vibration while motoring

[Jeff Kaplan]

i have posted before that #219 has a very noticeable vibration when cruising around 2000 rpms. the vibration osilates up than tapers of. 1800 or less rpms or more than 2200rpms and vibration seems to subside.  had a mechanic on board last season. found nothing loose, only thought was the shaft might not be straight. took it apart end of last season. had shaft straightened, balanced the prop, new cutlass brg, packing and coupler. engine is m25. this season, same thing. can't find anything loose to cause vibration, so my next thought goes to the hump hose. i have done a lot of reading about this in past posts, and saw that this hose could cause engine vibrations. there is no hump hose on this engine, only a 6" piece of straight hose between riser and muffler. am i on to something here. open to all suggestions. thanks...jeff

[Stu Jackson]

Good try but no banana, Jeff.  The hump hose simply connects the exhaust riser to the muffler, and avoids some vibration going TO the muffler lips.

The engine is causing the vibration, if you're sure the whole shaft / prop arrangement is, excuse the pun, straightened out.

So, what else could it be?  Engine mounts come to mind...

[prh77]

I had a terrible vibration between 1600-2200 rpm. Had to advance throttle quickly to get out of this range,it was so bad. Performed a really good engine alignment [ first time on this boat ], vibration in this range 99% better. A thought...

[Ron Hill]

Jeff : I'll guess at engine mounts and or engine alignment.  When was the last time you had your prop scanned for pitch and balance?  Ever replaced your injectors?   

[sedelange]

While the majority of my experience is the balancing of aircraft engines and propellors, vibration over such a small range usually indicates a harmonic problem.  This could be mounts that are no longer able to dampen vibration over this range or flexible items attached to the engine (hoses, etc) that are deveoping a harmonic vibration.  An out alignment or balance problem will generally cause increasing vibrations with RPM increases.

Another consideration, is the vibration you feel at the frequency of the propellor, the engine, or some frequency higher or lower.  Vibration at the propellor frequency would indicate balance or alignment issue.  Vibration at engine speed could indicate an internal engine discrepancy.

Vibration higher or lower  than engine speed will genrally occur at even harmonics of the engine speed.  These vibrations are resultant products of combustion and tend to cause items such as hoses, exhaust pipes, throttle cables to shake noticeably.  If these accessory items contact the engine or structure during there motion it will transmit noticeable vibration into the structure. 

Operate the engine with all the covers removed and observe the engine and components during the transition through the annoying range.  If it is the mounts, you will likely see a definite increase in lateral engine deflection.  Also observe the shaft and coupler, and notice any deflections.  Lastly observe the hoses, cables, wires, and exhaust pipe to see if they are shaking substantially.

Steve E. DeLange