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Old 09-06-2018, 04:23 AM   #125
SeahorseMarineDD54201
Veteran Member
 
City: Zhuhai
Join Date: Jun 2018
Posts: 71
Progress Update - Approach to Fixing the Vibration

In Post #124 I gave an update on finding the cause of vibration. This (belated) post is meant to give an update on the progress of fixing the vibration.

However as I have a very specific question on the approach to fixing the vibration, hopefully to bring some suggestions back, I will make another post after, to summarize the progress of the work.

Seahorse wants to replace the 2” shaft with a 2-¼”. In order to accommodate the thicker shaft, Seahorse built a reaming tool to enlarge the Stern Tube. It consists of:
1. a 10” long pipe diametrically (2.5”) precision fitted into the as is Stern Tube
2. a 2.75” diameter reaming knife at the tail of the pipe
3. a 2.5” diameter reaming knife at the head of the pipe.

The precision fitting between the pipe and the Stern Tube means the tool is self-aligning (see below for details). IN ADDITION there are other critical features and components to further enhance alignment (but these are not needed for discussion here). The process:

1. The Reamer would be pushed in from the Prop end of the Stern Tube, head first.

2. The Head Reaming Knife at exactly the diameter of the as is Stern Tube, will clean off any imperfection. After all something must be in the tube when Whip Bearing No. 2 was pushed in from the Prop end, with the Shaft in place (checked to be straight back in 2016).

3. The 10” precision fitted pipe will align itself with the cleaned up Stern Tube, so by the time the Tail Reaming Knife reaches the Stern Tube, it would enlarge the tube precisely aligned.

4. For the Prop end, only the very first 10+” inches need to be precisely reamed. That is the location of the Cutlass Bearing (8" long). But the Reamer will continue on until it gets near the location of the Whip Bearing.

5. The Reamer would then be pushed in from the Gear end of the Stern Tube, head first. The same process (2,3) above would happen. However precision reaming must be maintained for (my estimate) about 2 ft, to reach the planned location of the Whip Bearing (which is a bit forward of the current location, as the current location dividing the shaft into equal halves, will cause harmonics and “beat”).

The reaming tool was built and tried over a short section of a dummy Stern Tube, and the angular error over that short section (presumably the length of the Cutlass Bearing or about 8”) was less than 5 thousands of an inch. The Seahorse engineer is very satisfied with the trial results, and is adding an intermediate reaming knife to ease the load off the Tail Reaming Knife.

My concern is at least theoretically, the sag of the Stern Tube. If there is already a sag, no matter how precise the Reamer is aligned with the bent Stern Tube, the most critical sections of the reamed tube (ie. at the location of the Cutlass Bearing and the Whip Bearing) would still have an axial angular error, which may be significant to have caused the vibration in the first place. Of course a stronger shaft could withstand the stress better, nonetheless unnecessarily.

The sag obviously cannot be observed by the naked eyes, otherwise Seahorse workers would have stopped work back in the yard. So I am wondering if there is another way to check for the sag before we start reaming. For example we could stick a long straight 2.5” pipe through, or we could use a surveyor’s scope, or something. I would very much appreciate some suggestions from the community.
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