Plug vs bilge pump.

[spdracr39]

Since it is now our off season and boredom has set in and seeing a related left the plug out post I have a question. I have wondered this for a while and hopefully some rocket scientist can do the calculations. My bilge pump output is almost the same size as my bilge plug. What size pump does it take to pump water out as fast as it is coming in with the plug missing. I hope to never need to find out the hard way if it will keep up but as least I will have some idea.

 

[agallant80]

Re: Plug vs bilge pump.

I have always wondered this one too. How big of a pump would I need to keep up with the water that will come in through the plug hole if it fell out.

 

[Watermann]

Re: Plug vs bilge pump.

Too many variables to say exactly.

Best bet is to fix the leaks in your boat, cover your boat when not in use. I don't see the need for all of these plug threads. I take my boat plugs out when I winterize once a year, not after every time on the water. I could see that if you don't have a bilge pump and you boat leaks water.

 

[smokeonthewater]

Re: Plug vs bilge pump.

depends on the size of the boat or more importantly the depth of the plug in the water... I can tell you that a 1000 gph rule with a 1 1/8" output doesn't even come close to keeping up in a 26' carver and that was with the engine running so higher voltage to run the pump..... water was over a foot deep in 5 minutes...... I's say 3000 might have kept up

 

[dingbat]

Re: Plug vs bilge pump.

It depends on how far below the surface of the water the plug holes is. The deeper it goes the more flow you get

At 12" below the surface of the water, assuming a 1.1" hole, your looking at roughly 19.7 GPM (1,187 gallons per hour).

The problem as noted above, your bile pump provides nowhere close to the performance the rating would lead you to believe. You'd be lucky to get 1/2 it's rated performance

 

[phiny1134]

Re: Plug vs bilge pump.

Here is a chart that shows you how fast water will be entering your boat based on depth and size of the hole Flooding.html. In theory so long as you have a bilge pump rated higher than this rate it should be able to keep up. However, according to this my 1100 GPH pump should have been keeping up with my plug being out of my 21' Four Winns. In practice however it was not even close. I bet water was entering more than twice as fast as the pump was removing it. My guess is that once you factor in the loses for pump head height, and resistance from the tubing used many pumps will actually be putting out less than half what they are rated for.

 

[jasoutside]

Re: Plug vs bilge pump.

You'd be lucky to get 1/2 it's rated performance

I've heard this before ^^^.

So here's my question....

Why are pumps rated for basically twice what they are actually pumping?

Or maybe a different question...

How are bilge pumps rated?

There's obviously a discrepancy between the rating and the legit pumping eh?:noidea:

 

[DBreskin]

Re: Plug vs bilge pump.

Another factor is the height of the pump discharge. A pump with a discharge 18" above the bilge will move more water than one with the discharge 24" above the bilge. The GPH rating for a pump is usually based on zero height change.

For example, a typical bilge pump rated at 750GPH using a 3/4" ID hose is reduced to 625GPH if the discharge is 3 feet above the pump and only 450GPH if the discharge is 6 feet above the pump. This assumes 13.6VDC (engine running) and does not take into account any bends or restrictions in the discharge line that might further reduce output.

With the engine off and the pump powered at 12VDC output is reduced to 660GPH at zero height, 480GPH at 3 feet and 345GPH at 6 feet.

 

[limitout]

Re: Plug vs bilge pump.

as smoke and a few others said, it depends ...

the weight of the boat increases the pressure of the water being forced in so that affects the flow rate.

we had a hull rupture (6" long stress crack) in the hull of a 16ft peanut shell type boat from the 60's with a v4 evinrude 90hp and a 3500 gpm pump (borrowed from my uncles boat who was with us) was just enough to break even (thank god) as we were 8-12 miles offshore at the time. the crack wasn't open but it was a clean split in the fiberglass you could put a knife blade thru.

for what you are asking it wouldn't be too hard to find out if you do this, put the boat in the water then pull the plug and time it for exactly 10 seconds then put the plug back in. now take the boat out of the water and measure to see how many gallons came in and multiply that by 6 and you have the flow rate of how many gallons per minute comes in thru the plug hole.

 

[crabby captain john]

Re: Plug vs bilge pump.

