new Jabbsco Impellor trashed after 48 hrs solid running

[tinkrman69]

Originally Posted by EveningTide

Just to let you know, these pumps should self-prime but don't always work as they should. Mine (a Johnson pump) is very subject to air locking. When I change an impeller I must fill the input side hose to the pump and open the top of the strainer and let it overflow to be sure no air is in the system. Then everything is fine. As others have said, running the pump for long without water probably will quickly destroy the impeller. There is lube that comes with a new impeller but who knows how long it will work if water is slow in reaching the pump. I have never destroyed an impeller and have had no problems with water flow since I worked out the above system.

Those pumps "are' self priming. even the johnson made pumps. if you have no air leakes in the suction side, when you change the impeller the air in the suction side of the punp to the strainer will get pumped out and the water will be drawn in, hence priming the punp. if you have to prime the pump manually, then the clearances inside the pump are out of spec. usually the cover plate, also known in past times as the wear plate. it doesn't take much of a groove in that plate to allow air that is already in the pump cavity to slip by when the veins are folded down by the cam, hence "not" priming. also causing the pump to be less efficient, when it is primed. Water molecules are larger than air, so it takes more clearance to pass by. you'll also go through impellers faster. and in some engines that lack of efficiency will cause the engine to run hotter at higher RPMs.

[SteelCruiser]

Thanks for all of your replies / suggestions.
I'm heading down to the yacht next week.

Motor is Lister STW3
Fresh water cooled
Sea water only to cool exhast

The impellor is off the pump that drives this exhaust cooling, and thus any 'broken bits' could only end up in the hose to the Vetus LP45 water lock, the waterlock itself and then the exhaust hose.
My thinking is that this is thus not a problem - it's if the water galleries of the engine block are 'downstream' that there are issue?

I've distilled the received wisdom to :

I will check both the pump chamber and the face of the cover plate for scoring / pitting.
I will lightly sand both with fine wet sandpaper.

I have purchased 2 x new Jabsco impellors. I will install one and have a spare.

I have purchased a Vetus LP45 water lock and will remove and replace the one on board.

Inlet partially blocked by barnacles is definitely possible. She was left afloat without bottom paint by the previous (*) for about 12 months and had HUGE growth when I took her out. I see one suggestion is to remover the hose and check the water flow. Another possiblility - dive over with a screwdriver & mask and poke around the inlet hole?

No 'white smoke' seen from exhaust.

I will check for air leak somewhere between the impeller and the strainer:
eg: the paper gasket on the seawater pump cover, hose clamps (especially above the waterline), and the seals on the seawater strainer (especially the cover to service the strainer basket).
Or , less commonly leaks inside the sea water pump, cracked pipe fittings, bad seal in the seawater pump shaft, and an intake hose that is internally collapsing.

I won't stop when I find the first problem as there well may be multiple items slightly askew adding to the issue.

[tinkrman69]

the fresh water in a freshwater cooled engine needs to be cooled. either you have a keel cooler for it or you have a heat exchanger in the saltwater system. So unless you have a keel cooler type system, you'd be wise to follow the passages of the salt water from the pump to the mixing elbow. impeller peaces that have made it past that point won't be clogging anything.

as far as the exhaust hoses? if one is bad, replace that whole section. cutting out a section and coupling in a newer piece is not advisable, as the hose barb is smaller than the hose diameter and causes back pressure in the exhaust. buying a full length of hose verses buying a hose + hose barb isn't going to save you much money, not to mention the extra costs due to the restriction in the hose...ie...more fuel consumption, carbon build up in the cylinders and more... in an emergency? by all means it will work till you get to a point to fix it proper. if it's about saving money, it won't save you a dime, in fact it will cost you more. Being cheap is a very expensive way to live. Frugal and cheap aren't even close in definition.

No smoke, but lots of smell? is the smell down below or in the cockpit. inside; leaky exhaust...ie... bad hose, loose hose clamp,leaking exhaust manifold gasket... outside; leaky injector, bad fuel and/or slightly worn rings consuming a small amount of oil and even leaky valves. putting clean fuel in does not say clean fuel system. sh&7 in the tank will mix into the "clean" fuel. can't reach the exhaust port? get in the dinghy or put a clean rag on a stick and hold it down to the exhaust for a while. when you retrieve it, feel it, if it feels really oily, then you know you have fuel and/or oil getting by. you won't always "see" the oiliness till it gets really bad...ie..oil sheen in the water or smoke. if you'r running down wind you'll smell fumes. an inexperienced person will think "bad" experience will tell if its too much or normal. oiliness in the exhaust will say too much. is the transom collecting soot? there again too much. the water in the exhaust isn't for smell. it's to cool the non-metal parts and to help silence it.

an exhaust system is designed to have a certain amount of back pressure. some engines more than others. like a yanmar verses a westerbeke of the same horsepower. the yanmar has a larger diameter exhaust system, hence less back pressure. if you go less back pressure than an engine is designed for, you'll burn the exhaust valves, but increased horsepower, for a time. and too much back pressure will cause carbon build up and higher fuel consumption along with decreased horsepower and overheating. both are high cost and unreliable.