RawWater Pumps

Passage Maker - - Contents - Steve Zim­mer­man


Al­most all ma­rine en­gines rely upon an en­gine-driven pump to pro­vide the sea­wa­ter needed to main­tain proper en­gine tem­per­a­tures. Taken for granted un­til they fail, th­ese sim­ple pumps can bring your cruise to an abrupt end. Ser­vic­ing a fail­ing im­peller­driven pump is one of the skills the pru­dent cruiser should have. Once you suc­cess­fully trou­bleshoot your sea­wa­ter im­peller, you will be un­der­way again with en­gine temps where they be­long and with your boat blow­ing soap bubbles out the stern—more about that later.

Two com­pa­nies, Jab­sco (a Xylem brand) and Sher­wood (a Pen­tair brand) dom­i­nate the mar­ket, but JMP Ma­rine (a Korean com­pany with a U. S. di­vi­sion) is mak­ing gains. The fol­low­ing in­for­ma­tion ap­plies to all of th­ese brands.


The sea­wa­ter pump moves wa­ter from the in­take strainer into the en­gine’s heat ex­changer. The en­gine spins this pump by means of a belt or gear. Know­ing which type of pump you have is crit­i­cal. If it is belt driven, it likely shares this belt with the al­ter­na­tor. If the al­ter­na­tor seizes and you cut the belt, then you lose the sea­wa­ter pump and there­fore the en­gine. If it is gear driven, how­ever, it might be pos­si­ble to cut the al­ter­na­tor belt and keep cruis­ing.

The belt or gear spins a flex­i­ble im­peller in­side the pump hous­ing. The im­peller con­sists of neo­prene vanes at­tached to a cen­ter hub. Neo­prene, a syn­thetic rub­ber, re­tains its flex­i­bil­ity over a wide range of tem­per­a­tures. Neo­prene has good chem­i­cal re­sis­tance and sea­wa­ter poses no prob­lem. Chang­ing the im­peller might ex­pose the neo­prene to other chem­i­cals, how­ever, and we will dis­cuss that shortly.

It might seem ob­vi­ous that spin­ning vanes will in­duce a flow of wa­ter, but that alone does not suf­fice. In­side most pump hous­ings a cam—a thick metal or plas­tic in­sert—rests be­tween the in­let and out­let ports. The flex­i­ble vanes ride over this cam, which bends them away from the in­let open­ing where sea­wa­ter en­ters. When the vanes pass over this open­ing, they spring back, cre­at­ing a low-pres­sure area that draws the wa­ter in. The spa­ces be­tween the vanes fill with wa­ter. When the vanes come to the out­let port, the cam com­presses them, forc­ing the wa­ter into the out­let port.

So far we have iden­ti­fied the pump hous­ing, im­peller, and cam. A cover plate and gas­ket or O-ring pro­vide the seal needed to con­tain the wa­ter and main­tain pres­sure. The cover plate is more than sim­ply a cover: The edges of the im­peller vanes ride against the in­side of the cap, form­ing a seal. The same hap­pens on the other side of the im­peller where it rides against a wear plate. Fi­nally, we have a seal or two. Gear-driven and belt-driven pumps have a seal that fits around the pump shaft to keep wa­ter from leak­ing out. In ad­di­tion, gear-driven pumps have an­other seal that keeps oil from leak­ing out around the shaft.


Pru­dent pre­ven­ta­tive main­te­nance calls for re­plac­ing the im­peller an­nu­ally. Sure, it can last longer, but why take the risk? As it de­pends not just on hours of use but also a range of other con­di­tions (how long it sat un­used, abra­sives in the sea­wa­ter, tem­per­a­tures, and other fac­tors), im­peller longevity can­not be eas­ily pre­dicted. Im­pellers can also suf­fer from ex­tended pe­ri­ods of not be­ing used. How long is too long? That de­pends on many fac­tors, in­clud­ing tem­per­a­tures and im­peller size. While sit­ting un­used for sev­eral months (through the win­ter, for ex­am­ple) should not be a prob­lem, if the im­peller has not moved in a year or more it should be re­placed.

Many boaters go years with­out re­plac­ing the im­peller, but the pru­dent path would be an­nual re­place­ment or, at a min­i­mum, an­nual in­spec­tion. Pre­ven­ta­tive main­te­nance aside, you’ll want to know how to re­place an im­peller in case of an over­heat­ing en­gine. If your en­gine tem­per­a­ture starts to in­crease and the strainer is clean, the im­peller would be the next place to look.

