Mic wrote:
53 was a wild guess the 585 model avg. 62

That's a WEE bit of a stretch...50 is actually closer to accurate.

It seems that many prefer this option to the standard config. on a
retro fit for thoses vessels less than 28ft and for casual boating
seems to make sense, practically and cost wise.

Yep...and I often recommend a self-contained system for smaller boats.
No moving parts to maintain, no plumbing except a vent and pumpout
line...and with the addition of a y-valve and pump, even be dumped at
sea. No tank occupying a major portion of valuable storage...and it
costs a lot less.

Although in
outfitting an ocean going vessel this option is not desirable in heavy
seas?

There's no more problem with it than any other tank in heavy seas
....the tank is sealed, so the contents won't end up all over the boat.

The volume created using the conventional system is considerably more
per use than the thetford system as are the maintenance issues and
cost.

You keep referring to it as "the Thetford system"...as if they're the
only mfr who offers a self-contained pumpable/dumpable system. The
SeaLand 965 MSD is 5.6 gal system...and other portapotty that has a
model name or # that includes an "MSD" designation. Actually just about
any portapotty COULD be converted by simply installing a vent and
pumpout fitting in the tank...'cuz that's the only difference between a
portable and pumpable portapotty. The "MSD" versions just already have
'em installed.

Which system would you advise on and why? I guess it would relate to
the size of the vessel as well.

Very definitely related to the size of the vessel...but as much to meet
buyers' expectations as practicality. A portapotty is a good choice for
any vessel under about 26-28'...but today, the owners of larger boats
want a "real" marine toilet, not a "camper" toilet. Over 30'--often even
smaller--they have to have the push button convenience of electric...as
the size of the boat increases, the toilet also has to be increasingly
closer to something "just like the one at home." So it's not just
practicality that determines the "right" toilet for any boat, but human
emotional needs and a toilet that fits their image of the "right" one
for their boat. Even small boat owners..you'd be amazed at how many
owners of 24-26' sailboats cannot be talked out of replacing a perfectly
good self-contained system with a "real" toilet and holding tank.


--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://shop.sailboatowners.com/books...ku=90&cat=1304