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  1. #1
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    Effect by shortening the shaft of an 2 stroke outboard

    Dear all,

    i tried to find an answer on the internet but can't get one.

    Wich effekt does it have on a 2 stroke when i shorten the aluminium housing (shaft) where the exhaust pipe is sitting in. The exhaust pipe ist just cylindrical with an open end. So no tuner there. Does the motor extremly change in power when the backpressure is influenced by a modified housing with less volume?
    I see i.e. mercury racing motors with ultra short shafts. They should have a similar problem...

    Dont start loughing, but i like do this to a 3HP Yamaha Malta.

    DIY dyno is already finished;-)

    https://www.youtube.com/watch?v=W6F1K8IYDhM

    Any answers are highly appreciated!

    Thx
    Oliver

  2. #2
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    Mostly for lower C/G on fast boats with heavy motors. If your 3 hp is on a canoooe it will flip.....

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  4. #3
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    No laughing here. You sure seem pretty passionate about what you got going on..... Wish I could be more help. I'm no engineer, nor do I know what the phuck I'm lookin at in the video, but I know MY 3 HP. won't flip MY canoe. Interest is interest and you appear genuinely interested!

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  6. #4
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    I did not look a video just saying high C/G will tend to unstable canoooe.... Story here...... at a boat race me and someone.... both drinking watching race boats. I was in front watching and Canoooe wiggled..... other drunk fell out of canoooe with out captsizeing... I stayed dry. Canoooe is just a longer canoe .... or I just mis/spell......

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  8. #5
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    I don’t think the weight of a 3hp will cause your canoe to capsize unless you hold it over your head while standing and under way..
    Josh Peterson

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  10. #6
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    I don't think he has a canoe..... I started that.... C/G thought. Maybe he has a Min=maxx.... who knows.....He might not have a boat. Needs to visit we can give him a ride.....

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  12. #7
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    That's a lot of work for a dyno pull on a drill,

  13. #8
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    Thx for your answers. I am doing this just for fun!

    i do have a Bayliner 285. My small rubberboat with approx. 6 feet length takes me to the coast when anchoring. On this dinghi a light weight Yamaha 3HP dos the job.
    I would like to get the dinghi to plane. (For fun)

    i did a lot of work to sqeeze more HP out of it but i found out that without a Dyno it is almost impossible to have progress. Boat doe not plain jet, so no speed comparerism can be done.
    thats why i did the dyno.

    The Motor dos have Cfk Membran, 11:1 compression ratio, larger kart carburettor, digital inner rotor PVL ignition, slightly changed porting and and and...

    Parallel to this work i think i have to change the motor position as well. For that i will build a combination from a set back and Jack plate. When i do this, the shaft will be way to long.

    There my question comes from. What will happen to the power of an 2stroke outboard when the volume of the area (shaft) where the exhaust gas of an open ended pipe are blown into? (Due to cutting i.e. 100mm out from the shaft length)

    best regards Oliver

  14. #9
    Join Date
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    From a performance perspective there is very little if any difference. The main gains are as stated above. Center of gravity and less weight.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

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  16. #10
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    Trying to get 3 hp to plane off.......going to need a Merc lab 5 blade..... HeeHee....Boat might have a hook....going to need some balsa core here.... Smiles...

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  18. #11
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    When you are dealing with only 3hp a 20% gain is not much in terms of power but might be noticeable? These kind of projects are fun.

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  20. #12
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    More fun would be kg7 with Mk 7 covers......smoking as it goes buy all rich guys yachts....With a R tail...

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  22. #13
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    3HP does not work, but 6 does

    Click image for larger version. 

Name:	Twin Malta.jpg 
Views:	31 
Size:	120.1 KB 
ID:	422947

    Anf if i don't reach 6HP out of one, i still have a bottle of NOS laying around...

  23. #14
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    How about this Malta (stage 3 or so) ? Looked, let me say interesting similar to a phallus symbol. But the first section of the tuner was a lot to short.

    Click image for larger version. 

Name:	fertsch totale v re.jpg 
Views:	39 
Size:	66.9 KB 
ID:	422951
    Last edited by ostsideend; 11-09-2018 at 03:36 PM.

  24. #15
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    Seen as you're in Germany get an ignitech P1 programmable CDI and hook it up to that pvl. Pvl have no ignition curve as far as I know. With a programmable ignition curve you can tune it for way more torque out of the hole and find the best timing for top speed and power.

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