Neal, Guy's suggestion is very close to one I've had great success with and thought I'd share my installation for you to consider. By the way, both electrical and mechanical pumps (macerators) work just fine.
I've installed a number of head systems in previous boats, friends boats
and in our current one that don't involve any Y-valves at all. Go to page 527
of the 2005 West Marine catalog to see their "Our Favorite Method". It's
the one in the center of the right side of the page.
My installations are identical to their favorite with one big
difference. Mine eliminates the Y-valve marked as 8 in their drawing.
Y-valves are prone to breakage (snapped off plastic handles) and aren't
really needed in that schematic if you think about it.
I happen to have a macerator but a manual pump will work just as well.
Study the illustration and you'll soon see that you can pump overboard by
simply opening the seacock. To evacuate the holding tank from on deck,
simply remove the deck plate and pump away. You can see that a Y-valve
serves no purpose assuming that the O-ring seal in the deck plate is in good
condition and lubed with a bit of silicone grease. That's the simplest and
most efficient method of plumbing I've been able to come up with.
When ready to empty the holding tank, I simply open the discharge
seacock in the hull and pump overboard if I'm three miles or farther from
shore. If I choose to use the pump-out at the marina, I remove the deck
plate and do it that way. Note that the waste will follow the path of least
resistance regardless of there being a Y-valve in place. As long as the
overboard seacock is closed (must be by law, see below), the waste can only
go up and out of the tank.
I might add two more things here. USCG regulations require the seacock
for the holding tank to be closed at all times other than when in use and
they also require it to be defeated in such a way as to avoid the possibility of accidentally hitting the valve handle and opening the seacock to the sea.
I keep my seacock closed as required and have checked with the Coast
Guard to determine that removing my handle satisfies the law, it does. That
said, I keep the handle tied to a short line close by which they also approved of as meeting the spirit of the law.
One more thing I'd like to share. As far as keeping the head system
clean and odor free, I do the following. Once a year I flush the entire
system with muriatic acid as follows. (Muriatic acid is NASTY stuff so be
careful to mix it out of doors. The darned stuff actually smokes when you
take the cap off)!!
A. Make 50/50 mix of muriatic acid and water, (2 quarts in a plastic
bucket).
B. Pour 1 qt. into a dry bowl and let stand for 15 minutes and flush
down.
C. Repeat with second quart as above.
D. Flush the toilet and tank thoroughly at least 2-3 times to remove all
traces of the muriatic acid. Fill the holding tank completely and then
empty it at least twice. Include removal of any any trace that might be
left in the bowl too. This cleaning removes any calcium that might have
built up inside the entire circuit. Those calcium deposits are like a sponge and they hold odors like crazy. If calcium is present, the water in the bottom of the toilet will bubble like Alka Seltzer when you add the mix.
Finally, use 1 ounce of Odorlos nitrate head treatment (page 537, same
catalog) for every ten gallons of holding capacity of your tank as follows:
Pour it directly into the toilet bowl and flush it down with about 1
quart of raw water. Do that every week and head odors will be a thing of
the past, honest.
Apparently Odorlos differs from every other kind of waste treatment in
that it out gasses non-odor bearing nitrogen while the others out gas
odor-bearing oxygen, it's that simple. Look at the price of this stuff and
you'll see that your maintenance costs are pennies a day. Note that if you
have a 10 gallon tank, a 40 ounce measure bottle will last you 40 weeks and
at $14.99 that comes out to 37 cents a week or a little over a nickel a day. My tank is 13 gallons and I only use 1 ounce a week and have done so for at
least the last 5-6 years with no odor in the boat at all so you can play
with that ratio a little.
I hope this helps, Glyn