How Long Will Your Bilge Pump Last in Emergency?

[Steadman Uhlich]

I would like to pose a question that I think is good to consider. I hope you will answer based on the simple scenario outlined below, and post your answer in comments below, and please restrict your answer to the scenario and the conditions outlined in it. I have selected a single high volume bilge pump model (recommended by others in another thread in another CF Forum) and a single battery, to keep this scenario simple.
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The Emergency Scenario: Taking on Water: "Start the Bilge Pump!"

Assume your 35 foot boat is taking on water through a 2 inch hole, 3 feet below waterline. That hole puts 4,152 Gallons Per Hour (GPH) into the boat.

Assume your engine will not start, after several failed attempts. It is not usable in this emergency. You have no generator, no gas powered pumps, and you have no time to charge any batteries by using solar panels (which you don't have).

Remember this scenario condition: You only have ONE 12V battery available now to run your electric bilge pump.

You have only a single dedicated 12v Starting Battery (an ordinary wet cell that is a few years old, but still holds enough charge to start the engine when needed) that you have just been using to try to start the Diesel engine. You tried to start your engine about 8 times with no success. You are now concerned about discharging it further as you need it for your electric bilge pump. Your other "house" battery is not available or charged, due to previous discharge during the stormy night and then some short due to rising water in the bilge that rose over the batteries which were stored low in the boat.

You want to use a high volume bilge pump to dewater the boat ASAP, or as much as possible given these limits. The pump is a Rule Evacuator 4000GPH model pump, one you selected based on recommendations in a CF thread about high volume emergency bilge pumps. It is attached by alligator clips to the Start Battery, when needed, which is NOW. So, while your legs are wet as water sloshes over the cabin sole, you connect the alligator clips for the emergency bilge pump to the partially discharged engine start battery, and start pumping.

The pump has the following specs:
Rule Evacuator Pump 4000GPH uses 20 Amps, 1.6 HP, 12 volts.

How many Watts does that take? You found a formula and calculated:
I (Electrical) HP = 746 Watts. Therefore P(Watts)= 746 X 1.6 HP = 1193.6 Watts.

Big Question: How long do you think that pump will run (at rated speed, not a trickle) if you have only that one (start) battery available to use?.

Please post your guess below.
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Why is this scenario written as it is?

The scenario is written as it is to restrict the pump's use to a single battery that may be usable and movable to a higher position if needed during water rising over the sole.

And while some boats may have large and very heavy battery banks, I think it is best to consider what one would do in a case where you have the minimum available to you (i.e. A single battery), and, in this case a single electric pump.

What is the correct answer?
Because we don't know the actual charge of that battery and its condition, it is the big variable. So, there is no "correct answer" to this question above. Instead, I hope you will make an honest and sincere guess, based on your experience with boat batteries and the situation described in the scenario. More importantly, this post was made as a "challenge" for you to consider what you would do if you were faced with a similar scenario in real life.

[nhschneider]

The big variable here is the battery's capacity. For example, a 4D 12v battery would have about a 200 amp-hour capacity and if your bilge pump was drawing 20 amps, you'd be able to run that pump about 5 hours before the voltage dropped too low.

Also left out of the parameters on your scenario is the height of the bilge pump's discharge hose. The 4,000 gallons per hour is based on no 'head', which virtually never exists in any installation.

It's a dire situation but hopefully the resourceful skipper would have stuffed rags, cushions or anything else in the hole to stem the water's flow before attaching the bilge pump.

Fair winds and calm seas.

[Steadman Uhlich]

Originally Posted by nhschneider

The big variable here is the battery's capacity. For example, a 4D 12v battery would have about a 200 amp-hour capacity and if your bilge pump was drawing 20 amps, you'd be able to run that pump about 5 hours before the voltage dropped too low.

Also left out of the parameters on your scenario is the height of the bilge pump's discharge hose. The 4,000 gallons per hour is based on no 'head', which virtually never exists in any installation.

It's a dire situation but hopefully the resourceful skipper would have stuffed rags, cushions or anything else in the hole to stem the water's flow before attaching the bilge pump.

Fair winds and calm seas.

Thanks for adding your comment to the discussion.

The voltage drop is an important point for all to consider. Good you added that.

About the 4,000GPH numbers, I know what you mean and that is something to consider too.

[a64pilot]

I said before and I meant it, this is a silly scenario, partially discharged single battery that is a few years old, but holds enough charge to start the engine?
I have another one for you , I'm low on fuel how far can I go assuming I burn 1 gl an hour?
Neither of course is answerable

[snort]

How long will my arms keep going until they're too tired?

[Shrew]

How much head is there from the pump to the discharge? It's my understanding that the capacity of these pumps is measured with zero rise. This means that 4K GPH is not a real-world number either.

In this scenario, it's not a matter of IF, but WHEN. The ingress is still greater than the egress, as a result the levels will rise above the batteries. The race between whether the batteries will die from draw or from drowning is academic. The vessel is going to sink regardless.

