How much poo

[sundog]

How much poo do you reckon 8 people would produce over the course of 24 hours? I'm helping put together a converted shrimpboat that will be used as a kayak mothership... we'll be operating under a six-pack + 2 crew and we're trying to figure out how big our holding tank needs to be. Trips probably won't be over 3 days but a week could be possible. This is for black water only. Surely there must be some general equation for this??

Thanks!

[GordMay]

Some experts have estimated that the average effluent (sewage and flushing water) per person per day may be as low as three gallons; in many cases it will be higher.

FWIW:
“How many flushes will my holding tank hold?”
Holding Tank Efficiency Data

[knottybuoyz]

Fit the biggest tank you can possibly get in the space you've allotted to the holding tank.

[Hud3]

Sundog,

I have a VacuFlush head on my boat, and I think Gord's referenced link is pretty accurate on the volumes. It uses a lot less water per flush. You might want to consider installing one. The ladies, in particular, will appreciate the convenience of not having to pump a manual head. In fact, my wife made it a condition of buying our boat that the brand new manual head had to go, in favor of a new VacuFlush. Note, however, that the VacuFlush uses fresh water, not seawater. That, by the way, means you won't have the typical stinky head that comes with using seawater.

I'd like to make another point that some may consider sexist on the surface. The number of flushes will go up in proportion to the number of females on board. I've sailed with all male crews of four and a two-couple crews of four. For apparently valid biological and/or anatomical reasons, males require less water for flushing. When my wife is onboard, the two of us use 10-12 gallons of fresh water per day, for all purposes, including her daily shower and my off-the-stern wash-down. If it's just me, water consumption goes down to about 3 gallons per day or less.

[sundog]

Thanks for the input, I'll check out the vacu-flush. Thought about a lectra san too but I'm not sure if it can handle the volume...

[Roy M]

Look at the Lavac/Blake if you are going to have charter crews. Moving parts, electrical requirements, macerator pumps, etc., have real life consequences of failure. Check out the posts of real users, here on the forum, who own and praise the Lavacs. Also, if at all possible, create the holding tank in such a way as to allow gravity discharge at just above the waterline. When you get underway you can discharge in bluewater (outside the no-discharge limits), or leave the discharge valve open and have, in effect, direct discharge. No chemicals, no electrical failures, no blockages. Charter skippers will especially appreciate these factors.