20 Aluminum Boat Build

With Mig, less with tig, there will be voids, aluminum oxide weld spatter and inconsistencies in the weld, while it may look fine from the top, the bead depth will vary, ( less with a robotic welding)
Even changes in the distance that the tip is away from the material, fluctuations in the weld arc can cause various depths of the bead

By back cutting the weld down to sound, homogenous material, you are guaranteeing that your next weld will be better than just welding both sides and leaving a void or notch.

Without going into a lot of detail on localized work hardening of aluminum due to stress concentrations created from inconsistencies in a weld, crevices etc. a brief description is as follows

Take a 1 foot by 1 foot by 3/16 inch thick piece of glass which by virtue of the manufacturing process creates a smooth surface. It will take a reasonable amount of force in bending until it breaks. Merely scratch the surface and the glass will crack easily.
The reason is that the notch created by the scratch concentrates the stresses at the bottom of the notch and hence exceeds its breaking strength. In this case while the rest of the glass is experiencing a stress below its ultimate strength, the bottom of the notch exceeds it.

In a more work hardening susceptible material, aluminum, the flexing of the material well below its yield stress, ( the point that a material will if stretched, strained, is reached where it does not rebound to its original length) IF A NOTCH OR VOID exists, the stress at the bottom of the notch or void will see a concentration of stresses, begin to harden, then crack. The crack will have an even sharper notch effect and the crack will propagate. And hence cracked welds in aluminum beads are often seen.

Boats flex constantly and most aluminum boat designers will try to limit flex for this very reason.

Maybe 20 years ago now, Hawaiian Air or Aloha had a jet service between the islands, still do, and their jets might take off and land dozens of times per day. Eventually, a section of the side of a plane blew off . While the air frame did not reached its design life in hours, and many of the same model of plane had thousands of more hours on them, it was discovered that
this particular plane in question due to its number of landings with subsequent pressurization, and depressurization, had exhibited work hardening of the aluminum until it failed.

Another plane in I think Pittsburgh had an engine leave a nacelle and they grounded the fleet and found that other planes had experienced this crack formation due to work hardening.

Adhoc might know if this process is required for aluminum boats due to class requirements

 

Do yourself a big favour and buy the Book "Boatbuilding with Aluminium" by stephen F Pollard.

https://www.amazon.com/Boatbuilding-Aluminum-Complete-Guide-Amateur/dp/0071443185

 

While this ‘sounds’ like the objective of a simple home build, it is not recommended. A plate that is offered up to the frames of a hull must be formed/bent/rolled into its desired shape before being offered up to the hull frames.

If you pull the flat sheet onto the frames (even if fair) the plate is now in tension from the load being applied to pull the sheet into a shape that is no longer flat. This introduces residual stresses and when welded, the welds are under the same if not greater degree of built-in residual stresses. This means the as-welded strength has been reduced. Thus any high loads will make these welds fail. In a nut-shell.

Never force aluminium into place.

It is not the weld that workhardens.

Admirable effort Barry..but you are confusing the 2 issues as they are separate.

A notch is, as described, and creates simple localised stresses as shown:



The reasons why on a virgin sheet of ally, a notch/scratch will cause a failure is because of what the notch does on a microscopic scale. In a nutshell, the flexing of the ally causes the atoms to pass over each other on what are called 'slip planes'. Ally has many slip planes because it is a FCC atomic structured material and is easy to slide/glide on an atomic scale and thus susceptible to fatigue. The constant movement creates intrusions and extrusions that slowly propagate. The very basic mechanisms of fatigue (without going into details).

It starts by:


and then becomes:


Ally has no 'fatigue limit, unlike steel:-


Thus any stresses from a continually applied flex no longer has an effect is not applicable to ally. Thus the more flexing the lower the stress for the alloy to fail.

Fatigue requires an initiation point, a void, to start. A notch or scratch etc provides this 'initiation'.

Rewedling or poor welding etc, mearely reduces the localised hardness of the alloy. The lower the hardness the lower the yield stress.

 

I too know many yards that do this, but, it is not the correct procedure. Aluminium must be rolled into the correct shape, so it is offered to the framing system unstressed. If a yard elects not to do this, that’s fine, but it is does not produce long lasting hulls. Many may quote, aaahh.. shipyard XX or YY has boats lasting decades. Well, that is most likely because the plating and framing is over engineered and thus, the residual stresses do not cause any significant issues. Can’t get away with it on large builds, only smaller ones.

