Effect by shortening the shaft of an 2 stroke outboard

[ostsideend]

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

[Dave S]

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

[rochelle howell]

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!

[Dave S]

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......

[nitro_rat]

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..

[Dave S]

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.....

[Capt.Insane-o]

That's a lot of work for a dyno pull on a drill,

[ostsideend]

:D

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

[rock]

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

[Dave S]

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...

[rude tim]

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.

[Dave S]

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

[ostsideend]

3HP does not work, but 6 does :)

422947

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

[ostsideend]

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.

422951

[Umassot50]

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.

[ostsideend]

Thanks all of you for your comments!
I will shorten the shaft after i finished tuning HP.

The original CDI from the malta just adds pre ignition timing per rpm and does not stop it. The PVL i got, does have a curve which degreses in higher rpm. A free programmable version is available from PVL as well. I will see later how to organize one ;-)

[ostsideend]

I did start with shortening the shaft. My dinghi has 38.5cm of transom height. To level the cavitation plate (and not having the motor above my head:)) i have to cut out approx. 60mm. Tried to find the best position for the cut.

423171


Disassembeld on of the shafts:

423170


How to get so much debris in such a small shaft?

423172


An old machine but better than doing it manually:

423169


Simple leveling the shaft parallel to the gear housing backstop.

423168


Ultra short shaft Malta ;-)

423167


From the cut out i started to do a kind of centering flange

423166


The other section of the shaft will slide on (till it touches the cutting edge)

423165

I will put screws through it until the guy who can weld it will return (Or if i like it, it will stay as it is) so that i can proceed with shortening the driveshaft, pipe and shift leaver.

Anything i have missed so far?

[Dave S]

Great work...... lucky you don't live in the states.......you would be making 2.5 mercs for tenders....dingies...

[ostsideend]

Shaft is fixed, but i will get it welded later on. Looks like the belt of one of Alice Coopers trousers ;-)

Anyhow it will work.

423221

Of corse i double checked leveling in both directions

423220

While working on the shaft i saw the gearhousing with the water inlet laying around. I thought a low water pickup will probably not needed, but i want to have one ;-)

So i disassembled it and checked for a way through the hosuing.


The pre owner did forget to change the oil "last" year ;-)

423224

The clip (you almost can't see is smaller in reality than on this pic) gave me a lot of "fun" to get it out while i was thinking about how nice it becomes to get it in again later.

423223


The gear housing does not realy allow to feed a pipe or hose through it. I will search for pic from you guys and hopfully get an idea which i can copy.

423222

[ostsideend]

This evening i did hand over the shaft for welding to a buddy of mine and i hopefully optimized the gear section.

Did change almost all edges to smooth radii. Polished would look great also ;-)

423374


Changed the angle of the oil drain screw by redrilling to optimize the position with the smallest influence in turbulence.

423373

Down left of the gear case. Almost not visible...

423370

Made the guidance channels wider and a little deeper, smoothen the area chanel to bore

423371


423372


Question:

There are 6 bores working as the waterpickup. They lead to one bore with 5mm diameter only. Would it help to drill this bore to i.e. 6mm in diameter, or is there a risk that the impeller pump will not raise up the water due to the larger bore diameter ?

[Dave S]

Nahhh once the pump is wet it is GOOO suck all the H2O ....Ya something ...for doing this.....