Doug’s Workshop: Building in Aluminum

Part Three in a series by Douglas Westover.   Read the others in this series.

In my varied experience with boats, I have never seen a Shanty boat barge or other vessel built in aluminum.

Aluminum is a very good material. It does not corrode(easily…but yes it can if close to an electrolytic source), it requires no blasting, it is very malleable, and it can be cut easily with common power tools you might have in your woodshed. On top of that it is light and  looks cool, if you  like that shiny look.

Probably the most alluring aspect is the lack of maintenance required of an Al. hull. But don’t be fooled. There is ALWAYS maintenance in some form or another.  But it rates very high one the LM scale(low maintenance).

Yes, aluminum does have many great properties, not one of which is cost. This is one of the more expensive materials to build in. Having said that , if you add the blast costs of steel and the “conventional”  paint systems needed for steel, as well as the lifting devices, and cutting equipment, its likely you might break even.

Aluminum comes in a variety of extrusions. An extrusion is a process by which an aluminum Ingot is forced into a die using very powerful hydraulic actuators and comes out like spaghetti does when it is made.

The result is like  your “Playdoh” extrusions you might have played with as a kid. You know the star shapes, and the diamond shapes squeezed through a type of plastic press. Well, this is just like an extrusion in aluminum.  But in this case it is in the shapes of parts that make it easier to build a boat.

There are many shapes too! There is a type of a chine log that you can get, that actually slides into the mating surfaces of the topsides and the bottom plate to better weld it and to insure accuracy. This has the added advantage of building in pre-formed lift strakes for powerboats.

Aluminum, Comes in many varieties for a home builder. The type of aluminum varies, from the less expensive series to the more expensive types. There are 1000, 2000, 3000, 4000, 5000 and 6000 series, but the most common in boat building is usually the 5000 and 6000 marine series.

of these I believe that 5052 is the most common, followed by 5086,

photo from Glen-l.com

  aluminum - spaceaxe@gmail.com - Gmail.clipular

note the above in “E” allowing the plates to lock into the chine log. 

5052, 5083, 5086, 5454, 5456, and 6061 are all marine grades to be specific.

There are subtle differences in the series as well, 5052 is not quite as strong as 5086. But less expensive.

For instance if you were going to buy  tubing in aluminum you might use a lesser grade, than if you buy an extrusion for boatbuilding.  Of course these shapes come with a price.
You must also know your quality of aluminum. some aluminum types are easier to form curves with.  Heat bending is not uncommon  in a typical vessel with developable surfaces.

The typical 6061 series is commonly used for hull stiffening which means that it is not as easily bent, or formed , as the lesser series.
Shell plating is generally 5052, or 5086.

Ok so now that I have bored you into a stupor, I wanted to mention why a barge hull may not be suitable for Aluminum, and why maybe it is.

Aluminum would be ideal, for a barge hull, since its low weight will allow for a greater carrying capacity on a shanty hull. And the low maintenance makes it a Joy.

The trouble is, costs and weldability.

You see, it takes a LOT of skill to weld up an aluminum boat. I’m not saying it cannot be done by an amateur, but you will need some intestinal fortitude to attempt it if your welding skills are not up to par.

In fact welding Al. is messy business. The welds can look terrible and BE terrible in the wrong welders hands. If the welds are not made properly , you risk a crack in the weld. Some guys heat the Al. up before they work it, very much like you would heat cast iron before and after a weld is that shrinkage does not pull the weld apart. Having poor looking  welds is one reason there are not many unpainted Aluminum hulls around. Sure, glare is an issue in the sun reflecting off a shiny bright Al. surface, but they look so cool when unpainted!

Penetration is a problem too. It is hard to know just how good your welds are. Of course steel can be like this too. The problems of welding Al. is that its is so soft that it Is easy to burn through. Getting your heat settings correct is one issue. Distortion the other.

Choosing the right weld process is very important. I prefer a tig torch for this and the learning curve on this is a high one. Don’t even try to stick weld the stuff. It is very very difficult to do with a stick. Mig is ok, but still requires a furlong into bettering your skills. I personally would choose a tig over Mig for quality and aesthetics. If your new to welding Al. forget even trying up hand or down hill welding. And it is for this reason that I think Al. is not used by many homebuilders.

The one exception for using a  Mig would be  that the process is much faster than Tig. Likely in the area of 2x’s as fast or more. But a Tig will not produce the amount of distortion. A good welder, can cleanly and without distortion weld two razor blades together with a Tig. This is not achievable on a Mig or any other process commonly used by the home builder.

Another nice thing about Al. is that you can take a steel design and  convert it easily to Al. Of course you should have some knowledge on how to do this and there are many good books available. But its not too hard  for instance to convert a steel design to Al.

A general rule of thumb is that you add about 3/16th of an inch more to the plating than steel and the same adjustment  for stiffening.

I.e. lets say your barge hull calls for 1/8th plate, then you would increase your hull scantlings to 3/16ths. The same with stiffeners. If the plans call for 1/8th steel FB then you might even go with 3/16 angle for more stiffness. Note that I am not a Naval Architect, I have gained this through trial and error and owning a LOT of plans which are done in both materials, so a comparison is easily done.

this taken as an example, from the ultra awesome Glen-L “Fred Murphy” Tug of which I own a set of plans for. the first box is steel, the second is Al. substitutions.

Frames 3″ x 1/4″ flat bar, 225 lin. ft.

 

Frames (see Aluminum Frame detail) 3″ x 2 1/2″ x 1/4″ angles

As a side bar, The FM is a great project. It is well designed and well thought out. It would make a great personal tow vessel and I hope to outline why a tug/barge combo is better than a single vessel, in upcoming articles. just something to think about…
                                                                     ~
Ok back to the grist mill.
The only downside for Al. as far as strength is concerned is the abrasion  resistance, and for this reason I am not partial to it for a Tug or a Barge, if you are going to beach it.

Nowadays even jet boats are using a special (and expensive) material calledUltra-high-molecular-weight polyethylene. or UHMW for short.
Its basically the same stuff used to make plastic kitchen  cutting boards out of.

This is usually bolted to a jet boat hull. This material resists abrasion extremely well. In the past, Jetters would grind away a good portion of their plating if they were doing a lot of rock crunching. adding UHMW to the adds insures great impact and abrasion resistance.

I would recommend,  if building a hull in Al, and if your going to beach it much, that a thin layer of UHMW is used for the bottom.

Al. is very light, so when building a barge type boat, It makes it easy to  put the plating into place.

There is nothing worse than lifting 500 lb. plates into position when the wind is up. And to paraphrase  Designer and Builder, Thomas Colvin in his book, boatbuilding in steel”   It makes a serviceable  guillotine!

The light weight of  Al. makes this a much easier job.

So in closing,   If your welds are good, you can build a very good looking hull, which will require very  little maintenance ,if at all, and will last indefinitely.  An Al. hull has one thing going for it other materials don’t; great resale. If you are going to sell the boat and don’t want to take a hit in the pocket book, it can often resale for better than the build price if the hull is sound. This make it unique in that dept.

I think a shanty hull in Al. makes for a great boat.

My arguments for not using Al. in a shanty hull is basically simple; that it is not really appropriate for a shanty hull since there is now wood/epoxy techniques that are simpler, and probably just as robust and  long lasting, if done right, but are much less in costs than  AL. steel is also easier to use, and for a barge hull where aesthetics are not important to many people, it just doesn’t make sense. But to each their own.

…for those who can weld, and have the means- I would love to see a nice shiny, Al. shanty boat built.

what say ye?

Douglas J. Westover

Originally posted 2016-01-02 08:56:44.

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