Anybody have a rule of thumb on holding tank capacity to potable water tank capacity. I'm redoing a 34 ft Nautaline with twin engines and genset that had no holding tank. I think I can get in 100 gal potable and about 45 gal holding for black water all under the flooring on either side of the keel stringer, with easy access to the tanks by floor panels. I'm trying to keep from building in too many fixed things like dinettes, etc as we would like to have as much open floor space as possible in a small boat. Two of us plan to use the boat to weekend on and on occasion maybe do a two week river run from the Nashville area to somewhere.

Water tank and waste holding tank have little if any relevance to each other, since most (all, unless the toilet uses pressurized water) of the fresh water doesn't go into the holding tank, it goes overboard.

When it comes to fresh water, the only thing that matters is whether the supply is adequate for the average use of the boat--number of people typically aboard, length of time away from source of refill etc. Water lasts longer if it's conserved...it's very easy to tell the difference between seasoned boat owners and landlubbers who just happen to own boats by how much water they waste (also by how much toilet paper they use, but that's a different issue).

The size of the holding tank is dictated by 1) available space, 2) cost/availability of pumpout facilities, 3) how many people are usually aboard, and 4) how long you want to be able to stay out without coming in to pump out. All of the above can also influence the choice of toilets. Electric toilets that use raw water (sea, lake or river water) can require up to 3 gallons/flush, which can fill up a holding tank VERY quickly. Otoh, toilets that are designed to us pressurized water use much less water--some as little a quart/flush, which extends holding capacity.

All that said, 100 gallons of fresh water and 40-50 gallons waste holding capacity should be adequate for your boat. The only real question is where to put both tanks, and there are several factors to consider, starting with ballast--both waste and water weigh 8.33 lbs/gallon. So the water tank when full will weigh over 800 lbs...the waste tank when full, around 400.

The waste tank is the trickiest one...ideally, it should not be further than 6' from the toilet and the run should be as straight as possible. Reason: that's as long as anyone will keep flushing...and when flushing stops, waste stops moving in the hose...and it's waste left sitting in the hose that permeates it and causes the hoses to stink.

You'll find more information aboout how to install it properly at http://www.boatsafe.com/nauticalknowhow/phall.htm But I can tell you that you're gonna have a hard time installing one properly on that boat without any built-in settees or dinette seats to put it in, 'cus there's no place to put it AND a water tank under the sole...and, unless you're planning to the toilet somewhere other than the "standard" location in the boat, any other location is too far from the toilet.

Btw...your best source for water and holding tanks is Ronco Plastics http://www.ronco-plastics.com They make TOP quality thick-walled rotomolded poly tanks for a VERY reasonable price, and have more than 400 shapes and sizes to choose from.

Give me shout if I can help you spec out your plumbing.