Opinions for Newbie on Boat Build w/ AL 6061-T651 sheets 4'x10'x1/4"

I found a good deal on 4'x10'x 1/4" thick Aluminum 6061-T651 plate/sheets. I'm thinking about buying these sheets for a boat build or some other future project I may think of. I need to make a quick decision as the material may sell quickly.

I would love to build a simple open skiff (rear outboard Tiller steer) for use as a coastal/bay fishing boat on Lake Superior. Although I don't yet have specific boat plans, I would assume something like a deep V fishing boat with a minimum 18' to 20' length is necessary for the Great Lakes.

What are your thoughts on 6061-T651 as a boat building material? I only plan to use the boat in fresh water, but who knows, maybe someday I'd spend the winters in Florida and take the boat with me.

Is 1/4" sheet AL too thick for a boat of this size? I perused several other past posts and 3/16" thick 5052 Aluminum was selected for their boat project.

Since the sheets are only 10' long, I'd have to weld multiple pieces together to accommodate a hull length of 18+ feet. Although not ideal, what are the negatives of building a boat in this manner. Is this a show stopper?

Any ideas or recommendations would be appreciated for the newbie boat builder?

 

I ve heard 5083 is very good, high end material and 5086 even better.

 

goodwilltoall has it right
and
1/4" a bit thick
6061 not a good alloy - 5086 better. 5083 is okay but a bit more brittle. 5052 is a bit soft
10' = more welding = more work

 

According to this book 5086-H116 is best because it retains it's strenght after welding. The 6061-T5, T6 looses almost half it's strenght welded but are considerable cheaper and good for areas where you don't need that much weld strength.

So if you don't use it for hull and stringers it might be a good deal. But for deck the thickness might be even more excessive.

 

I would not recommend 6061 for hull plating. It has a high %'age of copper and is prone to corrosion issues with fixings/fastening much greater than other alloys.
The silicon in the alloy requires a degree of knowledge and skill to ensure quality welds too., compared to 5XXX series.

It is why 6082 was developed and is the prefered alloy of choice for extrusions, it has 3 times less copper than 6061 alloy.

The temper T651 is not a common one either, it is a sign of being stress relived after rolling/stretching. Most 6061 tempers are T4/5/6.

A H116 is a strain hardened temper, therefore it will loss strength after welding, it will return to its anneal O temper strength at the HAZ.

The loss of strength is only an issue if you have not designed or allowed for it.
Other than O temper, all alloys will loss strength after welding.

 

Well now you confused me. The book "Boatbuilding with Aluminum" says:

"As indicated by the -H temper designation, the 5000 series alloys are strain hardened for strength. because they are strain hardened instead of heat treated, the will retain their high strength when treated by welding".

5086-H116, H32 before welding tensile strength 40.000 psi, after welding 35.000 psi. Which is the same as the strength of 5086-0.
The 5083-h116 is 44.000 before and 40.000 after welding.
So you're right but it retains *almost* it's full strength with welding.

But regarding OPs question, if it's not saltwater and doesn't need to be optimized for weight/strength the 6061 should be fine?
I'd still compare the prices again because this is the cheaper material, so maybe it's not a that great deal after all.

 

The book is incorrect.

Strain hardening is increasing an alloy's strength by manipulation of the arrangement of atoms in the crystal matrix along the slip planes. Basically you are introducing obstacles, or dislocations, to movement within the crystal matrix.
This increases the strength. The more work hardening the higher the strength as more dislocations are introduced.

This is done when the alloy is cold. There is hot rolling too..but for now, we can ignore this as it is done at a temp below melting anyway.

However these dislocations remain in the unwelded alloy UNTIL, heat is introduced. Once heat, welding, is introduced, the dislocations become freed up and so the allow reverts back to its original state, that of O temper, and a lesser strength.

That's it in a nutshell.

 

Thanks. So the good welded strength of 5083/5086 simply comes from the untempered / annealed strength and has nothing to do with "strain hardening".

One thing I'm curious about: If an aluminium boat flexes over it's lifetime does that create work hardening and make the flexing parts eventually harder and more brittle?
So you need to design your welds / your boat be very very stiff and absolutely not flex?

 

Well, yes and no.
The base strength comes from the elements in the alloy and whether the alloy is a heat-treatable or non-heat treatable alloy.

You should read ProBoat magazine no.147, Fe/March.as there is an article all about this subject.

For example:


You can see the unwelded strength of high strain hardened alloys in the H321 temper of say the 5383 alloy, has a yield strength of 220MPa. Yet when welded it is the same as the base O temper of 145MPa.
The strain hardening is bit of a red herring really. Since one designs to the "as-welded" strength as this is the weakest link. Unless of course you can design a welded joint that can mitigate this, which is the reason for extensions, or the 6000 series alloys. Their higher yields strengths can be realised even with the major drop of strength. Since if the strength drops by 50%, the welded joint can be designed to have twice as much area/metal, thus gaining back what has been lost and so the "whole" region can utilise the un-welded strength. In an extrusion this is easy to do, as you design the extrusion to the shape you want so an increase in metal at the welded joint. Thus, horse for courses....

This is a more complex situation.
Firstly aluminium is a ductile material, not brittle. It is almost impossible to get a brittle failure in aluminium, unless under extremely high restraint and very restricted in movement locally - extremely rare. It also depends upon the alloy, this is true for 5000 and 6000 series, but other grades can experience brittle failure more readily, but again, not common at all.

