As Grant mentioned, tank size and placement will likely vary from
boat to boat.
Even ours (hull built by a professional yard, one of a series), may be unique as the
builder would customize per customer request.
That said, Panope has what I believe to be 'standard'
tanks (2) under the
saloon settees. They are welded into the
aluminum hull (hull skin is part of tanks) and are spec'd at 50 gallons each but I seem to get only about 45 gallons of USABLE capacity (90 total). I use these tanks for
water.
When she was schooner rigged and set up for tropical
offshore cruising,
fuel was in a
single tank of 30 gallons mounted in the lazarette. Currently, the boat is more of a
motor sailer and set up for the inside passage/Alaska and can carry 90 gallons of fuel in two tanks under the pilot house settees.
Using a section of the
keel box for a
fuel tank has been done on some hulls. Size will vary depending on the amount and placement of ballast. I have fantasized about adding this to my boat and rough calculations indicate that about 50 gallons are possible in the (empty) aft end of the
keel.
We have never had a
holding tank.
Steve