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I have a 24 ft Sport Cat (Twin Vee) powered by twin Johnson outboards, a 2 battery (switched) electrical system, which has a gremlin that has me stumped. In short, all is well as long as I only power the low draw electrical items, such as depth finder, GPS and VHF radio, (all or some). The problem is if I operate any of the pumps, (either bilge pump, raw or fresh water), after approximately 30 seconds I lose all electrical power to the entire boat, then after about 15 seconds the power restores itself. No individual circuit breakers pop, nor is there any observable voltage drop can be seen on either volt meter, and it does not matter which pump is used. This to me has all the characteristics of a self resetting circuit breaker/current limiter leading to the main power bus, which has gone bad and is opening well below rated current. The kicker is I can not find anything that resembles such device. I am not a boat nor electrical genius, so I thought maybe someone here has seen this type of problem before. Any inputs welcomed.

Rock

Corrosion can allow a little current to pass but block higher draws. Any place where you have a loose connection that allows arcing may also be a culprit.
I assume you cleaned up the battery terminals already ?

If you've got a low-voltage cutout circuit somewhere on your electrical system and your batteries are tired, you might see that. If your batteries aren't holding a full charge or have an internal short, the voltage drop might trigger the circuit to kill the power till it climbs above the threshold again which it would do after a short 'rest'.

As a means of diagnosing the problem, I'd start by clamping my test meter leads at the battery terminals, and then watching the voltage as you run the pumps. That will give you some indication of the battery condition, if it holds voltage through the test, or quickly starts to fade.

Then you can start testing downstream from there to find the spot where the voltage goes away.

Thanks for the inputs, the batteries seem in good repair, I've watched the voltage while cranking the engines and it holds steady at 10.5 under load for at least 1 minute, (not all at once). I've cleaned the terminals at the batteries and battery switches, and all the conectors up to the center console. Perhaps I missed one under the deck somewhere. I apprieciate the help, needless to say this is quickly becoming one of those "joys" of boat ownership, FWIW airplanes are no better!

Rock

10.5 volts seems low to me

The 10.5 volts while cranking the engine sounds about right. With electrical current being drawn from the battery, the voltage drops off. Think of it like water pressure. With the faucet closed you get 12 PSI supply pressure (like voltage) but when you open the faucet the supply pressure drops down to a lower value, depending on how much flow (like electrical current) there is. The more flow, the lower the supply pressure. Say you measure 12 volts while not drawing current from the battery, the voltage will drop a small amount for a small current. The voltage will drop more for a greater current.
I too would suspect mechanical connections in the supply wiring, check them for corrosion.

I had problems with my 21' factory boat.

Everything worked,but the VHF wouldn't transmit properly.

Turned out that the cheap manufacturer wired the boat with a small crappy fuse panel that the fuses would fall out of ( that I replaced with a good Blue Seas panel),and small feed wiring that was adequate for the factory lights,pump etc but not more accessories that I had installed like the stereo,VHF,GPS,etc.

I ran much larger feed wires, and a better panel,and the problems were solved

Are both pumps connected to the same battery or is there one pump per battery? Either way temporarily wire from the pump switch to the pump. That will eliminate any problems with wiring that you can't see or connections below deck that you may have missed. Check to see if the pumps have been replaced. If so maybe they need higher current to operate and just need to have a larger gauge of wire run to them. Let us know how you make out.

That really sounds like a thermal breaker tripping. A little corrosion in them, and they get hot, once they cycle once the problem is gone until they are idle for some time again. Another culprit can be connections on fuse links. If the cycle time is a regular interval, you can almost bet on a thermal failing.

From valuable experience gained through many years with vintage British vehicles.... it's going to be easier to just replace the suspect circuit.