I know I can do a bit of bitching and complaining when working on my boat; however, that is not the type of venting I am talking about right now. The venting I’m talking about is tank vents, so basically I am venting about vents!
As with most boat projects, the project I start on may not be the project I end up working on. It seems every project is only the beginning of a long road, which often goes far from where I started. On this day, I thought I would work on installing a full tank light in the new aft holding tank I had installed.
It seemed simple enough to install a float switch and run some wires to the light in the head compartment. Almost as soon as I started work on the tank, I noticed that the tank appeared to have some positive pressure inside. Not something you want in a tank, particularly a holding tank, as this is a sure way to stink up the inside of the boat. With a properly vented tank, the foul fumes would exit the boat through the vent.
When I installed the new tank, I used an existing vent and hose left over from the old leaky holding tank that was previously removed. Yes, I carelessly missed the step of verifying that the vent actually worked, and now I was finding out that it was completely clogged and not allowing the tank to vent.
Clogged tank vents are a surprisingly common problem, particularly on older boats. I have encountered this on my boat and many others I have inspected or worked on. A blocked vent can cause all sorts of problems, both with the tank itself and with equipment using the tank. Vents can get blocked by dirt, insects, corrosion, and problems with the hose connecting the tank to the vent. Hoses can become blocked by dirt, large loops filling with water, and kinks in the hose itself.
Blockages and restrictions can fully block airflow into and out of the tank, or they can be just enough to let some air in but not enough to keep up with the fluid being removed or put into the tank. This can sometimes cause mysterious problems. This is often noticed when filling a tank. You may have to pause filling to prevent the tank from burping with fluid coming back out the fill.
Problems with engines not running properly because of clogged or slow tank vents can often be hard to diagnose. Engines can lose power and not maintain RPM when they cannot pull enough fuel out of the tank because of a vacuum in the tank caused by a blocked or slow vent. Mechanics, when troubleshooting, will often overlook this, thinking the problem is a bad fuel pump or a clogged fuel filter. This can often be hard to diagnose as the engine may have to run for a while before the symptoms appear. How much fuel is in the tank can also affect this. A full tank will have less air, and even a small vacuum will have a large effect. I have even seen a fuel tank collapse after running the engine for a couple of hours when no air could get in to replace the fuel being removed by the engine.
A water tank with a bad vent can also cause problems with a water pump, causing it to cycle often and have poor flow. Low air pressure in the tank can even damage the pump by causing the check valves in the pump to be damaged by excessive suction. Waste systems can malfunction because of high air pressure when the waste is being pumped into the tank. This back pressure can also damage pumps and make them slow. These symptoms can be hard to diagnose when the vent is partly blocked, as by the time troubleshooting starts the pressure has equalized and the problem is not noticed.
As you can tell, a clogged or slow tank vent can cause many issues that are often hard to troubleshoot and properly repair. Rather than wait for an issue to arise, it is often better to check the vents on at least an annual basis. It is always good to visually inspect tank vents when your boat is hauled out for bottom work. This makes it easier to check the outside of each vent for dirt or bug nests that can block the vent.
Almost all through – hull vent fittings have a fine mesh screen covering the opening. This screen is there to prevent flames or sparks from entering the vent hose, specifically in gasoline boats. It also helps prevent dirt and bugs from getting into the vent. The small holes in most vent fittings make them a perfect home for many insects, from wasps to spiders. A screen can stop bugs from getting into the vent hose, but does not always stop them from making nests on the outside of the vent. The fine mesh of the screen can also be clogged with dirt or bits of wood fiber rubbed off of a dock.
Be careful when cleaning the mesh that you do not damage it. Should the mesh be damaged or missing, pushing a small amount of bronze wool into the opening can help prevent bugs from getting into the vent. If the vent fitting is in bad shape or corroded, it might be time to just replace it. The biggest issue with replacing a vent fitting is often accessing it from the inside of the boat. Unfortunately, many builders do not make it easy to get to vent fittings from the inside. Many builders seem to have gone out of their way to make it nearly impossible to access the backside of vents. In some cases, an access plate may need to be installed to get at the vent.
If possible, check the hose going from the vent to the tank. You want to make sure the hose is in good condition and has no kinks that could restrict airflow. The hose should loop up above the vent fitting to help prevent water from getting into the tank from the vent. After this loop, the hose should have a steady downhill run to prevent any fluid from being trapped in the hose. A loop that holds fluids can block airflow, and fuel sitting in a hose can damage the hose by softening the rubber. Any hose used on a fuel tank should be USCG rated for fuel.
Removing the hose from the top of the tank to check air flow is a good idea. I like to use an air pump for my inflatable dinghy to push some air through, while at the same time noting any back pressure. There should be no blowback from the hose after removing the pump. Have a friend check the vent on the outside as this is done. They should hear and feel the air coming freely through the vent. If it does not seem the air is flowing freely, the restriction should be found and repaired.
If you need to replace the vent fitting itself, I would suggest looking into some of the newer designs available. I like the Attwood stainless steel flush vent as it has a removable cover that will allow for removal of the screen so that it can be cleaned. Using a flush vent will help prevent damage to the vent from a hard docking. For fuel tanks, you could consider using one of the fuel surge protector-type vents that either block the fuel from coming out of the vent when the tank is full or causes the auto-stop of the fill nozzle to cut out.
Keeping your tank vents in good working condition is important to the operation of systems connected to the tank. It could also save your tank from being damaged from excess vacuum. If you are having issues with pumps or your engine not running correctly, do not overlook checking the tank vents as a possible cause. Remember to do a quick visual inspection of the vent fittings whenever you get a chance. It only takes a few minutes and could save you some aggravation.
By Capt Wayne Canning
