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Boat handling & design factors for operating powerboats in heavy weather

Chapter written by Frank Kowalski

Above: ‘Thunder Child II’ being sea trailed during a storm in preparation for her Transatlantic voyage across the North Atlantic at the entrance to Cork Harbour, Ireland

I think it will be useful to give some background as to where most of my experience in handling boats in heavy weather has taken place and the kind of seas and conditions that prevail there. The entrance to Cork Harbour situated on the South coast of Ireland can produce some extreme sea states during the winter storm months. There are two main factors that influence the sea state at the entrance, the first being the ebbing tide, the second being shoaling waters over the Harbour Rock, this is situated at the entrance to the Harbour off Roches Point lighthouse, right in the middle between the Western and Eastern channel entrances. Although every stretch of challenging water will have its own unique features, Cork Harbour is pretty representative of any area of water that has a strong tidal influence, such as a headland or where an estuary or river runs out into the sea, or where a bar exists with shoaling waters creating surf conditions.

On a spring ebb tide, a large body of water from the inner harbour and estuaries is forced between Roches Point and Weavers Point, a distance of approximately ¾ of a mile between the two headlands. On a spring tide the speed of the ebb can reach 2 ½ kts. This ebb causes a tidal race at the entrance, which even in moderate conditions can create very rough conditions. In a storm, the entrance can become nothing short of a maelstrom. Big, steep, closely spaced breaking waves occur across the entrance. The worst period being approximately between 1hrs after the turn of the tide and 3hrs before slack water. In a full blown storm of force 10 and above which has been blowing for more than 12hrs, the entrance can become un-navigable during the middle of the ebb, as we have seen breaking waves 10m high stretching right across the entrance.

When out on sea trials in such conditions sometimes the only option is to wait for the tidal run to subside a bit. When it has, we are then able to run out through the centre of the harbour entrance over the Harbour Rock, taking the breaking seas head on slowly, then turn around in a lull between sets and run back in through the Eastern channel, where conditions are generally not so violent. However when you do commit to transversing over the Rock, you are 'committed' as turning around can be too risky until you clear the entrance and race, and move into open water.  The difference in conditions that exist in a tidal race between the ebb and flood can be like night and day, I would estimate a Force 6 during a spring ebb in the race creates waves that are harder to navigate through than a Force 8 in open water, such is the dramatic effect the tidal race has on a sea state.

Over the years in the course of undertaking our trials we have developed an intimate understanding of the sea state and conditions that can exist at the entrance in a storm and having the local knowledge, understanding of the topography and experience of the conditions that can prevail at your home area of water is very important.

Apart from the tide, the second factor that influences conditions at the entrance is the shoaling water over the Harbour Rock. The Harbour Rock is approximately ¼ of a mile wide West to East, and has a water depth on average at datum low water of 22ft, with its shallowest area, a small pinnacle being 14ft. When the wave height exceeds 6m with a long period, the waves feel the bottom and break very heavily like surf on a beach, with near vertical faces and heavily plunging crests, and I imagine it is pretty representative of any stretch of shoaling water that might be perfectly safe to navigate through in calm weather, but very dangerous in a gale or storm. This ‘surf’ wave is a different wave than caused by the ebb in the tidal race, which although steep and violent tends to be a more ‘spilling, & tumbling’ type of breaker. It is the ‘heavy’ surf breaking wave that forms in shoaling waters that is the most dangerous type of wave. It takes a quite prolonged storm that has been blowing hard for many hours with a long fetch to build up the high, long frequency swell for these waves to occur here. But when they arrive, conditions can be quite spectacular to observe, and indeed challenging to navigate through. The swell tends to break in quite a predictable area and manor, however that been said, nothing at sea is completely predictable, and suddenly out of nowhere, a breaking sea can rise and catch you out.

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