If you do want too take the Port Townsend ferry with your rig, get on the Washington State Ferry website and make a reservation for the sailing you want.
Ferry ReservationsWithout one you may have to wait several sailings to get on.
If you make a reservation, and don't cancel or revise it if needed there is a "no show fee" of $30 charged, but otherwise the fare is based on your overall length.
I have compared the cost of this ferry vs driving around and it is a winner, even without considering Seattle traffic!!
Bigdog wrote:
If you are coming south and want to miss traffic on I-5 and have the great trip also, come down I-5 and cut across to Whidby Island and take the Port Townsend ferry. There are some really nice places to see on the island and then you pick up 101 south of Port Townsend. The ferry is gonna run you about $70 if you are in the 40'-50' range. It actually can be a wash between going south on the 5 with mileage,time and Seattle drivers as against taking the ferry and having a nice relaxing ride. Besides, Coupeville and Port Townsend are two not to miss places.