Brian says:

I've seen tanks made the way you suggested, but with the tank top oversize
so it extended an additional inch-plus on either side. This was for
suspending the tank by the lip. With all the pounding that a boat goes
through, I feel concern that the top weldment would be prone to fatigue and
cracking.
OTOH, I do know lots of people do it this way (1/8" sides/bottom,
3/16" top). What do you think?


I'm not exactly fond of that method. As you say, too risky. If you want to
hang something make up a bracket properly sized, and then all you have to
replace is the bracket if it fails. THat's the same reason why my stereo
system is component rather than "boom-box" style. I don't have to replace
everything coz the turntable dies (yes, I'm that old ;-))

I love things that work, and can be depended on. That includes gravity. If
gravity will hold something in place when static, then a simple strap to take
care of the dynamic situation is all you need. And it's cheap!

Steve

Steve, or an aluminum expert:

In general bends are better than welds (in aluminum) since you don't have
the heat treatment hardening of the tank in the area of bends, but do in the
area of welds. What if the aforementioned lips on the side, the sides, and
bottom, were all made from one sheet that was bent to make the shape. The
end caps would be welded on, as would the lid, but the welded-on lid would
not be carrying the weight. Is there any issue with bends becoming weak?

Brian


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

I've seen tanks made the way you suggested, but with the tank top
oversize
so it extended an additional inch-plus on either side. This was for
suspending the tank by the lip. With all the pounding that a boat goes
through, I feel concern that the top weldment would be prone to fatigue
and
cracking.
OTOH, I do know lots of people do it this way (1/8" sides/bottom,
3/16" top). What do you think?


I'm not exactly fond of that method. As you say, too risky. If you want
to
hang something make up a bracket properly sized, and then all you have to
replace is the bracket if it fails. THat's the same reason why my stereo
system is component rather than "boom-box" style. I don't have to replace
everything coz the turntable dies (yes, I'm that old ;-))

I love things that work, and can be depended on. That includes gravity.
If
gravity will hold something in place when static, then a simple strap to
take
care of the dynamic situation is all you need. And it's cheap!

Steve

My theory is that the least amount of welding the better. It weakens
the material much less and is a whole lot less expensive.

Brian D wrote:
Steve, or an aluminum expert:

In general bends are better than welds (in aluminum) since you don't have
the heat treatment hardening of the tank in the area of bends, but do in the
area of welds. What if the aforementioned lips on the side, the sides, and
bottom, were all made from one sheet that was bent to make the shape. The
end caps would be welded on, as would the lid, but the welded-on lid would
not be carrying the weight. Is there any issue with bends becoming weak?

Brian


"Stephen Baker" wrote in message
...

Brian says:


I've seen tanks made the way you suggested, but with the tank top

oversize

so it extended an additional inch-plus on either side. This was for
suspending the tank by the lip. With all the pounding that a boat goes
through, I feel concern that the top weldment would be prone to fatigue

and

cracking.
OTOH, I do know lots of people do it this way (1/8" sides/bottom,
3/16" top). What do you think?


I'm not exactly fond of that method. As you say, too risky. If you want

to

hang something make up a bracket properly sized, and then all you have to
replace is the bracket if it fails. THat's the same reason why my stereo
system is component rather than "boom-box" style. I don't have to replace
everything coz the turntable dies (yes, I'm that old ;-))

I love things that work, and can be depended on. That includes gravity.

If

gravity will hold something in place when static, then a simple strap to

take

care of the dynamic situation is all you need. And it's cheap!

Steve




--
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

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

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

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

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...."

Brian,

Something a tank builder in RI suggested to me was mounting the tank on
something like UHMW material attached to the tank with 5200. That way no
water can get between the mounting device (UHMW pad) and the Al tank.
That would take care of keeping the tank from going any lower (gravity
again) and I guess straps across the top, but there you go again with a
water entrapment problem. Maybe more UHMW pads and a strap over them?

I'm leaning more and more toward a plastic tank, looking for a small one
but it has to fit under the cockpit and behind the mizzen step of o my
Kenner Privateer 26, a bit over 10 gals would keep me quite happy.

And I found out just the other day that Todd no longer makes the small
tanks, apparently only larger ones.

Do the makers (Tempo and who else) put baffles inside these plastic tanks?

Dave Cannell
--
In article %L4kb.805694$Ho3.219150@sccrnsc03, Brian D wrote:
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