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Ideas for core drilling a long 3" diameter aluminum rod requested

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
 

ferretlegger

Aluminum
Joined
May 15, 2009
Location
San Jose, CA
Hi Wesg,
I had considered that. The issue is that the wall thickness when the OD is turned would be around 10 or 11 mm. The OD in the area of the cutlass bearing will be welded with a heavy bead to the outside of the hull. If I sleeved it, I might want a sleeve of at least 3 mm wall. That would leave about a 7 or 8 mm wall thickness before the sleeve is shrunk in. Being a paranoid, I was concerned that about the sleeve wall thickness versus the tube wall thickness, and effects of welding.

You make an excellent point though. Have you any suggestions as to the wall thicknesses of the shaft log versus shrink sleeve? would adding some loctite (603?) to seal the interface between the sleeve and the tube be a good idea? If this workes it would sure make the job easy, as I could use the 1/2" wall tube I can easily get.

Thanks for the input!
Michael
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
Don't use Loctite, if you want to ensure the sleeve is sealed weld a fine bead over the gap (joint line) at the end of the tube after the sleeve is properly shrunk fit. But do that before any final machining of the bearing bore is done.
 

ferretlegger

Aluminum
Joined
May 15, 2009
Location
San Jose, CA
Thanks, Milland!
That makes sense. I am very leery of trying to stick aluminum to things, especially underwater. Do you have any thoughts as to the thickness of the tube with respect to the sleeve? I was worrying about the change in the inner diameter of the sleeve due to compression of the tube, but it occurred to me that I might use a hone with a drill and some lube to clean up any irregularities and to bring it to final size if needed. I know I am overthinking this (a lot) but I have been pretty isolated since March, and without knowledgeable people to kick this around with I have gotten a little wild-eyed. ;)

Michael
 

BT Fabrication

Stainless
Joined
Nov 3, 2019
you could always find a good but very expensive gun drill, either way that would be a few thousand just to source one from a tooling rep.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
I'd keep the sleeve at around 3mm, but I'd consider 2mm safe too. Honing after shrink fitting might be fine, try a kerosene or other very light oil as a lube (keep a fire extinguisher at hand).

You do want to ensure the hole and sleeve are round, any "egg shaping" from poor setup or material may be tough to get back to cylindrical with a hand-held hone.
 

mjr6550

Aluminum
Joined
Oct 7, 2012
Location
Lansdale, PA
The company in the link below seems to show 3 inch diameter tubing with some wall thicknesses greater than .5 inch. I don't know if they really have that, but if they do it would be another option.

5086 Aluminum Tube
 

ferretlegger

Aluminum
Joined
May 15, 2009
Location
San Jose, CA
Hello Mjr6550,
Thanks for the link to Tork. I had never heard of them before. I called them and sadly, their little drop down selector system contains material that they do not stock. In particular there was a 3" OD tube with .594" walls which I would have loved to get, but apparently it doesn't exist in real life. Oh well...

I am starting to see the best way to solve this problem. Using 3" OD tubing with 1/2" wall, easily obtained (if you can find an untended armored car with lots of easily grabbed bags of cash) from Aluminum Distributing, using the sleeve idea proposed by Wesg, and fleshed out by Milland, I think I can get everything I need in a way that is straightforward to machine and robust. If I can convince the shipyard to bore out the tubing and heat shrink in a sleeve 5.5" long (the length of the cutlass bearing), and then weld the outside lip of the sleeve and tube, then the inside diameter of the sleeve can be made to give the required interference fit (.001" to .002") with the bearing. That dimension can be either turned before inserting the sleeve or finish turned after it is mated with the tube.

There are other details for the shaft log, but they are pretty trivial in a big lathe. I think that the advice and ideas you all have given me will get the job done. Many thanks to everyone. I am retired now, and no longer can run down to the lab machine shop and bounce ideas off the wonderful machinists I associated with for so many years. I was really overthinking this, and having your input really helped me get my head screwed on the right way.

Thanks, and All the best!
Michael
 
Just o.ut of curiosity since I have no personal experience-

Given that sailboats have a lot of added mass down low anyway, is there reason to not make the log out of thinner wall stainless tube?

PS: I have found gun drills in the 30” to 40” aprox 1” dia range and smaller including re-sharps to be cheap from online eBay surplus sellers. My use is for coring wood, but that is irrelevant. For the log, find one any diameter large enough to follow with a line boring op.

smt
 

Modelman

Titanium
Joined
Sep 12, 2007
Location
Northern Illinois
Before re-engineering the job, do a search for gun drilling shops. Gun drilling is a specially process with dedicated shops that do nothing but. From what I know of it (I send injection mold bases out to have the water lines gun drilled) the accuracy come not just from the drill, but also from the machine that drives it. This shouldn't be a hard job for a shop that specializes in this work.

Dennis
 

ferretlegger

Aluminum
Joined
May 15, 2009
Location
San Jose, CA
Hello Stephen,
My boat is made from 5086 aluminum, an alloy that is extremely corrosion resistant to seawater. In fact, the boat hull uses no zincs, and is doing very well after 30 years in the water. Using stainless for the shaft log would, however be a disaster, as the log would be in contact with the rest of the hull, and would cause spectacular galvanic corrosion. The prop shaft is made from Aquamet 22, a very corrosion resistant type of stainless steel, but the entire shaft and propeller are isolated electrically from the engine, and thus the hull. the only zinc on the boat is on the prop shaft.