Whether per­form­ing a pre­ven­ta­tive re­place­ment or trou­bleshoot­ing an over­heat­ing en­gine, the first step is to close the raw wa­ter in­take sea­cock and place a re­minder tag at the key switch: You don’t want to go through this whole process only to start the en­gine with the sea­cock closed and toast the new im­peller.

Next, you will have to re­move the cover plate to ac­cess the gas­ket. This seem­ingly sim­ple task can of­ten be far too chal­leng­ing. In many cases ac­cess can be shock­ingly re­stricted, leav­ing you to cuss the en­gine man­u­fac­turer or boat­builder in dis­be­lief.

And once you gain ac­cess, not only can the small screws be dif­fi­cult to re­move but they are also far too easy to drop in the bilge. Speed­seal (speed­ of­fers an af­ter­mar­ket cover plate with larger knurled fas­ten­ers to fa­cil­i­tate easy re­moval.

With the cover plate re­moved, the next step is ex­tract­ing the im­peller. The fit can be snug and the im­peller dif­fi­cult to re­move. Pry­ing out the im­peller with a screw­driver should be avoided as it will dam­age the pump hous­ing. Im­peller re­moval tools are avail­able for pur­chase, and they work much the way a corkscrew works to open a wine bot­tle. If you do not have the cor­rect tool on board, you should first spray a lu­bri­cant into the hous­ing and onto the im­peller. Then bump the starter once or twice to force the im­peller to ro­tate—the lu­bri­cant will now be dis­persed be­tween the im­peller tips and the metal hous­ing, mak­ing re­moval eas­ier. Grab the vanes with a pair of pli­ers and pull straight out. On higher horse­power en­gines—roughly 300

horse­power and higher—many of the im­pellers have a threaded in­sert that al­lows you to use a bolt or spe­cial tool to ex­tract the im­peller from the hous­ing.

Once the im­peller has been re­moved, you must de­cide on the next step. Now that you have gone this far do you re­ally want to put the old im­peller back in, even

if it looks fine? If it looks okay, the old one might make a good spare. If limited ac­cess or skills make re­place­ment dif­fi­cult, re­place it. If you are head­ing out on a long or chal­leng­ing pas­sage, re­place it.

If you want to try to re­use the im­peller, a close in­spec­tion should be the next step. The im­peller vanes will be bent over

a bit but should re­turn to some­thing close to straight within a day—if not, you need to re­place the im­peller. The ends of the blades should be round and not worn flat. Flex the blades and look closely where they meet at the cen­ter—no cracks should be vis­i­ble. And, of course, no pieces of the im­peller should be miss­ing. If the im­peller passes all th­ese in­spec­tions, the im­peller re­mains ser­vice­able.

If the im­peller is bro­ken or worn, you will need to in­spect the rest of the sys­tem

for as­so­ci­ated is­sues. If pieces of the im­peller have bro­ken off, you will need to find them down­stream as they will re­strict the flow of the sea­wa­ter cool­ing loop. You may find them lodged in the dis­charge port at the pump, at the in­let side of the heat ex­changer, or at the fit­ting where the raw wa­ter in­jects into the ex­haust el­bow.

If the ends of the im­peller are shiny or cracked and some vanes are miss­ing, it likely ran dry at some point. If the blades

are in­tact but have worn tips or im­prints from the cam, you might have a block­age on the dis­charge side cre­at­ing too much back pres­sure. Look at the in­let side of the heat ex­changer for an ac­cu­mu­la­tion of an­ode pieces or im­peller bits.

Fi­nally, if the im­peller vanes have pieces miss­ing from the mid­dle of each tip, you might have cav­i­ta­tion in­side the pump from too much vac­uum on the suc­tion side. Closely in­spect the cam and pump hous­ing for pit­ting and check the pip­ing be­tween the sea­wa­ter strainer and the pump for re­stric­tions. If this prob­lem has hap­pened more than once you might have to in­crease the di­am­e­ter of the in­let pipe fittings and hose.

Once th­ese in­spec­tions have been com­pleted, it’s time to in­sert the new im­peller (or the ex­ist­ing im­peller if you have de­ter­mined it is still ser­vice­able). The re­place­ment im­peller should match the part num­ber for your pump and en­gine. Sea­wa­ter pumps should use im­pellers made of

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