Aluminium plate sheet orientation question

When building plate aluminium boats I see that most designs have the long side of the sheets orientated horizontally. If the rear half of each hull of a 40 foot catamaran has no change in the profile would it make sense to change the orientation of the sheets so as each sheet is bent up in the required version of a U shape so as it "wraps" around the frames in one continuous shape and so several sheets could all be bent the same way and then butt welded together? (assuming that the design was done with this in mind so as the ribs etc all suited this?). It just seems like a way to work smarter.

 

It is very likely that, if you do not take great care of the welding sequences, many deformations will occur in the welded plates, giving the hull an unsmoothed appearance. For the rest, I don't see any problem with doing what you propose.

 

OK that is something that I hadn't considered, but hopefully the new laser welding will help to overcome that as it puts far less heat into the material than either mig or tig does - it is something that I am looking forward to using actually.

 

Indeed, you are right, the way you build your boat must take into account the material and human resources at your disposal.

 

If you do this, you need to consider how you will sequence the welding of the entire vessel, and also how you can take into account deformation/shrinkage in the long.t direction too.

 

That is going to be interesting to work out!

 

If building a smallish aluminium boat, smaller than 40 foot anyway, I was taught to get all of the plates from one batch only, keep the stack of plate together in one orientation, so as to mark one face and one short side edge, where the shear consecutively cut it off of the bulk at the plate rollers. You may see a curvature in the stack to note. So in future use, the sheets could be used with the same end orientation bow to stern, and top or bottom of stack facing out or in, taking advantage of any natural curvature, so when bent around to form a hull's sides and bottom, they bent equally and symmetrically both sides, as near as you could get in a low tech environment anyway.
So your suggested orientation, vertical, will influence your shapes. Plates are rarely perfectly flat in any direction, so you could end up with many shallow scallops or undulations in the side and bottom plating using it vertically. It would be worth checking that out before doing it. Probably a welder, builder, or NA can tell you more about that probability. It could make a new hull look bad.

 

That is interesting and something that I hadn't considered - will keep an eye out for that! Thanks!

 

Shipyards usually have two machines that are the first to "receive" the plates to start working with them: a machine to blast the surface and a machine to flatten the plate.
If you do not flatten the plate you may have serious problems in subsequent manufacturing processes, for example, when cutting pieces by CNC. Therefore, the plates should NOT have any curvature due to their stacking and, if they do, it must be eliminated before starting to work with them, imo.

 

I suspect that Tansl is referring to steel wrt to a blasting machine. (I have never ran across a "blasting machine for aluminum)
The large aluminum distributors will received the aluminum (plate) up to at least 3/16 in coils. The coil is unwound and ran through several sets of rolls to straighten the plate.
This permits a client to order long sheets as long as can be transported to the build site.
Certainly, smaller aluminum distributors carry the standard sizes: 4x8 or 10 or 12, 5 x 8 or 10 or 12, etc For out 21 foot boats, we would order 22 foot sheets, 23 foot 24 etc.
The issue though can be freight as putting a 24 foot sheet on a 44 foot high boy, uses up quite a bit of trailer deck space. Normally, a manufacturer will provide quite expensive
crating to ensure that the sheets get to where they are going without damage.

For a nominal amount, and if you are not going to paint the hull, you can order sheets with plastic on one side or the other to reduce surface scratches, to a point.

In order to reduce distortion, as other have said but it extremely important especially with thinner sheets, keep the sheets as long as possible with as few butt welds
as you can design to. Heat distortion can be significant. Usually, but not always using maximum length sheets, the joint between sheet may occur on an
inflection interface, ie change of direction at the weld joint. This "kink" (discontinuity) will provide a higher resistance to distortion and "follow" the lines of the boat.

 

@Barry, you are very right, I didn't realize we were talking about aluminum plates.

 

As an aside, these two comments bring up two seemingly trivial things about thin aluminum plate...until they unexpectedly bite you.

In manufacture, aluminum billets are hot formed by continuous casting. To get to a specific thickness, the billets are then rolled (either hot or cold) and below a certain thickness, coiled. This does two things, primarily it elongates the grain in the rolled direction and secondly it stresses the grain structure. These changes in the metal grain structure have two effects...it changes the material properties of the alloy in both the rolled and cross rolled directions, and it changes the weldability of the base alloy. That said, these changes can be mitigated by post forming heat treatment, but you had better know what you are ordering because these post forming treatments add to the cost of the plate...sometimes significantly. In the naval yards we did our own receipt inspection for every lot and also sent a sample to the weld shop for process testing (i.e. to verify final properties after welding). I think a lot of the issues small fabricators sometimes have is the lack of deep pockets to ensure 1) they got what they needed, and 2) they used the right process for the material they have.

Temper Designations for Aluminum | Metal Supermarkets https://www.metalsupermarkets.com/temper-designations-for-aluminum/

Edit to add: I forgot to add that the above comments about changes to material and welding also apply to forming after the plate is in the yard. A lot of either hot or cold forming to get it on the hull can significantly change the final post welded strength of the heat affected zone.

 

In welded metal constructions, the mechanical properties of the material after welding are applied. Therefore, what you point out is already taken into account in the calculations.

 

The hot rolling or cold rolling depends upon the alloy and temper being obtained....
The grain direction, on a sheet of plate, can be identified by an experienced plater. They can "see" the rolling direction on the plate and will ensure the bending is done with the rolling direction, not against it.
The coil is not stressed after being correctly rolled and the correct temper is applied. The last 'minor' rolling process will be used as a final dimensional thickness tolerance check.

That's not strictly correct and is misleading..
The "change" in alloy structure is the process of tempering during the production of the alloy and is the reason for whether the temper and the grade of ally is hot rolled or cold rolled.
But the final result, is stable and is exactly as ordered.

One never heat-treats ally once it is finished fabrication...and if one is having to apply excessive work to the plate to "fit-it-in" then it is poor fabrication and poor QA procedures.
The ally plate must always be offered up stress-free...and never apply force or heat.
To do so, is poor steel work methodology being applied to aluminium fabrication.

Exactly.
One always uses the as-welded properties of the alloy.

 

Yes, yes, yes, guys...you know that, I know that, and if everything is perfect that is how it is. But....RSD may not know that...such is the nature of his original question. As far as heat treating after fabrication, it is common in certain deep submergence PV applications. Otherwise you buy a very expensive billet and then throw 95% of it away; and yes, most yards that do small vessel fabrication don't have that ability....nor need.