Two Bilge Pumps on the Same Hose

[chala]

Originally Posted by Ziggy

That's interesting. I'm surprised the manual pump can suck through the electric... What kind of electric do you have?

Most leisure craft submersible bilge pump are of a basic centrifugal type see Google.
Properly sized when not operating they offer little resistance to the flow of water feeding a manual pump. They are equipped with a strainer so there is no need for, and it will save the weight, of a nice bronze strum box. The electric pump is sized for the full load not half load, the second electrical pump is of the same size and work as an instant back up working together they pump a greater volume of water. An electrical current reading of each pump give an indication of how well they share the load and of the amount of water pumped. Non return valves are part of the pumps and are included in the rating capacity of the pump if properly maintained they are reliable and do not leak sufficiently to impair on the capacity of the pump. I use a manual pump to vacuum my water tanks for a quick fill. Whale pump do hold a vacuum other brand do not, good to check at the chandlery. The regulation for my region says that I should have a manual pump able to pump 100 l/min. The pump that I can purchase is only rated at 80 l/min by having two pump in series I create a double acting pump and easily satisfies the regulation. Other regulation demand that a manual bilge pump should be able to be operated from the cockpit and also, should discharge into the cockpit witch is good. I like to be able to see easily how much water is getting pumped out. Interestingly the regulations say nothing about having the manpower on board to operate the pumps. I doubt that a mobile bilge pump would be suitable in heavy weather. It can be estimated that a 51mm sounding transducer 1 meter below the water line popping out can let up to 300 l/min into the boat. Test conducted by a boating magazine came up with the figure of 220 l/min, as the boat think the water inside the boat slow the ingress of water. Trying to pump that amount of water with a manual pump would by hopeless. As pointed out.

Originally Posted by Ziggy

If the electric pump is functioning and water is flooding in, I'm better off occupying myself with damage control than sitting at the manual pump. Even with adrenaline flowing, I could only operate the manual pump for a limited time anyway.

When using the electric pump, the manual pump in the cockpit can be easily bypassed by the use of a ball valve easing the flow of water. Like many cruisers in this forum I do have the possibility of pumping the bilge with the Main and or the Generator engine so starting one engine would be one of the priority with the advantage of getting a higher voltage to the electrics pumps.
At 220 l/min my boat would sink at the rate of 8.5mm /min after 70min of doing nothing it will be time to get the life raft ready.

Originally Posted by bewitched

The important thing is that you are comfortable with what ever system you go with.

I agree.

[Blue Stocking]

What is the total opening area of the holes in a suction strainer.

[amarf]

Chala,

So basically what you are saying is that I can mount a whale "super sub smart 60" in series with my manuel pump, and it will work?

[chala]

Originally Posted by Blue Stocking

What is the total opening area of the holes in a suction strainer.

If designed by a competent engineer a suction strainer will allow for some blockage of the orifices and satisfies the specifications given. In a submersible electric pump the suction strainer is the inlet of the pump. Good to stick to reputable brand.

[SURV69]

I don't like the idea of 2 pumps on one line.

I plan on having 2 pumps . . . and hopefully 2 lines.

I am going to install a small(250 gal or so), for the bilge, and a big (2000+gal battery killer), a few inches higher for the flood . . . real danger.

If I do use one hose, the pumps will be switched separately.

[chala]

Originally Posted by amarf

Chala,
So basically what you are saying is that I can mount a whale "super sub smart 60" in series with my manuel pump, and it will work?

I do not know and have never tried a whale "super sub smart 60" I have read the specification for a 650. I note: Open Flow Rate (per hour 650 Gals* (40 ltrs/min) Hose Connections 19mm (3/4")
You have not specified the capacity of your manual pump. In contrast I have two Johnson L4000 in parallel totalling 566 l/min open flow rate for a maximum manual pumping demand of 150 l/min with a minimum hose connection of 38mm.

[chala]

Originally Posted by GordMay

Double a hose's diameter, and you quadruple it's capacity.
Hence, a 1" diameter hose will have a capacity equal to 4 hoses at ½" diameter.

In fact if you take in consideration the pressure drop in the 4 hoses versus the pressure drop in the bigger hose, the ratio is much greater than 4.
a 26mm tube is equivalent to 6.6 12mm tube, 3.7 15mm tube, 1.8 20mm tube.