Originally Posted by bhenry
I tested the sinks with a magnet.
The Scandvik- no magnatism on the bottm but increased as I went up the side.
The economy bar sink- some magnatism on the bottom and decreased as I moved up the side.
My Kitchen sink. Three years old and supposedly high quality- No magnatism found.
The magnet test is not a reliable one, some of the high performance corrosion
resistant stainless steels are VERY magnetic - such as the duplex ones commonly used for propellor shafts (my own propellor shaft is VERY magnetic).
304 (which sinks/basins are normally made from and is fine for boats) and 316 are usually magnetic where they have been worked. So for sinks/basins one commonly finds them to be magnetic at least in the corners, waste penetration, etc where they have been formed and smaller and deep sinks/basins commonly more magnetic than larger ones by nature of there being more forming in them. The same reasons apply to the likes of screws, etc as they are likely to become magnetic from the manufacturing process.
As Gord says I would avoid a trap. Further to what he says regarding venting, if you have two sinks/basins with traps draining into the same sump the sump vent needs to be quite large else when you pull the plug
in one sink the displaced air is likely to blow the other sink's trap contents back up into that sink. Also, if the vent is into the boat, or the sinks do not have traps (which is in my view least problematical) then you will get smells from the sump back into the boat whenever you drain into it.
Personally, I would do everything possible to avoid use of a sump where food
scraps are concerned (in fact I would never have one on anything other than a very large boat) and if just for bathroom grey water
have the sump as small as possible so that it is constantly flushed by the pump constantly cycling - in a bathroom unless the boat has low freeboard it is usually only the shower
pan that forces the need for a sump there (in which case the basin might as well go into it too).