outboard motor 50hp.johnson?

You have a short . Check wiring and starter for this.

Are you considering and electric outboard motor, or an electric trolling motor vs the petrol outboard motor ? Electric outboard motors are made for continuous use , and can deliver up to 5 hp. Electric trolling motors are not designed for continuous use , and will cut out after prolonged use {I've experienced it in less than 1/2 hr} at full power , due to heat . Electric outboard motors are expensive , and require a lot of battery {36-48 volt} . Your average 20-30 lb thrust 12 volt trolling motor is giving out less than 1 hp . I wouldn't want to be in any kind of wind or sea with that kind of power. There isn't any reserve there to get you out of trouble either . I've tried the trolling motor route with a 20 ft lifeboat . In the end I had over 200 lbs of thrust in a 36 volt system , with 12 batteries. Still not enough . You get into a vicious circle . To get the endurance you want , you need more batteries , which creates more weight , which demands more thrust, which requires more batteries , which creates more weight, which demands more thrust .etc. No , petrol is your only reliable option.

pull off either battery terminal and with the key off, put a test light between the terminal and the battery..if the light come on you have a short or power drain. most likely its the key switch..or un plug the harness at the motor..put the fuse in and jump the starter solenoid...if it still blows, its the engine wiring, if not its there forward..

Check the wiring from the ignition back for corrosion.

follow your hot lead from the ignition switch back to the battery and you will probably find a spot where it has been rubbing against metal somewhere.

make sure that the fuse your using is what the engine calls for if it is check to see if your getting draw from anywhere else

theres a short some where