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Old 01-12-2015, 12:06   #1
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Placing through hulls at the rail

I'm replacing a bunch of through hulls, seacocks, vented loops, and hose. It occurred to me that if the outlet (e.g. bilge pump) through-hulls were up at the rail, then there would be less hose needed and less need for vented loops. The seacocks would then be more accessible, being high in lockers instead of down in the bilge under cabinetry. Has anybody done this? Is there a problem with it? Thanks,
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Old 01-12-2015, 12:26   #2
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Re: Placing through hulls at the rail

I can think of a couple, one pumping uphill you will lose some flow, the aesthetics of course, and I would think stains would show from the thru hull all the way to the water line.
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Old 01-12-2015, 12:28   #3
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Re: Placing through hulls at the rail

I think it is application / device dependent. You do not want some thru-hulls flooded when the boat heels, etc.

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Old 02-12-2015, 04:47   #4
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Re: Placing through hulls at the rail

We have some up high but our boat is steel and they are flush with the hull. If they were plastic, they would get damaged against wharf pilings etc.

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Old 02-12-2015, 06:00   #5
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Re: Placing through hulls at the rail

I put one through hull up near the rub rail on the stern, for the bilge pimp. No need for a vented loop this way. And if the boat floods from the bilge pump outlet there are already bigger problems to worry about.


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Old 02-12-2015, 06:07   #6
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Re: Placing through hulls at the rail

Another consideration with having the outlet extra high is how much water will backflow into the bilge from the line once the pump has cut-out. Sometimes this is enough water to set the float switch off again and the repeating cycle will kill a battery within a day.
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Old 02-12-2015, 06:12   #7
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Re: Placing through hulls at the rail

I suppose I should add our primary bilge pump is a diaphragm pump, no back flow, built in check valves, mounted almost as high as the discharge. The centrifugal pump sits much higher and is only activated when the diaphragm pump is overwhelmed, or when I put a garden hose in the bilge and turn off the primary pump.


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Old 02-12-2015, 06:23   #8
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Re: Placing through hulls at the rail

The reason for vented loops is to prevent back siphoning. If there is any chance that the through-hull could be underwater (sailing at an extreme angle of heel, for instance) then you NEED a vented loop!
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Old 02-12-2015, 09:25   #9
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Re: Placing through hulls at the rail

Very bad idea, placing the bilge pump outlet so high that there is no room for an anti-siphon loop above it.
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Old 02-12-2015, 09:30   #10
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Re: Placing through hulls at the rail

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Originally Posted by Terra Nova View Post
Very bad idea, placing the bilge pump outlet so high that there is no room for an anti-siphon loop above it.

What is the point of an anti siphon loop if the discharge is always above the heeled waterline?

My install may be unique, if water spends enough time up there to flood the boat, we've already sunk.


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Old 02-12-2015, 09:38   #11
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Re: Placing through hulls at the rail

The discharge will not always be above the heeled waterline.
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Old 02-12-2015, 10:01   #12
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Re: Placing through hulls at the rail

We actually did this with our ws43. Worked just fine. Never had a backflow problem, and we were heeled over about 48 degrees a lot of time in the southern ocean. Both sides had separate bilge pumps. Our old schooner had a sea chest where all intakes and discharges were routed. No through hulls, nada(but then again we did not have even an engine). So it worked for us.

BTW, we routed cockpit drains out the sides at cockpit level, kind of like our lobster boats do it up here.
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Old 02-12-2015, 10:03   #13
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Re: Placing through hulls at the rail

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The discharge will not always be above the heeled waterline.

The discharge on the transom at the rub rail is always above the heeled waterline


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Old 02-12-2015, 10:07   #14
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Re: Placing through hulls at the rail

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Originally Posted by IolantheSF View Post
I'm replacing a bunch of through hulls, seacocks, vented loops, and hose. It occurred to me that if the outlet (e.g. bilge pump) through-hulls were up at the rail, then there would be less hose needed and less need for vented loops. The seacocks would then be more accessible, being high in lockers instead of down in the bilge under cabinetry. Has anybody done this? Is there a problem with it? Thanks,
FYI Catalina 42 when it was introduced in 1989 came out with a similar bildge pump , no discharge loop. After two of the first hulls sunk since the running pump had primed the discharge hose and following seas just filled the boat with water and sunk it, the coast guard issued a recall and Mr Buttler himself send me a one way valve in the mail. Bottom line don't sink your boat by trying to improve on original engineering.
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Old 02-12-2015, 10:08   #15
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Re: Placing through hulls at the rail

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The discharge on the transom at the rub rail is always above the heeled waterline


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Not when you bury the rail. For heavy cruisers, not uncommon to bury up to the doghouse. 50 knot winds will do that.
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