Re: Holding tank size
Don't forget that a very large holding tank (like 38 gallons) will also require considerable structure to hold it in place in rough seas (or in the worst possible case, a roll).
A 38 gallon tank will weigh ~400lbs when full.
Personally I'd rather have useful storage than another 18 gallons of crap...
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior