Prop shaft banging thru hull log

bcollis · Mar 19, 2016

I have the same issue on my 88 H30, but only when I shift into gear (F or R) and give it a lot of throttle. When I put it in gear, and slowly allow the speed to build, the shaft won't bang. It's been that way since day one, and the surveyor was the one that told me what was going on. If you don't feel any serious vibration or banging when you get up to speed, I'd guess your mounts and everything else is acceptably aligned.

whatfiero · Mar 19, 2016

And I thought it was just the transmission making that bang

walmsleyc · Mar 19, 2016

I have had this issue in the past, and still do under some conditions. My primary issue was a bad engine alignment. A little hard to check on the hard (as boats flex, and alignment may shift), but if you can put on goggles and inspect from the outside while the boat is in the water, you may be able to see that the shaft it not centred in the log. For my boat, I did that inspection on the hard (when replacing the stuffing box with a drip-less, and thus the shaft and log were well exposed for inspection.

For my boat, the shaft was significantly off centre in the log, and thus, when the engine was running a little rough, or while motor sailing with a little bit of heel, the shaft would bang on the log. If you throttle up hard and fast, I find my boat/engine/prop shakes a bit until it reaches speed, and that knocking would also happen. It seems the hull amplifies the sound and it can be quite disconcerting.

When re-aligning, I found that I had to adjust the motor on it mounts to the limits of the oblong holes in the mounts to come close to proper alignment. Makes me think the boat was never setup quite right from the factory (strut, log, engine where never quite lined up right) or something has shifted over time.

Note that when aligning/centring the shaft the log, you will have to re-align your engine and coupling using standard procedures. You may find that you have to alternate the two alignments until they are both good.

Doing the alignment(s) significantly reduced the problem for me.

Edit: My mounts are old and soft (or have sagged), and likely contribute to the issue. Under load/torque, I've sure the mounts defect more that new ones.

Chris

whatfiero · Mar 19, 2016

Chris now you also must go back in water and check strut alignment

walmsleyc · Mar 19, 2016

Yes, always do the final engine/coupling alignment after the boat has been in the water for a week or two with the rig under full tension.

Chris

Blitz · Mar 20, 2016

Once you have exhausted adjusting your alignment if severely bad, and making sure none of your mounts are not cracked or metal on metal, seriously consider the transmission.

I had a similar banging; didn't happen all the time, depended slightly on weather warm or cold, and my boat's age is similar to yours. The transmission cone gets glazed and causes the banging as you describe. Had transmission rebuild (really just seals) and the cone deglazed - which is similar to honing of cylinder walls in an engine. Also changed damper plate while out. This solved it immediately, even with bad mounts that were later changed.

kito · Mar 20, 2016

Do you know when the engine mounts were changed last......if any? My 1979 ysm12 Yanmar had the original mounts. It made a banging noise too. I changed out all the mounts and found them to be in horrible condition. One actually had a broken stud where it was just bouncing on itself. There is not much clearance between the shaft and the log so I am sure bad mounts will cause some banging there too.

Donnybrook · Mar 20, 2016

To check motor mounts: Use a crow bar or 2x4 and a fulcrum to lift the engine at each motor mount. Make sure you can see movement. Release pressure. If the motor clunks in either direction, your mounts are shot.

Ralph Johnstone · Mar 20, 2016

datasailor2 said:
Is there a good DIY way to check condition of mounts?

I had a motor mount in questionable condition when the boat was only about eight years old. A microscopic fuel leak had been dripping on the mount for years and damaged the rubber. Almost impossible to see the damage until I removed it.

If you're determined to DIY inspect the mounts, first check each one to see if it's bottomed out and made metal to metal contact. Bounce the engine around by hand or get someone to goose the engine under load and see if anyone bottoms out. Like my rear starboard mount, you may have to remove to inspect. Each mount can be removed individually (and then replaced) without too much trouble ........... especially if the engine's not running

. Make sure you mark all threads on each mount so that you retain the rough alignment and then do a complete alignment when you've returned all mounts.