Brian says:

Is there any issue with bends becoming weak?


No, there is an issue with bends BEING weak. ;-)
Take a sheet of aluminum, bend it 90 degrees, then bend it straight. Now try
to rip it.
See what I mean?
Welding will lose you a lot of strength, yes, but it is a well-documented loss,
and folks will generally use only strength figures for the welded condition.
If you use bent plate, and calculate for the un-welded strength, you _will_
suffer for it.

Steve "not an aluminum expert...."

I must disagree. Especially with non-heat treatable alloys like 5052
and 5086 used in marine applications. These alloys are work hardened.
You are confusing bent strength with fatigue cracking. If you bend any
metal back and forth it will crack but if you bend aluminum once it gets
stronger. Aluminum can be bent and stretched more than almost any
metal. A beer can starts of as a flat disk.

Aluminum sheet usually comes hardened to some degree. The hardness
comes primarily from the rolling process. The amount is indicated by
the H number that follows the alloy number. For example H0 is anealed,
the softest and weakest. H1 is hardened only. H2 is hardened and
partially anealed. H3 is hardened and then stabalized. The second
number is the amount of hardening and the third number defines hardnes a
little colser. H116 is between 1/8 and 1/4 hard only. H34 is half hard
and stabalized. When you bend them the hardness and therefore the
strenght goes up slightly. When you weld any of them they drop back to
the anealed H0 condition and are up to 20% weaker.

Also if the weld is not done exactly right or it is a little windy when
it is done blowing the shield gas away you run the risk of hydrogen
enbrittlement which results in cracking.

Stephen Baker wrote:

Brian says:


Is there any issue with bends becoming weak?



No, there is an issue with bends BEING weak. ;-)
Take a sheet of aluminum, bend it 90 degrees, then bend it straight. Now try
to rip it.
See what I mean?
Welding will lose you a lot of strength, yes, but it is a well-documented loss,
and folks will generally use only strength figures for the welded condition.
If you use bent plate, and calculate for the un-welded strength, you _will_
suffer for it.

Steve "not an aluminum expert...."

--
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

We have a local guy here that teaches welding at the college and he's a
persnickety perfectionist and very very knowlegible about all types of
welding. I know another guy up in Homer, Alaska that is also extremely
top-notch and an excellent welder. I'll do my homework with both these guys
on the tank design, and then will spec it out. I'm hoping the local college
guy will weld it up for me ...he's such a perfectionist, not just with the
welding but all the structural issues. His students pass the certifications
at very near the 100% level, one of the highest rates of successful
certifications in the country. I don't know if he's willing to do custom
work but I sure hope so. I just want to get all the answers I can before I
go chase him down.

Having the tank suspended by the longitudinals means free open air under the
tank and prevents corrosion. I guess I could put a support stringer under
the tank and weld on a sacrificial strip of aluminum that'll rest on it.
Something like 1/4" thick...take a few lifetimes to corrode that away,
assuming you seal-weld the edges and don't trap water between it and the
tank via capillary action. Education is ongoing ...I'll keep looking into
this stuff. And I *do* own the full set of ABYC and USCG specs ...need to
go read the fuel tank stuff too.

Dave Gerr has a couple of articles, a 2 part series, in the current
Professional Boatbuilder and the last one (Number 84 and 85,
August/September issue and October/November issue, 2003).

Brian

--
My boat project: http://www.advantagecomposites.com/tongass


"Stephen Baker" wrote in message
...
Brian says:

Is there any issue with bends becoming weak?


No, there is an issue with bends BEING weak. ;-)
Take a sheet of aluminum, bend it 90 degrees, then bend it straight. Now
try
to rip it.
See what I mean?
Welding will lose you a lot of strength, yes, but it is a well-documented
loss,
and folks will generally use only strength figures for the welded
condition.
If you use bent plate, and calculate for the un-welded strength, you
_will_
suffer for it.

Steve "not an aluminum expert...."

