Users can control the amount of flush water an manual toilet uses by using the dry mode to do more than just remove the last of the water from the bowl.
Most people don't know (because they've never bothered to read the specs in the owners manual) that any manual toilet that working anywhere close to factory spec can move bowl contents up to 6 linear feet or 4 vertical feet in the dry mode. So as long as the tank or thru-hull is within 6' of the toilet and/or it's a downhill run, the dry mode will get it there. A couple of pumps in the wet mode to rinse the hose behind it are all that's needed.
"But what if my tank and/or thru-hull is more than 6' from the toilet or it's an UPhill run...or both???"
A fairly easy plumbing
modification solves that problem: reroute the toilet discharge line to go UP and over a loop immediately from the toilet (aim the discharge fitting straight up)...the loop only has be an inch or two higher than the inlet fitting on the tank. This will allow you to pump in either mode only long enough to push the bowl contents over the top of it...gravity will get it the rest of the way.
You'll also use less water--and have a much cleaner bowl--after solids if you add water to the bowl ahead of use...a couple of beer
cupfuls from the sink is enough.
If you ask most people what the joker valve's function is, they'll say "prevent backflow"...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 why it should be replaced at least annually:
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 less and less vacuum, till finally the bowl contents simply move up and down a bit, but don't go anywhere. So if it's been more than year since you replaced your joker valve, you'll find that replacing it will cut the number of times you have to pump in either mode quite a bit.
Btw...all of this is explained in a lot more detail in my book (see link in my signature).