2007 merc 40 hp 4 stroke

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I was out fishing today and on way back to dock. I was at cruising speed. With no real warning and not beeps it locked up and stalled. When I tried to restart it would not turn over when starter engaged. I let it sit for a few minutes and tried again. It finally turned over but felt stressed when it started running. After I tried to get up to speed it stalled again. Same thing would not start. I was towed back to marina and tried again to start. It would idle and I could drive back into ramp at slow speed but sounded stressed.
what might be wrong?

 

[galamb]

First two things I would check is oil in the crankcase and oil in the lower unit. If either have leaked out it could describe your issue.

However, it could be quite a number of things. After checking both of the above, I would start with a compression test - make sure you haven't toasted a cylinder, then a spark check to rule out ignition.

It does however, sound like a purely mechanical issue if the starter couldn't turn over the flywheel - either pistons seized, problem with the crank or gears (seized and not letting the driveshaft move).