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OWT? (Old Wives Tale)?

Old 10-14-2020, 01:37 PM
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Default OWT? (Old Wives Tale)?

In another (discredited) thread it was discussed how water may have entered a 4-stroke engine.... and after some reading.... I became a subscriber to the theory that at high-speed... if the engine suddenly died that the relative water-motion of the still-forward-moving boat ...[[in the discredited thread a quad-installation had #3 engine die while 1,2 and 4 continued to propel the boat]].... That the relative water motion would spin the prop backward and cause the engine to suck water up via the underwater exhaust and into the cylinder when the exhaust valve opens. Since the engine is being spun in-reverse the exhaust valve would open and the descending piston would suck water up/into the cylinder causing a “hydraulic lock” and damage the engine severely.

Having sworn beer and other liquor off for 6- months in a weight-loss program I’ve given my previous endorsement of that theory some “sober thought” and I’ve come to the conclusion that theory is B.S. ...or an OWT.

Why? Because, in the case of a clockwise / right-hand prop...being spun by the engine to propel the boat forward... the passing water would hit the front-face of the prop .... and tend to STILL FORCE THE PROP CLOCKWISE..... not reverse as the former theory imagined.

Now I am better able to understand the disbelief others expressed in that goofy thread. I had thought it pretty plain to understand but now realize it’s B.S that the prop will turn the engine backward if the boat contlnues forward.

In addition, the check-valve in the exhaust system would UN-seat due to lack of exhaust-gas-pressures and, instead of sea-water coming back up from beneath the surface via the prop.... it would open the above-water vent and suck air EVEN IF the (dead) engine were to miraculously be forced into reverse-rotation (presumably by the operator shifting into reverse while the other engines were still forward.) TWO reasons it’s an impossible theory of how water gets into a cylinder from the exhaust of a dead engine.

THERE! My conscience is now clear since I’ve revoked my previous belief in an OWT.

Last edited by Fishinado; 10-14-2020 at 01:43 PM.

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10-14-2020, 02:32 PM
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I've taken 4 different boats for test drives and tried this. Not one of them has sucked water into the engine. One sales person broke his nose on the port side grab rail, one motor went into limp mode, two times my wife said, NFW am I testing another boat with you (note it took two times), and the 4th boat had duo-props and it actually stopped the boat in 1 boat length and started going backwards at 4500 RPM on a plane with my dumb ass now in the bow with my feet in the air and a the broken steering wheel in my hands.

I'm trying to find a marina within 500 miles that will let me test the next boat with Sharrow props.
Old 10-14-2020, 01:49 PM
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Oh boy, here we go. I tried for about an hour to sort this very thing out myself. Just did not make sense, but I was berated into believing it was the water rushing forward from the boat slowing down that spun it backwards. Like when you slow a boat down quickly and the water rushes up into the splash well.
Old 10-14-2020, 01:53 PM
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So what was the outcome? I quit reading at page 273 😂
Old 10-14-2020, 02:10 PM
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The outcome was that the OP was a douche.
Old 10-14-2020, 02:13 PM
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The prop never spins backwards in the scenario you just provided. You are correct - if the boat continues forward, the prop will continue in forward motion with it.

However if the shifter were inadvertently moved into R while the boat was rapidly moving forward, then the continued forward rotation of the prop could result in a reversal of rotation of the driveshaft, and thus the engine.
Get it?

Last edited by TIMarine; 10-14-2020 at 02:24 PM.
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Old 10-14-2020, 02:17 PM
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I thought the "theory" was that the engine was accidentally shifted into reverse while the other engines (or the boat) maintained momentum, thus being in reverse gear, the water flowing over the prop then stalls the motor and sends it spinning backwards, "sucking" water up the exhaust, simply turning a motor off or having it cut out while still in forward gear is not the issue.
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Old 10-14-2020, 02:22 PM
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Originally Posted by noelm View Post
I thought the "theory" was that the engine was accidentally shifted into reverse while the other engines (or the boat) maintained momentum, thus being in reverse gear, the water flowing over the prop then stalls the motor and sends it spinning backwards, "sucking" water up the exhaust, simply turning a motor off or having it cut out while still in forward gear is not the issue.
Yes, I think this is how you had explained it in the thread. I think I just miss referenced it. I do think though, the boat slowing the water is pushed forward towards the prop. So I would think that would exacerbate it, right?
Old 10-14-2020, 02:32 PM
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I've taken 4 different boats for test drives and tried this. Not one of them has sucked water into the engine. One sales person broke his nose on the port side grab rail, one motor went into limp mode, two times my wife said, NFW am I testing another boat with you (note it took two times), and the 4th boat had duo-props and it actually stopped the boat in 1 boat length and started going backwards at 4500 RPM on a plane with my dumb ass now in the bow with my feet in the air and a the broken steering wheel in my hands.

