Propeller Entanglement
If there is a sudden slowing or stopping of the engine while motoring it is possible that you have picked up some floating line, hopefully not from your vessel. Should this occur, it is imperative to shut off the engine immediately to prevent overheating, damage to the transmission, or worse! You can test this theory by putting the gear into neutral. If the engine runs normally but then stalls when shifted into gear, more than likely the prop is entangled.
Depending on many factors, especially where this is happening, you need to gain control of the vessel by sailing or anchoring. In a busy harbor, this can be a major problem. Immediately post lookouts to warn other vessels. Call for help on VHF Channel 16 if needed.
If a crew member is capable of diving down to investigate and remove the entangled line with a sharp knife, try it. For precaution, take the keys out of the ignition so there is no possibility of the engine being started should some distraction occur. For this reason, a dive mask and snorkel should be a part of your on-board inventory. Many times you can get lucky by shifting the gear into reverse. Regardless, dive the prop if possible.
The line entanglement problem is too often a reality, especially when anglers, ski boarders, or some commercial fishermen regularly abandon lines.