Log Strike - Vibration

[dbsea]

Hi All,

I had the misfortune of striking a log in Admiralty inlet that was totally submerged at cruising speed (18kn) on Sunday. The boat started vibrating very violently, and I immediately put the boat in neutral and stopped the engine to assess for damage. No hull damage / water intrusion, so I was mildly less panicked. I restarted the engine, put the boat in gear, and tried to accelerate, but the vibration kept returning above 1200rpm. I limped the 1/2 a mile across the inlet to Boat Have Marina in Port Townsend, and we overnighted there. We were fortunate enough to get a haul out on Monday, and the prop seemed undamaged, which I confirmed with the prop shop in Mukilteo. We also checked the shaft and transmission coupling, all are true / within spec. At this point, it's a bit of a mystery. I'm going to take the prop back to Port Townsend tomorrow and get it all put together and back in the water. If there is still vibration what should I be looking at?? Fingers crossed there is not.

 

[Ralf H]

Hi Dave,

Not sure what you mean by cavitation. Cavitation is not a result of prop strikes. Are you asking about vibration? If there is still vibration at 1200 when you get the prop reinstalled, etc. If the shaft and coupling are indeed true, I would then look at engine alignment to make sure that's not what is causing the vibration. Engine vibration issues can start at any rpm, depending on how far out of alignment it is. Ask the shop that has helped you with this so far. They should know how to check and perform an alignment if necessary. Hope this helps.

Cheers,

Ralf

 

[dbsea]

Thanks Ralf - Yes I mean vibration... I misused the word cavitation! The shop (Gold Star Marine) said if it's still vibrating after it goes back in the water Friday and we do a quick sea trial with the mechanic on board, he can make adjustments to the engine alignment while it's in the water at the dock, and it's actually preferred to do that in the water vs on blocks.

Thanks,
Dave

 

[Cessna172]

Another inspection point would be the rudder. It may have been knocked loose or Kattywhompus from the log strike.

Gary

 

[allanrs74@gmail.com]

Dave,

I had a very, very similar experience on my 2016 R-27 on my journey from Anacortes to Seward, AK in June 2018. I was cruising along at about 3100 RPM and 13.5 Kts, watching for and maneuvering around debris in the water. We were just South of Butedale, BC some 120 miles South of Prince Rupert. There was a very loud 'BANG', we did not see any debris ahead of us and after putting the boat in neutral I quickly looked astern and saw nothing in my wake. I did the same as you, checked for water coming in and then put the boat in gear with severe vibration in the engine and transmission. The higher the RPM the more severe the vibration. We limped along at an idle for a few minutes until I hailed a passing Grand Banks who towed me into Butedale for the night. At this point we could only assume we had hit a submerged dead head and bent the prop and/or shaft. I shut off the engine without closing the sea cock which allowed my D3-200 HP engine to be hydra locked during the tow. I learned a very expensive lesson that day. The next day we spent the day behind a 45' tow boat from Prince Rupert. In Prince Rupert, we did exactly as you, the prop was checked and was fine. The shaft was true. So when putting it back together, the Tech who pulled the prop, shaft and coupling commented that the engine was not in alignment and he recalled when he disconnected the coupling, the shaft - transmission jumped a little. After reassembling the shaft, prop and aligning, no more vibration. After thinking about this and talking to a few people, I expect that one or more of the engine mounts lower locking nuts backed off from normal vibration and the engine hung in alignment until it dropped and caused the loud 'BANG' and the misalignment of the engine and shaft. Good news is that after blowing the water out of the engine and changing oil and aligning the engine-shaft I've put some 700 hours on the boat with no ill effects.

I have marked each of the 8 engine mounting locking nuts(top and bottom) with a white line and a corresponding line on the engine mounts to let me easily(with a mirror and flashlight on the under nuts) check that the locking nuts have not moved. I check the motor mounts each Spring.

Best of luck getting back out on the water and I hope your vibration is a thing of the past.
Bob Allan

 

[dbsea]

Dave,

I had a very, very similar experience on my 2016 R-27 on my journey from Anacortes to Seward, AK in June 2018. I was cruising along at about 3100 RPM and 13.5 Kts, watching for and maneuvering around debris in the water. We were just South of Butedale, BC some 120 miles South of Prince Rupert. There was a very loud 'BANG', we did not see any debris ahead of us and after putting the boat in neutral I quickly looked astern and saw nothing in my wake. I did the same as you, checked for water coming in and then put the boat in gear with severe vibration in the engine and transmission. The higher the RPM the more severe the vibration. We limped along at an idle for a few minutes until I hailed a passing Grand Banks who towed me into Butedale for the night. At this point we could only assume we had hit a submerged dead head and bent the prop and/or shaft. I shut off the engine without closing the sea cock which allowed my D3-200 HP engine to be hydra locked during the tow. I learned a very expensive lesson that day. The next day we spent the day behind a 45' tow boat from Prince Rupert. In Prince Rupert, we did exactly as you, the prop was checked and was fine. The shaft was true. So when putting it back together, the Tech who pulled the prop, shaft and coupling commented that the engine was not in alignment and he recalled when he disconnected the coupling, the shaft - transmission jumped a little. After reassembling the shaft, prop and aligning, no more vibration. After thinking about this and talking to a few people, I expect that one or more of the engine mounts lower locking nuts backed off from normal vibration and the engine hung in alignment until it dropped and caused the loud 'BANG' and the misalignment of the engine and shaft. Good news is that after blowing the water out of the engine and changing oil and aligning the engine-shaft I've put some 700 hours on the boat with no ill effects.