The scenario is like sitting on the deck of the Titanic asking "Should I jump now, or later?"

[Steadman Uhlich]

Originally Posted by a64pilot

I said before and I meant it, this is a silly scenario, partially discharged single battery that is a few years old, but holds enough charge to start the engine?
I have another one for you , I'm low on fuel how far can I go assuming I burn 1 gl an hour?
Neither of course is answerable

You are entitled to your opinion, but I don't share it.

The use of a scenario such as this is to get one thinking of what one would do when faced with this situation, and to consider the variables, the knowns, the unknowns, and the possible actions to take.

As with any scenario, there are variations in how people respond to the challenge, and that is part of its purpose too.

For that reason I don't see this scenario or challenge as silly at all.

As I see it, it is part of good seamanship to consider what could happen and to anticipate limitations.

[Kelkara]

Originally Posted by Steady Hand

The pump has the following specs:
Rule Evacuator Pump 4000GPH uses 20 Amps, 1.6 HP, 12 volts.

I just want to point out that 20A at 12V is 240W, and nowhere near 1.6HP.

[Steadman Uhlich]

Originally Posted by snort

How long will my arms keep going until they're too tired?

I think that is a good point that most people overlook, when someone mentions "Scared Sailor with a Bucket."

I suspect that most people underestimate the difficulty of moving a lot of water up and over the side of a boat, and overestimate their own capacity or stamina to bail (using their arms) and also overestimate the effectiveness of their bilge pumps (electric) too.

[Steadman Uhlich]

Originally Posted by Shrew

How much head is there from the pump to the discharge? It's my understanding that the capacity of these pumps is measured with zero rise. This means that 4K GPH is not a real-world number either.

In this scenario, it's not a matter of IF, but WHEN. The ingress is still greater than the egress, as a result the levels will rise above the batteries. The race between whether the batteries will die from draw or from drowning is academic. The vessel is going to sink regardless.

The scenario is like sitting on the deck of the Titanic asking "Should I jump now, or later?"

I think you brought up some good points.

The reduction in pump efficiency due to a high head (need to lift water higher to pump it out of the boat) should be considered.

The race between the batteries/pump and the water level is part of the issue here too.

Thanks for adding to the discussion.

[Steadman Uhlich]

Originally Posted by Kelkara

I just want to point out that 20A at 12V is 240W, and nowhere near 1.6HP.

I copied that "specs" from the Amazon FAQ page for the pump, it was posted by someone else. It could be wrong. I do not have the pump myself.

https://www.amazon.com/Rule-Evacuato...6SGF8G14ZMNKGD

[tkeithlu]

Forget the pump and the battery. Priority one is stopping the water coming in. If you can't do that, you're going to sink for all the reasons proposed above. If you can do that, then you have several options for getting the water out of the boat.

[Steadman Uhlich]

Originally Posted by tkeithlu

Forget the pump and the battery. Priority one is stopping the water coming in. If you can't do that, you're going to sink for all the reasons proposed above. If you can do that, then you have several options for getting the water out of the boat.

Unfortunately some real cases have happened where the source of the water coming in could not be found. Most recently the 2016 ARC boat that was abandoned for that very reason shortly after leaving the Canary Islands.

Part of the method and value of a scenario like this is to place limitations on what one can do, or use, so one is forced to consider alternatives to one's normal procedures.

Thanks for adding your comment. And, please don't think I disagree with you.

[Paul Elliott]

Originally Posted by tkeithlu

Forget the pump and the battery. Priority one is stopping the water coming in. If you can't do that, you're going to sink for all the reasons proposed above. If you can do that, then you have several options for getting the water out of the boat.

It's not an either/or situation. A working bilge pump can buy you time while you search for and plug the leak. Even if the pump can't keep up with the leak, it's going to reduce the rate of flooding. Let's assume the pump is automatic, so it's going to be running regardless. Until the battery goes dead, anyway.

[Pete7]

Originally Posted by Steady Hand

Assume your 35 foot boat is taking on water through a 2 inch hole, 3 feet below waterline. That hole puts 4,152 Gallons Per Hour (GPH) into the boat.

You are making the assumption that over the hour there is a linear flow of water with a 3 foot of head through a 2" hole.

That may not be true in the case of a yacht. A hole in a dam yes, but on a boat well there are all sorts of interior objects that are going to reduce the free area even temporary from allowing the water to flow throughout the hull. Thus whilst the failed seacock might start 3 foot below the water after some minutes the water will have risen but the hull probably won't have sunk by an equal amount, so water level inside the hull rises in comparison to sea level slowing the flow.

Also 4152 GPH sounds an awful lot of water, have a look at this, its not exactly pouring in even accepting US gallons are a bit on the short side compared to other countries gallons.

https://www.youtube.com/watch?v=bRhcXBtmPQs