Wheel and boat shuddering at times

[Feluka]

I have just motored my sailboat down the Erie Canal and about Brewerton and Lake Oneida, I encountered lots of weed in the water. I noticed after this that the wheel was shuddering and when sitting in the cockpit you could feel the occasional jerk as if the rudder was hitting something.
I assumed that there was weed around the prop so tried reversing the engine etc. but the problem didn't clear. It continued onto the end of the canal but at certain times was less noticeable. I wondered if a piece of rope was wrapped around the prop shaft and was knocking against the rudder. When I reversed the boat for a short way the shuddering wasn't present. I couldn't access my ladder as the mast was supported on the swim platform and the Canal was very dark so I wasn't able to go below to take a look until we were docked in Catskill Creek.
I only found a small amount of weed around the prop shaft, and nothing trailing that would affect the rudder.
Anybody else experience anything like this before?