I have marked each of the 8 engine mounting locking nuts(top and bottom) with a white line and a corresponding line on the engine mounts to let me easily(with a mirror and flashlight on the under nuts) check that the locking nuts have not moved. I check the motor mounts each Spring.

Best of luck getting back out on the water and I hope your vibration is a thing of the past.
Bob Allan

Thanks Bob - very helpful. The shop told me they would go out on sea trial with me before cutting me loose, and if there's still vibration, they will check the engine mount alignment.

 

[BB marine]

We also checked the shaft and transmission coupling, all are true / within spec.

I have seen vibration issues after an impact of logs or bottom strikes. When inspecting the prop there were no visual issues seen. I always will ask for a prop scan and balance. Sometimes this brings up a unseen issue. If not a full inspection of the shaft is done. I use a dial indicator and inspect the run out of the shaft with the prop removed but the coupling still attached to the engine coupling. I am looking for the maximum runout of no more than .003. The next check is the cutlass bearing clearance. Place the indicator on the bottom of the shaft and lift with a good amount of force and then pull down on the shaft. I want to see less than .012 deflection on a small shaft( 1.5 inch dia.) The next check is engine alignment. When an impact occurs this can make the engine shift, especially if there are a few mounting lags that are not tight. It can also cause the keel extension to move. It doesn't have to move much. A few thousands of an inch in movement can amplify the misalignment enough to cause a shake.

Interesting enough, a few weeks ago I was cruising down the inland rivers with a fellow looper. The boat he was in was a 2022 Nimbus 40' with twin inboards. The rivers are a bit low now and he cut a channel marker close and bumped bottom. He then noticed a bad vibration just above 1000 rpm and it would get worse as throttle was applied. The boat was lifted at a marina about 100 miles down stream. He ran on the Starboard engine to get there. When the boat was lifted the prop and shaft showed no damage after inspections. He had purchased another set of props when he purchased the boat. The new props were installed and a sea trial was done. Still a vibration. When inspecting the prop shafts after the new props were installed and a vibration was still present. I asked if the struts and engine alignment was checked. The answer was no. The technician then looked at the shaft log and noticed the shaft was not running in the center of the log and that the engine alignment was off. The strut had a small bend in it. Minimal but enough to place the shaft line off. The alignment could not be achieved and a new strut was needed.

A Cutwater does not have a strut. It has a keel extension. There is a seam along the keel were the extension meets the keel. Inspect this seam to see if there are any cracks along it. Then inspect the alignment and all mounting hardware for tightness. You want the coupling alignment to be .003 or less.

Hopefully when you take the boat out you have no issues.

 

[dbsea]

Thanks Brian. The only thing yet to be done from your list is the engine alignment, which they will do if there's still vibration on the sea trial friday AM.

 

[dbsea]

We also checked the shaft and transmission coupling, all are true / within spec.

I have seen vibration issues after an impact of logs or bottom strikes. When inspecting the prop there were no visual issues seen. I always will ask for a prop scan and balance. Sometimes this brings up a unseen issue. If not a full inspection of the shaft is done. I use a dial indicator and inspect the run out of the shaft with the prop removed but the coupling still attached to the engine coupling. I am looking for the maximum runout of no more than .003. The next check is the cutlass bearing clearance. Place the indicator on the bottom of the shaft and lift with a good amount of force and then pull down on the shaft. I want to see less than .012 deflection on a small shaft( 1.5 inch dia.) The next check is engine alignment. When an impact occurs this can make the engine shift, especially if there are a few mounting lags that are not tight. It can also cause the keel extension to move. It doesn't have to move much. A few thousands of an inch in movement can amplify the misalignment enough to cause a shake.

Interesting enough, a few weeks ago I was cruising down the inland rivers with a fellow looper. The boat he was in was a 2022 Nimbus 40' with twin inboards. The rivers are a bit low now and he cut a channel marker close and bumped bottom. He then noticed a bad vibration just above 1000 rpm and it would get worse as throttle was applied. The boat was lifted at a marina about 100 miles down stream. He ran on the Starboard engine to get there. When the boat was lifted the prop and shaft showed no damage after inspections. He had purchased another set of props when he purchased the boat. The new props were installed and a sea trial was done. Still a vibration. When inspecting the prop shafts after the new props were installed and a vibration was still present. I asked if the struts and engine alignment was checked. The answer was no. The technician then looked at the shaft log and noticed the shaft was not running in the center of the log and that the engine alignment was off. The strut had a small bend in it. Minimal but enough to place the shaft line off. The alignment could not be achieved and a new strut was needed.

A Cutwater does not have a strut. It has a keel extension. There is a seam along the keel were the extension meets the keel. Inspect this seam to see if there are any cracks along it. Then inspect the alignment and all mounting hardware for tightness. You want the coupling alignment to be .003 or less.

Hopefully when you take the boat out you have no issues.

Brian, if the coupling alignment is already .003 (they checked this), does that mean the engine is not out of alignment?

 

[BB marine]

If they unbolted the coupling and did a face to face alignment and measured no more than .003 of coupling face to face misalignment and the prop shaft turns freely while riding in the packing gland and cutlass bearing you should be good. If the boat is out of the water while checking shaft movement it helps to spray little water into the cutlass bearing and then turn the shaft. If the shaft turns hard and not freely there may be some misalignment. It sounds like they have done there checks and you should be fine.

 

[dbsea]

All is good, boat is back home in Seattle. Thanks for everyone’s input! It’s a bit of a mystery what the actual cause was, perhaps a piece of the log jammed in the prop that ultimately freed itself. Glad it’s all good and the boat is back home before the rain comes.