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Boat shakes when on gear at 1000-1200 RPM

standsy

New member
Hello,
I'm new on this forum and I have a very strange issue with my boat.
I have a small trawler with Penta MD21 and direct drive. Hull speed of the boat is 6 knots. The engine runs perfectly when not in gear with reasonable vibration on any RPM (700-2000). All 4 cylinders work, no misfire. Engine mounts are new and tight. Prop is balanced, engine and shaft are aligned to .0003"-.0005", shaft is straight.

When I engage the gear the magic begins.

1. Idle at 700 - 900 RPM - engine runs smooth and boat vibration is within limits.
2. 1000 RPM - boat vibrates a little bit rough but still ok
3. 1100 RPM - the boat starts shaking back and fore like something push her at stern with a frequency of 1-2 pushes per second.
4. 1200 RPM - shaking frequency increases, the whole boat begins pumping hard.
5. 1300-1600 RPM - Pumping/shaking amplitude increases, the whole boat is like in fever. The boat continue shaking until he reaches hull speed of 5.5-6 kts, then suddenly the shaking disappears and boat runs smoothly. Now I can reduce RPM back to 1300 and boat runs well - no pumping. Increase RPM back to 1600 and more - no issues: smooth running boat. Back to 1300 RPM - no issues.
6. Slowing down to 1100 - 1200 RPM - while the boat slows down, the shaking increases proportionally to the reducing of the speed and continue pumping until I increase RPM back to 1600-1800 RPM and boat gets hull speed.

I noticed, that when engine is cold, such effect is much lighter, may not appears at all. The shaking is less likely in cold May water than in August. Sailing against current reduces shaking RPM range.
The pumping occurs only in forward gear, reverse is fine and smooth in any RPM.

Please advice.

Thanks.

Alex.
 
Last edited:
Really hard to diagnose, but some quick thoughts about possibilities:
1) Broken engine mount(s)
2) Unbalanced prop blade angles (blade a bit bent etc)
3) Prop tip too close to hull (what is the tip to hull clearance?)
4) Bad gearbox shaft bearings
 
Had exactly the same situation when I had a Atomic 4 with a 5 ft long7/8 shaft with a gori prop.
Finally changed out the coupling to trans and that fixed the problem. Thinking back a vibration
coupling might have been another good idea. It was suggested that harmonics due to the length of
the prop shaft might also be in play. This might be mitigated with a change of the cutlass bearing.
Finally make sure your packing nut is properly adjusted, just incase.
Now for a long shot guess.. if the vibration is variable with temp.. check the waterpump. Sounds crazy
but mine finally seized and burned my belts. The boat only vibrated after a hot run, then gradually smoothed
out at idle. After changing the pump the symptoms never reappeared.
 
Really hard to diagnose, but some quick thoughts about possibilities:
1) Broken engine mount(s)
2) Unbalanced prop blade angles (blade a bit bent etc)
3) Prop tip too close to hull (what is the tip to hull clearance?)
4) Bad gearbox shaft bearings

1. Engine mounts are new
2. Have to check after the haul out
3. 4" min I guess estimated by picture.
4. No noticeable play by hands in any direction

Thanks for the reply.
Alex
 
What you are describing sounds a bit like what might happen with the prop too close to the hull. You need goo tip clearance so the turbulence from the prop doesn't 'hit' the hull. From memory the rule of thumb is something like the clearance should be a minimum of 1/3rd of the prop diameter, but you would need to check that.
 
What you are describing sounds a bit like what might happen with the prop too close to the hull. You need goo tip clearance so the turbulence from the prop doesn't 'hit' the hull. From memory the rule of thumb is something like the clearance should be a minimum of 1/3rd of the prop diameter, but you would need to check that.
Thanks, I was not aware on that.
High RPM suppose to hit the hull stronger, but there is no vibration
 
Did you fix this problem and if so, what was the fix? I have the same issue in a Yanmar 4 cylinder. In my case it seems completely temperature related. It does not occur when cold, but when it heats up it shudders enough to knock it off the mounts and I can not push past the shudder. If the engine is cold and or upto 70 degrees C, it is fine. Shudder comes on instantaneously.
Thanks.
 
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