Manifolding Multiple Bilge Pump Discharges

[GordMay]

Two Bilge Pumps - One Through-Hull Discharge:

The plumber in me, would never recommend discharging two (or more) bilge pumps through a single through-hull outlet, there are practical issues that might mitigate in favour of manifolding the discharges (ie: reducing the number of hull penetrations).

If you must:

1. Each Bilge Pump must have a properly sized & separated discharge hose, from the bilge to the discharge location (transom).
2. Check valves are NOT permitted.
3. The two separate pump discharges are combined only at the transome thru-hull location.
4. The larger combined final discharge hose & thru-hull are large enough to accommodate the combined pump discharge flows..

For example: (1) Electric and (1) Manual Bilge Pump

(1) 1500 GPH (25GPM) Electric Pump, plus (1) 25 GPM Manual Cockpit Pump

1-1/8" diameter (free area about 4 sq. in.) hose from Electric Pump to Transom,
and
1-1/2" dia. (free area about 7" sq. in.) hose from Bilge Strainer to Manual Pump, then from Pump to transom.
then
2" diameter (free area about 12.5 sq. in.) hose from (2 - 1) hose manifold fitting to through-hull. (*See ABYC note below)

Take each Pump hose up to an anti-siphon loop, then turn down.
Connect the two hoses with an appropriate “Y” fitting (1-1/8" & 1-1/2" to 2"), so both discharges point down.
Continue the 2" diameter hose down, to the through-hull discharge.
I recommend locating the discharge above the waterline.
A Seacock is only required when the discharge is below the heeled waterline - but I recommend installation of a seacock for all through-hulls (secured in “open” position for B.P. discharges).

*ABYC Section “H-22.8.8"
"If the discharges of several pumps are manifolded to discharge through a single thru-hull fitting, the system shall be designed so that the operation of one pump will not back feed another pump, and the simultaneous operation of each pump will not diminish the pumping capacity of the system. A check valve shall not be used in the manifold discharge system.

This requires that the free area of the final combined discharge hose (at least) equal the free areas of both Pump hoses.

The area of a circle, hence the capacity of a pipe, varies as the square of the diameter. Double a hose's diameter, and you quadruple it's capacity.

[SkiprJohn]

Here is my attempt at a bilge pump/cockpit drain manifold. The two 2" discharges are on port and starboard of the transom about two feet apart. As you can see I have a black radiator hose connected to the lower end of the manifold for cockpit drains (both sides). The upper (bridge) contains one 1 1/2" drain and two 1" drains. On each side I have an additional 1 1/2" drain for cockpit seat drains (probably overkill).
I invite comments, criticism and questions.
Apologies for the photo in cramped quarters.
Kind Regards, --JohnL--

[GordMay]

Nice assembly. Are the rigid pipes PVC or ABS?
It looks as though your main discharges may be smaller in capacity, than the sum of the individual drains feeding them. The area of a circle, hence the capacity of a pipe, varies as the square of the diameter. Hence, A 2" dia. pipe has almost enough capacity to handle 2 pipes at 1-1/2" dia (2 x 1-1/2" Acutually requires 2-1/4").

[Boracay]

The effectivness of this set up is dependant on the volume of water to be drained.
Others in this forum may have better knowledge but I would start by assuming that a pooping sea would fill the cockpit /bridge deck area every X min.(the area must be drained by the time the next pooping wave arrives) so your drains would need to be able to clear this volume of water at the same time as having maximum volume from your pumps!
If this cannot be done then you boat would be in danger of foundering in a heavy sea!
Others on this forum are better able than myself to comment on the interval between pooping waves. This would also depend on your freeboard.
There is probably a standard for this somewhere.

[SkiprJohn]

Aloha Gord,
It is PVC, not ABS.
The cockpit drains into the assembly are 1 1/4" x 2, the cockpit seat drains are 1 1/2" x 2, the bilge pump drains are 1 1/2" x 1 and 1" x 2. There are two 2" thruhull discharges.
Unless my three bilge pumps are at full capacity and my cockpit is full of water at the same time I think the two 2" should handle it. If getting full bilges and full cockpit occurs at the very same time with water still coming aboard I've got some serious damage control to think about and I'll have to have a large bucket and arms like a gorilla.
Could happen I guess but I plan on putting some foam in places for extra flotation.
Thanks for the comments. They are appreciated.
Kind Regards, --JohnL--

[SkiprJohn]

Chris,
Thanks for your comments. I believe the two 2" bilge/cockpit thruhulls will handle it except for a very extreme situation where I get a continuous pooping with no break where the cockpit will not be able to drain. In that event I'll be in some seriously deep poop. I've purposely made my cockpit a bit smaller than most 42 footers for that specific reason but will measure the cubics just to make certain it can drain quickly enough.
Further comments are welcome.
Kind Regards, --JohnL--

[Boracay]

I'd reckon that your drains would handle it but if you are bored one day with nothing better to do you could try blocking your drains and filling the cockpit about one third full with water.
Pull the plugs out and time how long to empty.
We'd probably all like to know the answer!

[SkiprJohn]

This is my solution to plumbing cockpit drains to the bilge drain manifold that I posted a photo of earlier in this thread. There are four 1 1/4" drains in the corners of the cockpit leading into the piping you see here. These two pipes are 1 1/4" each which lead to the 2" thruhulls pictured. I've been working on bracing for this system this week so was under the cockpit. Very tight space!!! Thought I'd take a photo since I don't want to hang out there very often.
Comments, opinions, critiques are always welcome.
Kind Regards, JohnL

[SkiprJohn]

Hey, this posting of photos is kind of fun!!