pstephens46; propeller and motor rotation

[boscoe99]

Does this help?

First image is the motor turning clock wise (normal) which turns the propeller counter clockwise. Motor is in reverse. Motor is driving the propeller which propels water forward, to move the boat in reverse.

Second image is when water flow moving aft (boat going forward) is turning the propeller clock wise which then causes the motor to turn counter clockwise (abnormal) because of the reverse gearing.

[pstephens46]

Yes. I was struggling with the concept of larger 4 and 6 cylinder 4 strokes being turned by the force on prop against their higher compression ratios. I would have never guessed this was possible.

[boscoe99]

The concept is not intuitive. Takes some thinking. Kind of goes against the natural thought process. I initially was a non-believer as well. Praise the lord I have seen the light.

Next time you go boating put your hand in the water. At about 20 MPH I would say. Bet you can't hold your hand flat against the oncoming water.

[pstephens46]

The motor would not spin backwards very long. Flash in the pan sorta thing I would guess.

Chop the throttle in a boat and speed bleeds very quickly.

[boscoe99]

The motor would not spin backwards very long. Flash in the pan sorta thing I would guess.

Chop the throttle in a boat and speed bleeds very quickly.

Something about water filling a cylinder that inhibits the rotation just a bit. Water does not compress as much as air.

[pstephens46]

Something about water filling a cylinder that inhibits the rotation just a bit. Water does not compress as much as air.

Hmmm Ya think?

[rodbolt17]

aint no way water on the prop is going to turn the engine backwards.
simply not gonna happen.
it will make a ratchting sound as the prop rotaes and the clutch hops in and out, but the drive shaft wont rotate.
however you just went from a very high pressure in the ex stack to nothing and the back wash wave is going to rise and start filling cylinders.
works that way in stern drives as well.
but the engine is NOT going to rotate backwards by the prop.

[99yam40]

I guess they cut the angle of the dogs that way on purpose to help prevent this from happening.

[rodbolt17]

the back draft cut on the dogs/clutch are to keep it locked together under load.
that is why most modern high HP engines use a shift interrupt switch.
you don't belive it try to shift without one.

[boscoe99]

aint no way water on the prop is going to turn the engine backwards.
simply not gonna happen.
it will make a ratchting sound as the prop rotaes and the clutch hops in and out, but the drive shaft wont rotate.
however you just went from a very high pressure in the ex stack to nothing and the back wash wave is going to rise and start filling cylinders.
works that way in stern drives as well.
but the engine is NOT going to rotate backwards by the prop.

Why then did Yamaha design certain motors to detect reverse rotation and to take precautionary actions if and when they might? Which you say can't happen.

If Yamaha is incorrect about what they say in their service manual, what else might they be saying that is incorrect?

Have you reported this error to Yamaha US so they might convey it to Yamaha Japan so that a service manual error can be corrected?

[boscoe99]

More information about stuff that cannot happen. This time from Honda.

Some say the earth is flat. Some say it is round. Some say the US put a man on the moon. Some say it never happened.

I don't know what to believe or disbelieve in this world any more. I guess just pick and choose what anyone wants to believe and they can find a supporting position.

[rodbolt17]

in certain situations the engine can backfire and attempt to run backwards. inboards and sterndrives try it all the time.
old outboards could run backwards quite handily.
ever seen the old OMC pulse pack/point ign syatem that the anti reverse spring either wore out or got left out?
modern electronics simply help prevent engine run on or dieseling.

[TownsendsFJR1300]

aint no way water on the prop is going to turn the engine backwards.
simply not gonna happen.
it will make a ratchting sound as the prop rotaes and the clutch hops in and out, but the drive shaft wont rotate.
however you just went from a very high pressure in the ex stack to nothing and the back wash wave is going to rise and start filling cylinders.
works that way in stern drives as well.
but the engine is NOT going to rotate backwards by the prop.

Humm, sounds familiar from another thread....

POST #5: http://www.yamahaoutboardparts.com/f...n-th29468.html

Quote: "Let's presume a guy is moving at cruise speed and a walrus surfaces right in front of the boat. In his haste to retard the throttle, being the critter lover he be, he over shoots neutral and puts the motor into reverse gear. The engine dies. The water flow against the propeller will then turn the motor backwards. The intake becomes the exhaust and the exhaust becomes the intake" End quote..

[99yam40]

OK
I just went out and checked my little C40 and that prop /clutch dogs do not slip when motor is in rev or forward when I turn the prop by hand in either direction. The motor will turn CW and CCW


Everone go out and check to see if their props lock in one direction and not the other when in gear
Maybe that was the old 35 Johnson I ran for years I remember doing that.

I remember some thread talking about lower unit noise when backing off the throttle suddenly, seems it was pointed to the clutch dogs

[boscoe99]

OK
I just went out and checked my little C40 and that prop /clutch dogs do not slip when motor is in rev or forward when I turn the prop by hand in either direction. The motor will turn CW and CCW

Everone go out and check to see if their props lock in one direction and not the other when in gear
Maybe that was the old 35 Johnson I ran for years I remember doing that.

I remember some thread talking about lower unit noise when backing off the throttle suddenly, seems it was pointed to the clutch dogs

Good for you.

Nothing wrong with being skeptical. Those that are sometimes need to see for themselves. It is what I do if and when needed.

I did not believe my Mama when she said the pot was hot. I 'spearmented to see for myself. She was right. Made a believer out of me.