Bilge pump dump below water line - SailNet Community
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post #1 of 6 Old 04-25-2008 Thread Starter
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Question Bilge pump dump below water line

Why on earth would a sailboat designer/manufacturer put the thru-hulls for both the manual and automatic bilge pumps below the water line? Seems like asking for unnecessary risk. This is exactly what we have on our 42 ft CC Beneteau, made in France. In addition, the associated seacocks are accessible only by removing panels held in place by 4-6 screws. I can imagine the scramble to get to them should a hose or clamp fail. One is accessed through a lazerette, and the other is beneath the propane locker via the aft cabin. The only defense would seem to be keeping the hoses, seacocks, and clamps in pristine condition at all times, despite the inconvenient access.

Does anyone know if a significant number of other boats have underwater bilge thru-hulls? What could possibly be the rationale for the arrangement?
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post #2 of 6 Old 04-25-2008
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Originally Posted by boydgatlin View Post
The only defense would seem to be keeping the hoses, seacocks, and clamps in pristine condition at all times, despite the inconvenient access.
That's exactly what they want the boat owners to do...

No actually, they may be trying to make hull look good all times... Like no water marks on the hull from a above waterline outlet...

They might figured that if the bilge pump doesn't allow siphoning water back in they should be ok...

As I said, if the bilge pump has a oneway outlet and a high loop, then a seacock on the throughhull, it should be fine... You said access was covered, that doesn't make any sense...

I've seen lots of through hulls, those might be bilge pump oulets, below water line...

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post #3 of 6 Old 04-25-2008
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All my boats were above water line both power and sail so Im no help...I agree with both your concerns...My bilge pump lines are thin wall and not designed to keep water pressure out...I would have to take a guess that they at least put a vented loop and a check valve in that design?....But on a nice boat such as yours I would personally change both issues if it were me...
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post #4 of 6 Old 04-27-2008
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The head discharge is below the waterline so that one does not have to smell the effluent. Bypassing the holding tank should only be done where it is legal. If the bowl of the head is still above waterline when heeling, then siphoning is not a problem.

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post #5 of 6 Old 07-04-2009
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I have recently come accross this below-water-line bigle discharge arangement for the first time while refitting a 37 Endurance for a client. I can not even begin to imagine a suitible justification. The problem here is that the pump in this arangment must be primed to function, and it uses sea water coming through the seacock in order to meet this requirement. Hence, a check valve is not possible. A one way valve inline between the seacock and the pump would render the system non-functional. Therefore the pump it's self, or rather the checkvalve within the pump meets the requirement of stopping the sea water from flowing back thru the sytem and filling the bilge. This is absolutly ridiculous and dangerous.

If the pump does not require priming this would not be such a major problem, even if still an unnecessary risk. It is the sea water priming of the pump, making a one way valve application impossible that makes this system so dangerous.

Debree entering the pump is enough to cause this system to fail. A hose clamp giving way between the seacock and the pump would sink this 120,000 euro 15 tonne vessel in a matter of minutes.

Yes there are many seacocks in such boats, but should one, or more, of these thru-hull fittings or systems fail, you automatic bilge pump is there to save you day; giving you the neccesary time to also man other pumps while stgemming the flow. But, this IS the bilge pump if it fails in this manner (while you comfotable snoozing in the bunk, or getting groceries, by the time you wake up, you're underwater.

I can't understand this engineering. Modify this systme to an over the waterline discharge, or add a second auxilery automatic bilge pump with a high float switch and an over the waterline discharge which will activate in case of failure of the main pump.
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post #6 of 6 Old 07-06-2009
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One of the features the builder of our custom-designed steel cutter has is a single standpipe with "tee-offs" for engine intake, A/C, head and "extra" (which is probably going to be for a watermaker). It's a welded 4-inch capped pipe that goes well above the waterline, and the t-fittings are all seacocked and double-clamped. It is accessible via the saloon companionway stairs. All the inlets rise fairly quickly to close to the waterline to inline strainers, and in the case of the engine, to a large Perko "jam jar and cheese grater" type strainer. All are easily accessible.

It is possible when hauled to jam a wooden plug in the standpipe inlet, and to winterize the engine, the head and the A/C via the standpipe (just pour in antifreeze) or via the sink (T fitting at the drain).

There is a Marelon seacocked drain outlet below the waterline for the head drain (starboard) and the galley drain (port). These are closed when away from the boat.

All other outlets are above the waterline, and I am considering seacocking (is that a verb?) my starboard waterline exhaust (it's not out the stern to reduce noise and the length of the run) so that I eliminate the chance of taking in water into the waterlift when on a steep starboard heel. Basically, when the engine's off, I close the exhaust hose and hang the engine key on the handle! Alternatively, I can wire a sensor to the ignition so I don't make an unfortunate error.

Bilge, head and A/C outlet are above the waterline and are looped and vented. I put in a one-way, "Joker" valve on the vent of my older sailboat's holding tank because I found I was getting some water into that tank on certain heeling situations.

Boat plumbing and hydraulics, particularly in exhaust systems, can be arcane and, to me, not always intuitive, but a general set of rules I have found applicable in all situations: 1) The fewer holes in a hull, the better. 2) Have backup means to plug holes should a seacock or a clamp fail. 3) Have good access to all through-hulls and inspect them (tightening clamps, servicing seacocks and checking hoses for wear) regularly. 4) Keep a clean bilge, because debris kills one's pumps, one of which should be a large volume manual pump should you take on water while your electrical system is compromised (such as after a lightning strike that blows a dime-sized hole in the can save the boat in the absence of electrical pumps, but only if another crew buys you the time to find the leak and plug it!).

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