boatdreams wrote:
Guys, Dave Gerr (Elements of Boat Strength, pg.186) and others have
written volumes about 5000 vs. 6000 series aluminum alloys. They all
recommend 5000 series for marine applications, but caution that it's NOT
taboo to substitute 6000 series for use other than hull plate--including
non-intregal fuel tanks.
6000 series can be used but with limitations. First it does not weld
anything like as well as 5052 or 5086 and second it is much more
susceptible to corrosion than the 5000 series.
Welded aluminum sounds horribly weak from the drift of this discussion.
Nobody has said welded aluminum is "horribly weak" but it will loose
about 20% of its strength in the area of the weld which has to be taken
into consideration when designing the structure.
I guess that's why aluminum tankers supply the fuel to our local gas
stations? Think about it. That tanker you passed on the freeway this
morning held 5000 gallons of gasoline enclosed inside an aluminum
skin--0.180" sides/top and 0.220" belly--and it bridged 30' kingpin to
center of axles. The girth weld, 15' from the nearest bearing support is
a simple butt joint and was probably hand mig welded by a kid who didn't
finish high school.
Only about 20% of those shiny tankers are aluminum. Most are stainless.
Aluminum tankers must meet DOT406-AL standards. They have a
longitudinal structure, either chassis rails and cross members or
internal webbing to prevent excess flexing of the tank. They are also
built with 5000 series material. The design is fairly standardized and
carefully refined and that kid that welded it has 2 or 3 years of
training, uses a $5K TIG welder and lays the same bead 8 hours a day 5
days a week. The guy I am working with can hardly write his name but he
lays a beautiful full penetration bead. :-)
Forget the semi trailer, the 50 gallon round or rectangular saddle tanks
hung off brackets bolted outboard from the truck chassis were most
likely welded aluminum and at 0.161" to 0.250" plate thickness--thicker
than they need to be--for added protection against puncture. And they
are hung--suspended by fabricated straps and/or U-brackets.
So please don't malign welded aluminum tanks. They've been the fuel tank
of choice for 50 years where light weight, low maintenance and cost
effectiveness are appreciated.
It is very hard to compare automotive tanks to marine tanks. While
automotive tanks have to deal with vibration they don't get the constant
cyclical loading of marine tanks. They also don't have to contend with
a constant exposure to a highly corrosive environment.
Again, nothing wrong with aluminum tanks but they are not the ideal tank
for all situations. They do weigh between 20% and 50% less than mild
steel but they are more than twice as expensive. They are also more
susceptible to stress cracking than mild steel. Removing weight from
consideration I would rank the material for diesel tanks under about 40
gallons like this:
#1 Mild steel
#2 5086 aluminum
#3 5052 aluminum
#4 Cross linked polyethylene (XLPE)
#5 stainless steel (either 304 or 316)
#6 FRP.
Over 40 gallons XLPE drops to fifth place because of the difficulty in
adding baffles and the limited selection of stock shapes.
Now, before Lew gets to upset, ;-) because fiberglass can be molded on
site to the required space it is the most efficient for the use of
volume but it is almost impossible to keep diesel from permeating site
fabricated FRP. (duck'n and run'n) :-)
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:
http://www.rutuonline.com
Shameless Commercial Division:
http://www.spade-anchor-us.com