90hp nissan plugs in top cylinder smashing?

Hello I have searched this forum before posting hopefully someone can help. I have an older 70's 17'aqua-sport with a 90hp Nissan on it. I don't know the year of the Nissan. It will start and idle and run fine for about three trips. After that it will bog down and the top spark plug will be smashed( not fouled )the electrode will be touching the ceramic. I originally thought it was over tightened. So i put another in a little less snug . Still smashed. Then I tried putting an extra washer in between case and plug and it wouldn't fire. what causes this? I only have a couple thousand into the boat motor and trailer so i wasn't expecting perfection. If it cost a lot to fix I will continue bringing spark plugs with me on my trips until it blows.

Thanks in advance . I hope this is something I can tackle.

I think you have a serious problem in the top cylinder ----------------Loose piston / loose wristpin bearing / loose big end bearing.-------------------Start it up and put a stick / screw driver near the top cylinder and listen for the knock.-------------Pull the head off to inspect.-------My guess is ---------EXPENSIVE !!!!

Also you can remove the plug from that cylinder and turn the fly wheel until you see the piston through the plug hole. Keep turning the flywheel slowly until you start to see the piston move away from the plug hole. Now take a long screwdriver put it against the top of the piston and push lightly. If the piston moves and the flywheel does not, you have a problem.

You probably have a serious problem with #1. Racerone is right in what the possibilities are, but you could also have some debris that is slamming around -- though that usually gets blown out the exhaust. Determine your model, as year is unimportant for servicing, but model/serial is critical. Get compression figures, and use the "stethoscope" method to listen for a knock. Likely uneconomical to repair, as you may be looking at replacing the crank/rods assembly (and possibly pistons and wrist pins/bearings). If you have an NS90A, the crank kit alone is: 3B7000300M CRANKSHAFT $1514.23.