Rule 3700 bilge pump air lock

Add a float sw

Can you post a picture of the pump and float? That type of pump shouldn't get an airlock. It is designed for its impeller to be immersed when running. There is no suction side therefore no "airlock" can occur unless there is not enough water. More likely the switch is failing in the on position.

How big is th sump box? It s a pretty big pump to be used in a sump unless the sump is big enough.

When you see the pump running and running how much water is in the sump.

As others mentioned check th float switch. If this is a shower sump, switchez often get sticky with hair and soap scum

Finally, make sure there is no check valve in the line. Many builders or yards use check valves limit back flow and cycling instead of routing the hose properly. Check valves may prevent bill pumps from starting

How about this. While sitting at the dock, the water in the sump box remains level, switch is only on with incoming grey water. Nothing to pump unless the float is tripped from incoming grey. Head out, the same amount of water remaining sloshes indicting more water than is actually there, the float switch turns on the pump, the pump tries to send the water up a 4' head, water leaves box, switch turns off, no water makes it out, water goes back down to the box. Switch trips, do the circle again. The switch too low and sensitive when away from the dock..

The pump will run and run if the switch is too low and there is not enough water to make it over the hump and reduce the quantity in the box. The pump does not know if it is getting water, it just follows orders from the switch.

Or some slight variation thereof...

Well I assume it is a rule float switch you are using and if they haven't changed it I believe it is a simple mercury switch in there so I don't see how that can hang up as it is all gravity. Is the switch housing hanging up? binding against the side of the box? or as Pascal mentioned, hair and scum. You have not indicated the position of the switch and water in the box while the pump is running dry.

While the pump is running dry, is the switch up and water holding it up? Or just up, semi-stuck and not enough water for the pump.

I have seen the Rule 3700 GPH pumps airlock easily and a lot. I've had several boats with 3700's do it. Your float switch is too low and underway when the sump box is tilted the switch doesn't shutoff prior to the pump sucking air. You can cut a block out of starboard and screw it down then screw the float switch to that, a piece of 3/4" starboard would probably solve the problem. An anti-syphon valve just after the pump will probably help as well. As what gets pumped out, doesn't return to the sump and slosh around.

Or, you could change the float switch to an ultra switch jr. The cylinder type float switch (which is less sensative to rocking and rolling) and hose clamp it directly to the pump and adjust height as needed.

Last ditch resort, I drilled a small hole on the discharge nipple of the pump, very close to the pump so the hose doesn't cover it. (make sure to do it angled downward and to the side a little otherwise it will spray all over in there.) This allowed the pump to not airlock.

3700 may be a bit large for a small sump.

Also, if you're using a smart or electronic switch, with a delay, you will have this problem.
Never put any fancy electronic switch in a small sump.

If you have ever tore a pump base apart, you will find a small nick in the upper ring to let air out and help prevent air lock. Sometimes it works. Sometimes it gets clogged by soap or junk. Sometimes it just will not prime and pump.
After years of experimenting with lots of shower sump problems in different boats, There is one easy fix; Put a 1/2 to 3/4 inch starboard block under the float switch. The pump base will always have water in it now.

Rcrapps. The sump might be fairly large. I think Carver did one large sump for the whole boat on some of their boats and it was fairly large. I'm also thinking they chose a 3700GPH pump (which is way overkill considering the boat's freshwater pump probably pumps a maximum of 400gph), because the sump has a lot of head to the discharge.

You shouldn't have to cut a wire to shut down the pump... Or even disconnect the wire. You should have a circuit breaker or a fuse somewhere to protect the float switch and pump.

In a normal instal, each pump has two CB. One for the manual feed at the helm(s) and one for the float switches. Ideally the CBs for the float switches should be on an auxiliary panel near the battery switches, but sometimes it could be an in line fuse near th battery

An alternative to screwing blocks into the bottom of the sump, or hull , is to mount the switch on a small L bracket and use hose clamp to attach it higher on the pump body....

Hello we had a similar problem in our air con sump box. Check your one way valve isn't sticking, trace pipe work to skin fitting. Should be a one way valve inline.

Ours also air locked frequently but we also found a sticky one way valve to be an issue.

Good luck.

AND, I had a case where hail had restricted the hose. Pump never could over come the restriction to get primed real well. Just like Mikeyersir was describing above, any restriction may be giving you more problems.
Monthly doses of genuine Liquid Plumber took care of that.

How does hail get in the hose ?

I thought the same thing. I think he meant to say Hair.....

Oh hail, yes I ment hair.