Sorry Barry, you’re confusing the issues again.
Workhardening, or also referred to as Strain hardening too, is a mechanical means of increasing the strength of an alloy.
Ageing, uses the metallurgical effects of heat on the alloy.

A weld uses heat, the heat is changing the atomic structure and its material properties. The heat also introduces foreign materials, usually unwanted, into the alloy matrix. Constant reheating of ally lowers the localised hardness, which lowers the localised strength. Thus, all these conspire to i) lower the strength, so it fails at a lower than designed value) and ii) reduces the fatigue life as many sources or sites of crack imitation have been produced.

A stress concentration is merely that, a change in localised stiffness of the weld bead above that of the surrounding area.

This influences the stress around said SC so that higher stresses are experienced at the SC, the oversized weld bead.

Not at all, im sorry if you took it that way. Merely applauding your efforts to describe in simple terms, a very complex mechanism.

A void does not cause a stress concentration! It creates a site of crack initiation…which leads to failure by fatigue. It is a totally different mechanism at play.

It is not the stress per se, but the strain.

It is the strain amplitude that is the important feature in movement along slip planes regarding fatigue.


It leads into complex issues like cyclic strain-strain curves and shows that deformation takes place at roughly constant values of stress and leads into persistent slip bands (PSB) etc etc…but I digress…

 

You are planning to pull down the 1/4" bottom plate while sitting on a metal frame I guess, so you want to bend it to shape before offering it up to the aluminium frame for tacking.

While this ‘sounds’ like the objective of a simple home build, it is not recommended. A plate that is offered up to the frames of a hull must be formed/bent/rolled into its desired shape before being offered up to the hull frames.

If you pull the flat sheet onto the frames (even if fair) the plate is now in tension from the load being applied to pull the sheet into a shape that is no longer flat. This introduces residual stresses and when welded, the welds are under the same if not greater degree of built-in residual stresses. This means the as-welded strength has been reduced. Thus any high loads will make these welds fail. In a nut-shell.

Never force aluminium into place.
==============================================================================

I WASN'T recommending anything. I was RESTATING what the OP was obviously intending to do, because he had already built the metal steel formers. - hence the "I GUESS" comment.

How the last sentence "bend it to shape before offering it up to the aluminium frame for tacking" made you think that he was being told to "force aluminium into place.", I cannot fathom

Likewise, it's obvious that the OP WASN'T going to FORCE the plate, BECAUSE he had built steel formers.

 

Well, let's take a look:

Hmmm...if that aint forcing into place..i don't know what is!!

 

You are planning to pull down the 1/4" bottom plate while sitting on a metal frame I guess, so you want to bend it to shape before offering it up to the aluminium frame for tacking.

Yeah, just ignore the "while sitting on a metal frame I guess" - the metal frame being the former he was talking about, and the "I guess" asking for confirmation of his methodology - NOT recommending AT ALL.


apply pressure with timber lengths and compression straps until the plate conforms to the metal jig.

Likewise, you are missing out the start of that sentence, "The normal way is to simply apply pressure with" followed by posting pictures of a commercial build doing just that, with brute force pulleys. And if that's not "normal" enough for you read the book I use for a manufacturing guide. They step by step tell you how to work your way along the bottom sheet using clamps as you bend the sheet to meet the frames.

 

You can repeat this as much as you want, but it is not the correct way to plate up a hull, despite small home builds doing this. Why, because they dont have the required plate rolling/bending machine (nor most likely the expertise that goes with such..) in order to crate the shape of the plate BEFORE it is offered up to the hull.

It is easy with the correct equipment and essential in any 'normal' build:



As shown above.

Perhaps you need to read up on normal and modern method of builds, like that described in ProBoat magazine issue 147, Feb/March 2014 which describes this sequence exactly.

 

Great advice, I just ordered a sheet of 20 by 8 aluminum. Good thing is that I will not have to butt weld the bottom of the hull together since it's such a long sheet.

The plan is the use the template and cut out the aluminum. Once that is done, the plan is to cut out a peice of 2 by 10 of timber with the correct curvature of the hull bending the aluminum hull over the timber. While the timber keeping the curvature correct, I will then place aluminum sheet on frame with timbers attached. Finally welding in to place.

What do you guys think?

