try that 23 Mirage plus and see what happens.

I would say a 21P with a 502 is way to small. Remember, the less blades, the faster. The less the blades the more you need to up the pitch. That's how Ive always seen it anyways. How does the bow act? Is it digging in the water or out? Id almost bet your blowing that prop out being at 5k WOT and no trim in the drive.

I think your right Matt. The Mirage I ran was a 23 and the Bravo was a 24. I also ran a 5 blade on my Check. Don't rememer the make, maybe a quicksilver??? Either way I never experienced any porpoise just a lil chine walk at wot and trimmed out with tabs all the way up.