Having had the problem recently, and getting a great suggestion too late from a plumber friend, I would suggest renting
a jetter. It's a high pressure plumber's tool that is used to clean sewers. It might be strong enough to ruin halyards & wiring
, so get them out if you can. Ideally you could run it on a hot water pressure washer.
I had a quote of $500 for a plumber to come out & do it, but renting
one was $120-$150/day using a cold water pressure washer. No one I found would rent me one to run on a hot water washer. Be sure it has a long enough hose. Many are 25' long.
Halyards will come clean with hot water--preferably using a pressure washer.