Breached hulls, swamped hulls, and bilge pumps

You see them at just about every boat show. Sometimes it's a six-metre fishing boat, sometimes it's a luxury cruiser with a six-figure price tag. But there are always a few boats with something terrifying lurking under a hidden access hatch in the stern: a "bilge pump" that would barely suffice for aquarium duty in my wife's Red Oscar tank. Sure, it'll get rid of rain water and the occasional bit of spray that seeps down there, but that's not what a bilge pump is for. Its main function is to keep you afloat if everything goes to pot, and frankly, most pumps just aren't up to the job.

Let us remember, of course, that no bilge pump ever achieves the flow rate stamped on its casing. They're typically rated when pumping horizontally (ie, no discharge hose) and often with an operating voltage that assumes your engine is running and the alternator is cranking away nicely. Add a discharge hose rise of a metre or so, a bit of gunk in the impeller and bearings, maybe a touch of corrosion on the terminals and run it on a 12 V battery with the engine off, and you'll be lucky indeed to get half the nominal rating.

With that in mind, let's look at how much water a pump might be reasonably expected to deal with. If you leave out a half-inch drain plug when you back down the ramp, it'll be letting in almost 380 gallons per hour by the time the keel is half a metre underwater. That's more than enough to overwhelm a "600 gph" pump, and an 800 gph unit can just barely keep the boat from flooding. If a large through-hull fitting were to fail, the resulting flow rates are terrifying:

Graphs of flow rate through a hole in a hull

We can easily pull some other examples off these charts. Say a 1.5" through-hull fitting, an engine intake perhaps, is a metre down on the hull bottom. If it fails, it could let in as much as five thousand gallons an hour- you'll need pumps rated at eight, maybe even ten thousand gallons an hour to keep the compartment from flooding while you find and patch the leak.

David Pascoe's article on bilge pump selection and installation may strike some folks as being rather too conservative. If anything, we've seen here that his suggested pumping capacities are minimums.

It's tempting to think that smaller boats, having fewer and smaller through-hulls, can get by with less pumping power. But when we move to smaller hulls, especially planing hulls, the risks change. A ruptured hose or through-hull is no longer the main concern: boarding seas are. Picture this: You're doing 10-20 knots in a following sea. For whatever reason, the engine cuts out. Your boat's own wake and the following sea rush towards you, meeting your transom at perhaps 10 knots or so. If that wave clears your 1.5 m wide transom by just four centimetres, it'll dump 300 litres per second into the boat. In just one second, you've gained almost a third of a tonne, all of it in the stern, and your engine just happens to be dead.

To clear that water from your bilge in a reasonable time- say, two minutes- is going to take a lot of pumping capacity. We don't have to lift the water very far here, so the pump will come closer to its stated specs. But even so, clearing 300 litres in two minutes calls for a pump that can move nine thousand litres per hour. An emergency pump rated north of 3000 gph would be called for in this scenario.

Those figures may sound a bit extreme, and for some, they are. A light boat like my runabout, Sunset Chaser, that rises to a following sea is not likely to ship quite so much water. But there are a great number of fishing and fish/ski designs out there with wide, low-cut transoms and heavy engines that could easily find themselves with similar (or worse) boarding seas. Installing a giant, kick-ass emergency pump should be a top priority for anyone with such a boat.

I won't touch on the electrical side of this issue today, other than to point out that a pump is only as good as the batteries and wiring that power it.

Key Observations

  • Larger boats need to contend mainly with hull, hose and fitting ruptures, which can let in surprisingly large amounts of water.
  • Smaller boats may be vulnerable to boarding seas, and if so, their pumps have to be sized accordingly.
  • The common 500 to 800 gph bilge pumps are OK for routine dewatering, but cannot handle emergency situations and must be backed up by an independent, much larger pump.
  • If the hull is breached, stemming the flow has to be a top priority. If you have below-the-waterline fittings, keep an assortment of tapered foam plugs handy, and a supply of Stay Afloat emergency putty certainly won't hurt. Anything that can slow the leak- cushions, bedding, balled-up rope- will suffice in an emergency, at least until you can get your bearings and think for a minute or two about how to fix it.
  • In small boats, especially outboard-powered ones, a full transom is good; wide cutouts are not. Designs that present an easy opportunity for a wave to come aboard should be avoided in favour of designs that offer good protection from a following sea.

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