What's the technical term for...

[skynyrdcat]

when a propeller turns slightly while in neutral?

From my experience it's fairly common, but what causes it and what's it called?

Thanks

skynyrdcat

 

[JB]

Re: What's the technical term for...

It is caused by the circulation of oil in the gearcase. The circulation is caused by the turning of the engine driveshaft.

I don't know if there is a term for it.

 

[rodbolt]

Re: What's the technical term for...

surface tension
all fluids have a certain amount, the thicker or more viscous the fluid the more tension usually.
as the propshaft passes through the gears and the gears rotate anytime the drive rotates the oil will try to make the shaft rotate with one of the gears occasionally.
its common, some cases do it more than others just be aware that 15" rotating SS prop can be dangerous even turning free.

 

[steelespike]

Re: What's the technical term for...

I'll try to explain; the forward and reverse gears run free on the prop shaft and are allways in motion this sometimes causes the prop shaft to turn.
There is a clutch dog that is splined to the prop shaft between the gears when it ingages one of the gears it drives the prop shaft.I don't know if there is a technical term to describe the neutral prop shaft spin.

 

[F_R]

Re: What's the technical term for...

The technical term is friction. You have two gears spinning in opposite directions, with the prop shaft running through both of them, but not connected to them.

When running in gear, the prop shaft is connected to that respective gear and the other gear is still spinning on the shaft--but it is spinning in one direction and the shaft in the other direction.

This spinning in opposite directions at slow speeds is not a problem. That would be the case when running in reverse.

However, when running in forward gear the shaft and reverse gear are spining in opposite directions at very high speed. That requires a very good bearing in the reverse gear.

So: The forward gear has a cheap bearing because it spins in the opposite direction only at slow speeds. However the reverse gear has a good bearing because it spins in the opposite direction at high speeds.

Back to running in neutral: Remember, we have two gears spinning in opposite directions on the prop shaft. But the forward gear is spinning on a cheap bearing, while the reverse gear is spinning on a good bearing. The cheap bearing has more friction, translated as drag against the prop shaft, so the shaft tends to go along with it.

I tried to make this understandable. Is it??

 

[dingbat]

Re: What's the technical term for...

However, when running in forward gear the shaft and reverse gear are spining in opposite directions at very high speed. That requires a very good bearing in the reverse gear.

So: The forward gear has a cheap bearing because it spins in the opposite direction only at slow speeds. However the reverse gear has a good bearing because it spins in the opposite direction at high speeds.

Back to running in neutral: Remember, we have two gears spinning in opposite directions on the prop shaft. But the forward gear is spinning on a cheap bearing, while the reverse gear is spinning on a good bearing. The cheap bearing has more friction, translated as drag against the prop shaft, so the shaft tends to go along with it.

I tried to make this understandable. Is it??

You lost me back with you started with this good bearing and bad bearing thing.

There are lots of forces at work to turn the prop. Here is one.

Momentum Diffusion

 

[rodbolt]

Re: What's the technical term for...

wow, I am amazed that cheap bearing can handle the thrust load of a large V6 up to 300 HP without disentergrating.
actually most outboards have a ball bearing or some a plain sleeve for reverse as there isnt much load and a torrington or a tapered roller for fwd.

 

[JustJason]

Re: What's the technical term for...

on newer outboards you don't even have to shim reverse anymore.

the technical term for the prop spinning in neutral is called "thick oil"

don't believe me???

Drain out your "thick oil"

replace it with something light... like 5w30...

see if it still does it...

 

[TOHATSU GURU]

Re: What's the technical term for...

The phenomenon is caused by the oil current flow, but it has no official name...Until now! I hereby name it____________.

So lets all pick a name and become famous. I kind of like "Gear Case Fever".

 

[F_R]

Re: What's the technical term for...

wow, I am amazed that cheap bearing can handle the thrust load of a large V6 up to 300 HP without disentergrating.
actually most outboards have a ball bearing or some a plain sleeve for reverse as there isnt much load and a torrington or a tapered roller for fwd.

I KNEW IT was going to happen. Try to make a simple explanation that can be understood by the novice and somebody is going to challenge it.

