wrote in message
...
I'm about to begin the installation of plumbing components on a 30 ft
sailing cutter (new, "holeless" hull) with a basic complement of
fixtures which require through-hulls & seacocks:
Galley Sink Drain
Galley Salt Water Foot Pump
Engine Raw Water Intake
Head Vanity Sink Drain
Head Toilet Raw Water Intake
Head Overboard Discharge pre Holding Tank
Head Overboard Discharge post Holding Tank
My basic questions a
Which - if any - of these fixtures can share a common seacock, and
what are the standard or typical hose sizes for each of these
functions?
For example, can the galley sink drain and engine raw water intake be
'Y'd' off of the same seacock?
Likewise the toilet raw water intake and vanity sink drain?
Obviously, I'd like to minimize the number of holes I need to drill in
the hull, but don't want to violate any traditions... 
Appreciate any thoughts.
Mike Worrall
Los Angeles
Mike, since you're starting with a clean sheet of paper with this design,
this would be a good time to consider building what's called a "sea chest"
for most or all of your water intake needs.
A sea chest, in this pleasure boat context, consists of a pretty good-sized
throughhull and seacock, followed by a good strainer, leading to a
compartment, so to speak, off of which lie a number of sea water outputs
that you then distribute to all the things on the boat that need a source of
sea water. This could be a section of pipe, or a box-section fabrication.
To be more specific about this, I have a 41' sloop for which I need the
following inputs in the after parts of the boat: raw water for the engine (a
Perkins 4-107), sea water for the refrigeration system, sea water for one of
the pumps at the galley sink, and (possibly some day) a desalinator.
Aft of the engine I put in a 1 1/4" through-hull, leading to a Groco
integrated seacock and water strainer. The output from the strainer comes
off the top of the unit, and a short 1 1/4" hose leads horizontally to a
rectangular box (the sea chest) about 4" x 6" by a foot high. The top of the
box is just a little bit above the waterline. Off the bottom of the sea
chest are four tubes, sized appropriately for whatever it's feeding through
that hose. Heavy rubber hoses take the sea water wherever it needs to go. On
top of the box is a removable lexan port covering almost all the top, so
that I can inspect it for obstructions or remove it for cleaning.
Off the sea chest, then, goes a 1" hose to the engine's raw water intake, a
3/4" hose to the refrigeration system's raw water pump, a 1/2" hose to the
galley sink, and the fourth output is plugged off for now. All of these are
short and without significant bends, with the engine intake being the
longest at six feet of straight run. None of these is a very demanding
consumer of sea water, and it's never the case that they're all drawing at
the same time, so the 1 1/4" intake is large enough to feed them all without
starving any one of them. I'd have gone up to 1 1/2" if there had been room
in the engine compartment, just to be on the safe side, but I'm satisfied
with the sizing as built.
The outputs from the sea-chest are not individually valved in my system,
just because I had to squeeze it into a fairly small space. But if you had
the room you could put ball valves inline to individually shut off the
inputs if you ever needed to. I didn't see the need to do that.
So this takes care of all the input needs aft. Forward I need sea water for
the head, and a head discharge. They have their own through-hulls and
seacocks which I saw no need to try to integrate with the sea chest, and of
course the galley and head sinks drain through their own seacocks and
through-hulls. Bilge pump outlets and the refrigeration raw water outlet are
in or above the boot-top. Engine exhaust is high up on the stern, about a
foot and a half above the water line. Up forward, I tee'd off of the head
intake to feed the washdown pump, and sized the through-hull for the rather
demanding draw of that pump. This is the only tee'd intake on the boat, and
if I'd had room forward I would instead have built a similar sea chest for
those two intakes.
Good things about this scheme: one hole through the boat for all sea water
inputs connected to the sea chest, there's only one seacock to maintain, and
everything is well-strained before distribution. This Groco strainer is easy
to clear, and the Lexan cylinder makes it easy to inspect. I added to the
sea chest a fitting that lets me flush the engine's raw water system, which
means that I could charge it with antifreeze in the winter, or just flush it
with fresh water to discourage the critters.
Actually, on a ship, I believe that a sea chest would just be a box welded
to the hull, open below with the top of it above the waterline, and pierced
for as many water intakes as the ship needs, with valves to control each of
the inputs. Or something like that. I haven't personally seen any of these
but that's what I've heard.
With regard to general principles, it's not a good idea to send out through
a through-hull anything you would not want to draw back into it. So you
might not want the engine raw water pump drawing from the same through-hull
that the sink drains into, lest your raw watter impeller have to cope with
solid waste that escapes down the sink drain. And it's not uncommon to put
the head discharge on the other side of the boat from the head intake. Under
way this doesn't matter, but it might if you were sitting still.
Tom Dacon