"Jim Woodward" wrote in message om...
This is in response to an e-mail from Julian. The return address on
his e-mail is , hence the reply here.


Jim,

Thanks so much for taking the time to explain both Fintry's and
your earlier Swan 57 sea chest set ups, and sorry about the email
address, I forgot that I'd recently changed it on my newsreader.
I have a few comments regarding your email below...

If you go to our web site, you'll find a piping diagram:
http://www.mvfintry.com/pix/piping800.png
and lines drawing:
http://www.mvfintry.com/pix/flines800.png

If you're interested, I can send you bigger versions (ie, more
legible) of either -- just say how big is OK. The originals are in
AutoCAD, if any of those formats are helpful.

The stuff on your website is perfectly legible thanks.


The sea chests are at frame 8 and 17 (of 45 total, 20" spacing), so
they are well aft of midships, but they are also well down under the
hull, where there is little deadrise. This is a disadvantage when she
is in very shallow water or sitting in a mud berth. Her sister,
Amazon Hope (see link on site), which is way up the Amazon in Peru,
has recently been modified to avoid this problem.

(The drawings at the bottom of http://www.mvfintry.com/details.htm all
show frame locations).

The chests are just steel boxes welded to the inside of the hull, with
grating holes in the hull plating -- no special shaping at all.
They're maybe a foot square and 6" high and have two valves mounted on
the top for seawater intake and weed clearance (if you look at the
piping layout, valves M19, M20, and M21 allow you to inject pressure
seawater into a seachest with the intake valve closed to blast away
anything that might clog the chest. Obviously, this trick works only
if you have more than one seachest.)

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.

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.


On the intake side, there's a large (although not particularly fine)
strainer with a valve on either side to isolate it for cleaning. All
the piping is galvanized steel, which requires attention from time to
time, but has the advantage that it won't burn and sink the boat in
case of an engine room fire.

The fundamental advantage of a sea chest system is minimum holes in
the hull. On a smaller scale than Fintry, when we bought our Swan 57,
Swee****er, she had ten seawater intakes (engine, genset,
refrigeration, 3 air conditioners, 2 heads, washdown, watermaker) and
ten intake seacocks. If you started flooding, you'd have to close ten
seacocks, several of which were very difficult to reach, in order to
eliminate a bad hose as the cause of the flooding.

As part of the preparation for our circumnav, we put in one 2"
seacock, and manifolded everything to it. On top of the seacock was a
tee, with the manifold connected to the side and a pipe plug in the
top (think of the tee on its side with the straight through part
vertical). By closing the sea cock, removing the pipe plug, screwing
in a three foot length of pipe (long enough to be above waterline),
and opening the seacock, we could push a rod down through the pipe,
the tee, the seacock, and the hull and clean the intake.

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.

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.

Does anyone see any significant problem or improvement for the above?

- Julian