Sam Fortes­cue finds out why your bilge pump is prob­a­bly not up to the job

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Sam Fortes­cue looks at how bilge pumps ac­tu­ally per­form in the real world

With­out much re­flec­tion, I re­cently re­placed my bro­ken bilge pump with a slightly larger model. Af­ter all, I thought, surely an 800 gal­lon-per-hour (gph) pump will out­per­form the pre­vi­ous 500gph unit? Well, yes, but that’s no rea­son to feel much safer, as I soon dis­cov­ered.

The rea­son is that no pump is de­signed to de­liver its rated out­put for long—at least not in real-world sce­nar­ios. And even a mod­est breach in the hull is likely to over­power the pumps sup­plied with most boats—just look at ingress rates on the op­po­site page in the Fig. 1. Small won­der that the Coast Guard at­tended over 1,100 dis­tress calls last year to ves­sels tak­ing on wa­ter.

Part of the prob­lem is that boat­builders sim­ply aren’t re­quired to sup­ply a sys­tem that can deal with dam­age con­trol. Be­tween the strict re­quire­ments of the Eu­ro­pean Union’s Recre­ational Craft Direc­tive and the Amer­i­can Boat and Yacht Coun­cil’s (ABYC) guidelines, boats only need to be equipped with a sys­tem to re­move “nor­mal ac­cu­mu­la­tions” of wa­ter in the bilge—mean­ing oc­ca­sional spray, rain­wa­ter or slow leaks.

As ABYC tech­ni­cal stan­dards spe­cial­ist David Broad­bent says, “Our con­struc­tion stan­dards are to cre­ate the safest pos­si­ble boat, and we cur­rently do not go into emer­gency pro­ce­dures. That be­ing said, other stan­dards such as H-27 for sea­cocks and through-hulls have spe­cific re­quire­ments such as a 500lb test to help pre­vent is­sues like a sheered sea­cock.” In other words, to keep wa­ter out of the boat.

If, how­ever, a through-hull fails or a sea­cock shears off, you may sud­denly be deal­ing with tons of wa­ter en­ter­ing the boat in a mat­ter of min­utes. Fig. 1 shows what to ex­pect ac­cord­ing to the size of the hole and how far be­low the

wa­ter­line it is. (In short, you’re bet­ter off with a smaller hole far­ther down than a larger hole closer to the sur­face.)

These fig­ures make stark read­ing. Mod­ern yachts just aren’t equipped to deal with these flow rates by their man­u­fac­tur­ers. One pro­duc­tion 33ft cruiser I know of, for ex­am­ple, has a pump rated at roughly 520 gph. This would only just keep up with a small hole at the wa­ter­line, in the least se­ri­ous of our sce­nar­ios. Sur­pris­ingly, I’ve also come across 50ft off­shore cruis­ers with al­most iden­ti­cal pumps.

And dire as they are, these fig­ures only tell half the story. That’s be­cause bilge pumps rarely de­liver their rated out­put. Specif­i­cally, a pump rated for 2,000 gph will only do so when hooked up to an elec­tri­cal sys­tem run­ning at 13.6 volts and op­er­at­ing on the level with no bends in the pip­ing: what Claire McCrea of Whale Pumps de­scribes as a “best-case sce­nario for the pump, usu­ally based on lab con­di­tions.” “When in­stalled in a ves­sel,” McCrea says, “there are many vari­ables at play, which some­times can af­fect the per­for­mance of the pump.”

In­deed, in re­al­ity most in­stal­la­tions will be us­ing bat­tery volt­age of around 12.5V, in­volve a pip­ing run full of bends, el­bows and joints, and de­liver the bilge wa­ter to an out­let well above the pump it­self. All this chips away at the pump’s per­for­mance.


Most sail­boats op­er­ate on a nom­i­nal 12-volt sys­tem, us­ing lead-acid bat­ter­ies. The chem­istry of these bat­ter­ies means that they run at 13.6 to 14.4-volts only when hooked up to shore­power or when charg­ing on the en­gine’s al­ter­na­tor. Oth­er­wise, the volt­age in the sys­tem drops ac­cord­ing to the bat­tery’s state of charge—12.7V would in­di­cate a fully-charged bat­tery in mint con­di­tion, drop­ping to 12.4V at 75 per­cent charge, 12.2V at 50 per­cent and so on.

Lower volt­age means lower per­for­mance, so in an emer­gency, even with your bat­tery fully charged, your pump is al­ready about 6 per­cent off its stated out­put. Old, tired bat­ter­ies could fur­ther re­duce out­put by as much as 10 per­cent—all be­cause of the lower volt­age.

In ad­di­tion if your ca­bles are too small for the length of the wire run, the pump can suf­fer yet more volt­age drop caus­ing the loss of yet more pre­cious out­put.

Man­u­fac­tur­ers are aware of this is­sue, but only Xylem Flow Con­trol ap­pears to be tack­ling it, re-rat­ing the pumps in its Rule line based on likely out­put at real-life volt­ages. “Hope­fully, our com­pe­ti­tion will fol­low suit,” says prod­uct man­ager Michael Irv­ing.


Next, you need to con­sider that bilge wa­ter col­lects be­low the wa­ter­line, but you will usu­ally want to dis­charge it above that same level. The height above the pump that the wa­ter must be raised is known as the “head,” and ev­ery ex­tra inch sees grav­ity in­crease the pressure at the base. This fights the op­pos­ing pressure ex­erted by the pump, re­duc­ing its out­put.