I'm trying to find a marina within 500 miles that will let me test the next boat with Sharrow props.
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Old 10-14-2020, 03:19 PM
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Originally Posted by From the Helm View Post
I've taken 4 different boats for test drives and tried this. Not one of them has sucked water into the engine. One sales person broke his nose on the port side grab rail, one motor went into limp mode, two times my wife said, NFW am I testing another boat with you (note it took two times), and the 4th boat had duo-props and it actually stopped the boat in 1 boat length and started going backwards at 4500 RPM on a plane with my dumb ass now in the bow with my feet in the air and a the broken steering wheel in my hands.

I'm trying to find a marina within 500 miles that will let me test the next boat with Sharrow props.

This man is my kind of scientist!
Old 10-14-2020, 05:40 PM
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Originally Posted by Capt. Fred View Post
This man is my kind of scientist!
Oh yeah I believe that! Just don't let him sea trial or in the same state as your boat, (P) CF . Kind of makes me think of Capt Caca researching the effects of pressure on unvented bladder holding tanks for some reason.
Old 10-16-2020, 11:04 PM
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Ok, I will take a stab.
First, cut out carbs and get back on the booze, especially if you are married. :-)
Second, I can't see the prop turning in reverse from the backwash.
Nevertheless, we are talking about the prop exhaust of an outboard or stern drive, right? Even if the prop was pushing water, couldnt water flow into the exhaust at the center of the hub due to enough backwash?
Finally, I dont know anything about exhaust check valves. I have had inboards for 14 years since my last stern drive.
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Old 10-17-2020, 06:23 AM
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Originally Posted by Fishinado View Post

Why? Because, in the case of a clockwise / right-hand prop...being spun by the engine to propel the boat forward... the passing water would hit the front-face of the prop .... and tend to STILL FORCE THE PROP CLOCKWISE..... not reverse as the former theory imagined.

.

It only “works” if you put the transmission in reverse. Of course the prop turns the same direction, but with the gears reversed the engine turns the opposite.
Old 10-17-2020, 06:30 AM
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Originally Posted by TIMarine View Post
The prop never spins backwards in the scenario you just provided. You are correct - if the boat continues forward, the prop will continue in forward motion with it.

However if the shifter were inadvertently moved into R while the boat was rapidly moving forward, then the continued forward rotation of the prop could result in a reversal of rotation of the driveshaft, and thus the engine.
Get it?
Yes, that is quite understandable. But in the referenced thread it was not a matter of shifting into reverse. It was a matter of an engine...in Forward Gear....suddenly shutting-down for some reason. In that situation it is not simply unlikely... it is Impossible for the dead engine in forward gear to be rotated backwards by water pressure of a forward-moving boat.

Yes... at high speed... if an operator should attempt an emergency-stop by shifting into reverse.... (I.E., bringing a high-speed forward-throttle quickly to idle-position, then into reverse/idle-position...and even further into an attempt at engine-powered-reverse... might result in the water-flow spinning the RH prop continuing in it’s RH rotation...but (the engine gear-train having been selected into reverse) might overcome the engine’s ability to rotate normally (since it’s momentarily at idle)... and the engine can “die” and be spun in reverse-rotation. That might allow an engine to “suck” whatever is in the exhaust system backwards into the engine (as the reversed piston descends while the exhaust valve is open in the obverse of typical 4-stroke cycle.). BUT that scenario is one of an operator attempting a high-speed emergency-stop shift into reverse. And that scenario ignores the exhaust-system check-valve which exists in many designs specifically to avoid such potential damage. Nevertheless, it would be good to know your system if one would avoid such hazards. It would be wiser yet to be mentally and observationally Ahead of your boat when at-speed in order to avoid such emergencies.

Some I/O designs (my old Volvo AQ125 comes to mind) have a simple rubber flapper at the underwater exhaust to prevent water-backflow into the exhaust system. It’s incumbent upon the owner/operator to inspect/maintain that flapper.
Old 10-17-2020, 06:55 AM
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when do you turn on the conveyor belt? like the airplane thread

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