Bad thumping noise BV1505

Mike · Member

Bad thumping noise BV1505

Farron · Senior Member

Hello Mike,

I do not think the problem is in the gear box.

If the shaft and propeller are straight and true and the cutless bearing is good-

I would next examine the flexible coupling between the gear box output flange and the prop flange. The flexible coupling is designed to fail and protect the drive components if you experience a sudden heavy loading on the prop shaft.

IE: Wrapping a dock line around the shaft and hard sudden shifting of the transmission.

If the flexible coupling checks good I would next look at the damper plate between the engine and gearbox.

Like the flexible coupling, the damper plate is designed to fail and protect the drive components if you experience a sudden heavy loading on the prop shaft such as you describe occurred.

Both the damper plate and the flexible coupling will fail safe and still allow you to motor but it will exhibit some degree of rough or noisy operation particularly at lower RPMS.

Best regards,

Farron

Mike · Member

Hi Farron,

You are right ! Spent most of the day checking on the problem, First fully decoupled the shaft from the gearbox. Ran the engine in forward and reverse and it ran smoothly in both directions. Would have expected some vibration/ noise if the problem was in the gearbox ?. Turned the shaft manually while decoupled and could feel a definite hit when turned to go forward. The end of the shaft was right up against the flexible coupling. Moved it back the required 10 mm... no improvement. Removed the flexible coupling and connected the gear box output flange and the prop flange directly to one another. No improvement. You mention a damper plate. I don't know what this is. Is it upstream of the gearbox? I have had many theories on this problem. Now considering castellations/dogs damaged or the damper plates you mention. In any event it's now looking like a lift out

Farron · Senior Member

Hello Mike,

When decoupled from the gear box the prop shaft should turn free and easy with no "definite hit" being felt. If not, I would inspect the shaft/prop first.

The damper plate resides in the bellhousing between the front of the gear box and the engine flywheel.

Best regards,

Farron

Mike · Member

Hello Farron,

Thanks for the info. There appears to be a definite hit when turning by hand. It does not turn freely and I can hear what sounds like the squeaky resistance of rubber. I checked the engine mounts this morning and one of them seem to have moved. A few fairly gentle taps from a lump hammer and tightening the mounts seem to have greatly reduced the vibration. With a bit of luck I should get to the end of the season and then have everything inspected/repaired on lift out'

Best wishes

Mike

Farron · Senior Member

Hello Mike,

With the fouling of the line and vibration that followed and then to find a loose mount, I would recommend checking the alignment and shaft straightness at your very first opportunity.

Best regards,

Farron

Mike · Member

Hi Farron,

It very much looks like the engine was not well mounted and this has resulted in wear on the cutlass. I am satisfied that it is now reasonably well in line and with careful watching will be ok until late Sep when I will bring her to the local Beta agent for proper mounting, alignment and cutlass bearing replacement.

Thanks for your valuable advice,

Mike

Mike · Member

Hi Farron,

The saga continues one year later. By now engine has been re-bedded, cutlass bearing replaced, Gland and alignment checked. New mechanic now on the job. Stripped gear box replaced cones. reinstalled gearbox and still the noise. He checked everything and discovered wrong prop fitted. Checked PRM manual and looked at prop. He is definitely right. A right hand prop is fitted. Now looking at another lift out and a new prop. Do you think this is a likely cause. Only thing left after this methinks is the damper plate

Farron · Senior Member

Hello Mike,

A right hand turning propeller is required for your engine and marine gear. The proper maximum engine speed is 3000 RPM, in gear, in calm water and with a clean bottom.

It sounds like you needed to straighten a few things out and have done so.

Yes, it is entirely possible that the damper plate was damaged when you fouled the mooring line back in 2010. I am thinking and you have stated that all of these issues started at that time.

This is what the damper plate is supposed to do. The element twists and fails as a sacrificial link in the drive train. This sacrificial failure is designed to save many very expensive drive train parts forward and aft of the element. The element fails safe in order to allow you to continue motoring but it will no longer be effective as a damper plate.

Unfortunately the only way to inspect the element is to remove the marine gear.

The damper plate would have been very easy to see and inspect when the marine gear was a removed from the engine. You can see it just inside the bell housing.

The mechanic should have inspected the damper plate when the gear was removed. This is standard procedure any time a marine gear is removed from an engine.

Look for any deformation of the elastic element. The lobed edges of the element should be symmetrical around the center of the damper plate face. If they are not symmetrical the element has failed and the entire damper plate will require replacing.

Mike · Member

Hi Farron,
Thanks for the quick response. So a RH Prop it is. I guess by now the problem has to be the damper plate. The only thing that still surprises me is that I can see the gearbox slipping when I slowly engage forward gear. Does this indicate anything ?

Farron · Senior Member

Hello Mike,

The gear should not slip when forward or reverse is engaged. Slipping of the internal cone clutches will cause them to be damaged.

I suggest that you check the adjustment of the shift mechanism and shim as specified in the PRM 120 workshop manual.

Also: Slowly engaging a small mechanical gear is not advised. It is better to solidly follow through when shifting into gear. A smooth but positive shift allows the clutches to immediately and solidly engage.

Mike · Member

Hi Farron, I inspected the shift mechanism some time ago but I will have themechanic inspect it again and also the shim. I engaged the gear slowly only when looking for problems. Also I haven't checked for slippage since cones were replaced as I am out of the country at the moment. Can the damper plate be replaced with the engine in situ ?

Mike · Member

Hi Farron,

Appears there is definite slippage when going forward. Slippage and noise stops when revs are increased. Damper plate inspected and looks ok. Is it true to say that problem cannot be damper plate as there is no noise when going astern ?

Mike

Farron · Senior Member

Hello Mike,

If the issue is with the damper plate you would probably hear it in FWD and REV. The damper plate and input shaft turn the same direction regardless of which gear you are in.

If the gear is slipping after new cones have been installed you can figure improper adjustment to the shifting mechanism or that the FWD gear has a damaged female cone and this is not allowing the clutch cone to engage completely and properly.

Mike · Member

Hi Farron,

In the end the gearbox was the problem. Some expensive polishing and everything is now definitely ok.

I need to up the idle speed . I think I saw somewhere that 850 RPM is correct I can't find anything in the manual on how to adjust. Can you point me in the right direction please ?

Mike

Farron · Senior Member

Hello Mike,

A small screw with a locking nut is used as a low idle stop. The screw can be found near the pivot of the throttle arm. This screw sets the low idle position of the throttle arm.

If you loosen the lock nut, the screw can be turned in with a small screwdriver. Moving the screw in will move the throttle arm stop to a higher RPM.

Set idle to 850 - 900 RPM and re-tighten the lock nut.

Mike · Member

Thanks. Found it. Hard to get at