Engines, Shafts, Steering, Struts, Rudders, etc.
With dual engines, Marinette used a "battery jumper cable" set up to boost a low battery. The momentary switch on the dash triggers a special type of solenoid switch (that can stay ON without overheating) which cleverly joins the low and charged batteries to get the motor(s) started. Slick idea.
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington