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Go Back   SailNet Community > On Board > Gear & Maintenance > bilge pump discharge thru hull
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Thread: bilge pump discharge thru hull Reply to Thread
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Topic Review (Newest First)
01-02-2011 11:53 PM
montenido Hi all, interesting thread. I am getting ready to replace my primary pump and float switch, and will also add a larger "de-watering" pump. I did this on my last boat, adding a discharge thru-hull high up on the port side. I also tied in a 12v horn that would sound when the big pump went on. I like the idea of having the larger pump discharge into the cockpit, but have one reservation. If you get pooped will the additional water in the cockpit flow down the discharge hose and down into the bilge? I guess you still need to loop it above where it discharges into the cockpit.

Does this sound reasonable to you guys?

Thanks, Bill
12-03-2010 12:15 PM
RichH jrd - all this is probably based on 'averages' because there will probably be that for an immersed discharge, there will be an equal average time when such discharge is above the water. Also if there is a 'siphon loop', and because the static head is from the *bottom of the bilge to the waterlilne ---- INSIDE the discharge hose* the relative 'relative static head' will remain quite 'constant'. When the discharge is above the waterline, it will take MORE energy to pump (because of the reduction of the 'siphon effect).

If the system is 'sized' correctly the velocity of the flow through the pipes/hose should be 'non-turbulent' and therefore the friction/resistance due to the flow in (excess) hose/pipe would be at a minimum (approaching whats known as essentially frictionless 'laminar flow'. That usually means (as a wild-assed guesstimate), using the next larger pipe/hose diameter and plumbing components than the discharge nozzle diameter of the pump, omitting any sharp bends or 90 degree 'elbows' whenever possible. When the 'velocity' of the discharge becomes 'high' and the flow in the hose is 'turbulent' there arises a lot of needless 'pressure drop' in the piping due to this 'friction', ... the (common centrifugal) bilge pump begins to 'slip' and automatically reduces its volumetric flow vs. increased discharge pressure. Most 'bilge' pumps are centrifugal ... and by this argument would favor a more 'constant displacement' type pump such as the old fashioned diaphragm or vane bilge pumps (ignoring their vulnerability/ poor performance when loaded with 'trash'/debris).
I use 6 ft. /second velocity as the max. limit to avoid 'turbulent flow' as a 'guesstimate or 'go-by' instead of doing laborious calcs. of each component, etc. 9 ft./sec. usually results in full turbulent flow --- high resistance and corresponding loss of centrifugal pump capacity.

"....... I kind of like the idea of the bigger second pump discharging into the cockpit".
Me too, I run my 'last chance' high capacity pump to the cockpit ... I want to SEE that the hi cap. is ON ... when you SEE it, your 'sphincter' tingles a bit 'faster' and encourages you into 'action' a little faster than waiting to be surprised to see the hatches in your sole floating by.
12-03-2010 12:18 AM
jrd22 RichH- I understand what you are saying but isn't that all based on when the boat is level, not heeled over one way or the other? You got me thinking about why Brewer designed our discharges to be a foot or so below waterline. If the discharge is at the water line when level it will be quite high when heeled to the other side, depending on the angle, which would increase the total head by the number of feet above the heeled waterline (assuming the pump to be in the center of the boat). If the discharge is about a foot below waterline when level it will be app. that same foot lower when heeled over, increasing performance. Conversely, when heeled toward the discharge it will be lower in the water which doesn't seem like it would decrease performance much if at all. I agree with you that there must be some increased resistance discharging into the water as opposed to into air, but it seems like it would be minimal at the flow rates that most bilge pumps put out and the relatively large diameter of the hose (I could be wrong about that, I'll have to ask my brother the pump expert). The biggest disadvantage I can see to being below water line is the potential for growth, like you said, to block or partially obstruct the opening which could really decrease performance. The only other thing that I see as a possible negative to having thru hulls at or just above water line is the potential for damage from hitting objects (logs here in the PNW) in the water which generally are floating at the surface.
Interesting thread. I kind of like the idea of the bigger second pump discharging into the cockpit.
12-02-2010 10:33 PM
Omatako On my boat the bilge pump pumps into the cockpit drain about a foot above the waterline. I've never had a problem with it. No chance of a syphon because the drain is open to atmosphere in the cockpit.
12-02-2010 10:00 PM
RichH 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'.

;-)
12-02-2010 04:12 PM
LakeSuperiorGeezer 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
12-02-2010 02:55 PM
raayres18 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,
12-02-2010 10:49 AM
mitiempo The only problem with that is the electric doesn't lerave the bilge as dry as it could if it was mounted lower.
12-02-2010 08:06 AM
jjablonowski 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.
12-01-2010 09:54 PM
Hudsonian 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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