Bilge Pumps Versus Holes In The Hull

[tkeithlu]

Someone awhile back questioned my concern that a popped seacock could sink a boat despite the presense of bilge pump(s), so I decided to dust off my 50 years ago fluid dynamics course notes and also scrounge around the internet on the issue. I actually found one formula on the internet that has gallons per minute and per second confused.

The starting point model I'll use is a 1" popped seacock located 3 feet below the waterline. That computes out to 34 gallons per minute. That calculation is easy and pretty robust; it's the lift by the pumps and particularly the friction loss in the hose and fittings that gets hazy.

Pumps are rated with no intake obstructions and no hose attached, so 1600 GPH = 27 GPM, 2000 GPH = 33 GPM and 4000 GPH = 67 GPM (Johnson 4000, the largest I could find on the ready market). Even at rated capacity, only the 2000 GPH and up are going to keep up with our 1" hole.

When BoatUS looked at them, they could not find one that did better than 75% of its rating at 12.0 volts. Let's assume 13.4 volts and give them full rated output. But, these little centrifugal pumps only put out about 6 PSI, so you don't have much to work with either for pure lift (goes to zero PSI at about 14 feet of lift) or friction loss (varies with hose diameter, smoothness, bends, and length).

At an assumed 4 foot lift, through 10 feet of smooth hose and no sharp bends, the answer is about 60% of the rated capacity for the smaller pumps. Part of that is head , which costs you about 2 PSI, and part is friction loss in 3/4" hose. Standard friction loss tables for lined fire hose say that at 30 GPM the friction loss is 10.5 PSI per 10 feet, so you see where we're going - the larger the pump, the greater the friction loss percentage. Even a very big pump won't put 30 GPM through a 3/4" hose 10 feet long. 1" hose helps immensely, dropping friction loss at 30 GPM to 2.6 PSI for 10 feet.

Then put the stream through a necessarily constricting thru-hull (think of it as a nozzle), and you get less than 40% of rated output. These are not fire pumps. A different assumed model used by BoatUS and actually tested discharged 26% of its rated capacity.

Let's call it an optimistic 50% and move on. That gets you 13, 16, and 33 GPM. Only the Johnson might keep up with our 1" hole, and it better have 1" hose, but the others may slow things down enough to give us time to get to the seacock. The 1600 optimistically slows it to 20 GPM into the hull.

That leads to a second but I think the central question for cruisers. How many gallons can enter your hull before the tops of your battery bank are reached? Once that happens we don't have electric pumps. Given a popped 1" seacock, how many seconds do you have to realize the problem and stop it? I know that for mine, the answer is "not very many" if the hole in in the forward cabin with the battery bank, but I only have two seacocks, and they're in the engine room. One stays closed (sewage) and the other can be closed by pulling a wire in the main salon above it. I do need to remember to keep the engine room door dogged shut, and to exercise my gasoline OMG pump frequently.

I'll have to agree with BoatUS on this issue - They say the priority must be stopping the leak, because the pumps we have are not likely to keep up, and pump only a fraction of their rated capacities.

Thoughts?

[vjm]

Nigel Calder has a whole table on the issue, with the gallons per minute based on different hole diameters. It's incredible how quickly water comes in, and what a low percentage of their rating pumps are actually capable of pumping (he addresses that too, IIRC). I basically think you install and carry the largest pumps possible. And I mean that in a "that pump is so big it is hilarious!" way. Much like anchors.

[Dockhead]

I believe you speak the truth.

I don't know whether it was you or not, but some years ago I asserted that my two Rule 4000's plus two Whale Supersub 1100's could keep up with any given blown-out through hull on my boat, and someone on here proved that I was wrong. Most of my through hulls are 1 1/2" and a couple are 2".

As a direct result of that argument, I acquired a massive 230v trash pump and roll-up 3" fire hose. It weighs 33kg (!) and will pump 42 000 liters per hour.

[Viking Sailor]

I believe that standard battery boxes are designed to have an air pocket to keep the terminals dry when submerged and level. Thus, allowing the batteries to continue providing power while being underwater.

