Positive control astern - Sail Magazine

Positive control astern

Because a sailboat without a bow thruster lacks positive directional control when going astern at slow speeds, many skippers choose one of three options when it’s time to go into a slip. They go into the slip bow first; they stop at a right angle to the slip and then use dock lines to pull the stern in by hand; or they back down with enough speed on to maintain control.The first option is
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Because a sailboat without a bow thruster lacks positive directional control when going astern at slow speeds, many skippers choose one of three options when it’s time to go into a slip. They go into the slip bow first; they stop at a right angle to the slip and then use dock lines to pull the stern in by hand; or they back down with enough speed on to maintain control.

The first option is easy, but if the slip has a short side dock, the middle of the boat may not end up alongside the dock. If there isn’t a side dock, and you’re simply pulling in bow first to a wall or transverse dock, the bow will be the closest part of the boat to shore, which makes it hard to board or leave the boat. Also, in both cases, it will still be necessary to maneuver in reverse when leaving the dock.

The second option is awkward and can be risky; it can take a lot of effort to turn a boat by hand if there is any wind, and there’s a good chance it may collide with adjacent boats.

The third option is also a bit risky because most rudders aren’t quick to respond when going astern. A miscalculation when backing down at speed could damage the boat or dock and may risk injury to crewmembers trying to fend the boat off the dock or another boat.

However, there is another way. If you do it correctly, you can have as much control as you would with a twin-screw boat or a bow thruster. But to be successful you must fully understand the principles behind prop wash and prop walk.

Prop wash

When a sailboat is put in forward gear, the prop sends a powerful jet of water back against the rudder. When the rudder is turned in one direction, that water jet imparts a strong lateral force on the front of the rudder, causing the stern to move immediately in the opposite direction. This effect—which works only when the boat is in forward gear—is called prop wash. When a sailboat is not moving, prop wash can be used to create directional control.

Prop walk

A rotating propeller also produces a lateral force, much like the wheel of a car spinning on ice, with the prop pulling the stern in the same direction the prop is turning. This action, called prop walk, works in forward gear too, but here the effect is masked by prop wash.

When a boat is slowly moving astern, prop walk becomes the primary lateral force on the stern. A prop with a right-hand rotation (the most common sort) turns clockwise in forward gear and counterclockwise in reverse. This means that when the prop is in reverse, prop walk will push the stern to port.

In general, heavy-displacement cruising sailboats are slow to start moving forward or backward, but they will rotate quickly in response to lateral forces. It is this differential inertia that gives rotational movement a chance to precede—often by several seconds—any forward or backward motion. The trick is knowing how to make use of this delayed response by alternating between forward and reverse and placing the rudder at appropriate angles to produce controlled lateral movement.

You can use these principles to keep a boat motionless in a breeze or current, to spin it on its axis when turning in close quarters and, yes, to spin it while keeping it centered and under positive control as you back into a slip. Here’s how it works:

Approach the slip with just enough headway to maintain positive steering control. After checking the wind, current, and neighboring boats, determine where to stop in front of the slip; the correct position will depend principally on the wind direction relative to the slip’s centerline. Keep a minimum distance between the boat’s stern and the outer slip pilings, again the exact distance depends on how hard it is to maneuver the boat to the correct angle relative to the slip while avoiding adjacent boats. Situations A and B are optimal, while C and D will be more challenging (Fig. 1).

In all four cases, once the boat has stopped, the bow will start moving to leeward; the larger the angle between the wind direction and the boat’s centerline, the quicker the bow will want to move. In cases A and B, the boat must rotate more than in C and D, and it’s most important to make sure the wind is on the correct side of the bow. Because the wind is blowing from the slip in A and B, there is no risk the boat will be blown into adjacent boats. Here you can stop the boat with the bow close to them. If the wind is blowing into the slip (C and D), keep the bow as far away as possible from other boats. Also keep the angle between the wind and the boat as small as possible. That means it will take longer for the bow to cross the slip’s centerline, which gives you more time to enter the slip.

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