I was NOT talking about the thrust bearings. I was talking about the bearings inside the gear that support the gear on the prop shaft. It is impossible to include the makeup of every outboard on the market in a few short paragraphs, but I am sure you will find that your V-6 has a bushing inside the forward gear---a "cheap" bearing. And in this case, it doesn't even have a bearing in the reverse gear, the gear instead being supported by the bearing carrier. So, there is no gear drag at all from the reverse gear acting on the prop shaft. But there IS drag from that bushing in the forward gear.

 

[dingbat]

Re: What's the technical term for...

The phenomenon is caused by the oil current flow, but it has no official name...Until now! I hereby name it____________.

So lets all pick a name and become famous. I kind of like "Gear Case Fever".

Humm.... let call it Momentum Diffusion.

Momentum Diffusion refers to the diffusion, or spread of momentum between particles (atoms or molecules) of matter, usually in the liquid state.

In the case of the laminar flow of a liquid past a solid surface, momentum diffuses across the boundary layer which forms at the boundary where the solid meets the liquid. The gradient in this case occurs between the liquid in contact with the surface, which does not move at all and has zero momentum (see no-slip condition), and the liquid far away from the wall, which has momentum proportional to the speed at which it is flowing. The rate of transport is governed by the viscosity of the fluid and the momentum gradient.

 

[kenmyfam]

Re: What's the technical term for...

when a propeller turns slightly while in neutral?

From my experience it's fairly common, but what causes it and what's it called?

Thanks

skynyrdcat

Lets make it really simple.......Normal !!!!!

 

[mickarch]

Re: What's the technical term for...

The same thing happens on my motorcycle when it's on the center stand in neutral. The viscosity of the oil between the clutch plates creates drag. Since all the bearings are in good shape, the drag is enough to turn the rear wheel.
On a motorcycle forum I frequent, the phenomenon is referred to as a "viscous coupling".

Googling that turned up links to all-wheel drive car differentials that work on a similar principle but use a oil that changes thickness based on pressure, creating more or less drag.

HTH,
mickarch

 

[F_R]

Re: What's the technical term for...

Humm.... let call it Momentum Diffusion.

Momentum Diffusion refers to the diffusion, or spread of momentum between particles (atoms or molecules) of matter, usually in the liquid state.

In the case of the laminar flow of a liquid past a solid surface, momentum diffuses across the boundary layer which forms at the boundary where the solid meets the liquid. The gradient in this case occurs between the liquid in contact with the surface, which does not move at all and has zero momentum (see no-slip condition), and the liquid far away from the wall, which has momentum proportional to the speed at which it is flowing. The rate of transport is governed by the viscosity of the fluid and the momentum gradient.

Well he DID ask for a technical term. Say what??

I prefer the following one that calls it viscous coupling. The spinning forward gear/bushing drags on the oil which is a fancy word for sticky goo, and the sticky goo drags on the prop shaft. There, now is that understandable?

 

[jamesdeanj]

Re: What's the technical term for...

Fluid Dynamics. The surface tension that the rotating oil around the free spinning shaft overcomes the potential energy of the shaft sitting still, thus causing it to move. It's the same basis by which a torque convertor in an automatic car works. The spinning fluid applies pressure on the opposite fins.

 

[jtexas]

Re: What's the technical term for...

I really like "viscous coupling". The term, I mean. For the prop spinning in neutral. Oh forget it!

I want to call it "skinning the cat".

 

[Frank Acampora]

Re: What's the technical term for...

You fellow ALMOST have it correct: The prop shaft is carried in a bronze bushing in the forward gear to eliminate slop or play. The reverse gear is usually carried in a separate bearing and has clearance on the prop shaft to eliminate friction drag while in forward gear. (because the shaft and reverse gear are turning in opposite directions) When in neutral the Hysteresis of the oil in the bushing has more drag than the oil in the clearance between the reverse gear and the prop shaft, so the prop slowly spins in the forward direction.

Now before you get to arguing, the bushing in the forward gear is not the tapered thrust bearing. It is there to eliminate steel on steel contact when the engine is in reverse. When in forward, the shaft, gear and bushing all turn as one. there is no relative movement between them. Got It?

 

[ebry710]

Re: What's the technical term for...

I am a simple person and as of yet not torn into an outboard, so I'll called it "neutral prop spin".

If we were voting, I like kenmyfam simple explanation "because its normal" and anyone who said in "technospeak" as the engine spins so do the shafts below it even if they are not engaged.