I replied earlier in haste, but another clue here is the forward-reverse thing
as well as the low rpm banging only.
The shifter moves a bronze clutch back and forth, this has a conical surface on
each each end that mates with one mating female cone for forward, and the
other mating female cone for reverse. In neutral, the conical clutch sits
between both and engages neither. Forward is used a lot more, therefore it
glazes first.
As to rpm- these boats do not have thrust bearings so the prop thrust pushes
forward on the engine/trans and ultimately the mounts. It does so via the
trans, so the thrust overcomes the slip at a certain point and the banging
stops.
Before long it will slip irrespective of rpm, and get so bad that you will
think the strut will be sheared off the hull.
Per instructions in the internet I used valve lapping compound. Did not
rebuild the gearbox, only lapped the cones. the halves use no gaskets, just
silicone.
I used a punch to mark the orientation of the cone and any other parts and was
very careful to clean any residual lapping compound out of the transmission.
Worked perfectly.
Make sure you secure the prop shaft so it can't fall out.
Dave.
Date: Sun, 8 May 2016 15:40:53 -0700
From: Andrew Means <andrew.cnc...@gmail.com>
To: cnc-list@cnc-list.com
Subject: Stus-List Soft thumping/banging when in gear at low-rpm?
Message-ID: <etPan.572fc098.5095526e.13b@worsley.local>
Content-Type: text/plain; charset="utf-8"
Boat: 1977 C&C 34
Engine: Yanmar 3GM30 (F)
Prop: 3 Blade Max Prop
Recently I noticed that when engaged in forward-drive, at low RPM, it almost
feels like something is banging against the bottom of the hull, like a piece of
kelp or rope tangled in the prop. Once the RPMs get above 1400 or so the
roughness goes away completely and everything feels very smooth.
Sent from my iPhone
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