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Prop for MX14 / cobra with 115 merc inline

G

gtrumpet

Member
What type of prop would you run, i've been told to get either a 21p or 23p for this setup. I have run the boat with 19p aluminum up to around 2/3s throttle, after doing so I noticed that the prop no longer had any paint on it and the blades were warping. So I figure I need some sort of stainless prop. The motor has to be trimed in all the way of the boat porpoises badly at mid speeds, due to excessive bow lift. The previous onwer mounted his gas and battery up front to keep the bow down, I'm also going to mount the battery there, and probably sand bag it also.What type of props have you guys found to work well on these small hulls.

Thanks,

Will
 
Kary he has more problems than just what prop to run........he needs to completely change the boat set up to make the problems he has go away.....I tried to help but what do i know lol........Viper
 
WOW!!

I didn't read the sandbags, gas tank, and battery.
When I come and look at your MX16, can I look at the MX14 and see what you have got goin with it? You need to redo your whole center of gravity. You can move that stuff around, but in a 14' boat, those are DRASTIC measures. Viper1 knows what it takes-set it and forget it.
 
What year is that 115? If it's pre 81 (like mine) it's really only about 90hp by modern standards. Mine won't even spin a 19p up to 5200 rpms on a 17' hull.
 
I just saw your post in another thread. The 8" of setback sounds like way too much on a 14' boat, but I think you already know that. Your inline should turn at least 5500 at WOT. Don't lug it. Don't turn the max timing up too high either. You have to detune the old ones to run modern gas - Max timing 21*. That will bring rpms down also if it's at 23* now.
 
big difference between his little MX14 and your VMATE 17ft.....set up right his mx14 should handle and run real nice in the low 60's
 
I run both a 21p and 23p on my mx-15 with 140 evinrude (equal to 115hp today) and i like the punch of the 21p, doesn't rev over 5600 rpms with this setup and the 23p gains a few mph. I am sure i could run a 24p and loose more punch. Mine also has a bow lifting problem and I always wanted to try a cleaver type stern lifting prop to see if it helped but I never have. I too have the battery and a sand bag up front. -shane
 
I removed the jack plate over the weekend and the boat ran much better, although it it still has to be trimmed all the way in. Yesterday I ordered a 22p stainless Quicksilver prop at one of the local marinas. 362 for a prop seemed a ok price. I hope it works
 
Shane you did a very nice job on your mx15.......i think if you look at the ser. number on the back you might find your mx15 to be a 1971.......also if you still have the checkmate trim tabs it would make a difference in the way your boat handles........the 1500lb's sounds really heavy for that boat.....most mx15's came in under 500lb's for the hull with seats.....plus it would run a little better jacked up about 1.5 inches.......if you were keeping it this is the information that would make a difference in the way your mx15 ran......plus the cleaver would do nothing for this boat........Viper
 
viper 1 said:
big difference between his little MX14 and your VMATE 17ft.....set up right his mx14 should handle and run real nice in the low 60's
Click to expand...

No kidding. My point was just that I wanted to see how he had the motor tuned and what he was shooting for as far as Max RPM. There are some differences between the best way to set up an inline vs the common way. Original spec for mine was 5200-5400 rpm, but the inline gurus say to shoot for 5500+ if the Max timing is right. There are some good sources of info out there.
 
No kidding .........but most people don't mess with the timing and so called tuning ......and if there motor is in good running condition 5500 to 6000 rpm is not a problem..as long as they use good oil and fuel....to start with his biggest problem is with how the boat is set up and what prop would be the best for his package....Viper
 
The merc is a 82 or so I'm led to believe by the serial number. I've got two different tachs for it but neither of them are installed, one is the mercury racing tach that was in the boat the other is new teleflex universal tach that I was going to put on my old mx-16 prior to buying the cobra. the problem is I can't figure out how to wire it properly the harness coming out of the controls is the four prong variety. my question is what wire goes where? none of the marinas up here want to tell me how to wire it, they all seem to want me to go and pay them $70 a hour to get it hooked up.

