Prop spins quickly in neutral

[DennisG01]

I've been meaning to ask this for a while now. I've had this engine for a few seasons and it's always been like this... When in neutral (out of the water), the prop will spin quite fast - at least like when you would put the engine into gear at an idle speed. I CAN stop the prop and it doesn't take much force (I used a piece of wood and slowly went "with the rotation" of the prop). But I've never had an engine where the prop spins as fast it does with this one. Is this normal with this era of a Yamaha? It will still spin when in the water and in neutral, but much slower.

I have, on a number of occasions (aside from when I change the gear oil, annually), pulled the drain screw on the lower unit to check the fluid for water intrusion, but I have never found any. Obviously, my fear was that the seals are bad.

This video shows the issue in a different way. We're just floating along here, and I noticed the prop spinning easily from the current.

https://youtu.be/9jAvxuBIxTE

[robert graham]

It's normal...friction of 80-90 gear lube with the gears....no problem there!

[zenoahphobic]

You have smooth bearing and little seal drag, or your oil is to thin. At a guess the prop should turn slower, but turn, with 90 oil.

This video demonstrates the danger of getting near the prop even when engine is not running. Particularly boarding this vessel!!

[TownsendsFJR1300]

In that video Dennis, you have a pretty good amount slow of water spinning that prop alone (look how fast the grass is floating by!!)

Also, what I do when I have passengers (or myself) boarding (or anchored up actually) from the swim platform, leave the engine OFF and IN GEAR.

It locks up the prop from free spinning.



If the engine is running and in gear a disaster in the making for anyone in the water.

[DennisG01]

I use the Merc Synthetic stuff, so definitely not a 'too thin' issue. And the water is only about 60*, too. There was definitely a good current, which was the exact reason I made sure to get some of the floating weeds in the video so someone else would have a reference point.

The 'smooth bearing/seal' comment... Is that something I should be worried about? It seems to me that it might be, since the seal may not be "tight" enough? But I could just be thinking too much...

With all of the different boats/engines I've ever had, I just have never seen a prop spin this easily. But since there is no/zip/zero indication of water intrusion, I wanted to get some other opinions from people that have more experience with big Yamaha's (I've never even owned a small Yamaha, before).

I've had plenty of Mercruiser's... Bravo's will never spin because of the way the shifting point is setup. But Alpha's shift into and out of gear similar to the Yamaha and the gearcases are relatively the same size. But I've never had an Alpha spin this easily.

[boscoe99]

My Mercury 25 HP two stroke, Mercury 175 EFI two stroke and Yamaha F200 would all turn the propeller shaft when the motor was in neutral and being run on flush muffs. Completely normal.

The propeller shaft is not coupled/connected to anything when the lower unit is in neutral. Water current can cause it to turn if the boat is stationary.

If being run on flush muffs or the garden flush fitting, gear oil inside being turned by the gears can cause the propeller shaft to rotate just like a torque converter on an automobile used transmission oil to turn the input shaft to the transmission.

[DennisG01]

Boscoe, about how fast would your props turn when on a garden hose? Mine turns VERY quickly. Although I can easily stop it, it spins as though the engine is in gear. It absolutely is NOT in gear, but that's 'roughly' how fast it appears to be spinning - or at least how I can try and describe it for you guys.

[boscoe99]

Boscoe, about how fast would your props turn when on a garden hose? Mine turns VERY quickly. Although I can easily stop it, it spins as though the engine is in gear. It absolutely is NOT in gear, but that's 'roughly' how fast it appears to be spinning - or at least how I can try and describe it for you guys.

The 25 HP with a light aluminum propeller would turn very fast. Not sure about the gear ratio but I suspect it was 1.85/1 or so. The blades would be a blur.

The F200 with a heavy stainless propeller and 2/1 gear ratio would not turn as fast.

[TownsendsFJR1300]

About 45 seconds into this video of my F150 on muff's, cold start up, the prop does not spin at all (SS prop too- a bit larger than yours and heavier);

1st Link, Flushing F150 Video by SRT-60 | Photobucket

With allowed tolerances, etc, your's may have been shimmed up on the tighter end of spec's, but I really don't think you have a problem at all.

