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The BEST discharge is AT the waterline but with the thruhull 'just above' it to prevent 'growth'; and with a siphon break for when the boat is heeled .... all due to simple 'physics'.
Simple static fluidics will show that this arrangement will have the least restriction and resistance (ignoring wall friction of the tubing) because once the siphon loop is filled, the total resultant 'static head' (total net positive suction head) becomes the distance from the bottom of the bilge to the waterline (and no matter how 'complicated the piping' .... the line will begin to 'automatically siphon' the extraneous lines mathematically canceling all the 'head pressure' above the waterline once natural siphoning begins from a 'full' system.
Discharging to higher than the waterline (out the stern) results in greater static height to pump ... and you need an additional 1psi for every 2.3 ft. of static head.
Discharging below the waterline only resuits in more friction because the actual 'waterline' is INSIDE the discharge tube hence the part of the discharge tube below the waterline is 'omitted' from the 'total head calculation' ... and 'stuff' will grow in the constantly submerged hose.
Just simple physics ('hydrodynamics').
Simple Speak - the discharge AT the waterline (but with siphon loop) will require the LEAST amount of 'power' to operate and correspondingly will have the greatest amount of 'flow'.
Last edited by RichH; 12-02-2010 at 09:25 PM.