Can someone explain this?

[pstephens46]

Posted on THT:

On a four stroke, water ingestion happens when coming off plane too fast and slipping the engine into reverse accidently. If the engine stalls, and it can at speed, the prop rotation will reverse the engine's rotation and the pistons will suck water into the power head.

[fairdeal]

some interesting concepts there.

I have some vague recollection from my youth, that it was possible to "jump start" a car with an automatic transmission - providing you pushed it >30 mph....

[boscoe99]

I'll try.

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. Water gets sucked up into the exhaust section of the motor and makes its way into a combustion chamber. The piston coming up against water does not fare so well. Rods bend. Pistons break. Other bad stuff happens. One Yamaha guy did it twice. Upon his receipt of his second replacement power plant, on a good will, basis he was told if this happens a third time don't come back cryin' to Yamaha. You are on your own the third time.

Put your truck into reverse gear with the truck on a hill. Push in the clutch and let off the brake so that the truck rolls down the hill. Let out the clutch. The engine will turn backwards.

[pstephens46]

I'll try.

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. Water gets sucked up into the exhaust section of the motor and makes its way into a combustion chamber. The piston coming up against water does not fare so well. Rods bend. Pistons break. Other bad stuff happens. One Yamaha guy did it twice. Upon his receipt of his second replacement power plant, on a good will, basis he was told if this happens a third time don't come back cryin' to Yamaha. You are on your own the third time.

Put your truck into reverse gear with the truck on a hill. Push in the clutch and let off the brake so that the truck rolls down the hill. Let out the clutch. The engine will turn backwards.

So this is possible? On a F150 or larger motor? The force of water across the prop is enough to spin the motor in the WRONG direction? Seems like more THT billsheit.

Terrifying! I may not be able to sleep tonight.

[TownsendsFJR1300]

I'll try.

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. Water gets sucked up into the exhaust section of the motor and makes its way into a combustion chamber. The piston coming up against water does not fare so well. Rods bend. Pistons break. Other bad stuff happens. One Yamaha guy did it twice. Upon his receipt of his second replacement power plant, on a good will, basis he was told if this happens a third time don't come back cryin' to Yamaha. You are on your own the third time.

Put your truck into reverse gear with the truck on a hill. Push in the clutch and let off the brake so that the truck rolls down the hill. Let out the clutch. The engine will turn backwards.

The above bolded (IMO) is wrong. The OB engine turns the same direction whether in forward or reverse. Exhaust is STILL coming out the exhaust valve (or port).

Put that same engine(not running) in reverse. With plugs out, spin the prop, in gear, in reverse.. Which direction does the engine spin? Normally...

Throwing it into reverse at speed (and high RPM's) will definitely damage the LU.

.

[boscoe99]

So this is possible? On a F150 or larger motor? The force of water across the prop is enough to spin the motor in the WRONG direction? Seems like more THT billsheit.

Terrifying! I may not be able to sleep tonight.

Absolutely, positive. Has happened on all brands of four stroke outboard motors. Which is one reason why Yamaha and other brands to say to only shift gears when at an idle RPM.

[ausnoelm]

Has anyone tried to turn a prop with the motor in gear? It "ratchets" one way, so, how does the prop turn the motor backwards in ratchet mode??

[TownsendsFJR1300]

Has anyone tried to turn a prop with the motor in gear? It "ratchets" one way, so, how does the prop turn the motor backwards in ratchet mode??

It doesn't. That's why all this is BS.. The engine turns the same direction
NO MATTER WHAT GEAR IT'S IN.

[boscoe99]

[TownsendsFJR1300]

Absolutely, positive. Has happened on all brands of four stroke outboard motors. Which is one reason why Yamaha and other brands to say to only shift gears when at an idle RPM.

I call BS on "why Yamaha and other brands to say to only shift gears when at an idle RPM".

It's to keep you from destroying the LU.

With that thinking, I can be cruising at 10 knots, say 1800 RPM. Slam it into reverse and the engine is going to go backwards??? Absolutely BS..

[TownsendsFJR1300]

Yes that ^^^^ can happen BUT that's NOT what you posted earlier(post #3).

Quote: "The water flow against the propeller will then turn the motor backwards."End quote.

BS or not understanding..


That (engine turning backwards) IS NOT what is happening here (in the shifting post warning).

[boscoe99]

In the case of the Yamaha driver who sucked up water into his exhaust twice by shifting into reverse while going forward, both times the power plant was damaged. Beyond economical repair. His lower unit was just fine.

Life can be strange. Like a bus full of school kids going over a hundred foot cliff. First thought is that everyone has to be dead. No one could survive the carnage below. The bus is destroyed beyond recognition. Miraculously, no one is dead. Most are barely injured.

[TownsendsFJR1300]

In the case of the Yamaha driver who sucked up water into his exhaust twice by shifting into reverse while going forward, both times the power plant was damaged. Beyond economical repair. His lower unit was just fine.

Life can be strange. Like a bus full of school kids going over a hundred foot cliff. First thought is that everyone has to be dead. No one could survive the carnage below. The bus is destroyed beyond recognition. Miraculously, no one is dead. Most are barely injured.

The water pressure has to be higher than the exhaust pressure (which can easily happen in that circumstance). Agreed.

You posted the engine is turning backwards and sucked it up, that is NOT TRUE.

I have seen two strokes, when cranked backwards, fire up and run (no valves to deal with just reeds, etc). Four strokes CANNOT RUN BACK WARDS with the 4 working strokes needed to be in the correct order.

This is basic stuff....



.

[walleye1]

You are sso correct.

[ausnoelm]

A bit of confusion here, need to clear something up, no one is saying the engine is RUNNING backwards, it is "forced" backwards by the engine stalling at speed when shifting into reverse accidentally, the gear being in reverse and the water flow over the prop, rotates the motor backwards....so they say!