Friday, May 9, 2008

Shiphandling (Sternway and Thrusters)

When the ship is dead in the water put the engine half astern and back for a period of ten minutes to get good sternway. Take note the degree to which the ship follows her rudder, the ability or lack of ability of a ship to steer while going astern is important when moving in confined waters.
Collect the usual data and note the direction of the wind relative to the ship. Since the ship with sternway wants to back into the eye of the wind, it is important to be aware of the wind. Back first with the rudder amidships and then try to steer with the rudder after sufficient sternway develops.

Bow thrusters are becoming increasingly common aboard merchant ships and stern thrusters are also seen occasionally. The bow thruster has its advantages and disadvantages as does any other equipment.

Located at the extreme end of the vessel for maximum effectiveness.
Available at all times, unlike a tug.
Gives good lateral control without affecting headway.
Saves some expenses by reducing the need for tugs.

Becomes ineffective as speed increases.
Less powerful than a modern tug.
Cannot be used to slow a ship, or hold against a current from ahead or astern.
Requires continuous maintenance to assure reliability.
Unusable at very light drafts.

Thrusters are used much like a tug to move the bow and stern laterally, steer the vessel when going astern, hold the ship alongside a wharf or pier, and hold the ship into the wind at slow speeds and when anchoring. The thruster's uses are more obvious to the seaman than its shortcomings. The thruster is a useful tool to supplement the anchor and tug but certainly does not replace a tug in all cases.
Keep in mind that the thruster is most useful at speeds of 2 knots and less, and should not be relied upon at higher speeds. This is very important.

The bow thruster's effectiveness can only be determined by experimen­tation. The many diagrams that show this equipment being effective at speeds of 6 knots and more are a figment of some naval architect's imag­ination. More than one ship has a similar carefully drawn but incorrect graph posted in her wheelhouse. The person who developed this graph obviously never got beyond sailing model boats in a test basin and the graph only demonstrates the gap that remains between the theoretician and the real world.

Try your bow thruster first to the right and then to the left, making it a point to orient your ship so you bring her bow through the wind each time. This is an interesting maneuver for the helmsman since the data collected makes it possible to predict with confidence the thruster's effectiveness when steaming through a crowded anchorage or holding the bow into the wind while the mate drops the anchor.
Perform this maneuver first at 1 knot and again at 3. To see how far the theoretical data is from reality try the thruster again at 6 knots. It won't have the effect and it probably won't even be felt by the helmsman holding the ship steady on a course.

Prepare a graph of speed versus bow thruster effectiveness (change of heading in degrees per minute as measured by observation or rate of turn indicator, if that equipment is available) for both your own infor­mation and for the use of pilots. There is no doubt that the graph prepared aboard ship will be more accurate and useful than the one supplied to the ship at delivery. Remember that it is not so much the power of the thruster that is of interest in these tests but the ship's speed through the water at which the thruster is effective. It is better to learn the capabilities of your equipment during trials maneuvers