[C320-list] Knock, knock, who's there?

[Rick Sulewski]

Bruce, 
It sounds like you are experiencing the prop shaft knocking against the
hull's shaft tube under normal turning conditions which would point to a
worn cutlass bearing, or worn engine mounts, and/or the misalignment of the
engine to the prop shaft coupler, and/or the shaft is favoring one side of
the hull's shaft tube due to misplacement of the engine in its bedding/grid
(trusting that the shaft tube is true with the centerline of the engine
grid/hull).

To Jim and other owners who have experienced prop shaft knock and are
uncertain as to why, perhaps this explanation may be helpful: I  have
experienced a prop shaft knock when placing the hull under a rare and very
stressed condition when I make a sharp, pivot turn when the engine is
running at a consistent midrange or higher speed. I rarely make such an
evasive move under the extreme condition I described where I had to spin the
hull under speed. In that condition I believe the prop shaft began to
striking the edge of shaft log tube because the prop shaft was flexing under
the  rapidly placed stress as the stern section of the hull/strut area
withstood the lateral pressure of water flowing rapidly under the furthest
ends of the hull, both fore and aft of the keel (as stress was being placed
on both ends of the hull in opposing directions). When I first had to pivot
the hull at speed and experienced shaft knocking, my engine/prop couplers
where within tolerances/aligned, and the engine mounts were in good
condition (I checked). I suspected prop shaft flexing as the water was
momentarily forced to accelerate under the stern as it was sliding (as the
hull spins) before the forward motion of the hull ceased due to energy
transferred from forward motion to the lateral spin of the hull on its keel.
Sharply pivoting to starboard side induces pressure on the starboard side of
bow and then forces water under the portside side of stern at the same time,
or vice versa, if pivoting in the opposite direction. Under such a
condition, the prop shaft is the most flexible element at the time forward
energy is transferred to a spinning hull while the engine is under power,
and I surmised the prop shaft flexes (deflects) as the prop struggles to
bite. 

The other extreme condition I occasionally  induce also results in an
occasional shaft knocking condition. That occurs when there are very strong
cross wind conditions at my marina dictating that if I elect to leave the
dock I must rapidly back out of our slip to port in a tight fairway to
maintain rudder speed/directional control against a high wind speeds, and
then power up to move forward to rapidly spin the hull on its keel to
starboard as the prop digs in (creating flex stress on the shaft) as I
attempt to regain rudder speed and move forward to avoid being blown up
against the string of docks I just left. The knocking ceases when the hull
completes its spin and/or I back off the throttle as the hull accelerates
forward. I believe the rapid application of engine power combined with
spinning the hull causes that prop shaft to flex and the prop shaft begins
to strike the hull's prop shaft tube as the prop attempts to dig to move
forward. On the other hand, when I must return to my dock In high wind
conditions I may need to enter my slip under higher than normal speed  to
maintain rudder/hull direction and must rapidly throttle up after shifting
into reverse to avoid striking the bow against the dock, but have never
experienced shaft tube knock while revving the engine to reduce hull speed
while in reverse. This causes me to surmise that there was no lateral spin
of the hull at that time, and while in reverse, the prop is not biting with
the same resistance against the water. Therefore, the prop shaft under those
return to dock conditions was not forced to deflect (flex) and knock against
the shaft tube.  I also noticed that when we replaced the bronze shaft with
a stainless shaft, the incidence of shaft log knocking was greatly reduced.
Rick
 

-----Original Message-----
From: C320-list [mailto:c320-list-bounces at lists.catalina320.com] On Behalf
Of Jbrown5093
Sent: Thursday, May 14, 2015 7:03 PM
To: C320-List at Catalina320.com
Subject: Re: [C320-list] Knock, knock, who's there?

Does it also happen in reverse when in a hard turn?
Jim Brow

Sent from my iPhone

> On May 14, 2015, at 6:51 PM, Jim Sweet <jamesweet at frontiernet.net> wrote:
> 
> Bruce,
> 
> You have described , almost verbatim, a problem that we have had since
late last year.  The only differences are that the knocking doesn't sound
like it occurs with every turn of the shaft because it only seems to happen
at 1900-2000 rpm in a hard port turn.  The knocking  sounds like someone is
tapping the hull with a rubber hammer possibly in the aft lazerette.  I had
the boat pulled and the marina couldn't find anything and suggested that the
very minor play in the rudder meant that the bearings needed replacing.
Didn't feel right so I declined the service.  I have crawled around in the
lazerettes and can't find anything loose and I plan on pulling EVERYTHING
out of the storage areas to see if there is something loose.  I even checked
the aft water tank to see if there was any play in that.  Someone suggested
the transmission but I was below with She Who Must Be Obeyed driving to
produce the noise and it didn't seem to be anywhere near the tranny or shaft
coupling.  Even my gearhead friends are stumped.  If anyone finds a solution
or what the problem is please post it.  Thanks.
> 
> Jim Sweet
> TGIF (Thank God It Floats) 902
> 
> 
> ----- Original Message ----- From: "B. Stumpp" <bstumpp at comcast.net>
> To: <c320-list at lists.catalina320.com>
> Sent: Sunday, May 10, 2015 4:40 PM
> Subject: [C320-list] Knock, knock, who's there?
> 
> 
>> We went out for our first Spring sail this afternoon.  Everything 
>> worked great BUT with the engine running and turning left I get a 
>> knocking sound with what seems like every revolution of the prop 
>> shaft.  It does not make the noise when going straight, turning right or
in either direction in
>> reverse.     If I speed up the engine the noise  goes faster.  I stuck my
>> head in the engine compartment both in front of and behind the 
>> bulkhead, the sound is emanating farther back towards the stern.  
>> It's not  a real loud knock but kind of like someone lightly tapping on
the deck with a
>> hammer as a surveyor does when checking for moisture.   Before we
launched
>> the cutlass bearing and strut all seemed fine.  Has anyone 
>> experienced this before?
>> 
>> 
>> 
>> Bruce Stumpp
>> 
>> Adventure, #647
>> 
>> Frog Mortar Creek, Maryland
>