The glycol closed circuit is referred to as the freshwater loop. This loop is circulated the same way as the coolant in your car. A pump half shell is mounted to the engine block which makes up the other half. The two halves sandwich a metal impeller which creates the driving force, works on the principle of centrifugal force, and needs no maintenance except for the upkeep of glycol/coolant chemistry (change it every 5 years). The temperature in this system is maintained by a thermostat which throttles the amount of water allowed to pass through the heat exchanger (radiator in your car).
The open loop coolant system has water that flows in from the surrounding body of water, cools engine heat exchanger (glycol loop), exhaust riser, exhaust muffler, and then back out to the body of water. This is equivalent to air passing over the radiator in your car. The water may be fresh or salt depending on the body of water but this can create confusion since the glycol loop is sometimes referred to as the fresh water circuit. And the glycol engines are called freshwater cooled. So for clarity the sea water loop can be called the RAW water circuit. It is the water which you see exiting the boat at the exhaust. It is also the water for which you have to open the through hull. Since it is RAW - air, sand small particulates, silt, etc. can get into the water and if the pump was not designed to pass this trash as well as self prime then the entire system would not be very reliable. As such a rubber vane impeller pump is used to provide the movement of water. These pumps are NOT centrifugal pumps but rather work on a positive displacement principle. Instead of drawing water in the middle of the casing and slinging the water outwards to build pressure, they simply create expanding volumes on the inlet side and contracting areas on the outlet. The rubber vanes allow for trash to be pushed through without damaging the impeller or casing. Since the expanding area on the inlet is a sealed area it creates a vacuum and will allow the pump to self prime (remove air). Unfortunately the impeller cover creates this seal by pressing on the impeller itself. The rubber vanes in contact with the case and the impeller face in contact with the cover create a high friction interface which will heat up quickly if not cooled or lubricated (something to check by hand when feeling the exhaust riser). In the off season the rubber can dry out and stick to the metal surfaces resulting in torn impeller blades and no raw water flow during the first start of the season. Placing the impeller in olive oil or dish soap during off season maintenance prevents this. As the impeller blades press on the metal casing they will wear and (assuming that no other fate shortens the life of the impeller) this will ultimately cause the impeller to fail to create enough suction to self prime. Eventually, even if it was primed, it would fail to keep the water in the contracting volume and subsequently not pump water. In short, there is rarely a rubber vane impeller pump used in the freshwater/glycol/closed cooling loop and as such it will not need maintenance. Just replace it when/if the shaft seal fails. Josh Muckley S/V Sea Hawk 1989 C&C 37+ Solomons, MD On Tue, Jun 9, 2020, 09:07 David Knecht via CnC-List <[email protected]> wrote: > Hi John- My manual makes no maintenance period recommendation for the > fresh water impeller. Not mentioned in maintenance at all. Does yours? I > am unclear on why the raw water needs periodic maintenance and the fresh > water does not. Is it a similar rubber impeller? Dave > > S/V Aries > 1990 C&C 34+ > New London, CT > > > On Jun 9, 2020, at 8:47 AM, John Conklin via CnC-List < > [email protected]> wrote: > > Just rebuilt my freshwater pump on my 3HM which was done aftermarket as > it covers the engine ID plate. > Was leaking just a bit but the bit caused some rust and ceased the engine > kill switch on that side of engine > Needed impeller change anyway ;) > > > John Conklin > S/V Halcyon > S/V Heartbeat > www.flirtingwithfire.com > > > On Jun 8, 2020, at 12:02 PM, Doug Mountjoy via CnC-List < > [email protected]> wrote: > > > David, > You have a circulating pump usually driven by a belt off of the crank > pulley, also this belt drives the alternator. This pump usually doesn't > require any maintenance. There is a weep hole below the shaft to signal > when a seal has failed. There is a thermostat that controls the temperature > of the engine, on the output side of the fresh water side of the cooling > system before the coolant enters the heat exchanger. Once again not much > maintenance required unless the engine is either too hot or not getting to > temp, around 180 deg f. > > Hope this helps. > Doug > > On Mon, Jun 8, 2020 at 8:47 AM David Knecht via CnC-List < > [email protected]> wrote: > >> I thought I might have had a problem with the fresh water side of the >> cooling system of my Universal M4-30 engine because it was heating water >> very slowly. I now think that it is just that the sea water is cold this >> time of year and it was taking longer than I expected. All was fine with >> lots of hot water after an extended motoring session at speed as opposed to >> idling at the mooring. However, it made me think about this side of the >> cooling system. Presumably, minimal fresh water circulates through the >> heat exchanger until the thermostat opens. My question is, what drives >> that water circulation. I know to check the impeller on the raw water pump >> side periodically, but I have seen no mention of doing the same for a water >> pump on the fresh water side. That pump appears to be pretty inaccessible >> on my engine anyway and nothing in the routine maintenance section talks >> about periodic replacement of an impeller there. So I am guessing it does >> not have an impeller that needs replacement, but I can’t find anything >> relevant in the manual. Thanks- Dave >> >> David Knecht >> S/V Aries >> 1990 C&C 34+ >> New London, CT >> >> <pastedGraphic.tiff> >> >> _______________________________________________ >> >> Thanks everyone for supporting this list with your contributions. Each >> and every one is greatly appreciated. If you want to support the list - >> use PayPal to send contribution -- https://www.paypal.me/stumurray >> >> > > -- > Douglas Mountjoy > 253-208-1412 > Port Orchard YC, WA > Rebecca Leah > C&C LandFall 39 > _______________________________________________ > > Thanks everyone for supporting this list with your contributions. Each > and every one is greatly appreciated. If you want to support the list - > use PayPal to send contribution -- https://www.paypal.me/stumurray > > <pastedGraphic.tiff>_______________________________________________ > > Thanks everyone for supporting this list with your contributions. Each > and every one is greatly appreciated. If you want to support the list - > use PayPal to send contribution -- https://www.paypal.me/stumurray > > > _______________________________________________ > > Thanks everyone for supporting this list with your contributions. Each > and every one is greatly appreciated. If you want to support the list - > use PayPal to send contribution -- https://www.paypal.me/stumurray > >
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