big electric pump needs loop?

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cormorant

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Hi, I have fitted a Johnson 4000 GPH electric bilge pump to my 38ft boat, (yes I know it's big) it's about 5ft down in the bilge, submersible, electronic float switch and dedicated Johnson switch panel. Manual or auto pump. I am bringing the 1 1/2 inch hose up to a seacock in the stern which is about 1 ft above the waterline. I suppose under some conditions it would be under water whilst sailing. I could fit a loop and gain an extra foot or so, should I fit a vented loop or a one way valve do you think? A simple one way flap valve is supplied with the pump but it fits on the pump outlet itself and of course there is probably about 10/12ft of hose between the outlet and the seacock. What is the safest thing to do given that I want this seacock open?
 
Personally I would loop the pipe as high as possible under the deck prior to the outlet. If it is the standard simple impeller pump without valves it is surprising how many people forget that the pump can save the boat , but it can also sink it, as it can take just as much water in as it can pump out in rare circumstances. I have come across near disasters because of the outlet too close to the waterline without a loop. The anti syphon valve will not prevent flooding if the whole of the outlet system is below the waterline at any time.
John Lilley
 
An adequate loop is essential if the outlet can submerge, but is not total protection. A back syphon can be established if, for example the pump is operated in this situation. A 1.5" pipe will then fill a boat very rapidly. An adequate anti-syphon vent will guard against this, but it would be better to raise the outlet position. Non return valves etc. should not be relied upon, in fact, may even induce a syphon.
 
I was on a delivery trip with friends many years ago, their new pride and joy was a 2 1/2 ton Hillyard, voyage West Mersea to Woolverstone. Not very far but full of surprises /forums/images/graemlins/frown.gif

The relevant problem was that once we were beating up the Wallet, we realised that there was lot of water down below. She was fitted with 2 pumps, one of which was an electric pump with the outlet not far above the waterline on the starboard side. This was well submerged under sail and rapidly filling the bilges.

As you might imagine port was the making tack, so we tacked and fortunately were able to hang the owners daughter over the side so that she could ram a cork in the outlet.

We then discovered that the Whale did not gush, for every pint that went over the side, at least 1/2 returned to the bilges. /forums/images/graemlins/frown.gif

We had left Woolverstone to drive to Mersea at about 10-00 and finally got back about 22-30. A memorable voyage.
 
Cant remember where I saw it, but a clever little skin fitting device looked like a strum block, with a ping pong ball inside it. When the pump was pumping, it pushed the ping pong ball downwards allowing the gap to open. When the boat heeled and water entered the skin fitting from the other side, the water forced the ping pong ball against the pipe end, thus forming a seal. Bit like an old fashioned snorkel.
 
Thanks guys but I am now even more worried than before, there must be a way of doing this. Yes, I have seen how much can backflow as I connected it up temporarily to a seacock below the waterline to test the pump, quite frightening.How about I just connect it up as is, but only leave the seacock open when I am on the mooring? I've got shorepower so at least some protection there when I'm moored up. And what about the Henderson mk5 I have then? Same story there?
 
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