Vibrations
A smooth running engine is great, but what to do when you experience some shaking and rattling? Vibrations can be annoying, damaging, and hard to find.
In this interview, our head of the Mechanical Department at Townsend Bay Marine discusses Finding and Fixing Vibrations (4.8 MB Streaming MP3) to keep the smooth in your cruise.
- Where to Look First
- Other Culprits
- Wear and Tear
- Fixing a Key-bound Prop
- Re-pitching
- Prevention
- Stern Tube
Where to Look First
The propeller is first place to look. Frequently, props are “key-bound”, which means the prop gets tightened against the keyway instead of the taper. Underwater prop collisions can bend or nick a propeller blade, setting up a vibration throughout the boat. After you’ve changed a propeller and there is still vibration, there are other possibilities, possibly a new vibration or one from another source entirely.
Other Culprits
Other vibration sources are engine, shaft, and transmission. Naturally, hitting something with the prop can also bend the shaft. A dial indicator measures the shaft run-out. A foreign boat might have been mishandled before installation, or maybe the proper material was not used.
Wear and Tear
Engine mounts can add to vibration, but they rarely cause it. Misalignment will wear cutlass bearings and other components by putting side loads. The cutlass bearing won’t normally cause vibration, although if it’s really worn it can cause some issues.
Fixing a Key-bound Prop
It is very easy to repair a key-bound prop—just remove a little material from the key. It’s also easy to check, slide the prop on the shaft against the taper without the key and measure the location. Next put in the key, put the prop back on, and measure again. If it's key-bound, it won't be able to seat as far on the taper when the key is in place as it will without the key.
Re-pitching
When a prop is re-pitched, a quality propeller shop will use a program to dial in all the blades to make them perfectly equal; a shop incapable of this may leave the blades slightly unequal in pitch or diameter thus creating vibration. Sometimes people will carry multiple pitch props to change the engine RPM, depending on the load.
Prevention
Drive savers and low-res couplers will not take care of misalignment, but they will prevent some noise and vibration from getting into the boat. First, align the shaft in the stern tube, and then line everything up to that. If the boat is in the water, you can only align the faces of the shaft coupling to the output flange of the gearbox.
Stern Tube
Lining the shaft up to the stern tube and aligning everything else to that is the ideal scenario. Sometimes though, you may not know where the shaft is, so you have to take the boat out of the water altogether to fix it. In many foreign boats the method of placement is very different from the standards here. On those occasions, it can be a challenge to find and fix the problems. Challenges are good.