Let’s take out the Rule 2000 pump as an ex­am­ple. Ac­cord­ing to Rule’s tech­ni­cal doc­u­ments, the pump will put out 1,620gph against a 1m (3ft 4in) head or 1,300gph against 2m (6ft 8in). In other words, you lose around 20 per­cent of your pump’s out­put when it has to lift the bilge wa­ter more than 3ft and 30 per­cent at 6ft 8in. (If

you have a smaller unit, chances are it’s pro­duc­ing less than half its stated out­put at 6ft of head, while the big­ger units are more pow­er­ful.)

Then there’s the fi­nal piece in this puzzle, the pipe run. Be­cause ev­ery bend or kink adds fric­tion to the sys­tem and re­duces out­put, you want as sim­ple a run as pos­si­ble be­tween the pump and the out­let, us­ing smooth hose, not the cor­ru­gated stuff of­ten sold as bilge pump hose. Some ex­perts also ad­vise against us­ing non-re­turn valves. In­stead, put a vented loop in the line that will al­ways be above the wa­ter­line, even when the boat is heel­ing hard. The vent pre­vents wa­ter si­phon­ing back into the bilge, elim­i­nat­ing the need for a valve.

An­other pipe-run is­sue arises if you have

RE­SOURCES Attwood Marine Prod­ucts attwood­ma­ Bos­worth the­ John­son Pump/SPX Flow Plas­timo plas­ Rule/Jab­sco/Xylem xylem­flow­con­ Seaflo Shur­flo/Pen­tair pen­ Whale Marine

mul­ti­ple pumps shar­ing the same out­let. Ac­cord­ing to Rule’s Irv­ing: “If you plumb two pumps into the same dis­charge line, you run the chance that while one pump is run­ning, it may be emp­ty­ing back into the bilge through the sec­ond pump.”

Irv­ing adds that two pumps op­er­at­ing on the same out­let hose will also see their out­put fall as they fight for hose di­am­e­ter. “In 99.9 per­cent of in­stal­la­tions, we rec­om­mend in­stalling a sep­a­rate through-hull dis­charge for each pump.”


Faced with this re­al­ity, your first step should be to de­ter­mine what your ex­ist­ing pump’s real-world out­put is by mea­sur­ing the vol­ume it re­moves over a set time of say, one minute. Mul­ti­ply this num­ber up by 60 to get the rate per hour. Bear in mind that a com­bi­na­tion of lower bat­tery volt­age, dis­charge height and bends in the pip­ing could eas­ily re­sult in an out­put that is less than half the pump’s rat­ing.

Next take a look at the “wa­ter ingress” ta­ble to see what kind of sce­nario you want to pro­tect your­self against. Let’s say you have 1.5in di­am­e­ter sea­cocks sit­u­ated 3ft be­low the wa-

ter­line; you might want to be able to deal with 4,600gph—the rate at which wa­ter would flood in through a failed through-hull. Now al­low for the 6ft rise to the pump out­let, some in­evitable volt­age drop and fric­tion in the sys­tem, and you can see that you need around 8,000gph of pump ca­pac­ity on board.

Although Rule man­u­fac­tures an 8,000gph emer­gency pump, the Evac­u­a­tor, you might be bet­ter served by two 4,000gph pumps—a size pro­duced by all the ma­jor man­u­fac­tur­ers. Ask your­self where you would need them if the hull were breached—for ex­am­ple, at the low­est points of the bilge or on ei­ther side of any bulk­heads that don’t al­low free flow.

Re­mem­ber that big­ger pumps also need larger-di­am­e­ter pip­ing and skin fit­tings—up to 2in for large units. Don’t be tempted to skimp here, says McCrea, oth­er­wise “bilge wa­ter is be­ing pushed out through a re­stricted di­am­e­ter bore at a faster rate. That in­creases back pressure and can ac­tu­ally have a neg­a­tive im­pact on the pump’s per­for­mance.”

Sim­i­larly, if you can sim­plify your pump’s pipe run to re­duce fric­tion in the sys­tem, great, although, much pip­ing on to­day’s boats is bonded in and hard to reach. Also check that there are no su­per­flu­ous Y-valves, con­nec­tors or re­duc­ers in the sys­tem and that the skin fit­ting is no smaller than the pump’s out­let.

Fi­nally some sur­vey­ors rec­om­mend a du­alpump­ing sys­tem. Vet­eran marine sur­veyor David Pas­coe, for ex­am­ple, writes: “Con­trary to com­mon be­lief, the pumps them­selves rarely fail; it’s the elec­tri­cal sys­tem from which they op­er­ate that is usu­ally the cause of the fail­ure. Be­cause of this, one way to im­prove re­li­a­bil­ity is with re­dun­dancy, or in­creas­ing the num­ber of pumps to de­crease the odds of com­plete loss of pump­ing abil­ity.”

With this in mind, Pas­coe ad­vises boa­town­ers in­stall two pumps at each lo­ca­tion: one unit higher than the other. In this con­fig­u­ra­tion, the lower one is a smaller-ca­pac­ity pump on an au­to­matic switch that keeps day-to-day bilge wa­ter un­der con­trol, with the other only switch­ing on if the first is not keep­ing up. Take care with any au­to­matic pump, though, as an­other ma­jor cause of fail­ure is the float switch. Specif­i­cally, keep your bilge clean of de­bris and make sure that pipework or wires are kept clear, al­low­ing the switch to op­er­ate prop­erly.

Also note that a new breed of field sens­ing switch is also now ban­ish­ing un­re­li­able float switches to the his­tory books, although even here you should be care­ful, as there are re­ports of poor wa­ter­proof­ing around the elec­tri­cal con­nec­tions.

If all this seems like a lot of bother over an unloved part of the boat, just re­mem­ber that pro­por­tion­ately more sail­boats run into prob­lems at sea than power boats—and all be­cause just one pump re­ally isn’t enough. s

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