Eventually, everyone will wrap a line around the prop. Most of the time, the engine itself will give you hints as to what’s going on. This might manifest itself in a variety of ways – heavy clunking, extreme vibration or a complete engine shutdown. Either way, shut the engine down immediately, stop what you’re doing and think.
A lot of sailing yachts – particularly racing boats – have folding or feathering props, and – though it’s rare – there are instances where the prop itself is simply not feathered correctly. The boat, when put in gear, will vibrate out of control with any amount of rpms. If this happens and you don’t think you’ve hit anything, try putting the boat in forward and reverse, back and forth, to try and get the prop to un-stick itself. Sometimes this works. Other times, it’s something else entirely. Just recently I experienced the situation described above. It turned out that one of the blades of the folding prop had actually fallen off, thanks to a faulty set-screw. Indeed it wasn’t feathering correctly! A new prop solved that issue.
If it is a wrapped prop, try and maneuver the boat into a safe place if possible. Issue a Securite call on the VHF so that nearby traffic understands your situation (but never call a Pan Pan or a Mayday, unless that situation truly exists). Only dive overboard if you have a mask and fins and the water is clear enough to see anything. And the wave action is calm. At sea, sailors often carry old bike or hockey helmets if they have to go overboard in a swell, when they risk hitting their heads on the bottom of the boat as it heaves in the swell. Don’t jump overboard unless you’re attached to the boat and someone is watching after you.
Quite often, if you shut down the engine immediately, un-wrapping the prop is a simple if not lengthy solution that requires a good knife and a good deal of patience, but is rather straightforward. Wear gloves, and avoid at all costs getting up against any barnacles, which can give nasty infections if they cut you. If you manage to free it, gently test the engine and running gear to ensure no damage was done. Always have a backup plan – like a local towing company – and never panic.