Motor cranks engine acts as if locked

motor is a 1999 5.7 gsi ptwr.

motor is a 1999 5.7 gsi ptwr. my problem is this. motor starts to crank and then makes a clunk and starter disengages. it is almost as if the motor locks up. i have changed batteries and been through all connections. i have changed the starter. i have pulled out all spark plugs and turned over motor using starter. motor turns fine. i have turned engine using crank without any problems. i have verified that stern drive is not engaged. has anyone ever seen or heard of anything like this. im not a mechanic but i have never found a problem with a engine i cannot solve. this has me completely confused. any help would be greatly appreciated.
thanks
cj

Some history please... Did thi

Some history please... Did this engine ever run while you owned it... Is this an new boat for you? Are you trying to start this with the drive tilted up?

"Yes, in addition to Robert&#3

"Yes, in addition to Robert's first two questions..... does this (we'll call it a "restriction") appear to occur at the exact same "indexing" of the crankshaft each and every time?
With the spark plugs removed, you may not be able to notice any hydro-locking, if that were to be the case.
Were any of the spark plugs wet with water or E/G (ethylene glycol if a closed cooling system)?

."

Please keep us updated...
alt

Please keep us updated...
although we haven't troubleshooted this as much as the original poster has.

We just had similar symptoms yesterday on our 2000 Volvo Penta engine (one of two)

we are hoping and praying this isn't going to be a costly problem as we've had a murphy's law cloud over us lately.