Can anyone recommend a mechanic on East Coast - or what is this knocking noise?

F

frlrubett

Well-Known Member
Joined
2 Mar 2020
Messages
78
Location
Rochford
Visit site
Based on another thread on this forum, have found a new boat which is perfect... nearly.

The engine Yanmar 2GM20 'bangs' on idle. Couple more revs on it and it sounds perfect.

My immediate thought was engine mounts, but I am no expert. Anyone know someone who could diagnose this? Or what can I look for?

There is a 2019 survey on the boat but it does not include anything about the engine.
 
That sounds familiar, but it could be a variety of things. Hopefully my little anecdote might help.

I had a similar thing after a well respected yard had rebuilt and refitted my Perkins. The noise did not come to light immediately and I was off the coast between Ile d'Yeu and Sable D'olonne when I did first notice it.
At Sable D'olonne I got under the boat with a mask and had a good look around, then inside I checked all my through hull for the prop, checked gear box mountings and finally found that one of the "L" shaped brackets that hold the engine to the plinth was loose. One bolt goes vertically down into the plinth, one bolt horizontally into the engine block. The two studs were too long and the nuts could not be tightened on both. I cut some washers and banged them onto the studs so I could tighten the nuts up. It solved my issue.
I had no idea what I was looking for when I started the investigation, but when I found the culprit I knew I had identified the source of the knock. So, I recommend having a good look around and seeing if you can spot an obvious source like I was lucky enough to.

Don't go under the boat with the engine on!
 
Last edited:
If the engine maintains a good hot oil pressure and what you're hearing is not just normal diesel combustion knock then engine mounts or prop alignment might be the first place to start looking, because if the mounts are compromised or the prop has gone out of alignment there may be enough movement when the engine is idling for the prop to knock on the hull where it goes through.

Try levering the mounts up and down with a bar to see how much movement there is and look to see if bolts are tight and rubber hasn't separated from metal and with the engine running in neutral - obviously - hold the prop shaft and push and pull it as hard as you can in all directions to see if doing that makes the banging better or worse.
 
  • Like
Reactions: Obi
Boat is Wivenhoe way. On the Colne, in the water.

I have looked at a similar boat with the same engine and hear what the usual diesel knock is, this is a level above.

So if this is worn engine mounts resulting in out of align prop. Is that an out the water fix or can it be done in situe? The top of the engine is below the cockbit, so a verticle lift is not going to be simple.
 
Might ask if you have checked out the Flywheel nut ? if loose , only slightly , it can allow the Flywheel to move lightly back n forth when on low revs , but on higher revs its maintained in one possitiond due to forces ; happy hunting
 
  • Like
Reactions: Obi
Edit/Update/ just realised you do not own this boat. Sounds like an SEP to me.

Ignore the following, it is not your boat :-) I'll leave it here in case it helps anyone else.

Don't "jump the gun", get in there and check things first: Visually, then as suggested gently use a pry bar and see if there is play in between engine and prop. Fly wheel is also worth checking. There is no point causing distress or concern until about what it could or could not be, at least until you have eliminated a few first principle basics as outlined in this thread.

I was worried about mine, did some research and asked questions. All it did was increase my concern.

The best thing I did was look at my engine to prop. I gave the engine block a gentle push and discovered what I suspected to be the cause.

The actual fix took a few minutes of looking and thinking "how"? Then it took me no more than ten minutes with bolt cutters, washers and a spanner.
 
frlrubett said:
The engine Yanmar 2GM20 'bangs' on idle. Couple more revs on it and it sounds perfect.
Click to expand...

Could it be as simple as the tick-over is set too low?

What rpm does the manual specify for tick-over? Does it have a rev counter? If not, others here will advise what you'll need to measure it.

I have no knowledge of your Yammer, but believe at least some fuel pumps have an adjuster screw for the tick-over.
 
Cobra said:
As has already been said, get her down to Brightlingsea and get either French Marine Motors or DB Marine Engineering to take a look. Alternately, I have a 'tame' marine engineer who will usually take a look for drinking vouchers!!!
Click to expand...

They all do it for beer vouchers.. the only question is whether they declare their drinking vouchers to the taxman?
 
I used to have an annoying engine knocking noise from time to time.

It turned out to be a stainless steel u-bend in the exhaust riser (where it turns around from rising to falling if you get my meaning) was banging against a frame in the lazerette. A bit of creativity with some string(!) managed to stay it in a position away from anything. The right way to do it would be to properly attach some stays to the stainless steel ubend and screw them to sensible places.
 
According to more than one post in a SailNet discussion the idle speed of the 1GM (and also 2GM & 3GM) should be 850 to 900 rpm. You can probably download the relevant Yammer manual from the internet to check.

One person there posted the following advice on adjusting the idle speed of a 1GM:
'As you look at the front of the engine, on the left is the throttle lever. On the front of that is a screw (should have a locking nut on it). Undo the locking nut, and then turn the idle adjuster screw [anti-*]clockwise until you have the idle you like (should be between 850 and 900rpm). Make sure the engine is warmed up before making this adjustment. It's very simple. Remember to tighten the lock nut back up afterwards.'
*n.b. This poster was responding to someone who's idle speed was too high, so if your idle speed is too low you would need to turn it clockwise, not anti-clockwise, and I have therefore shown the 'anti' from the original post struck through.


Checking the actual rpm without a rev counter. Note I've not done the following myself, so hopefully others will advise the practicality of the following 'in principle' advice.

a)To check the actual rpm you can download a strobe rpm checker app for most mobile phones. (Some of these apps are free.) n.b. Do not use a strobe if you are epileptic or have other health problems that may be adversely affected by rapidly flashing lights.

b) You will need to put a strip of white or reflective tape on the driving pulley on the front end of the engine crankshaft (not the driven pulley at the alternator end), with the tape placed on a radius - i.e. to make a line from the outer edge of the front of the pulley and pointing towards its centre.

c) Start the engine and allow to run for a few minute to warm up. Beware of the moving parts of the engine! Set the strobe to 14 (or 14.2 if available) hertz (=flashes per second), point at the pulley, observe which way the tape appears to moving, if at all. Reset the strobe to 15 hertz and repeat. (You may need to close hatches or otherwise reduce background light, as the phone strobe light is not very powerful.)

d) With the engine running at the correct idle speed of 850 to 900 rpm (which equals 14.2 to 15 revs per second) the tape should appear to be turning one direction (or stationary) with the strobe set to 14 hertz (flashes per second), and the opposite direction (or stationary) with it set to 15 hertz.

d) If it turns in the same direction at both 14 and 15 hertz it is either too fast or too slow. The Yanmar 1GM turns clockwise viewed from the front, so the tape will appear to be turning anti-clockwise if the idle is too slow, and anti-clockwise if it is too fast.

e) If it is out of spec, reset using the adjusting screw as described above. Ideally to 875rpm (=14.6 hertz), but so long as it's going in one direction at 14hz and the other at 15hz it is within spec.

(As an alternative to a strobe, there are video tachometer apps for phones, avoiding the flashing lights and perhaps easier to use in situations with high background light levels, for which the advice above can be adapted.)
 
Had a 2GM20 .Knocking noise appeared on low revs. Worry.
In that case it was the flexible greasing tube to the stuffing box which had shifted so as to rap on the hull.
 
You must log in or register to reply here.

Share this page

Top