The new 22p lightspeed mercury prop came in today. supposedly the lightspeed is the replacement for the lazer II I had to remove the trim tab and cut it to accomodate the larger prop. it seemed to run great top end is something else, its damn right wicked. although I didn't have time to look at the speedo. It doesn't seem to porpoise nearly as much especially since I removed the jackplate. It also seems to have a better holeshot which is surprising since i went up in pitch, though the new prop does have those vents.

Luckily I had just slowed down and was on my way in when the Cop showed up. I've never seen a cop stare so long and hard at a boat, he even went around it to see how big a motor I had on it. thank god for 90 cowlings and a old face plate.

Thanks,

Will
 
The porpoising

The porpoising is due to the motor being able to push it beyond speed-wise. Horsepower plays a key role in prop selection, setup and balance. You still need to work on your center of gravity and balance the boat PROPERLY so zero trim is needed to achieve 3/4 top end. You have to have a tach to see where you are runnin. I don't care what you feel, I have been racin and runnin boats for years and the tach will help you to know where you are runnin-been there done that with seat of the pants feel.. As you eliminate porpoising, you can move weight back if you don't correct it properly. You want the boat to balance on a 4X4 with your weight equivalent in the driver's seat and loaded wet 5 feet forward the transom for that boat. You will have to load your weight in the center of the boat for balance purposes. That will get you close, then you can move weight accordingly after inital setup. After 3/4 top end, you can trim from there. If you don't have trim, then it is like testin props with passengers. Weight distribution and speed makes a difference. It is a pain in the ass to do, but the right steps taken for setup will reap lotsa rewards and safe handling.
 
viper 1 said:
No kidding .........but most people don't mess with the timing and so called tuning ......and if there motor is in good running condition 5500 to 6000 rpm is not a problem..as long as they use good oil and fuel....to start with his biggest problem is with how the boat is set up and what prop would be the best for his package....Viper
Click to expand...

If you run an inline 5500-6000 with the timing at the stock 23* setting you'll fry it. Then you'd have a setup problem.
 
??????????

If you run an inline 5500-6000 with the timing at the stock 23* setting you'll fry it. Then you'd have a setup problem

If you run it at this RPM, run premium minimum 91 octane
 
The top inline guys still wouldn't reccomend it. 91 octane still sucks compared to what gas was in the 70's (we have 93 octane readily available in the South, and that still isn't good enough). I'm not trying to be an ahole, but you can't change props trying to maximize your rpm if you haven't checked the motor settings. Just a slight decrease in the max timing settings lost 200-300 rpms on mine. Start at the beginning and work down. Adjusting settings is free, props aren't. I guess the easy way may be preferred if you have money to throw at it, but most of us don't.

I'm going through the same process. Obviously my boat is different, but the process is the same.

gtrumpet, I'll check my tach (teleflex) and let you know. I installed it last year but can't remember the wire colors.
 
you still don't get it? his problem had nothing to do with timing.......he never asked anything about timing........you started this timing thing.......he had a set up problem and wanted to know how to set it up and what prop to use.......i sent him a pm last week telling him to take the plate off and told him how to set his boat up right....the reason i sent him a pm was because stuff just like this....everybody wants to be a expert....as for me?....how do i know what i am talking about? well since I was the Largest Checkmate and Hydrostream Dealer also the largest 6 cylinder Mercury Outboard Dealer in the usa for many many many years till I closed my business down and retired and since i have raced mercury products since i was 16 years old in 1964 till now i think i might know a little more than your The top inline guys. as for 93 octane anybody that wants to run a hi peformace boat, car, motorcycle, should use it or better . i build a 540ci chevy with dual turbos putting out 1056hp at 6100 rpm and 17 lb's of boost on 93 octane. that motor is in a 22ft Checkmate Vision......and that is just a small sample of the boats i have built and raced. we were just trying to help this person with his handling problem and prop....as for his tack he can go to his mercury dealer and buy a tack harness for his racing mercury tack for about $10 or $15.00 and it comes with how to wire it in the package. he should also get a kill switch if he does not have one......thats it for me.....no more post's..everybody have a great 4th of july weekend..Viper:)
 
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