Piloting in a Following Sea - Part III

Displacement Boats

In a following sea the waves will be overtaking you and there is not a great deal you can do to influence where the boat is in relation to them. The bigger the waves the faster they travel, but under normal conditions the waves will probably be travelling at between 15 and 20 knots – almost twice your speed.

Small waves will have little impact: it is only when they are big enough to start lifting the stern, that you will have to drive the boat more carefully. The steering tends to become more erratic because the bow is buried in the water and the stern is in the air. The boat then tries to pivot about the bow, so that you have a sensation of over-steering.

Initially you will find that the steering has no effect but the boat could suddenly take off with a rush and may turn almost beam on before you have a chance to correct it. This is known as broaching and a serious broach can leave you dangerously exposed beam onto the waves. This means that in a following sea with moderate waves running you can find yourself working fairly hard at controlling the boat. Quick, responsive steering is a great help to keep the boat running straight.

When you find yourself working overtime on the steering, winding the wheel from side to side it is about time to look for an alternative course. Varying the speed may also help to improve control over the steering but do not slow down too much or you may lose steerage way. At what stage the steering on a big cruiser becomes difficult depends a great deal on the type of boat you have. If you have a displacement boat with a fairly fine bow, or even a semi-displacement boat operating at slower speed, you may well find that the steering becomes difficult to control in comparatively moderate seas. This is because the bow buries itself and acts as a fulcrum, which the boat tries to spin around.

A boat with a big full bow and cutaway forefoot will lift readily on the waves even though it is angled downhill and is far less likely to bury and thus as a pivot. The side of the rudder also has a bearing on the way the boat handles: a good sized rudder which has a powerful steering effect is better in this situation than a smaller, less effective one. In a following sea with a displacement boat, the solution is not to ease back the throttles, but rather to open them further because you get better steering control the faster you go.

However, the motion of the boat can become quite uncomfortable in these situations because of the sudden change in angle as the wave passes under the boat. You may find that the boat, when angled with the bow down, tends to rush forward because of the downhill slope and then virtually stops as the wave passes under the hull and the boat adopts a bow up position. If the motion becomes too uncomfortable you may have to resort to driving the boat on the throttles, opening them wide to drive up the slope and then easing off as the boat comes over the top.

Planing Boats

A planing boat is much easier to handle in following seas than a displacement boat. On a planing boat you have the option of overtaking the waves rather than having then overtake you. This enables you to dictate where you are in relation to the waves.

One of the most comfortable positions is to be gently overtaking the waves, so that if the waves are travelling at 20 knots you want to be travelling at around 25 knots. This will mean that you have enough power to climb up the back of the wave. There will be a sudden change in attitude as you go over the crest, when the boat runs downhill before it lifts once more to the wave in front. Because the boat is only overtaking the waves at a gentle speed, you should be in full control of the situation and the ride will be fairly comfortable except when the boat changes attitude as it overtakes the wave crest.

You have a certain latitude to vary the speed, perhaps to try to find a more comfortable ride. But in general, you won’t want to drop below the speed of the waves because the small rudder fitted to planing boats means that you may find yourself having difficulty maintaining steering control. The worst situation is to find you just matching the speed of the waves as the rudder can become virtually ineffective, leading to the possibility of a broach. With stern drive or outboard boats this would not apply and you have more flexibility both in control and speed because of the positive steering at all speeds.

In a planing boat you can run before a following sea even when the seas are quite big and it can be a very exhilarating ride. Although you feel fully in control of the situation you may not always appreciate that conditions are deteriorating.

If you find yourself in a freshening wind running before the sea, it is a good idea not only to keep a lookout astern but also occasionally to stop, turn around and head into the sea to get a better indication of how the conditions are developing. It is very easy when running downhill to be lulled into a false sense of security so turning around and checking the developing conditions early, can allow you the time to make necessary alterations, such as running for shelter or looking for an adjacent harbor.

A Comfortable Ride

In general terms the skipper of the boat is out to give the crew a comfortable ride. On a long coastal passage, this can be quite important, since tiredness can have a detrimental effect on your reactions and can affect your decision making. Keep a careful watch on the stamina of your crew, particularly on a planing boat where the motion is often more violent than on a displacement craft and where you need to have your wits about you because things happen much faster.

Darkness adds a whole new dimension to reading the waves and boat handling. Firstly, you can’t see any of the approaching waves very well, so to a certain extent you are driving blind. However, you should use the same techniques as in daylight. Of course, you will not be able to see the waves ahead, and the biggest danger will be from that larger than average wave which catches you when you least expect it or are going too fast. At night you will get little warning of the approach of these waves, so you should cut your speed back even more than during the day. This gives you a great safety margin should you have to deal with unexpected conditions.