So, the bending may be OK after all...but if the lips for hanging the tank
are bent, then a lid is welded in, the weld will run right along the bend
that makes the lip. Probably not so good again. There must be *some* way
to make an aluminum tank that can be suspended by stringers. I want the top
of the tank to be 3/16" so that together with the welded in baffles, it'll
make a stiff deck to walk on. I know lots of people who've built the boat
that I'm building have used tanks like these, but they let them 'hang by the
lid weldment'. The biggest tank that I know of like this is about 8 feet
long and contains 80 gallons of fuel, or about 60 pounds per foot ...30
pounds per weldment, pounding in the waves. Is that OK or are these guys
driving around with a failure waiting to happen?

Brian

--
My boat project: http://www.advantagecomposites.com/tongass


"Glenn Ashmore" wrote in message
news:h83kb.79220$sp2.58042@lakeread04...
I must disagree. Especially with non-heat treatable alloys like 5052
and 5086 used in marine applications. These alloys are work hardened.
You are confusing bent strength with fatigue cracking. If you bend any
metal back and forth it will crack but if you bend aluminum once it gets
stronger. Aluminum can be bent and stretched more than almost any
metal. A beer can starts of as a flat disk.

Aluminum sheet usually comes hardened to some degree. The hardness
comes primarily from the rolling process. The amount is indicated by
the H number that follows the alloy number. For example H0 is anealed,
the softest and weakest. H1 is hardened only. H2 is hardened and
partially anealed. H3 is hardened and then stabalized. The second
number is the amount of hardening and the third number defines hardnes a
little colser. H116 is between 1/8 and 1/4 hard only. H34 is half hard
and stabalized. When you bend them the hardness and therefore the
strenght goes up slightly. When you weld any of them they drop back to
the anealed H0 condition and are up to 20% weaker.

Also if the weld is not done exactly right or it is a little windy when
it is done blowing the shield gas away you run the risk of hydrogen
enbrittlement which results in cracking.

Stephen Baker wrote:

Brian says:


Is there any issue with bends becoming weak?



No, there is an issue with bends BEING weak. ;-)
Take a sheet of aluminum, bend it 90 degrees, then bend it straight.
Now try
to rip it.
See what I mean?
Welding will lose you a lot of strength, yes, but it is a
well-documented loss,
and folks will generally use only strength figures for the welded
condition.
If you use bent plate, and calculate for the un-welded strength, you
_will_
suffer for it.

Steve "not an aluminum expert...."

--
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

Now here I agree with Steven that tanks should be supported from below.
While the static load per inch may be low the dynamic cycling from
pounding through the waves can cause stress corrosion and cracking in
the weld. A thicker top could actually make it worse. Think about what
happens when 500 pounds of diesel bounces. The bottom of the tank
flexes pulling the sides together. The flange therefore pulls against
the weld. The top goes into compression and because metals are stronger
in compression than tension it deflects less so the weld has to bend.
Being the weakest point it will bend the most. Also having lost any
resistance to stress corrosion during the welding it will corrode faster.

If you are hanging the tank a better solution would be to at least add
angle irons across the bottom every couple of feet to reduce flexing
and possibly up the sides to mounting points on the stringers to take
the load off the weld.

Brian D wrote:
So, the bending may be OK after all...but if the lips for hanging the tank
are bent, then a lid is welded in, the weld will run right along the bend
that makes the lip. Probably not so good again. There must be *some* way
to make an aluminum tank that can be suspended by stringers. I want the top
of the tank to be 3/16" so that together with the welded in baffles, it'll
make a stiff deck to walk on. I know lots of people who've built the boat
that I'm building have used tanks like these, but they let them 'hang by the
lid weldment'. The biggest tank that I know of like this is about 8 feet
long and contains 80 gallons of fuel, or about 60 pounds per foot ...30
pounds per weldment, pounding in the waves. Is that OK or are these guys
driving around with a failure waiting to happen?

Brian


--
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

Keith wrote:

Did you check Ronco for a tank? They make more stock shapes than anyone I
know, and are very easy to work with. http://www.ronco-plastics.com/.

