Non-return valve on bilge plumbing or not?

[rwidman]

Originally Posted by S/V Illusion

Check valves fail, get clogged or both and you will never know until you need it. While opinions vary, facts do not. There are better ways of solving backflow which don’t/can’t fail. Why choose the bad option?

This is the correct answer.

[capt jgw]

Check valves can clog up, and worse they restrict flow, important in a fooding situation.
My 36' sailboat had one pump amidships and the 1 1/2" discharge hose ran all the way to the stern, probably 20 feet. The backflow was enough to restart the pump, it would cycle the same couple gallons of water back and forth all day. I fixed it by installing a second pump, the smallest I could find and ran the 3/4" hose the shortest route possible out the side. Less than 8 feet. I also mounted the big pump's float switch three inches higher so it won't come on with nuisance water. It hasn't come on since, the little pump keeps the bilge down to about 1/4". A "dry bilge" system could also be used instead of the little pump.

[Cheechako]

I would use a high loop in the hose somewhere. A valve will restrict some flow and can get clogged with hair or etc. IF you do use a valve, be sure to have the loop anyway, otherwise you can siphon water into your boat! Dont ask how I know this...

[Cheechako]

Originally Posted by dfelsent

We were half way from Newfoundland to Nova Scotia. The bilge pump outlet was on the transom. 4AM. The sea was rough enough that water was over the floorboards due to no anti siphon loop. Float switch didn’t exist. Running the pump continuously fixed that. SCARY.

YEP! I discovered mine because the boat motion seemed weird. I lifted a floorboard and the water was just to the top of the floor boards! Sea wasn't even rough, but we had been heeled for a long tack.

[Wotname]

Originally Posted by tronic72

I don’t understand how a thorough-hole above the water-line can siphon?? I’m guessing it’s because my boat is a cabin cruiser and many other boats are sailing yachts?

Many through holes are above the static waterline but can be below the heeled waterline; it can make a big difference!

[GILow]

Originally Posted by tronic72

I don’t understand how a thorough-hole above the water-line can siphon?? I’m guessing it’s because my boat is a cabin cruiser and many other boats are sailing yachts?

So...I'm going to out myself here.

As I learned the damp way, it turns out waterlines are dynamic.

One day I sailed in a blustery 25 knot beam reach across the Gulf St Vincent, a nice little 30 mile jaunt. Exactly half way my son says "Dad, why is the floor wet?"

By that stage I'd shipped 1.5 tonnes of seawater.

What was happening was the bilge outlets at the stern, well above the waterline when the boat was at rest, were both fully submerged when the boat was doing 7 knots. Some kind of bow-stern-wave thingy... not sure what it is called. Anyway, this had nothing to do with the boat heeling under sail, the outlets are centred on the canoe stern.

Anyway, I'm in favour of the tall anti siphon loop if practical. Aside from the risk of the one way valve failing, I don't like the idea of extra points of resistance or blockage snagging in my bilge pump hoses.