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251 Convincor Porpoising Help!

bigsbetter

bigsbetter

Active member
I know this is a common charactoristic... but what can I do to make it go away (the porpoising) and still be able to trim my bow out of the water?It's a 1988 251 convincor w/ 454-385hp. currently running a 23 pitch Mirage. Why don't I see hydrofoils like a Stingray or the Sport 300 on larger I/o's. I know they work on smaller boats. Has anyone tried them? Any help would be appreciated. Thank you, Charlie
 
Those hydrofoils slow boats down too much. I wouldnt use them on a performance boat. Just drop your tabs a bit, the porpoising will stop.
 
your tabs will help out but slow you down also, and then you cant trim the bow up...
i switched to a four blade prop on mine, i tried the hydromotive 22, lost about 2 mph but the handling was MUCH better.
i also was very impressed with the 23 rev4 by merc, seemed to run just as fast as the 3 blade mirage but much less porposing. i have, but havent tried the 22 bravo yet.

just a plug....

ill be selling all of these props, i went up in power and need a 27/28/29 prop.
 
Yes, tabs slow you down also. The big difference between tabs and a hydrofoil is that you can raise your tabs when you want to go fast. At higher speeds, the boat wont porpoise. Maybe I should say , shouldnt propoise.
 
I have a 23 pitch Hydromotive coming soon off ebay. I hope this helps! If not I'm going to try a hydrofoil. I'll take the slower top speed as long as I'm not bouncing. Has anyone tried a hydrofoil on a checkmate I/O?
 
You are making a mistake with that hydrofoil. Thats not the way to go for a performance boat. I have never seen anyine run one before.
 
hydrofoil

What Vinny is telling you is true. On an outboard with a hydraulic jack plate, you can raise the motor high enough on top end that the hydrofoil isn't even in the water. Can't do that with an i/o. But if you are willing, I have a brand new Stringray in stock for $45+shipping. boatman
 
Bigsbetter,
try the prop, youll probably find that the 23 hydro might drop your rpm 2-400, but try it anyway and see how you like the feel of it. with mine i was able to trim it "to the sky" to get bow lift.

where are you located?

i have to agree. dont waste your time or money on the hydrofoil thing. try the props!! i had the same problems when i bought my boat, you may even search some older posts here. a lot of good advice on this forum. i can set you up with a prop to try and if it works you could buy it!! cant get much better than that!! unless i just gave it to you!!! but that aint gonna happen..!

let me know, glad to help out
 
A common problem would be the X-dimension is too low on your boat resulting in the the drive being too deep in the water and creating your "bounce". If you're willing to spend the big bucks, purchase an Imco shortie drive and you should see some big improvements including higher top end speeds.
 
I'm located in Warrenton MO. I go to the Mississippi river alot. I hope the prop works. You guys know how it feels when you have your boat and you're parked out on the beach and your looking at all the boats out there, and you can't help but think yours' is the best one out there. It may not be the newest or the fastest or the biggest, but there's something about it that you like...or maybe ...love. It's knida like the family dog...everyone thinks there dog is the best! That's a GREAT thing. Well that's the way I felt about my 21' Scarab and that's the way I want to feel about this boat. I bought this Checkmate as apposed to another 28' Scarab I was looking at. I don't want to change the design of the boat with thousands of dollars. I will expeirement with props and other add-ons to figure it out until I find what works the best. If that doesn't work.. Then I won't have that feeling I want for the boat and it will be sold, and I won't buy another Checkmate untill I drive it first. I'm an auto mechanic, and I'll do my best to fix the porpoising. I just can't believe after all these years Checkmate hasn't fixed this common problem.
 
you know, ive thought of the drive thing too, wonder if anyone on the board has tried it on these boats and if it did them anygood??

biggerbetter,
try the prop, i know exactly what you saying, i was sooooooo disappointed in the way my boat handled, heck, im still not 100% satisfied!! but im not gonna give it up now. the boats are nice, but i can see exactly where your coming from here.

i feel as though my center of gravity is high too, causes alot of "roll" when following in a boats wake or trying to get around one!! ive decided just to give it more power to overcome that stuff!

kevin
 
I appreciate the comment Boatn70...so adding more power so I could cruise at a higher speed will get me out of the porpoising? Is that correct? I put a Weind 144 supercharger on my Scarab, and it was the best thing I ever did to it. I could get a 177 charger for this one new for 1900-$2000.00. It looks like a tight fit though.
 
it does change the boats attitude some, the porpoising will still exist at similat speeds but, yes, i think your cruise speed will increase to a better range.

i would think youd be ok, you may have to hop under the hatch and have someone close you in with a tape measure and light. measure from the block below the intake to the hatch....see what youve got. i think mine had an inch to spare without raising the hatch.
 
I think the height will be ok. It's the area in front of the engine. I don't know if there's enough room to bolt on another pulley to the crank and have another belt on the front. It sits real close to the rear seat back/storage area. I may have to do some modifying there. Did you install a supercharger? What kind/size, dual or single carb? I just got my 23 Hydromotive in the mail yesterday and it's going to be 75 tomarrow.....soooo...maybe I'll get out.
 
I have a 1990 251 GTX and It had the very short Bennet tabs. I extended my tabs to 15". All I did was attach it to the bottom of the old tabs and added stiffener to the extensions. I cant weld to save my life so I use counter sunk SS screws with locking nuts. Turned out better than I expected. And man did it make a world of difference. I also added Bennets TPI system to them. I cant express how much it help to know where they are positioned at all times. If I stuff the tabs I do loose some speed at cruise. But I usually don’t need to stuff them. With a little cross current or wind at about 30-40mph all I need to do is drop one tab a bit and it levels/ stops the pull and stops the proposing. At 40+ mph I have no proposing.
 
MikeSW Is the GTX the same as my Convincor? My tabs are 11 x 12 inches. I can control some of the porpoising with the tabs but then I'm shoving the bow in the water. Will extending them help? What prop are you using and what horsepower/speed. Does weight/position effect yours?
 
Yes same boat, 251 hull. I am running a 500hp 502. It had a 330/454 when I first got it.I have a 23p Mirage and a 23p Mirage plus. My Mirage I use for my shallow lakes and it is beat up bad. out on Lake Mi I run the Mirage plus. They are three blade props and work well with my boat. As for the tab thing mine where smaller than yours. I cant recall what they where originally but now are 11"wide by 15" long and work great. Weight/position I make sure where distributed evenly port to starboard and that’s it
 
OK I tried the Hydromotive 23. It's a little better, but I don't have the feeling I want to have for this boat because of the porpoising. I think it's a stupid design. The boat looks GREAT and runs excellent but rides like crap. I do not want it so it's for sale. I'm sorry to say this but I'm disgusted with it. I'm honestly thinking about buying 3 Stingray Hydrofoils and putting one on each trim tab and one on the outdrive. 1988 251 CONVINCOR 454 MAG BRAVO 1 Original trailer. 505 hours. 55 hours on new engine...not rebuilt.
 
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