 

Gnokd_Tad,

I second the recommendation to buy Stephen Pollards book. Invaluable and explains a lot of the welding and building procedures, do and don'ts...not that I ended up following all best practices.
I will not give much advice, as I made lots of errors building my first (and maybe only) aluminum boat, also a Glen-L design - a 25' Double Eagle.
What I might suggest is to read some of my threads (here and also under Power boats, on the Glen L site) to see sometimes what NOT to do, and how / why some of the experienced folks here showed me what was wrong, or what I could potentially do to mitigate the issues... My boat was built upside down, on a jig, but I did build the jig as a rotisserie, which really helped with welding a lot of it in flat position. (that idea came from Kevin Morin's many valuable posts).

re: thickness - my hull ( original at 23' called for 1/8" bottom and topsides. I stretched it to 25' but also changed to 3/16" hull and topsides and am happy with that decision. I am not sure what thickness you ended up ordering, but I hope it was 3/16". I know they use 1/4" for river runner boats, but unless the hull sections to be welded have very little shape/ bend needed, I can't imagine working with 1/4". It would have been much more difficult on my boat I believe.

re: cutting - get a worm drive skill saw - you can use metal blades but any blades will work fine if you use wax, or spray wd40 every few seconds on blade to keep it cool and keep the aluminum chips from melting, sticking to the blade and quickly ripping the teeth from the blade.
Note - at first I tried a regular skill saw, but unlike the worm drive, it had too much speed and too little torque and control - there was an absolute night and day difference between the two.
When using it to back chip welds, as Barry explained, do NOT use any wax / wd40 spray as you do not want to contaminate the weld - unlike cutting pieces, where you spray wd40 to cut much more easily and then use acetone to clean up well before dressing / filing edge to weld. Often I used a router with carbide bits to dress edges, for example to bevel thicker plate edges prior to welding.

What model design is it, and does it call for any longitudinal stringers, or is the hull welded directly to the frames?

Best of luck - it is rewarding, but challenging for newbies, like you and I...

 

I would definitely NOT use anything on the blade like a wax or WD 40. You are just contaminating the weld area. There is no need with a regular carbide tipped 40 - 60 tooth blade .

Dry and clean

 

Barry - do you mean specifically when backchipping welds?
Or, are you referring to any cutting of material with a skill saw prior to cleaning and welding?

As stated, I did NOT recommend spraying with wd40 when back chipping for sure.
But, I went through enough blades cutting material, that I can't imagine cutting material dry, beyond a foot or two in length. The blades quickly heat up and teeth go missing in seconds.

 

For all cutting, not just back chipping. I am surprised that you had problems with blades. On a 36 foot x 12 1/2 beam boat, I believe I used 2. But the blades were Freud or Diablo
probably a little more expensive than a Chinese knock off.
Certainly you can clean a weld but then you need a solvent to ensure absolute cleanliness.
Just a slightest bit of anything such as water, oil in the weld path even on material up to 1/4 inch, when the weld arc got there, often the contaminant would expand, water maybe
what is the magic number 1600 or 1700 times, when this happens, the weld bead can pretty much explode and blow a hole in the aluminum. And yet steel, it can be covered in rust, oil, mill scale and the rigidity of steel at close to the melting temp does not cause the same problem.
We were often asked to repair aluminum cast pieces and the biggest problem was minute contaminants. We had problems and over the years we had just told people that we were not interested
A guy brought in a water jacketed exhaust manifold, kind of a friend, we told him the issue, We did some grinding, cleaned it with trichloroethane III, and there must have been some residual
oil somewhere, when the tig arc hit it, an aluminum hole about an inch in diameter appeared with a small puff of vapor. Probably something inside the jacket that we were unable to clean caused the problem
I have seem this remark about wax on a blade in either Pollards, or Colvins addendum before. While we had shears, vertical bandsaws, hand held plasma and a NC plasma table, there are just some cuts requiring a skilsaw. I would think that a quality skilsaw blade would cut several hundred feet of 1/4 inch plate without issue.

If you enter say 1/8 inch at 90 degrees, though, without support of the sheet, sometimes the teeth will give the sheet quite an abrupt impact. But with experience, knowing that it is going to
happen, an extremely slow feed at the start takes care of that.

 

Following on from Barry's post.

Our QA system had the welder dye-pen their own welds, the root, before capping. This also applied to the opposite of a double sided butt, if no ceramic backer is used. The alcohol base of the dye and fixer is easily cleaned and any residual would simple atomise under the heat.

Welding castings is notoriously difficult.
The principal reason why is that without knowing exactly what the grade of alloy it is, the filler wire will most likely be incompatible. There is no universal filler wire like one can use with normal ally plate. The casting grade is key to the successful welding along with the correct amount of heat applied.