As you already know, Vented Loops are an integral part of any plumbing system that uses a thru-hull connection below the waterline, and has the potential for siphoning water into the boat.
The most common application of a vented loop I have seen is in the black water discharge for holding tank pump-outs. We all know that you should NEVER pump your holding tank into inland lakes or waterways. That would be disgusting! Take your boat out past the 3-mile limit before pumping. Some engine exhaust systems also employ vented loops.
Other applications for vented loops, less commonly installed, are engines and generators mounted low in the boat. Many installers don’t even give it a second thought – they install the engine or generator with the factory plumbing. Engines and generators come from the factory with the raw water pump outlet going directly to the heat exchanger using a short, often pre-molded, hose. Please don’t trust this setup if your engine or generator will be mounted near or below the waterline.
Recently a service client of Boundless Outfitters experienced a boaters worst nightmare – his sailboat sank at the dock, right behind his house! Now, he is not blind, nor inattentive – he just doesn’t use the boat much. The boat didn’t hit bottom. I say it “sank” because it flooded with raw water beyond the capacity of the bilge pumps (more on that later). Fortunately, our client saw the boat riding low on her waterline and investigated. He immediately installed a portable sump pump to remove the excess water and bring the boat back to her proper waterline. Then he called us to investigate the cause.
What I found was surprising. The entire interior of the boat was coated in black goo up to about one foot above the cabin sole. It took a while to determine what the mess consisted of. I set about looking into all bilges looking for water flowing into the hull. I found nothing! I then looked at the main engine (a turbocharged Yanmar, 4JH4-TE, installed as a re-power) and found water dripping from the air intake screen near the turbocharger. At first I thought maybe the water accumulated in the air intake when the water level rose, and was still dripping. So I pulled the screen off, and saw that the water was still coming from the crankcase vent hose! Drip-drip-drip. This can’t be good!
As it turns out, raw water had found its way past the raw water pump impeller and filled the water-lift muffler. It then proceeded to fill the engine through the open exhaust valves. (By the way, this is why you don’t keep cranking your engine if it fails to start – the muffler fills with water!) This particular boat is a center-cockpit ketch, with the engine below the cockpit sole, very low in the boat. Too low, as it turns out.
After filling the engine with raw water, the overflow ran into the bilge. This should have been no problem for the bilge pumps to keep up with – it was just dripping quickly as far as I know. Now it occurred to me what the black goo was – ENGINE OIL! Engine oil, being less dense than water, floats. When the engine began to fill with raw water, the oil was the first thing to overflow. Turns out bilge pumps hate engine oil. They failed. Now it was just a matter of time before the boat began to fill with raw water, carrying that slick of black, gooey engine oil on top, coating everything that got wet with a film of used engine oil. This was a total mess. Not to mention the damage to the engine from being filled with salt water for such a long time.
Root cause: The engine is installed completely below the waterline. This engine, that was installed brand new about five years ago, used the factory hose between the raw water pump and heat exchanger. The installer did not consider the consequences of water getting past the pump with the engine stopped. The water did not siphon into the engine, it just slowly filled because it was below the waterline.
Lesson learned: Install a vented loop! The loop needs to be in the portion of the system that gets pressurized when the system is operating. It cannot be installed in the suction side of a pump, as it will do its job and let air in. The hose connecting the water pump to the heat exchanger on a main engine or generator must be extended to a vented loop well above the waterline. Optionally, a vented loop can be installed after the heat exchanger if there is a hose between the heat exchanger and the exhaust elbow. Check your engine installations to see if the raw water pump and heat exchanger are near the waterline. If they are, install a vented loop downstream of the pump. If you can’t install a vented loop where needed, close the seacock every time you moor, and place the engine key near the valve handle as a reminder.
You, or a professional technician, should review all your boat’s plumbing installations on a regular basis. Inspect for worn, kinked or cracked hoses, rusted hose clamps, leaks and so on. Make sure all hoses are the proper type for the application. Exercise all your seacocks at least twice per year. If any appear questionable, replace them at your next haul-out. Don’t forget about your fresh water plumbing. That is a subject for another article…
Happy cruising,
Tim