ferretlegger
Aluminum
- Joined
- May 15, 2009
- Location
- San Jose, CA
Hello everyone,
I have an aluminum sailboat (Garcia Passoa 47) which is going to need the shaft log replaced at the next haulout. The shaft log is a 5086 aluminum tube which runs from the engine compartment through the hull and through which the propeller shaft runs. On the engine side there is a shaft seal which keeps water out, and on the water side is a rubber bearing, known as a cutlass bearing, which guides and supports the propeller shaft against side forces of the propeller.
Here is my problem: I will be hauling at a well known boatyard in Richmond, California. This boatyard has access to good machine shops in their sister operation, which is an actual shipyard. This yard will not let me weld, and will be doing most of the actual work of replacing the shaft log. Including, most likely, machining it. I DO NOT want to haul the boat without a clear plan on how to do this job, and all the materials in hand. I have been doing all the maintenance, fabrication, welding, and so on for this boat for 18 years and know it very well. They do not. My biggest single issue is getting the shaft log machined.
There are a number of interlocking issues which are proving problematic. My prop shaft is 35 mm diameter. Cutlass bearings for a 35 mm shaft all have a 50 mm OD. Cutlass bearings are designed to be press fit into the shaft log with a .001" to .0015" interference fit. This is critical for aluminum, because if there is a gap between the shaft log and the bearing, salt water will get in it and cause poultice corrosion (very bad). I have been unable to source aluminum rod with good starting dimensions. The OD will be between 2 7/8" and 3". The ID (for a 35 mm cutlass bearing) would be nominally 50mm -.0015" (excuse mixed units). I am only able to find 5086 hollow tube that is remotely correct in 3" OD by 1/2" wall (2" nominal inner diameter). This is from Aluminum Distributing in Florida. The alloy must be 5086.
So my question: The shaft log will be circa 3 1/2 feet long at the most (final dimensions will be taken when hauled out). If I were to start with 3" diameter 5086 aluminum rod with no center hole, can anyone suggest practical ways to center drill the rod with a hole between 45 and 48 mm in diameter. This hole would not need to be highly toleranced, as most of it would be clearance for the 35 mm shaft. At one end, the hole would be bored out for the 6" long cutlass bearing with the appropriate tolerance at 50mm.
I have been working with research machine shops for 40 years, and running "normal" size lathes and mills that entire time. Unfortunately, all my work has always been precision stuff in a smallish footprint. I have no personal experience in big lathes. My Hardinge HLV is too small to do this job. I want to be able to discuss this with the shipyard machinists intelligently. So what approaches to this problem would you people employ. Trepanning? Gun drill? long twist drill and flip the part? Any advice or experiences doing this sort of work would be very helpful.
Thanks,
Michael
I have an aluminum sailboat (Garcia Passoa 47) which is going to need the shaft log replaced at the next haulout. The shaft log is a 5086 aluminum tube which runs from the engine compartment through the hull and through which the propeller shaft runs. On the engine side there is a shaft seal which keeps water out, and on the water side is a rubber bearing, known as a cutlass bearing, which guides and supports the propeller shaft against side forces of the propeller.
Here is my problem: I will be hauling at a well known boatyard in Richmond, California. This boatyard has access to good machine shops in their sister operation, which is an actual shipyard. This yard will not let me weld, and will be doing most of the actual work of replacing the shaft log. Including, most likely, machining it. I DO NOT want to haul the boat without a clear plan on how to do this job, and all the materials in hand. I have been doing all the maintenance, fabrication, welding, and so on for this boat for 18 years and know it very well. They do not. My biggest single issue is getting the shaft log machined.
There are a number of interlocking issues which are proving problematic. My prop shaft is 35 mm diameter. Cutlass bearings for a 35 mm shaft all have a 50 mm OD. Cutlass bearings are designed to be press fit into the shaft log with a .001" to .0015" interference fit. This is critical for aluminum, because if there is a gap between the shaft log and the bearing, salt water will get in it and cause poultice corrosion (very bad). I have been unable to source aluminum rod with good starting dimensions. The OD will be between 2 7/8" and 3". The ID (for a 35 mm cutlass bearing) would be nominally 50mm -.0015" (excuse mixed units). I am only able to find 5086 hollow tube that is remotely correct in 3" OD by 1/2" wall (2" nominal inner diameter). This is from Aluminum Distributing in Florida. The alloy must be 5086.
So my question: The shaft log will be circa 3 1/2 feet long at the most (final dimensions will be taken when hauled out). If I were to start with 3" diameter 5086 aluminum rod with no center hole, can anyone suggest practical ways to center drill the rod with a hole between 45 and 48 mm in diameter. This hole would not need to be highly toleranced, as most of it would be clearance for the 35 mm shaft. At one end, the hole would be bored out for the 6" long cutlass bearing with the appropriate tolerance at 50mm.
I have been working with research machine shops for 40 years, and running "normal" size lathes and mills that entire time. Unfortunately, all my work has always been precision stuff in a smallish footprint. I have no personal experience in big lathes. My Hardinge HLV is too small to do this job. I want to be able to discuss this with the shipyard machinists intelligently. So what approaches to this problem would you people employ. Trepanning? Gun drill? long twist drill and flip the part? Any advice or experiences doing this sort of work would be very helpful.
Thanks,
Michael