bilge pump discharge thru hull - SailNet Community

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Old 12-01-2010
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bilge pump discharge thru hull

I'm looking for suggestions for the best placement for the bilge pump discharge thru hull? do you prefer the water line area or higher ? with a syphon break or without? tnx.
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Out the transom is probably the least problematic (except for the longer run)
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It would help if you said what boat you have. The boat design will often have an impact on where you can put the bilge pump discharge through-hulls.
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Old 12-01-2010
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definetely above the waterline ...higher the better
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If it just the discharge from a small centrigual pump, consider teeing into a sink drain to avoid an additional through hull. Dishwasher tees are commonly used for this purpose.
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Old 12-02-2010
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My Catalina 25 came with a factory-installed Whale manual pump routed out the transom (above the waterline).

To which I added---per Don Casey's writings---a second electric pump w/high capacity, installed a couple of inches above the intake for the manual one.

After contemplating the location for the Rule pump's discharge point---a second through-hull, a Y connection to the existing through-hull---and with the help of several SailNet suggestions, I decided to discharge the backup bilge pump into the cockpit, right about ankle level, and let it empty through the transom scuppers.

The reasoning? If/when the secondary leaps into action, it gets my attention right away.
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The only problem with that is the electric doesn't lerave the bilge as dry as it could if it was mounted lower.
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Thanks for yur reply's, I have a 1975 Cape Dory 25, I was just thinking about into the outboard well area? The sink drain is Teed into the cockpit drains.
that are below the water line about a foot,
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Old 12-02-2010
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You will get more back pressure discharging into water than air. The reason is turbulence, a little bit like having a dirty bottom. Also, marine growth might be an issue? I really like the cockpit idea although I think I would like it close to the scuppers
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Old 12-02-2010
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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.
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