I'd stick with regular steel for the tanks if you can't find one at Ronco. I
have a pair that are 17 yrs. old and fine. You are right about the inside,
the diesel is a good anticorrosion treatment, as long as you keep the water
out. The outside should be painted with a good coating of some kind, and the
bottom should be supported so that air can circulate under it. Obviously,
you've got to keep the water off the top of the tank as well. Be sure to
have the maker put a sump with a dip tube from the top in the tank so you
can suck out any water that might end up in there.

In a sailboat, maximizing tank size is going to result in an irregular shape
with the fuel pick up at the deepest point. A sump is an unnecessary
embelishment. Water is going to be of concern only with contaminated
supply in a well designed system. If water in the fuel causes sleepless
nights tee into the fuel pump discharge so a liter or so can be pumped
into a container for disposal.

I've never felt comfortable about hanging the tank. I'd use poly, but I can
get more volume with a custom aluminum tank. I'll support the tank, either
by shelves that run along the sides of the tank (extending beneath the tank
to carry it from below of course) or will put an on-edge stringer under the
tank. Either way, will use neoprene between the support and the tank. The
tank will be etched, primered, and epoxy painted to prevent external
corrosion and I'll lift the tank out every 5 years or so for an inspection.
I'll maintain the "use the top as a deck" concept and will place enough
baffles in the tank, welded to the top, and use a 3/16" top too, to make
sure it is a stiff deck. Might be a gas fumes sensor in the tank area since
the drain plug from that chamber will lead into a fish box, which means
it'll be closed until the boat's back on a trailer.

Thanks,
Brian

--
My boat project: http://www.advantagecomposites.com/tongass


"Glenn Ashmore" wrote in message
news:%79kb.79853$sp2.33451@lakeread04...
Now here I agree with Steven that tanks should be supported from below.
While the static load per inch may be low the dynamic cycling from
pounding through the waves can cause stress corrosion and cracking in
the weld. A thicker top could actually make it worse. Think about what
happens when 500 pounds of diesel bounces. The bottom of the tank
flexes pulling the sides together. The flange therefore pulls against
the weld. The top goes into compression and because metals are stronger
in compression than tension it deflects less so the weld has to bend.
Being the weakest point it will bend the most. Also having lost any
resistance to stress corrosion during the welding it will corrode faster.

If you are hanging the tank a better solution would be to at least add
angle irons across the bottom every couple of feet to reduce flexing
and possibly up the sides to mounting points on the stringers to take
the load off the weld.

Brian D wrote:
So, the bending may be OK after all...but if the lips for hanging the
tank
are bent, then a lid is welded in, the weld will run right along the
bend
that makes the lip. Probably not so good again. There must be *some*
way
to make an aluminum tank that can be suspended by stringers. I want the
top
of the tank to be 3/16" so that together with the welded in baffles,
it'll
make a stiff deck to walk on. I know lots of people who've built the
boat
that I'm building have used tanks like these, but they let them 'hang by
the
lid weldment'. The biggest tank that I know of like this is about 8
feet
long and contains 80 gallons of fuel, or about 60 pounds per foot ...30
pounds per weldment, pounding in the waves. Is that OK or are these
guys
driving around with a failure waiting to happen?

Brian


--
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

On Sat, 18 Oct 2003 12:39:43 -0700, w kensit
wrote:


In a sailboat, maximizing tank size is going to result in an irregular shape
with the fuel pick up at the deepest point. A sump is an unnecessary
embelishment. Water is going to be of concern only with contaminated
supply in a well designed system. If water in the fuel causes sleepless
nights tee into the fuel pump discharge so a liter or so can be pumped
into a container for disposal.

Disagree. Tanks that are less than completely full condense water
during thermal cycles, and water being heavier than gas or diesel, it
collects at the low point. This really ought to be a small sump, need
not be more than a tea cup - where it can be checked via a sampler.
Nothing stops an engine better than water in the fuel.

Brian Whatcott Altus OK

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.
Welded aluminum sounds horribly weak from the drift of this discussion.
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.
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.
Regards,
Boat Dreams

boatdreams lifted the trapdoor, peered around and
wrote:

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.

http://www.tek-tanks.com/

Would probably be my choice.