It's easy to find the best option with public transport if you use the travel planner that Kolumbus have on there web page.
The stop you need to input is: Hjelmeland kai.
Remember to press the info sign on the far right of the chosen route to see the full planning of the trip. As the trip starts with the ferry to Tau. It looks like this: +
By express boat
This boat is a great option if it fits your schedule. It takes you directly from "Fiskepiren" to Hjelemland Kai, which is a 200 meter walk from Hjelmeland Hostel.
"Fiskepiren" is the boat terminal where the Tau Ferry leaves from. Just ask for help in the ticket office and they will tell you what boat it is.