All outboard motors and sterndrives (I/Os) are assemblies consisting of a variety of metals; aluminum, steel, iron, brass, copper, bronze, and stainless steel. When a combination of any, or all, of these different metals is submerged in saltwater, two things happen:
Grasses and mollusks attach to that portion of the lower unit that is underwater. This "growth" also occurs inside the cooling water intake, water pump, and cooling passages. It solidifies there, and the next time the engine is started, its neoprene water pump impeller is scored, gouged and torn by the sharp salt crystals and small mollusks (shellfish) that are attached to the inside of the pump housing.
Additionally, as the water evaporates, cooling passages become clogged with the coarse, jagged, salt crystals that have accumulated there. In time, the salt corrodes the aluminum, eating through the soft metal to ultimately destroy the engine.
All of this happens if saltwater is not flushed from the outboard motor or sterndrive after every use.
Secondly, when dissimilar metals are submerged in saltwater, an electrolytic process occurs. The softer of the metals (least noble) begins to be eaten away, attracted by the other (nobler) metal. This is basically the action that occurs in a battery, and is termed "electrolytic corrosion". It can destroy an outboard motor or sterndrive in a matter of weeks, days, even hours. If it is necessary that the engine be left down, submerged in saltwater, the protection afforded by the flushing/mooring bag is highly recommended by manufacturers of these units.
Flushing with fresh water after every use in saltwater (no matter how diluted) is recommended by every manufacturer of outboard motors and sterndrives. It is imperative that the salt and growth are efficiently flushed away, inside and out, to ensure trouble-free service.
ENVIRONMENTAL & SAFETY CONSIDERATIONS REGARD-
ING THE USE OF FLUSHING DEVICES FOR OUTBOARD
MOTORS AND STERNDRIVES
The value of fresh water flushing of all outboard motors and sterndrive units after every use in salt, silted or muddy water is universally recognized and recommended by manufacturers and service technicians of those marine propulsion units.
Regular flushing of the internal passages, pumps and thermostats help to prolong the life of the engine and ensure the proper operation of all internal engine components.
A properly operating outboard or sterndrive powerplant will produce less pollutants than one that is overheating or running too cold. Engines are designed to run most efficiently at a given temperature range (usually from 140ºF to 210ºF). Operating at higher or lower temperatures means that they are operating less efficiently, with the possibility of greater pollution.
There are two primary styles of flushing devices on the market today; the clamp-on device and the flushing bag.
The use of inexpensive clamp-on devices for freshwater flushing allows a considerable amount of water to be wasted by leakage at the point of injection. Up to 50% of the fresh water may not enter the engine's cooling system, rather, it is sprayed outward from the suction cup-like attachment. It is, impossible to ascertain if a sufficient amount of water is being introduced into the engine's cooling system - It may well be that flushing in this manner is not fully efficient. It is also quite possible that this type of device may slip off, stopping the flow-of water into the engine. Additionally, when flushing an engine using a clamp-on device, pollutants and noise are exhausted directly into the atmosphere through the exhaust ports continuously while the engine is running.
The use of the higher priced flushing bag submerges the entire lower unit in fresh water. This ensures proper engine back-pressure and that there is little water wasted. The flow of fresh water into the bag can be controlled so as to eliminate excessive water use.
The engine's own water pump is supplying the flow of water through the engine and cooling system. Being fully submerged, it cannot suck air or leave any passage or component unflushed, Water can he pumped from the bag into a sump, sewer, or retained for use again.
The engine's exhaust is totally directed into the bag where oil and fuel residue will adhere to the inside surfaces of the bag. It may then be wiped out and properly disposed of at the completion of the flushing operation.
Flushing devices other than the flushing bag are contributors to noise pollution as well. When normally operating, the exhaust of an outboard motor or sterndrive is submerged. Noise is muffled by the water surrounding the unit. With clamp-on devices, the exhaust is open, unmuffled and loud. But as in actual use, the flushing bag puts the exhaust fully underwater, muffling the sound. The motor is flushed efficiently, cleanly, quietly and with minimum effect on the environment.
From Burgee Magazine, Volume XXV, Number 3, December, 1997
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