How to Prevent Vapor Lock

Marlin DIY: Maintaining seawater pumps on your boat

reader tips pump

reader tips pump

Seawater pumpMarlin

I’ve had several seawater pumps — feeding either the livewell, tuna tubes or ice makers — become air-locked after a spell of aggressive boat handling. Even though these pumps are self-priming and are mounted below the waterline, a heavy sea state or aggressive backing can sometimes lock these pumps up if they get too much air.

I got tired of having to break the discharge hose loose from the pump to “burp” the air out of the pump head — which blows saltwater everywhere — so I could reprime the pump. To solve the burping problem, I added a custom venting system to each pump’s discharge.

Here’s How to Prevent Vapor Lock:

First install a T fitting on every pump’s discharge outlet using a close nipple fitting . (A close nipple is a section of pipe with threads on both ends.) Now attach a hose barb on the horizontal part of the T to attach the hose that will feed the item needing sea water . On the vertical part of the T install a ball valve, using another close nipple fitting . Attach another hose barb fitting on top of the ball valve; this is where your venting hose will attach. Keep the ball valve in the closed position during normal operation of the pump.

You have two options of where to run the vent hose: You can "T" the venting hose into a nearby overboard through-hull discharge above the waterline, or route the hose in a U shape — up and then down — into a wet bilge area. With either option the goal is to be able to vent the air out of the pump by opening the ball valve. A turn of the valve allows the air to escape the pump head and then start pumping sea water, with little to no head pressure. By routing the vent hose overboard or into a bilge area, you won't get salt water all over the pump or engine room once the air is pushed out, and you can let as much water flow out as you want to ensure the pump is reprimed.

In the photo you can see I had several pumps mounted side by side, so I routed all of the discharges into a single venting hose, which feeds into an overboard through-hull fitting. I also drew a black arrow on the ball valve so anyone on board would know which way to turn the valve handle to open it.
Capt. Randy Baker
Destin, Florida_