Originally Posted by captnjack
The tank is grounded so 12.5 (at rest) to 13.5V (engine running) difference between the tank and the battery positive is totally normal.
You'd be better off switching to amps and testing the amperage in each circuit as you reconnect the individual devices to the battery. With each device (depth sounder, chart plotter, lights, etc) off at its switch the amperage should be zero. If you get current flow with the device off you have found your leak/short.
Thanks to all for the suggestions, i think im am starting to get the picture,
I've removed radio/lights from the circuits completely (separate loom) and bypassed the switch so its just battery and engine and I've still no power at the engine. Think ill look for a fuse engine side, and test where the power drops out?