During our stay in Porto a low pressure with winds up to 40-50 knots, was moving NE some 500 M off the coast of Portugal on Wednesday, July 24. During the afternoon the day after the passage, a 3-4 m swell from the low pressure reached Douro river and as long as the tidal level was low, the harbor wasn’t affected that much. I think the swell loses energy on the 6 m river bar. However, when the tidal level increased (approx with 2 m) during the evening, more and more swell pass the bar. There were no visible waves in the harbor, the only visible sign of the swell was that Sally and the other boats start dancing with the finger pontoon it was docked at.
Since we weren’t aware that the marina should be affected by a swell in this way, we needed to change the dock lines of Sally and experimented with some different setups, and asked others how they dock to finger pontoons when there was this big swell.
Some recommend loose lines while others said tight. We tried both tight lines and loose lines in different configurations and we found one configuration that was better than the others (more comfortable onboard Sally, less jerky, and thereby less force on the mooring cleats) in a situation with limited space around the boat. Since each boat only can dock to one finger pontoon it was not possible to keep the boat from bashing into the pontoon and a lot of fenders were needed along that side.
The boat moves both sideways and back and forth and rotates as much as it can. So the boat can hit both the pontoon and the bridge.
The figure below shows the final configuration of the ropes and dampers we had available.
For the moment we only have one dock line with an Unimer damper and we will prepare one more. It seems like a good strategy to have few turns around the damper. Then it will be less stretched and less turned during the stretch.
One of the local boat owners told me that one of his Forsheda rubber dampers (also 20-24 mm) broke during the night (it broke just behind the head). His boat weight around 18 tons compared to the 20 tons for Sally. The broken damper had three turns of rope and it was not the first time his dampers broke. He also told me that some of the winter storms give much worse swell.
Lessons learned
When ocean swell is expected to reach our harbor we will
- Ask local boat owners how the marina is affected by swell and check how they have docked their boats.
- Use robust dampers (not Bungy dampers) on the shortest dock lines or flexible rope like 8 braided anchor lines if you have too few dampers
- Use as long lines as possible and tighten them up
A note about Bungy dampers
- The Bungy dampers we used was to weak to use on Sally in this situation.
To avoid breaking them completely, we added an extra rope in parallel to the Bungy damped rope to limit the stretch of the dampers.
Maybe we use to few of them (4 dampers on each rope) - In our case, the dampers started to chafe the rope where the rope enter/leave the damper. The dampers are made for ropes up to 20 mm and the dock line we use are 20 mm
- The dampers have been bent during the use, the bend is still there after removal of the rope.
Sailing Sally 
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