Motor makes a loud grinding noise.

Hey everyone, I purchased a 1997 mercruiser maxum 2300. Has the 250 hp 305 inboard motor. I took it to the shop to get it checked and they rebuilt the carburetor, gave it a tune up, changed the oil and filter. I installed a new starter and replaced the belt. Me and buddy took the boat out to the ocean this past Saturday. It started fine we drove it around for about 35 minutes. Stopped and fished for about 2 hours. When I went to start it I made a lou grinding sound and it would not turn over. I used 5/8 socket and turned the crank shaft pulley maybe 1/4 inch, and then turned the key and it started up almost as If the motor was seized. Every time I shut it down or it would bog out i had to turn the crank shift pulley manually. It got really aggravating, can anyone please give me any advice or help to point me in the right direction. This is my first boat am I hoping to enjoy it before it breaks my wallet.

Thank you everyone for taking the time and patience to help me out. I appreciate any advice you all have.

Oh also forgot to mention at the end of our trip the belt started to sequel as we made our way back to shore. When I opened the motor compartment I noticed the belt had a small frey starting that was not there before. Not sure if that info helps any.

NEVER try to turn an engine with the crank bolt ,you wallet wont like it.
Evidently the 1 hour on water test was not performed

Pull the spark plug checking for water intrusion
pull the starter and inspect the bendix and the flywheel teeth
Make sure the starter bolts are the correct ones, not just ones that fit. If the starter is not lockd correctly in place it could cause a grinding noise

Sounds like you are missing a tooth or two off the ring gear.

Your motor is hydro-locking from water getting into the cylinder(s). The problem is: from where? The most common source is a leak in the exhaust manifold(s).

Check the oil to see if it looks 'normal'. If it has a vanilla milkshake look to it... Let's hope not.

Jeff

For future endeavors what exactly is the on hour on the water test, always looking to learn.

You find something you really like , pay for a launch usually non-refundable fee .Start the motors , check oil levels. how did they start and sound. Get to operating temperature, check all the gauges, check for water entering the bilge, check bilge pump operation. Check shifting BEFORE you leave the dock, if acceptable proceed for a run.
Slowly raise the throttle at what rpm does the bow start to drop and you get on plane. Most boats with adequate horsepower drop at about 3000. More than 3000 and your underpowered.
If you boat alone then just you, if you boat with 4 or 5 people bring them along for the weight.
What is the maximum rpm attained , must be within range or there are engine/drive/prop issues.
How does it handle in sharp turns,
Stop and wait 20 minutes, does the engine start normally.

If you cant spend the time or of the owner refuses keep looking. You want to find everything out before you buy and not complain about your purchase. Most boats for sale were purchased at boat shows watching a video . The show boats usually make less horsepower to keep the price low but sacrifice on performance. You may have saved 15K but then realize it`s not like the boat in the video you watched at the show

Thank you for the information above. I appreciate any information I can get. Thank you everyone for all your help.

X2 , what bt said

Thank you