If you want to seriously test whether someone gets seasick or not (or if you don't want them to come again), give them the job of clearing the blocked head while sailing. Heads, the nautical term for onboard toilets, can be one of the most vexed issues on a boat, depending on where you sail.
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Which head you will choose for your boat when you are upgrading will firstly depend on the regulations of where you sail, or where you are likely to sail in the future. Then it may depend on the facilities available where you sail. The following information ignores these various regulations, or proposed regulations and the available facilities, all of which should be checked thoroughly.
There are five types in common or not-so-common use: gravity feed, manual pump, macerating, vacuum and composting.
These work usually by having the unit positioned directly above a holding tank, which can be separate or integral to the unit. Porta Potties are a common example. Bowl contents drain directly into a removable tank which has to be removed and dumped in the appropriate receptacle ashore. Flush water (if provided) is supplied by an internal reservoir, and while most require no plumbing, some larger units can be plumbed to draw flush water from the vessel’s freshwater system to a deck pumpout.
One of the most simple of these that I ever saw was rigged off the stern of the boat and emptied directly into the sea. This was a sure way of ensuring the toilet never smelt and was never blocked.
These have a hand pump which brings 'raw' water into the bowl, where it mixes with the sewage and is then pumped clear. Installation requirements include a below-the-waterline skin fitting for the flushing water, a seacock, a holding tank and pumpout options, which will depend on the regulations in your area, as well as the various hoses, fittings, etc., needed to connect each one of them. Sometimes you are offered an optional electric pump.
These are designed to puree solid waste, much like a garbage disposal. This is typically accomplished by an electric-powered 'grinder' (aka macerator) that uses blades to simultaneously chop and pump waste clear of the bowl. Flush water can be raw or from the vessel’s onboard freshwater system. These are noisy contraptions, which may or may not faze you or your crew/guests. Some types of macerating toilets claim that the waste they put into the sea has been treated by electrolysis so that the matter is inert and not harmful to marine life.
These use an electric or manual pump to generate a vacuum in the system, which pulls sewage from the bowl to its final destination when the toilet is flushed, be it a holding tank or overboard depending on regulations and your own view of the world.
These very 'green' alternatives use peat moss to turn waste into compost. As they’re completely self-contained, they can be an attractive alternative to traditional toilets. No plumbing is needed; however, they do require you to install a fan and a vent pipe. While they are reputed not to smell, I have never seen one in operation which doesn't.
Correct installation is critical for all of these head varieties, and your choice will depend on the comparative convenience, the regulations where you sail or are likely to sail, your attitude to how 'green' you wish to be, and, finally, how far you have to travel each time you want to empty the holding tank.