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251 Convincor Porpoising Help!

I can understand your frustration. It would be a shame if you sold without solving the issue. The 251 can only be trimmed out at high speed (full speed). Tabs can also be critical to smooth ride. It can be made to run flat, at any speed.
It may not be the most efficient to keep it flat at cruise speed since you will not trim up much if any and some tab may be required (bow will be in the water some at cruise speed).
My 251 had 3 blade 23 Mirage + and stock (short) tabs. It loved 60+ mph but could be set up to run flat for giving rides to my 85 year old grandfather in 2ft. chop at any speed, 25 to 60 mph.
I've seen people bounce so bad in these 251's that it surprises me they would even boat at all and I am confident it can be overcome with the proper driving set-up.
That being said, Checkmates are custom boats so maybe there is something unique to yours that needs a modification to correct.
Keep experimenting and good luck !
 
Thanks for the info KellyO. I lengthend my trim tabs from 11" to 20" and haven't tried it yet. I can make it run smooth at any speed also but like you said it's not efficiant to have the bow in the water. I also bought a Stingray Hydrofoil and will try that after I try the tabs out. I don't know why nobody uses the Hydrofoils on these performance boats. If it slows me down 2-3 mph but eliminates the porpoising, that's ok by me.
 
TRY A BEAD OF SILICONE ON THE OUTER MOST CHINES. THE BEAD SHOULD BE ABOUT 1 INCH WIDE AND ALMOST AS LONG AS THE OUTER 2 MOST CHINES...... IT SHOULD START AT ITS TALLEST POINT FLUSH WITH THE BACK OF THE TRANSOM AND TAPER FORWARD ABOUT 1 INCH [SHAPED LIKE A WEDGE]..... HOPE THAT ISNT TOO CONFUSING. I KNOW THIS WORKS ITS A PROVEN FIX FOR THE 251. IM NOT SURE BUT I THINK THAT TIP ORIGINALY CAME THE MANUFACTURER. GIVE THEM A CALL AT CHECKMATE IM SURE THEY WILL HELP. GOOD LUCK
 
ROBZ 27... Could you please explain the silicon thing in more detail. Does the silicon go across the back of the boat, or does it go from the back forward? Are the chines the ridges in the bottom of the hull? Is there any way I could see a picture of it?
 
hang in there

The 251 is a performance boat that in my opinion is designed to run the best and most efficiently at top end. I think most on this board will agree. They sit on the water the best at top end. Most boats with this characteristic will porpoise at lower speeds unless you trim in or use your tabs. You can make it better with the right prop, but pretty much you just need to work your trim and tabs. You can’t just get on plane, get to cruise speed and hold your trim right there. Constantly have to adjust. My 270 (259) is the same way. You should be able to run trimmed out at cruise, but will need to adjust depending on what kind of water you encounter...Also, when you trim down at cruise, you’re not really stuffing the bow down as much as you think. These boats have some natural lift. Yeah, there are other brand hulls that don’t porpoise at cruise speed but they are slower on top end.
 
Craig...what you said makes me feel a little better. Whats the deal with your 270 but you relate to it as a (259). Do the 270 and 259 have the same hull just rated different? And also does anyone have some solid answers about using the Stingray Speed XP Hydrofoil? Or the Sport 300? Thankyou for all youre ideas. I think we're helping a lot of Convincor owners with this thread.
 
I have the same problem with my enforcer 24' with a mirage 21. As soon as I touch the trim at or below cruising speed it bounces like crazy even in the calmest of waters. Can you fix this problem on a checkmate?
 
All these "bouncing" questions can be addressed in numerous ways. Alot of the "porpoise" has to do with hull design. Boats that have alot of rocker in the bottom will tend to bounce at moderate speeds. The rocker is built into the bottom in an attempt to get the forward section of the boat to lift higher out of the water at top end speeds. Another factor re: bounce is how far back the lifting strakes (or chines) extend. Boat hulls that end the bottom strakes before reaching the transom will tend to bounce more than boats that carry the strakes all the way to the transom...boats that carry the strakes all the way to the transom tend to ride higher in the water at speed and sometimes want to chine walk. X-dimension (how high the drive is mounted on the transom) also plays a big part. Many people will play with drive spacers in an attempt to improve ride and speed. There is alot of trial and error in dialing in a boat just right. Good luck!!
 
Well...here's the new scoop. I've been runn'n the boat quit a bit and just keeping it trimmed down. I also installed the Stingray and it does help. I also installed/lengthened my tabs from 12" to 20". I guess it all helps a little but not drastically. Soooooooo, my Weiand 177 Supercharger should be at my door Friday the 16th. Maybe this will allow me to cruise at a faster speed with less porpoising. I also sold all my test/expiermental props and bought a 28 pitch Hydromotive to use after the supercharger install. I'm hoping to go from my current 55-56mph to 70mph. That's about the gain I got with my 21' small block Scarab when I put the supercharger on it. Anyone else have a stock 454 with a 177 blower? What prop are you running at what speed and rpm?
 
i think your on the right track but i feel the 28 hydro may be too tall for you to turn. hopefully you prove me wrong on that one and it all goes well.

ive been lookin for a 28 so if you need to sell it.......let me know.

good luck
 
I'm watching a Land and sea stern jack on ebay right now. It's up to 1260.00. for the complete kit. I can't spend that much right now. I offered the guy 700 when it was up to 250.00.
 
OK It's 1 am and I just got back in from the river today. I had to leave my rear seats out and my hatch off ( which weighs about 300lbs). The supercharegr is installed. I only gained about 8 mph. It ran 65 on the gps. Not real impressed yet. The 28 Hydromotive prop may be too small!? I was turning 4500 rpm. and would like to turn 4200-4300. The porpoising was almost completly gone!!! waaaaahoooooooo yyyeeeessssssss!!! Don't really know why....maybe because of all the weight off the rear at this time? I did not rejet my carb yet. I was going to and took the rear bowl off the Holley and it had #84 jets in it, so I thought those sounded big enough but now think I should go larger. The supercharger is a very tight fit! I had to cut out the structure of the rear piece that the seat bolts to and will have to cut out parts of my rear seat and raise my hatch but it's in there and it wines like a s.o.b.
 
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