Rule 3700 bilge pump air lock

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

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....

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.