"Julian" wrote in message ...

SNIP

There's no such thing as a new idea! I was considering a very similar
mechanism of 2 sea chests and building in the facility to back-flush
each sea chest if it became blocked. One thing that strikes me about
the Fintry flushing arrangement however (if I have understood it correctly)
is that, because it uses a seperate valve to inject water for clearing the
seachest, it probably won't always clear a blockage in the mouth of the
inlet valve (before the strainer) and might even make it worse. I was
considering setting up each seachest with just one seacock leading to
a strainer (like Fintry) but after the strainer then having a valve arrangement
so that if the input becomes blocked the valves can be set to isolate the
upstream water flow and allow water to be pumped out through the strainer
and inlet to try to clear any obstruction that way. It seems to me that blasting
water out through the inlet is the most effective way to clear any obstruction.

Your concern is quite correct, but I think your solution just
exchanges one problem for another. Your proposal tees the blast line
to the inlet line outboard of the shutoff in the inlet. There is
still a section of the inlet line between the tee and the inlet valve
that can't be blasted.

To some extent your concern is over an improbable event -- the grating
in the hull plating has much smaller holes than the inlet line, so
almost all of the time you'd be clearing either a plastic bag from the
grating or mud from the chest.

If I do it this way then I don't really see the need for a sea chest as
such and the simpler "sea chest" of your earlier Swan 57 would seem
sufficient, provided that the necessary calculations are done to ensure
that the inlet size and placement is sufficient to provide adequate flow
for everything manifolded off it.


The advantage of the chest is that the first strainer is the small
holes in the hull plating, so plastic bags don't get inside at all.



SNIP

That's a good idea. Actually, there is one other sea chest arrangement
I have heard of that incorporates this, in a way it's like Fintry's seachest
in that it's a foot share box welded (well, glassed, since it was fibreglass)
to the hull, but instead of being 6 inches high like Fintry's, it was about
3 feet high so that it cleared the waterline, and the top was clear Lexan
bolted on so that one could see any obstruction and unbolt an access
panel to reach in (probably with a stick) to clear any obstruction. One
drawback I see of a tall seachest like this is that it would compromise the
effectiveness of using a back-blast of seawater from the other sea chest
to clear an obstruction.

Well, yes, you'd use the stick rather than blast. An advantage with
this system is that you could use it as the manifold, putting a
seacock on each pipe leading off it. While that means that you'd have
to close all the seacocks to stop broken-hose-flooding, at least
they'd all be in one maintainable and accessible place, rather than
scattered around the boat. You've also got to find space for and
construct a mission critical box three feet high.

Using my philosophy of good engineering (carefully select everyone
else's good ideas and blend them together to create the perfect result)
my thinking right now is to fit 2 sea chests with each "sea chest" being in
fact a simple large through-hull like your Swan 57, with your rather clever
T arrangement and removable rodding-pipe as an emergency backup, but
with appropriate valves upstream of the strainer to enable back-flushing
through the inlet as the preferred method of clearing any blockage.

If you have the space, looks good, although it violates KISS. I'd pick
one or the other clearing method. I should add, that in our
circumnav, roughly 33,000 miles, we never had an obstruction in the
intake -- grass in the strainer that required cleaning eventually,
yes, but a complete obstruction, no. Thus planning two different
methods of clearing two intakes seems overkill to me.

One further thing to consider. On Swee****er we located the graywater
and blackwater discharges to port of the keel. (We used Lectra San,
but a voyaging boat must be able to pump it overboard.) The one
intake described above was to starboard. You don't want to be pumping
your own waste back aboard -- there were places (Cairns, for example)
where we were moored bow and stern in a reversing current, so half the
time the current was moving from stern to bow. At least in places
like that, I'd like the intake and discharges to be on oppostive sides
of the boat.

Jim Woodward
www.mvfintry.com