Modelman,
Your suggestion about ebay for gun drills is very interesting! I will look around and see what is available. I have never had occasion to use one, but knowing what is available would be helpful when I go to talk to the machinists in the shipyard about this job. I have found that when hauling a boat, just assuming that the yard machinists or other craftsmen will solve problems the way I would usually doesn't work very well. They are often very talented, but don't really have the time to work through all the possibilities and the implications of each solution, and unless you work with them to find the best solution for my boat (which is an unusual one) you may end up with a disaster. Aluminum boats are very robust hardcore offshore boats, but there are real penalties for taking shortcuts.

Would anyone like a picture of the boat?

All the best,
Michael
 

ferretlegger

Aluminum
Joined
May 15, 2009
Location
San Jose, CA
234.jpg

2016-07-20_8497_MoutonNoir_8147.jpg

Here are a couple of pictures of the boat I was discussing, S/V Mouton Noir. She is a Garcia Passoa 47, built in 1991 out of 5086 and 5083 aluminum. The first picture is at the start of one of the SingleHanded Transpac races from San Francisco to Kauai. The second picture is me finishing at Hanaleii Bay, Kauii. I am dropping sail just after crossing the finish line.

Many thanks to those who have responded to my request for ideas. Your comments have been VERY helpful.
All the best, stay safe!
Michael
 

Modelman

Titanium
Joined
Sep 12, 2007
Location
Northern Illinois
Modelman,
Your suggestion about ebay for gun drills is very interesting! I will look around and see what is available.

Errr... I'm not the guy who suggested E-bay for anything. I'm the guy who suggested finding a contract gun drill shop. I've found, if the material is expensive, best to let the guys who do it every day do it. I just googled "gun drill shop" and find dozens of hits for gun drilling and deep hole machining. One calls out up to 2" diameter and 72" deep. Ask them what kind of concentrically they'll guarantee. If it's not adequate, order over size round stock and turn the OD after the ID is drilled.

Dennis
 

Trboatworks

Diamond
Joined
Oct 23, 2010
Location
Maryland- USA
Dump the shaft and replace with a diameter that works better with off the shelf tubes and cutlass bearings is the first thing that springs to mind.

The second is dump the alloy shaft log.
Get some collars made up fit the fore and aft weldments and drop the tube of choice in them- again bought to play well with either existing shaft or a new one.

The third the is to just go larger with tube and step down with sleeve to the od of cutlass.

About the last thing I would do is make a tube out of solid.

Oh- not the last thing -I would never make the tube..
 

hanermo

Titanium
Joined
Sep 28, 2009
Location
barcelona, spain
Em..
I suspect gun drill shops for that deep and that large are probably pretty expensive.

But anyone with a big lathe can easily drill a tube for you, in less than an hour of work time.
Clearance hole.
Sleeve both ends, done in one.

Suggestion:
And perhaps use a boss and bolts and a gasket, instead of welding.
Zero distortion, will never break, will never leak, and can be dismounted at need, and won´t fail catastrophically.

You can make a 2-part boss if you want to weld to the hull.
2 plates, thickish, largish, turned for fit and d/t together, then weld the outer onto the hull.

For say 3" maybe a 5-7" D boss about 1.5" thick, x2, will dissipate the haz and distortions and make the other side dismountable.
And if You cannot officially weld in-situ, glue it in place with epoxy, and then tig it once you are free to weld.
If You want to.
Bolts and epoxy would hold it just fine.

Just wet-sand the alu under something liquid, a bit, before using the epoxy.
Plenty of industrial epoxies set under water, like distilled water.
Through bolt the boss for extra piece of mind, as many bolts as You feel like.

The good part about a 2-part boss is that it wont fail catastrophically, like a crack in the hull or the shaft log breaking under strain.
At most, something might leak, and this is very unlikely.
If you are bolting onto frp, use oversized holes tapped, or a backing plate (much better) with oversized generous holes pre-filled with solid expoxy, then re-drilled.

AND You can test sleeves and bearings and the shaft log for free running by hand - that will instantly indicate any binding.
Before final assy.
And after, before You are in the water, and before the prop is on.
Easy to adjust as needed.

Also, the hull flexes when in goes into water and comes out of it.
I would expect a 47 ft alu hull to flex around 15 cm at centerpoint.
This may or may not matter.

You probably knew all or most above, if You´ve worked on it for 18 years.
But since You asked for ideas, I figured I might try some ..
Most above are staples for profesional boatbuilders ..
 

Trboatworks

Diamond
Joined
Oct 23, 2010
Location
Maryland- USA
OH- #4

Just use the 1/2”x3”.
Bore over for cutlass and bush it.
I assume you are using non metallic shell cutlass- easy to turn and bond on a bushing.
 

boslab

Titanium
Joined
Jan 6, 2007
Location
wales.uk
Would have thought a trepanning tool on a boring lathe would eat that
Shame David Wilkes on YouTube is the wrong side of the pond, worth watching the videos though
Mark
 








 
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