Your Experience with HSS/SHS/RHS Hull Framing?

Questions:
How common is HSS/SHS/RHS hull framing?
What are possible benefits and drawbacks?
How does it mate/attach to plating (that's probably at a different angle)?
Is crevice corrosion still significant between 5000 and/or 6000 series aluminum plating and tube framing?

Background: I understand approximately why Dave Gerr, in Boat Strength, say "Water can get into or behind these shapes (HSS/SHS) where corrosion can take place, but you can't ever see it to inspect." But he still notes that indeed "Some builders use...square tube for hull frames."

Thank you for your consideration.

 

It is generaly not good practice to use inaccessable hollow shapes for primary structure. Big issues are fusion lines, mill scale, coatings, inspection and de-alloying if welded. Normal commericial construction is to skip weld, so voids are formed if they are used for frames (i.e face to shell). Additionally, alloys such as those used in high efficeicy shapes, contain materials (nickel, chrome, manganese, magnesium, silicon) that change thier electronegativity so that they are anodic to normal materials (this is true for aluminum or steel products).

 

I appreciate the info jehardiman. Had to look most of it up, but generally get it now. I'm really only interested in aluminum, so I'm curious about something. You say electronegativity affects aluminum, and obviously inspection would too. But do fusion lines, mill scale, coatings, de-alloying, and voids (ergo crevice corrosion)? Any thoughts would be appreciated. Thanks.

 

As JEH has noted, corrosion is one issue to address, especially if the flange of the RHS is wide. You can get pitting because the stagnant water can become very alkaline, or in other cases oddly enough waterline corrosion, for the same reasons. This being as the water evaporates, it leaves concentrated chloride (salts) in solution but in varying concentrations which creates an 'electric potential'. So bare metal to metal ally is not recommend. At times it is unavoidable, but as a rule, it is not something you should do. Doesn't mean it cant be done though....just requires care and attention pre- and post fabrication and good maintenance.

As I noted on your other thread....the stiffer the section the harder it is to bend too, and bend without fracturing.

 

Good points Ad Hoc; thank you.

 

With the general exception of mill scale, yes, everything else does. Most small aluminium closed shapes are made by extruding, and this leaves a noticable (chemical and metallurgical) difference at the fusion line (i.e. where the material has to flow around the support for the center of the die) as well as the possibility of voids and die lubricant contamination. Since the lubricant can be graphite or lead, this can lead to corrosion issues.

De-alloying and voids are welding/dissimilar metal issues as Ad Hoc points out and must be accounted for or avoided (i.e. not good practice but sometimes cannot be helped, closed shapes for frames is one of those that can be avoided though). There are severe restrictions of welding some of the higher strength aluminium alloys. It would almost be better to glue and rivet similiar to jon boat/aircraft construction to maintain strength.

 

Interesting. Thank you jehardiman.