Ask Sail: Vibrating Engine Drive Trains - Sail Magazine

Ask Sail: Vibrating Engine Drive Trains

Last season I repowered my Catalina 30 with a new M3-20B Universal diesel engine. Its power curve was substantially different from the original engine, so I also changed the propeller. I ordered a new shaft, a new coupling half (fitted at the supplier) and a new prop.
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Gerald Pritchett Lorain, Ohio

Q: Last season I repowered my Catalina 30 with a new M3-20B Universal diesel engine. Its power curve was substantially different from the original engine, so I also changed the propeller. I ordered a new shaft, a new coupling half (fitted at the supplier) and a new prop. Prior to the installation, a new cutless bearing was fitted. Alignment of the shaft coupling to the transmission coupling was kept well within 0.004in.

Now my boat only gains 0.3 knot of speed when engine speed is increased from 2,500 to 3,500 rpm. Maximum speed is 6.2 knots at 3,500 rpm, when calculations show it should be 6.7 knots.?I was expecting to have a nice smooth-running drive train. Instead there are vibrations at different levels throughout the operating range. At certain rpm levels, it sounds like someone is running a stick across a washboard. A thorough inspection was performed this fall and nothing seemed amiss.?Any suggestions for spring?

Nigel Calder Replies

 A: It sounds like you have harmonic vibrations at certain speeds. Vibration can absorb a lot of energy, which may be why you are not achieving your peak speed. It can also do significant damage. I am not an expert in this field, but here are some thoughts. If the vibration only occurs with the engine in gear, the problem is most likely in the drive train. If it is worse in forward or reverse gear, the problem is probably in the transmission itself. If the problem is in the drive train, but not in the transmission, typically the cause is a bent shaft, a loose or misaligned coupling or a damaged propeller. You may want to check all these again.

NigelCalder

Here’s another thought. If the propeller is not seated properly on its keyway, it could be a little off center. To check it, pull the propeller off, remove the key, slide the propeller up the shaft and mark where it seats. Then replace the key and make sure the propeller seats at the same point on the shaft. The propeller itself may be out of balance, so have it checked while it is off, or swap it for another.

If the vibration occurs out of gear, and if it is rhythmical, you probably have a fuel-injection problem. Given the fact that this is new installation, it may also be that the engine feet have not been adjusted properly. This can cause significant vibration and expensive damage. I once saw the crankshaft on a 2,000hp engine broken this way!

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