The flexing is not strain hardening, per se. Since what is strain hardening, it is manipulation of the slips planes to increase dislocations and thus get locked in strength. This is a permanent state.

Flex, like an aircraft wing, is just that, flexing between 2 conditions of load where upon relaxation, the load is removed, ergo, it is a temporary state, not permanent.
However, what DOES occur, is that the movement, flexing models the same behaviour as the strain hardening...the atoms move along the very same slip planes.



The problem here is that upon each relaxation, the slip plane movement creates intrusions an extrusions on a microscopic level - just take a ream on a table and press down hard on the top paper and slide the top sheet back and forth, what occurs?. This is the initiation of a the 3 stages of crack growth.



Does this help?

 

Thanks for explaining! Yes very helpful and interesting. I'm really just curiosity though and not planning to build with aluminium.

I was looking at the ultimate tensile strength before but I guess you really have to design for the yield strength since that leads to these intrusions and fatigue.
Converted to MPa the book apparently has a too high value for welded yield strength for 5083-H116 (165MPa instead of 125MPa). Kind of unsettling!

 

Maybe, maybe not. Since values of alloys and their tempers varying according to the source. You should always use values quoted from verifiable sources, such as min requirements from Class. Again, from the ProBoat article:
With increasing temper, the yield (proof) stress increases:



and shown graphically:



So if you wish to sue a value for design, use one from a verifiable source, such as Class rules. Which, you'll note the yield, proof stress of 5083 is:



.... 125MPa.

Sorry forgot to add (EDIT).

Then when looking at the welded strengths:



You'll see the only variable in the as-welded strength is the filler wire. So it is either 125MPa or 115MPs, depending upon the filler wire, and note, the tempers make no difference to the as-welded strength.

 

Wow, lots of great info for a newbie boat builder. Unfortunately, I'm an Electrical Engineer and not a mechanical/metallurgical engineer. It's going to take a bit of time for me to digest this great info!

In addition to the low price of 6061-T651 sheets I can get, another reason I contemplated using 6061-T6 for a welded AL boat build is from the following pdf produced by the American Boat & Yacht Council (2001). As shown, they do list 6061-T6 as suitable for "Welded Outboard Boats" for Hull (non-stretch formed) as well as for transom, decking, and gunwales. I'm not sure of the differences in the alloys to whether 6061-T651 can be substituted for 6061-T6. Nevertheless, I'm glad all the experts here are chiming in about the suitability of various AL alloys for boat building. I have lots to learn!

http://newboatbuilders.com/docs/T-01.pdf

As for cost, I'd be able to buy the 4'x10'x1/4" aluminum 6061-T651 sheets from a local private party for about $225/sheet or roughly $1.60/lb. Checking other commercial suppliers it looks like sheets of 6061 sell between $4.60 to $5.20 per lb.

A quick check on-line, sheets of 5052-H32 sell for about $5.20/lb while 5086-H32 would sell for around $6.00/lb. As you can see, my local supply of 4'x10'x1/4" of 6061-T651 is much cheaper in comparison to the other more preferred boat building AL alloys. But of course, safety should be my first objective rather than cost of materials.

Thanks for all your help as I continue to learn the skill and art of aluminum boat building.

 

Not saying you can't use 6061, as we did many years ago too. But lessons have been learnt and alternatives are now available. Most Classifications societies also do not allow 6000 series to be used as hull plate material, ostensibly owing to the corrosion issue. But also note, the T651 is stretched..!!

What a smorgasbord of alloys and tempers, what a mess!

Whilst a better grade, I do understand the cost equation.

Just one thing to note, i wouldn't recommend a high strain hardened temper for any small vessel, as there is usually plenty of rolling and forming required. Also those grades you note, especially the 5052, has a much lower as-welded strength.

 

Lake Superior isn't salt water right? Not some sort of fjord with mixed brackish / salt water?

If you don't mind the boat weighing more than it would have to, and if you don't normally use it salt water, from my limited understanding it should be perfectly usable.

You have to find the plans that take the material into account and you'd have to engineer the welds correctly. But I figure an 18' 5.5m boat with 6mm aluminum plates is monstrously overbuild anyways. So it shouldn't be hard to make it work.

Do you have boat building plans?

I'd definitely would test if you can bend them though like ad hoc mentions. That would be a bummer if you buy it, get good plans for it and then find out you just can't bend the plates into shape

 

Lake Superior is a fresh water lake in the US and is the world's largest freshwater lake by surface area and the third largest freshwater lake by volume. No saltwater anywhere to be found in these parts of the US ie Michigan. Since Lake Superior can get mighty rough, I need to ensure both the material and the design is sound and I have no problem if the boat is overbuilt. I plan on having the boat "trailerable" so I don't plan to keep it in the water for more than a week or so.

Currently, I do not have any plans for a boat design. I found a good deal on Aluminum 4'x10'x1/4" 6061-T651 plates from a local private party and that got me thinking and researching whether or not this material would work as marine boat material. If the material can be used in a boat building project, I'll probably buy them. Heck, even if I don't use the plate for a boat build, I'll probably find other building projects in which I could use the plate.

I agree that I should research and test to see how well this stuff bends. I'm an engineer by trade so I enjoy researching and thinking about building this kind of stuff.