5 liters/flush??? That's more than twice what you should need! The average flush water
volume that any manual marine
toilet that's working anywhere NEAR factory spec should need is only about TWO liters...and you can even reduce that by learning
to use the dry mode to do more than just pump the last of the water
out of the bowl.
When was the last time you replaced the joker valve(s) in the toilet(s)? If you're wondering why I asking, this should explain it...and why any toilet might actually need 5 liters of flush water:
JOKER VALVE 101
Most people think that the only thing the joker valve does is acts as a check valve to stop backflow from returning to the toilet or odor
from the tank from escaping through the toilet. But that's not a joker valve's most important function...in fact, the joker valve is THE single
most important replaceable part in a manual toilet.
Here’s how the discharge half of the pump works: On the upstroke of the piston, a vacuum is created in the area beneath the piston. This causes the joker valve to close tightly, and the flapper valve beneath the pump to open, allowing some of the contents of the toilet bowl to be drawn into the bottom half of the pump. Then, on the down stroke of the piston, the flapper valve is slammed shut, and the effluent is forced out of the bottom of the pump, through the joker valve, and off down the line. But when the joker valve becomes worn and/or there's a buildup of sea water minerals on it, it can no longer seal tightly on the upstroke of the piston...less vacuum is generated when you pump it. And as it becomes more worn there's less and less vacuum, till finally the bowl contents simply move up and down a bit, but don't go anywhere. Sometimes the flapper valve needs to be replaced too, which is why toilets should also be rebuilt at least every 5-6 years as PREVENTIVE MAINTENANCE
You prob'ly won't notice the loss of efficiency at first because it's so gradual...same as we don't see that we've gotten a little older than we were yesterday when we look in the mirror each morning. But I guarantee you that if it's been two years or longer since you replaced the joker valve, you need to pump the toilet at least 50% more times to move the bowl contents to the tank or all the way out the thru-hull....IF they're getting there at all any more.
toilets have moving parts
that require maintenance
. PREVENTIVE maintenance is called that because it PREVENTS problems that have to be fixed. You get to do PREVENTIVE maintenance on YOUR terms, when it's convenient for YOU...unlike repairs
, which NEVER are needed at a convenient time.
As for what size holding tank
...there really is no right answer. Boat size provides a "rule of thumb" when it comes to trying to come up with an adequate size tank for the number people the boat can carry. But just 'cuz a boat CAN "sleep 6," how often does it have more than 4 or even just two aboard for more than a day or evening? A tank that allows for max capacity would turn most boats into honey wagons!
So what it almost always comes down to is the size of the space that's available in a location that works--where it can be vented to keep it aerobic and with simple straightforward plumbing