Wave formation and Sea State for Powerboating.

No matter how sea-worthy your boat may be, before you leave the security of your home port you should always consider the kind of sea state you are going to meet, and how it will change over the period you are out on the water.

In Christchurch Bay we have two high tides every day, and you can leave the harbour entrance in a flat calm at 09:00 on the first high tide, but often face a very lumpy sea, especially rounding Hengistbury Head, at 18:00 on the way home. Many a novice in a small speed boat has had a hairy passage home over Christchurch Ledge because they didn´t allow for the way the sea gets whipped up by the effect of the ebbing tide against the afternoon south westerly force 4 wind that sets in most summer days across Poole Bay.

Knowing what factors effect the sea state are an essential part of planning a safe passage.

The sea state in for a Force 2

Wind and Waves

When listening to the forecast, taking a note of the predicted wind strength and direction will allow you to determine the kind of sea state you face. Powerboaters are affected by wind and tide as much, if not more so, than sailors. Wind makes waves, and waves can make for uncomfortable rides and heavy fuel usage.

Everyone knows the waves get larger and the sea state worsens as the speed of the wind increases. The pictures show the typical sea states one can expect as the wind speed picks up.

It is the surface water meeting the resistance of the air that creates waves, so it naturally follows that the greater the relative speed of the water against the air, the more energy is available to create larger waves. The most marked effect is of course when the wind is blowing in the opposite direction against the tide.

The sea state in for a Force 3

There are two additional factors that determine the state of the sea - Fetch and Tide.

Fetch

Fetch is the term used to describe the distance the wind has travelled over clear water. If the wind is coming from over an open sea, it has had the time to build waves to the maximum its strength will support. As a rule of thumb, the larger the fetch, the greater the distance between each crest.

In contrast, if a wind is blowing off the land, and it has had only a few hundred metres of sea to work on, then the actual sea state will be nothing like as developed. As already mentioned, in Christchurch and Poole Bays, a south westerly wind will quickly develop lumpy seas because it has blown up the Channel. Any wind with a northerly bias coming off the land, however, will leave the bays with very slight seas even if the wind strength increases to force 5.

The sea state in for a Force 4

Tide

The speed and direction of the tide has a bearing on the sea state because it affects wave form and pitch.

The form of a wave is its cross-section shape as presented to your boat travelling through it. If the wave is just one metre tall, but the face is steep, you will face an uncomfortable ride. If the wave form is shallow and takes on a more smoother symmetrical shape, even if the height is considerably greater, you will hardly notice your passage over them.

The pitch describes the distance between the crests of a series of following waves. If the pitch is short, the waves will be very close together, and once again this will mean an uncomfortable ride.

The effect of wind against tide is to shorten the wave pitch and steepen the face opposing the wind. This is why the sea state around most headlands change from relative calm to downright lumpy and dangerous within a matter of minutes as the direction of the tide reverses. The combination of strong rip-tides and shallow waters makes places like the Alderney Race and Portland Bill infamous.

The sea state in for a Force 8

Familiarity breeds contempt

If you have plied your waters for more than a few seasons, through that experience you will know the kind of sea conditions you will regularly meet and what your boat can comfortably deal with.

Our challenge to you is a simple one. Every time you slip the mooring, based on the expected winds and tides you of course have looked up, you should be able to predict the sea states you will meet once you hit the open sea and throughout the rest of your passage. This simple planning approach may just mean you have a much more pleasant day out on the water simply because you will avoid rounding a headland where you know the sea state will be rougher than you and your boat can easily cope with.

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