I gather the idle is NOT TOO high?

[DennisG01]

Thanks, guys. I do have an SS prop. I'm not sure of the gear ratio, although I suppose that doesn't make much of a difference since we're talking about neutral?

Correct, idle RPM (in the water) with the engine warm is 700-ish. On the muffs, of course, it stays a bit higher since it's a 2-stroke.

I guess as long as I'm not seeing water intrusion or an oil slick out back I should be happy that it spins so easily (less drag)?

[TownsendsFJR1300]

Yepper...

Its certainly NOT hurting anything, doesn't leak, works great, NO issues, go FISH!!!

[zenoahphobic]

Yepper...

Its certainly NOT hurting anything, doesn't leak, works great, NO issues, go FISH!!!

What an interesting thread, how tempting to tell DennisG01 there is something wrong with his motor. I think we are a bit jealous that his prop turns so freely!

Dennis you must have an extremely balanced prop, at first thought your prop must be extremely efficient and was put on backwards. But that can't be true because it looks normal and you say spins under earmuffs. So I have to be the devil's advocate and think a bit harder about this.
This is just food for thought, as I am approaching this as a problem, and therefore explore what is wrong with your motor.
One, if your dog has worn away it's prongs, the friction turning in oil would be reduced.
Two, if your seal is reaching the end of it's life, friction will be reduced.
Three, similarly if the seal has worn away your shaft, friction will be reduced and your shaft is becoming "faulty".

The seal seals by it's ability to reduce it's diameter following movement in the shaft as it rotates.
If the seal wears too much it will nolonger at some point keep contact with the shaft, and it then fails.

You could have the condition where the seal has worn and is cutting into shaft. It may also have created a neat groove in this shaft. The seal nolonger pressing flat against the shaft. This riding in a groove also reduces friction. Through very little contact but sufficient to maintain a seal.

So if you went to a thoughtful mechanic (also keen to make a buck) he may suggest the seal is not going to last much longer.
But if you change the seal you may end up with less of a seal because the new seal will nolonger line up with the groove in the shaft (however small, it is still there). So the shaft needs fixing/replacing.

One can reasonably say don't fix something that isn't broke. Nevertheless things wear.

Food for thought.

[TownsendsFJR1300]

The only seals that would cause less drag on that prop shaft ARE the prop shaft seals. the upper case seals wouldn't make a difference for the drive shaft.

Many if the above seals in question, the SS spring that provides tension on the seal IS VISIBLE (at least the outer seal).

Re-seal the case? re-seal the prop shaft seals, sure. It'd be a good precaution.

Pull the entire unit apart and look for worn dogs, re-check shims/ play, etc, sure. WAS IT SHIMMED too tight from the factory? Its been that way for years...

$1,000 to go that deep when he ISN'T HAVING ANY PROBLEMS, IMO, I wouldn't.

A waste of time and $ un-less you have $1,000 burning a hole in your pocket.

***And that's hoping you get a tech that knows how to shim up a case, etc. Might turn out worse getting that deep.

Seals, pretty safe and difficult to screw up. And yes, seals(parts wear).

One of my two upper drive-shaft seals had the SS spring fail, no leakage. The shop (prior to inspection) strongly suggested changing them as they WILL fail over time no matter the hours or how well you flush the engine.


Zeno, please post what you find with your tear down and maybe a before and after video.

[boscoe99]

My port motor idles at 775 RPM. My starboard motor idles at 745 RPM. Something must be wrong. I suppose I should tear into both motors and find out what is the problem. One seems to have too much drag or the other seems to have not enough.

Jeeeeze louise.

[DennisG01]

Thank you, guys. I appreciate all the responses... sarcasm and all!

This was just a new experience for me - having a prop spin so fast and never owning a large outboard (50HP and under, but also many stern drives and inboards where none of them spun like this one). The comment about the seal not sealing so well was my main concern, especially since the boat stays in the water for the season.