C34 MkII Low Speed Transmission Chatter & Damper Plate

[Colonel Butler]

Last fall I started this thread looking for some input on solutions for a low speed clatter I was hearing in the drivetrain of my 2006 MkII. The boat was new to me and I was just not liking the sound I was hearing when idling in gear. All of the investigation I was able to  undertake on this and the C36 forum pointed to the damper plate being the problem. The original spec was too stiff, not absorbing the power pulses at low speed, causing backlash in the gear box.

Last fall I split the coupling and removed the gearbox, heat exchanger and bell housing to get at the damper plate. It was a R&D Marine polymer style plate rated at 3 degrees deflection. I talked to the folks at PYI who are the US distributor and they recommended that I change it to the high deflection model, 22AM4, which has 30 degrees deflection. They also recommended a flex coupling 910-014 for the coupling.

In the course of dismantling, I managed to uncover a couple of other issues. The heat exchanger mounting brackets had broken and it was held in place by the hoses. Also, a couple of hoses had almost chaffed through so I managed to address these issues also.

The cutlass bearing had worn and required replacement. This required the acquisition of a Strut-Pro to change it without removing the rudder.

This spring I had to wait for the Sailing Club to get out of COVID lockdown to get to the boat to start reassembly but finally, was able to get it put back together and get it launched in late May. I did an in the water shaft coupling alignment.

I am happy to report that the chatter is no longer present and the drivetrain is very smooth. Here are a few photos of the finished installation and the Strut-Pro in action.[/img]

[Ron Hill]

Colonel : You're looking good!!! 

[andre]

Hi sir, I have changed my damper plate and rebuilt my transmission before to go for a long trip in the south So my question is when you install the flex coupling do you need to modify the length of your shaft and what is the thick of your flex coupling. I have a 1 inch diameter shaft I think is like you. Thank you

[Colonel Butler]

The flex coupling takes up some of the limited room between the stock stuffing box and coupling making assembly a bit difficult. I had enough steam tube protruding into the boat that I was able to cut off approx 3/4" and shorten the stuffing box hose by the same amount and still get two clamps on the hose over the stern tube. the prop is now sitting about 1 1/2" further from the strut but I don't think that is any problem.

[andre]

Thank you

[Breakin Away]

You should be OK with your prop sticking 1.5" further out from the strut, but mainesail has noted on his websites that Catalinas, in particular, can be prone to dynamic whip of their shafts, and advises that the shaft be sized to place the prop as closely as possible to the strut. As I recall, there are many factors that contribute to this: prop type, weight, and setback of blades on the hub (he ditched his brand new, heavy, folding prop for this reason), shaft material (bronze worse than SS because it's softer), distance from prop to rudder (hydrodynamic interactions between the prop and rudder can make this worse), and number of blades on the prop (2-blade worse than 3-blade because both blades interact with the rudder at the same time).

This is all from memory, because I am unable to find his post on the topic on either of his websites. Perhaps he can post a link and/or additional comments.

I posted this not to criticize, but merely to advise you that if you discover vibrations in your system, to consider your shaft extending the extra distance beyond the strut as a possible root cause.

[Ron Hill]

Guys : The normal distance between the prop and the strut is "one shaft diameter".  For a C34 that would be one inch.   

A thought

[Colonel Butler]

OK, thanks. Will keep that advice in mind if I detect vibration. Right now it seems smooth throughout the RPM range. I am using a Varifold 3 blade propellor.

[Breakin Away]

OK, thanks. Will keep that advice in mind if I detect vibration. Right now it seems smooth throughout the RPM range. I am using a Varifold 3 blade propellor.

I think that 3 blade is much less likely to suffer this sort of problem, which is why I went with a 3 blade Flexofold instead of 2 blade.

Although I could not find any reference on mainesail's websites, I did find this forum post. I recall reading additional details about bronze vs. SS shafts (and other contributing factors) somewhere else, but can't find them now:

https://forums.sailboatowners.com/threads/folding-prop-is-gone.141820/

[Jon W]

Colonel, lately I started hearing (or noticed) the chatter at low rpm's when in forward gear and remembered your post. Is the 22AM4 high deflection damper plate still smooth and quiet a year later? Which engine and transmission do you have? Thanks for the help.

[Ron Hill]

Jon : The C34 MK II came with a M35 BC engine.  Completely different damper plate than your M25XP engine!!

A thought

[Jon W]

Thanks Ron. I saw the silver engine in his photos, but you never know so thought I'd ask.