Hopefully, by now you have done the boat exercises in Part 1. If so, you know the “advance” and “transfer” associated with your favorite boat. You may have noticed that advance and transfer is different while turning to starboard than while turning to port. Maneuvers in forward may show only slight differences, while maneuvers with astern propulsion may be quite different. Part of this is the impact of wind, however, a great deal of the difference is attributed to the effects of propeller generated current on the boat.
Propellers and Rudders
It doesn’t matter if the propeller is turning in a direction to move the boat forward or astern, current is generated. Current moving across the blades of a propeller results in lower pressure on one side of the blade than on the other. The blade (with the boat attached) pulls itself into the lower pressure. Just like a sail pulls a boat forward into the wind. The turning propeller creates a current moving towards the propeller, referred to as “suction screw current”. Moving away from the propeller, in a much more forceful, focused direction, is “discharge current”. When you learn to manage these two types of current, especially the more forceful discharge current, and a third force known as “unequal blade thrust” your close quarter boat handling skills improve dramatically. These three forces, alone or in combination with each other, result in what we generally refer to as “prop wash” and “prop walk”.
Prop Wash is primarily a result of discharge current. The greatest example of prop wash is found while the boat is not moving through the water. Water flow across the rudder is required for steering control. Sitting still in the water the rudder has little impact except as a large inefficient paddle. As a boat begins to move through the water steering is gained and control over direction of travel is established. However, there is another way to get current flowing across the rudder. Because a rudder is mounted aft of the propeller engaging the propeller in forward produces a focused stream of water flowing directly across the rudder in the form of discharge current. Turning the rudder causes higher pressure on one side of the rudder than the other, resulting in lift, and forces the stern to move towards the side with lower pressure.
It’s time for a small detour. I often hear, and even say it myself, that “driving” a boat with a wheel is just like driving a car. It gives a new helms-person confidence to do it. The truth is, while the effect may seem similar (i.e. turn the wheel to the right and the car/boat turn to the right) the mechanics are totally different. A car steers from the front wheels, with sideways (lateral) resistance created by the rear wheels. A boat steers from the stern, with the “center of lateral resistance” located up at the keel (don’t worry; this isn’t a technical discussion of CLR). Here’s the difference, with a car the back wheels follow the steering wheels. With a boat, the bow moves opposite the stern, because the boat is pivoting on the CLR up at the keel. Driving a boat in forward is actually more like backing a car in reverse. The key point to know … a boat pivots on its CLR.
Back to the topic at hand. Here is an exercise for you. To test the impact of prop wash; pick a day with little or no wind. Use a boat that you know has port prop walk. In a slip use spring lines fore and aft from cleats located amidships, port and starboard. That’s four lines starting amidships, one traveling forward, one aft, on each side of the boat. Remove the bow and stern breast lines. Make sure the boat cannot drive forward and strike the dock, dock box, or fire hydrant, and that it cannot accidentally back out of the slip. In other words, make sure the boat is secure before starting. Center the rudder. Place the boat in forward at idle, and slowly turn the wheel to port. The stern slides to starboard forcing the bow to port. Now, center the rudder … not much should happen (assuming no wind). Turn the wheel to starboard until the “turn” stops. Center the wheel. This time, the stern may start to slowly move back to starboard due to the impact of unequal blade thrust which we will cover next. As you can see, prop walk has a large impact on a boat sitting still in the water.
Here is how discharge current, suction current, and unequal blade thrust interacted in this exercise:
- Discharge current was the main player. Water moving across the turned rudder caused the stern to move toward the lower pressure area, and the bow to pivot to the opposite direction.
- Unequal blade thrust. Tried to slowly move the stern in the direction of propeller travel. Was greatly overpowered by the discharge current while the rudder was turned. Became more important at the rudder straightened.
- Suction current. Water moving evenly across the hull and keel acted as a minor stabilizing force, however, was easily overpowered by both discharge current and unequal blade thrust.
Prop walk is a combination of unequal blade thrust and discharge current. The angle of the propeller shaft causes the “pitch” (in simple terms … the angle) of the propeller to vary depending on if the blade is descending or ascending. The blades have a greater pitch while on the down side of the stroke that while on the up side. In other words, a propeller turning clockwise is more efficient on the right side (down stroke) than it is on the left side, resulting in the stern wanting to “walk” to the strong side. Most propellers turn clockwise in forward, and counter clockwise in reverse. While in forward, the stern wants to go to starboard. While in reverse, the stern wants to go to port (the first half of port prop walk). The greater the shaft angle, the more pronounced the effect. Now add in the impact of discharge current. As the propeller turns counter clockwise (in reverse), the blades on the left are throwing water forward and down. The blades on the right are throwing water forward and up … right into the curvature of the hull, creating greater pressure on the starboard side than the port side, forcing the stern to port. Unequal blade thrust and discharge current both pull to port, and until you can get enough current over the rudder in the form of speed, you are not going to have control … unless you actually want the stern to move left (as in a fairway rotation).
Here’s part two of the earlier exercise. Center the boat in the slip with the rudder centered. Place the boat into reverse at idle. The stern will move to port. Turn the rudder to port. Not much happens. Turn the wheel to starboard. Again, not much happens. There may be a slight easing of the force to the left, due to suction current over the rudder, however, probably not enough to really make much difference.
It’s time to get out of the slip and into sheltered open water to put it all together.
Do four “fairway rotations”
- Forward turning clockwise
- Forward turning counter clockwise
- Astern propulsion turning clockwise
- Astern propulsion turning counter clockwise
Here are a few tips. While turning clockwise, prop walk is a major contributor. Balance it and prop wash for the most effect. While turning counter clockwise, prop walk will stop your turn. Eliminate as much of it as possible, and use prop wash as the major contributor. You can’t do a fairway rotation while moving backwards. Bring the boat to a stop with a centered rudder. Then turn the rudder in the direction you wish to rotate, using prop wash to start the process moving.
Docking with prop walk. Find a dock where you can safely conduct this next maneuver. Under normal Marina Bay wind conditions, I like to use the leeward side of the D dock pump out station, port side to the dock, facing out. Approach the dock at a 45 to 90 degree angle, slowly but with steerage way. Stop the bow of the boat within two or three feet of the dock with the port side bow cleat near a dock cleat. While the boat is resting in the water, the bow line handler should get the bow line around the dock cleat, and secure it back on the boat. Make sure the dock line is NOT over any lifelines or pulpit rails. DO NOT TRY TO HOLD THE BOW DOCKLINE by hand. You can’t. Secure it in a way it can be easily adjusted, and keeps hands and fingers clear!!! After the bow line is secure, place the boat into reverse and slowly take up slack in the line. Once the slack is removed, the stern will begin to walk slowly toward the dock. Use only enough engine speed to keep the stern moving. When close enough, get a stern line onto the dock, and secure the boat. This maneuver works great in tight spaces with a boat in front of and a boat behind your docking location.
Departing a dock with prop walk. Tie up starboard side to the dock. Set up the bow line so that it can easily be released from the dock cleat while still on the boat (e.g. doubled around the cleat and back to the boat). Center the rudder. Release the stern line. Engage reverse at idle, and allow the stern to prop walk away from the dock. When clear, release the bow line and back away. This is a great way to get away from the windward side of a dock with limited space fore and aft.