Outboard freewheels in high revs

[demonboy]

Developed a strange complaint on our 4hp Yamaha OB recently. If anyone can suggest what's wrong I'd be very grateful. I'm not very good with OBs (yet).

When in low revs the OB performs fine, but when I rev up it's like the OB goes out of gear and loses power, though the engine screams away.

Someone suggested that it could be a loose prop. Had a quick look at the prop and doesn't appear to be loose but perhaps I'm looking in the wrong place (I do know where the prop is though!). If it could be this what should I be looking for?

Could it be an incorrect oil/petrol ratio? I used a medical syringe to measure the oil so I'm convinced it's spot on. Again, can't see that this would have anything to do with it since it works ok in low revs and doesn't produce black smoke or anything.

Any suggestions gratefully received.

 

[sarabande]

May be the prop is on a rubber bush, and at higher revs the bush slips ?

Which model Yam ?

The yamaha site

http://www.yamaha-motor.com/outboard/parts/home.aspx

gives exploded diagrams for all their motors, so you should be able to check .

 

[Lakesailor]

It could be the shear pin in the drive shaft has sheared. At low revs the interference between the broken ends of the pin supplies drive but as revs (and torque) rise the grip fails and the prop stops driving.
Take the split pin out of the prop hub nut and unscrew it. Pull the prop off (it may be hard to remove) and inspect the pin in the shaft. You'll probably find three bits of pin. Two bits loose and one left in the shaft.

 

[demonboy]

The model is a 3A (thought it was a 4, but there we go).

I'm back in the boat yard tomorrow so I'll inspect the pin then. Thank you.

 

[LadyInBed]

Sounds like cavitation to me
Try altering the vertical angle on the transom

 

[mitchc]

Ventilation my dear boy Ventilation!

Prop breaks (or nearly breaks) the surface of the water and sucks in air, causing it loose "grip" on the water

From Wikedpedia:
"Cavitation can occur if an attempt is made to transmit too much power through the screw. At high rotating speeds or under heavy load (high blade lift coefficient), the pressure on the inlet side of the blade can drop below the vapour pressure of the water, resulting in the formation of a pocket of vapour, which can no longer effectively transfer force to the water (stretching the analogy to a screw, you might say the water thread 'strips'). This effect wastes energy, makes the propeller "noisy" as the vapour bubbles collapse, and most seriously, erodes the screw's surface due to localized shock waves against the blade surface. Cavitation can, however, be used as an advantage in design of very high performance propellers, in form of the supercavitating propeller. (See also fluid dynamics). A similar, but quite separate issue, is ventilation, which occurs when a propeller operating near the surface draws air into the blades, causing a similar loss of power and shaft vibration, but without the related potential blade surface damage caused by cavitation. Both effects can be mitigated by increasing the submerged depth of the propeller: cavitation is reduced because the hydrostatic pressure increases the margin to the vapor pressure, and ventilation because it is further from surface waves and other air pockets that might be drawn into the slipstream......."

If possible lower the height of the engine, or if on a RIB/tender sit further back/forward as needed to trim the boat correctly. Most engine's have "trim pins" that can be adjusted to alter the angel of the engine....

Good luck

 

[ChrisE]

I'd go with cavitation as well. Our Yam Malta was a s*d for that.

You can tell if that's what it is by looking at the prop. It is cav then the prop will be whirring around and will grip again if you ease off the revs. If you have broken linkages the reverse will happen ie the prop will be still (ish) at high revs and whirring at low.

As others have said try fooling with the angle of attack. We did all that and eventually accepted that under some conditions cav wld occur so we just eased off when it occurred.

 

[demonboy]

Well, I just went out for a quick hack in the dinghy, altering the angle of the OB, and not much changed. However it did occur to me that perhaps we are sitting too far back and the one time I leant forwards it managed to go a bit higher in revs without the problem occurring. Still couldn't get it to full revs so when we nip ashore this evening I'll experiment by sitting further forward.

I believe it is a Yam Malta since it says 3A on the engine. Doesn't say Malta anywhere on it but it's the engine we inherited when we bought Esper so it'll have to do for the time being. The bright red Plastimo flat-bottomed dinghy is just an embarrassment!

 

[VicS]

If the prop is immersed to the correct depth (or more but not less)and is below the bottom of the transom, it should not draw air down into it.

True cavitation is very unlikely. It would require a serious mismatch of prop with engine and boat for that to happen.

If the prop is driven by a sheer pin then check and replace that, Seriously though if that were the trouble I would expect all drive to the prop to be lost, fairly soon even if not immediately. You would be able spin the prop freely by hand.

I do not know if you have a rubber bonded hub on that engine but if you do that is a likely cause of the trouble. You might be able to turn the prop by hand without the engine turning.

BUT all this assumes that your engine is clutchless ie has a fixed forward only drive. Some have an automatic clutch that engages as the revs are increased.
If it has a manual or automatic cluch then this could be the cause of the problem.

 

[misterg]

I'd go for the rubber bush, too.

It happened to our Honda 8HP 4 str twin last year. Even though the bush was slipping, I could still turn the engine over by turning the prop. It wasn't until I took the prop off, and locked the hub on the workbench that I proved that the outer was spinning on the rubber.

A simple way to check for this would be to mark the prop relative to the driveshaft, take it for a spin (making sure the problem happens), and then check whether the marks still line up.

Castle Marine couldn't get a bush to suit our old prop, so I ended up getting a second hand one from Bill Higham.

I've always associated a sort of intermittent loss of drive with ventilation / cavitation (like the waa-waa-waa- noise that jetskis make /forums/images/graemlins/blush.gif) , but when the bush went, after a certain throttle setting, the motor revs just took off, and stayed there.

HTH

Andy

 

[KellysEye]

Typical symptoms of a failed rubber bush, at some point the prop has hit something or it has just failed. Take the prop off and take a look - the rubber should be splined, if it isn't...

 

[LadyInBed]

"If the prop is immersed to the correct depth (or more but not less)and is below the bottom of the transom, it should not draw air down into it. "

On an inflatable air gets to the prop by getting trapped under the hull at the bow end then traveling aft.

If as suggested by DBoy he is sitting too far back, then this will add to the problem as the boat is way out of trim.
It isn't said how 'tall' the transom is, but if the prop isn't seeing clear water then it might be necessary to take a \__/ out of the transom.
The anti cavitation plate should be below the bottom of the transom.

 

[VicS]

[ QUOTE ]
It isn't said how 'tall' the transom is

[/ QUOTE ] He says this is a recently developed problem. Transom height can't have changed and it is unlikely that he has changed the trim of his dighy by sitting somewhere different. He might have changed to a different dinghy of course but then he would surely have noticed that while the engine was ok on the old one it isn't on the new one!

If it's a recent problem and its is the same old dinghy it must be a problem with the engine, shear pin or rubber hub, which ever it has, is the most likely followed by an internal gearbox fault.

 

[demonboy]

Yeah, I've written off cavitation. Must take the prop off tomorrow and have a look....been really busy with forest fires here in Bodrum right now!

Thanks for the useful posts.

 

[Lakesailor]

Let us know the outcome.