Interesting point that the static pressure due to hydrostatic head
only. But Peggie also mentioned a hydrodynamic effect - shifting
the fluid around in a seaway - and that force *is* sensitive to the
mass of fluid.

Brian Whatcott Altus, OK


On Fri, 15 Jul 2005 16:07:21 -0700, "Robert or Karen Swarts"
wrote:

Just a small technical correction, Peggie. The weight of salt water is, in
fact, 8.33 lb/gal, but that is misleading. The pressure exerted is .444
psi/ft. So if the tank is 2 ft deep, the pressure is only .888 psi, which
would also be the pressure on the walls at the bottom of the tank. The total
force on the wall if the tank were 1.3 ft wide on a side (for about 27 gal)
would by 166 lbs.

As long as a true "weld" is achieved equal to or greater than the wall
thickness, its integrity will be as great as the wall itself. I have
certainly welded the bottoms of polyethylene buckets one foot deep without
any problems. I can't speak to the integrity of the tank itself, of course.

BS

"Peggie Hall" wrote in message
. ..


Robert or Karen Swarts wrote:
I have "welded" many polyethylene parts using a common electric soldering
pencil, anywhere from 25 to 125 Watts, and scrap polyethylene from any
source. With a little practice, you could do it yourself.

Welding parts together is one thing...but mending a crack in a tank adds
another dimension, because you have the weight of the contents--8.333
lb/gal for water and waste, which would be 227 lbs in a 27 gal tank)
pushing against the walls of the tank trying to pull the crack apart
again...if the contents are shifting while the boat is underway, they're
pushing even harder. Then there's quality of the original tank material
itself to consider...why it split in the first place. Even if the weld
holds, will the same forces that created the first crack cause a split
somewhere else?

So, as I said earlier, if it were a water tank he'd have little to lose by
trying to mend it. But when it comes to waste tanks, the wisest course is
replacement.


--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/store/custo...0&cat=6&page=1