[TurninTurtle]

Actually.... the batteries will keep providing power while submerged. You can prove this with a common LED flashlight. (use a cheap one) Drill it so when immersed the flashlight will fill with water. It will still work, but be a little dimmer.

Salt water will conduct more of the power than fresh and thus the batteries will deplete faster.

The dangerous part here is when salt water gets into a flooded lead acid battery the acid reacts to the salt and releases chlorine gas which will kill you.

[Steve Bean]

Another factor is that the rate of flow into the boat will increase as the boat gets lower in the water.

[Yeti]

You really should do the calculations using the correct diameter of hose from the bilge pumps. The 4000 gph rated pumps have 2.0" outputs and the next pump down is 3700 gph with 1.5" hose outputs. Those numbers will help tremendously with the friction loss over .75" hose.

Heck I might just spend some time looking this up and post it later.

[tkeithlu]

I'm glad to hear that. If you want to figure the friction loss for larger hoses, here is the link to Elkhart Brass figures for fire hose. Note the chart is PSI loss per 100 feet, so divide by ten for a ten foot run.

https://www.elkhartbrass.com/files/a...ion%20Loss.pdf

[barnakiel]

Yes.

It is all about finding and plugging the leak.

But you can only find the leak before it is under water (under the water that is, this one time, INSIDE the hull).

Hence your best bet is to have super powerful pumps and many of them.

Working on some "quality" yachts build by some renown boatyards I find many boatyard fit installations laughable. Even worse, some renown cruising designs have bilge sumps that are completely useless when it comes to fitting extra pumps.

In our own boat, the equivalent of our displacement can be pumped out in about 15 minutes. This takes two biggest rule pumps. So given a minor hole I hope for having maybe 10 minutes to locate the hole and start plugging it.

Sure, on some boats, such take is not viable BUT the point is that on most boats not even the slightest effort has been made (except boats where skippers are as anal and/or naive as I am).

On the whole, I think plenty of all this fuss is luck or lack of thereof and the rest is in our preparedness. And our preparedness is a personal trait and not an evenly distributed one. And so I respect all those who elected to sail without pumps.

Well, my 2 c.
Cheers,
b.

[sanibel sailor]

Boat size has a lot to do with it as well. Little boats need even bigger pumps as they fill quicker.

[Yeti]

So looking at the friction loss charts linked to, I think I can safely say friction loss in 1.5" and 2.0" hose isn't really going to be a problem if you keep your hose runs shortish. I did a quick search for output psi of the Rule 3700 and couldn't really come up with anything. I would imagine if it was truly important the info would be provided by the manufacturer for consideration.

The real deal to be concerned about is the elevation loss. The manufacturer does provide this info to you and you should take note of this.

[Cheechako]

Yep, the leak is going to win. However, if you maintain and inspect your seacocks, one failing is extremely rare. I don't think I've ever heard of one. There must be some though. I've seen failed gate valves, that were not maintained etc though.

[Lepke]

If you're over worried, keep some of this stuff. You can shove a wad down the thru hull or over the hole and it stops.
Stay Afloat is available online. There's a video.
Home

[Cruisingscotts]

Drills training and more drills...

I would be a little suspect of the duty cycle on a DC bilge pump dealing with any serious breakage. As stated above pressure in is going up with each gallon coming in. Add a foot of water in the bilge and see how many "treasures" appear to foul the pump.
Damage control kit "DC kit" and training on how to deal with the bad stuff, invaluable. Thru bolted proper bronze seacocks only way to go, and a appropriatly sized DC plug tied next to each one.

The obvious practice being a dry bilge always, any little bit of water is a immidiate red flag to anyone on the boat.

Interesting to note, we have a yanmar diesel dewatering pump at work which is pretty similar to the ones the CG drop, they are horrible to prime and will tolorate no debris.

[HopCar]

I always think of bilge pumps as time machines. They are there to buy some time to fix the leak.

I put a set of these on my boat in addition to tapered wood plugs.
Seabung Breach Control
Seabung - Breach Control Technology

If you're thinking of carrying an engine driven pump, consider having it converted to propane. Propane engines can sit unused for months and still start.

On the subject of batteries, I remember seeing batteries in fish tanks at boat shows when AGM and Gel batteries first hit the market. There were lights hooked to them to show they were still working.