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Old 02-12-2015, 09:12   #16

Join Date: Jan 2012
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Re: Placing through hulls at the rail

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,
Bilge Pump Discharge Installation.

To obtain optimum bilge pump performance, the discharge hose should be as short as possible, with shortest elevated height possible, with straightest run possible, with hose equal or greater than output discharge diameter, with fewest restrictions possible.

Often the best theoretical discharge location is in the centre of the transom, above the highest anticipated water level under all conditions.

If the location you have selected cannot possible go under water under any heeling (including abnormal) condition, then you are correct, you would not need an anti-syphon valve.

If the location could be underwater (but higher than the other end of the hose) you need an anti-siphon valve to prevent water ingress way. IF the location can be underwater, lower than the other end, the boat will fill with water under all conditions.

So here is the major issue with vessel side mounting. When you are heeled, depending on tack, the discharge may be very low in relation to the inlet(good, max output) or high (bad, reduced output).

When level (95% of the time for most boats) high on the rail will reduce discharge vs a lower mounting on the transom, in almost all cases.

Just a note that folks often never consider, should an emergency arise and the boat is filling with water, immediately sound the alarm, turn on the pumps and as soon as practical, TURN ON THE ENGINE.

This will increase the voltage supplied to the bilge pump (rated at 13.2Vdc) to around 14.2 Vdc, and it will output much more that if just running off batteries (that could be down around 12.2Vdc)

The few seconds to start the engine, could give you considerably more time to find the source of the leak and stop it, before the boat is lost.

With the engine started and all pumps running, and all crew (if any) pumping or bucketing water out of the boat, if a catastrophic leak can't be located or stopped, call a Mayday, set a course for a travelift or shallow water (which ever is closest), and prepare to abandon ship, if necessary.

Be aware that as soon as the water reaches the tops of your batteries (assuming all bilge pump wiring connections are watertight and properly made above this level, which few are) all 12Vdc electrics connected to ships batteries, including bilge pumps, will not operate.

Therefore, if you don't have a handheld VHF with fresh AAA batteries in your ditch kit, it would be very a very good Christmas present to the vessel this year.

Ramblin Rod
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Old 02-12-2015, 09:45   #17
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Re: Placing through hulls at the rail

Originally Posted by reed1v View Post
Not when you bury the rail. For heavy cruisers, not uncommon to bury up to the doghouse. 50 knot winds will do that.

If I bury the rail and doghouse, even laid over 90 deg, the stern of the boat is still above the heeled waterline. Even with a vented loop, with the deckhouse underwater there is nowhere satisfactory to put a loop. I'll take my chances with my setup.

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Old 02-12-2015, 11:03   #18
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Re: Placing through hulls at the rail

My bilge pumps exit just below the rail. Had a previous boat try to sink on me once when the pump failed and let water siphon into the waterline outlet.
The only problem with m my high mounted outlets is my rusty keel bolts were causing stains. I'm now replacing the bolts so that should fix that problem.
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Old 02-12-2015, 19:55   #19
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Re: Placing through hulls at the rail

A few thoughts, questions and random maunderings:

Unless one could place the vented loops amidships, they would be below the waterline when heeled regardless of where the through hulls are located.

With the pump having to lift all the way to the vented loop (due to the vacuum break it's a free fall after that so no help), just dumping overboard at the rail would be no different from the pump's POV, so long as the vented loop were placed that high anyway.

As mentioned, placing the through-hulls on the transom would add a lot of hose, and the drain-back to the usual centrifugal bilge pump would be substantial. As Sailmonkey says, a diaphragm type bilge pump wouldn't drain back. Just curious though - why do they frown on check valves in bilge pump lines? Would the same issues affect diaphragm pumps with their double check valves? Is it because the pumps use duckbill valves instead of fragile, cloggable flaps? How about a big bombproof duckbill valve in a regular pump's line?

Anyhow, thanks, all and red skies at night.

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