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Hi Brian. Although I will be going with the interior valve/piping design, I
like the idea of an exterior valve because of the ease with which you can put a
BIG valve on there, making the flooding stage quick.
Not having really investigated the design much, a couple of thoughts come
to mind.
First off, you gotta have through-hulls to operate the ballast system. One
to put the air into the tank, and another to control the valve. It could be
possible to have just one through-hull ( say a 1 inch ) and have two lines
( say 1/4 inch ) running through the nipple/through-hull with epoxy sealing it
all up.
The two lines could be: one for air into the tank, and the other a
hydraulic or pneumatic line to operate the exterior valve. The valve could be
pretty much any size. 3 or 4 inch would be plenty.
Now for the valve itself. A butterfly valve seems to me could possibly leak
as it gets older. I haven't much experience with them, but generally don't they
have a central shaft where the disk pivots and when closed one side swings out
while the other side swings in ? I think a mushroom or "poppet" valve would be
less inclined to leak, where there's a disk that pops up into the seat. This
type of valve could be fabricated easily and save a bit of money over buying an
expensive butterfly valve. The main difference is, a butterfly valve needs
power to hold it closed, where a mushroom valve is held closed by the pressure
behind it.
I have seen a butterfly valve where there's two halves hinged together and
both sides swing out against stops that seal, but the mechanism that opens the
two halves is like a shifter fork and is bound to give you trouble is there's
any pressure involved. I think Vance was going to use a mushroom valve type
system, and he may be able to provide a link or better information than I can.
Let me know what you find out, and let's get together for a beer again soon.
Frank D. |