In high water, small rowing boats could get through the natural canal, but larger boats had to sail around Moster. The Røyksund canal was made wider and deeper in the middle of the 19th century and was completed in 1859 so that larger boats could also use the canal.
The main reason for making it wider and deeper was that it would be easier and faster to get around without having to row or sail around Moster.
In the 19th century, it was rich herring fishing on the west side of Bømlo and this was probably contributing to the canal being built.
Recently, a new bridge has been made over this canal, and the pictures are taken both up from the bridge and down by the bridge fortress.
On the last picture, we can see the bridge from below at the Røyksund canal.
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From @siggjo