How far above waterline should vented loop be?

[mrybas]

Hi,
I have a bilge pump that is below the water line, but the exit thru hull is about 6'' above the waterline. Its a cat, so doesn't heel much, so the thru hull probably doesn't go under water. Is a vented loop necessary, if so, what is the recommended height? What about a one way valve, instead of a vented loop?

[UNCIVILIZED]

Depending on the hull design, & spacing, when underway you can consistently have a solid wave of displaced water easily 1' or more up the hull. And by solid wave, I mean that so long as you're on that tack, the water is continually immersing that part of the boat. And it's pretty common for the leeward hull to lose 20% of your static bridgedeck clearance when underway. Not to mention deeper hull immersion from loads onboard etc. So you decide

[denverd0n]

One-way valves on a bilge pump line are a very bad idea, generally speaking. The one exception would be where you have a very small bilge pump, down at the very bottom of the bilge, to get the last inch or two of water. And then you also need a much larger bilge pump -- without a one-way valve -- mounted above that to deal with any serious water.

Bottom line is that you want the vented loop to be high enough that there is no foreseeable condition where the vented loop would be below the water level.

[thinwater]

FWIW, a check valve is also a good way to sink a boat in freezing climates (a plug of water freezes the hose, and that's all folks). For example diaphragm sump pumps (PD pumps in general) are not useful in freezing conditions.

I have seen boats sink for this reason, one of them right next to mine.

[David M]

Check valves can seize up in the closed position....bad news.

[sailorboy1]

Originally Posted by mrybas

Hi,
I have a bilge pump that is below the water line, but the exit thru hull is about 6'' above the waterline. Its a cat, so doesn't heel much, so the thru hull probably doesn't go under water. Is a vented loop necessary, if so, what is the recommended height? What about a one way valve, instead of a vented loop?

Put the loop as high up in the hull as possible. And t me the check valve isn't as bad an idea as it is made out as long as you there is a strainer and you account for freezing. Sewage pumping stations have check valves and they almost always stick open not closed

[JOHNMARDALL]

If you use a vented loop to raise the flooding level of the bilge pump discharge hose, mount it as close as possible to the center line of the boat and tight under the deck so that no matter which way the boat is heeled, the effective high point of the discharge hose will stay above the waterline. (Same logic applies to an exhaust system goose-neck/swan-neck.)
As others have written, check valves are a problem as they can stick open or closed, but remember to mount the bilge pump float switch/electrode switch above the pick up level of the pump, so that when the pump shuts off and the water in the hose drains back to bilge, it doesn't cause the pump to turn on again so that the pump cycles endlessly pumping the same plug of water. This can also be a problem with a small sump in the bilge.

Good luck
John Mardall
Vetus Maxwell Group

[Lepke]

The problem with a check valve isn't just freezing. With a leak, bilge debris can be caught in a check valve allowing water to enter and require the pump to come on, again and again until the battery is exhausted. If you read sinking reports you'll see it's a common problem.

[sailorboy1]

If the check sticks open it's the same as not having a check valve, ........... oh my!

[denverd0n]

Originally Posted by sailorboy1

If the check sticks open it's the same as not having a check valve, ........... oh my!

Exactly. And if you were counting on a check-valve to serve as a substitute for a vented loop, then you are in very, very big trouble!

That's the whole point. The OP is thinking that with a check-valve he will not need a vented loop. Bad idea. VERY bad idea!