Every boat should have a minimum of 2 pumps, each to a separate battery.

 

[Splat]

Re: Plug vs bilge pump.

They rate them that way for two reasons. One is because they can, and it makes their pump look better than their competitor, and the second is there are many variables to actual performance. Voltage to the pump, head height, type of tubing (smooth or ribbed), numbers of turns, bends, fittings, and even the type of connectors used.

 

[limitout]

Re: Plug vs bilge pump.

I've heard this before ^^^.

So here's my question....

Why are pumps rated for basically twice what they are actually pumping?

Or maybe a different question...

How are bilge pumps rated?

There's obviously a discrepancy between the rating and the legit pumping eh?:noidea:

because they are rated at the output they have coming right out of the pump and NOT thru a hose or plumbing setup so that's a huge difference right there.

just purely guestimate of the water I see and pressure I can feel coming out of the pump I bet I lose a good 30% of capacity when I lift the hose up and over the transom vs laying it flat on the floor of the boat.

my uncle used to say whatever the pump says you really will get only half of that between the plumbing or low voltage from bad connections or weak battery.

they only exception I can think of to this is some of the self priming commercial high grade pumps will be made to hold pressure under load and not have this drop off.

 

[britisher]

Re: Plug vs bilge pump.

Is this before or after the engine runs out of gas and the battery runs flat because it's not being charged??
You are definitely bored!
How about how much water can you pull out with a hand baler compared to the bilge pump which stopped running due to the flat battery, because the engine ran out of gas, after the plug fell out?
That should stop you getting bored :laugh:

 

[dingbat]

Re: Plug vs bilge pump.

the weight of the boat increases the pressure of the water being forced in so that affects the flow rate.

It has nothing to do with the weight of the vessel. The flow of water at a given depth would be the same in a john boat or the Queen Mary.

The flow is determined by the Delta T between the water pressure on the outside of the hull at X depth (12" = 15.14 psi, 24" = 15.6 psi) and the atmospheric pressure (~1 bar (14.7 psi) at sea level) inside the bilge.

 

[spdracr39]

Re: Plug vs bilge pump.

It has nothing to do with the weight of the vessel. The flow of water at a given depth would be the same in a john boat or the Queen Mary.

The flow is determined by the Delta T between the water pressure on the outside of the hull at X depth (12" = 15.14 psi, 24" = 15.6 psi) and the atmospheric pressure (~1 bar (14.7 psi) at sea level) inside the bilge.

See, I knew we had a rocket scientist here somewhere

 

[bruceb58]

Re: Plug vs bilge pump.

See, I knew we had a rocket scientist here somewhere

Any scuba diver would know this too!

 

[limitout]

Re: Plug vs bilge pump.

Is this before or after the engine runs out of gas and the battery runs flat because it's not being charged??
You are definitely bored!
How about how much water can you pull out with a hand baler compared to the bilge pump which stopped running due to the flat battery, because the engine ran out of gas, after the plug fell out?
That should stop you getting bored :laugh:

you missed the point, he wants to know what size pump he should have that would cancel out forgetting to put the plug in so he has time to plug it before the boat fills with water.

its a smart question to ask IMHO and one few people ever stop to consider

 

[smokeonthewater]

Re: Plug vs bilge pump.

I've heard this before ^^^.

So here's my question....

Why are pumps rated for basically twice what they are actually pumping?

Or maybe a different question...

How are bilge pumps rated?

There's obviously a discrepancy between the rating and the legit pumping eh?:noidea:

the bilge pumps are rated a 0 lift and no hose... IE set the pump in the water and measure the flow out the outlet adding hose and lifting the outlet greatly reduce the flow

 

[izoomie]

Re: Plug vs bilge pump.

I don't think a bilge pump is intended to keep up with incoming water from a lost plug. I just handles accumulated rain or the odd splash over the bow.

 

[spdracr39]

Re: Plug vs bilge pump.

you missed the point, he wants to know what size pump he should have that would cancel out forgetting to put the plug in so he has time to plug it before the boat fills with water.

its a smart question to ask IMHO and one few people ever stop to consider

Exactly. I suppose the water pressure should hold any flat piece of plastic (i.e. credit card) over the hole long enough to get to the tow vehicle but has anyone ever tried this ?