Hopefully, by now you have done the boat exercises in Part 1.\u00a0 If so, you know the \u201cadvance\u201d and \u201ctransfer\u201d associated with your favorite boat. You may have noticed that advance and transfer is different while turning to starboard than while turning to port.\u00a0 Maneuvers in forward may show only slight differences, while maneuvers with astern propulsion may be quite different.\u00a0 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.<\/p>\n
\u00a0Propellers and Rudders<\/b><\/p>\n
It doesn’t matter if the propeller is turning in a direction to move the boat forward or astern, current is generated.\u00a0 Current moving across the blades of a propeller results in lower pressure on one side of the blade than on the other.\u00a0 The blade (with the boat attached) pulls itself into the lower pressure.\u00a0 Just like a sail pulls a boat forward into the wind. The turning propeller creates a current moving towards the propeller, referred to as \u201csuction screw current\u201d.\u00a0 Moving away from the propeller, in a much more forceful, focused direction, is \u201cdischarge current\u201d.\u00a0 When you learn to manage these two types of current, especially the more forceful discharge current, and a third force known as \u201cunequal blade thrust\u201d your close quarter boat handling skills improve dramatically.\u00a0 These three forces, alone or in combination with each other, result in what we generally refer to as \u201cprop wash\u201d and \u201cprop walk\u201d.<\/p>\n
Prop Wash<\/b><\/p>\n
Prop Wash is primarily a result of discharge current.\u00a0 The greatest example of prop wash is found while the boat is not moving through the water.\u00a0 Water flow across the rudder is required for steering control.\u00a0 Sitting still in the water the rudder has little impact except as a large inefficient paddle.\u00a0 As a boat begins to move through the water steering is gained and control over direction of travel is established.\u00a0\u00a0\u00a0 However, there is another way to get current flowing across the rudder.\u00a0 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.\u00a0 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.<\/p>\n
It\u2019s time for a small detour.\u00a0 I often hear, and even say it myself, that \u201cdriving\u201d a boat with a wheel is just like driving a car.\u00a0 It gives a new helms-person confidence to do it.\u00a0 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.\u00a0 A car steers from the front wheels, with sideways (lateral) resistance created by the rear wheels.\u00a0 A boat steers from the stern, with the \u201ccenter of lateral resistance\u201d located up at the keel (don\u2019t worry; this isn’t a technical discussion of CLR).\u00a0 Here\u2019s the difference, with a car the back wheels follow the steering wheels.\u00a0 With a boat, the bow moves opposite the stern, because the boat is pivoting on the CLR up at the keel.\u00a0 Driving a boat in forward is actually more like backing a car in reverse.\u00a0 The key point to know \u2026 a boat pivots on its CLR.<\/p>\n
Back to the topic at hand.\u00a0 Here is an exercise for you.\u00a0 To test the impact of prop wash; pick a day with little or no wind.\u00a0 Use a boat that you know has port prop walk.\u00a0 In a slip use spring lines fore and aft from cleats located amidships, port and starboard.\u00a0 That\u2019s four lines starting amidships, one traveling forward, one aft, on each side of the boat.\u00a0 Remove the bow and stern breast lines.\u00a0 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.\u00a0 In other words, make sure the boat is secure before starting.\u00a0 Center the rudder.\u00a0 Place the boat in forward at idle, and slowly turn the wheel to port.\u00a0 The stern slides to starboard forcing the bow to port. Now, center the rudder \u2026 not much should happen (assuming no wind).\u00a0 \u00a0 Turn the wheel to starboard until the \u201cturn\u201d stops.\u00a0 Center the wheel.\u00a0 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.\u00a0 As you can see, prop walk has a large impact on a boat sitting still in the water.<\/p>\n
Here is how discharge current, suction current, and unequal blade thrust interacted in this exercise:<\/p>\n
Prop Walk<\/b><\/p>\n
Prop walk is a combination of unequal blade thrust and discharge current.\u00a0 The angle of the propeller shaft causes the \u201cpitch\u201d (in simple terms \u2026 the angle) of the propeller to vary depending on if the blade is descending or ascending.\u00a0 The blades have a greater pitch while on the down side of the stroke that while on the up side.\u00a0 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 \u201cwalk\u201d to the strong side.\u00a0 Most propellers turn clockwise in forward, and counter clockwise in reverse.\u00a0 While in forward, the stern wants to go to starboard.\u00a0 While in reverse, the stern wants to go to port (the first half of port prop walk).\u00a0 The greater the shaft angle, the more pronounced the effect.\u00a0 Now add in the impact of discharge current.\u00a0 As the propeller turns counter clockwise (in reverse), the blades on the left are throwing water forward and down.\u00a0 The blades on the right are throwing water forward and up \u2026 right into the curvature of the hull, creating greater pressure on the starboard side than the port side, forcing the stern to port.\u00a0 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 \u2026 unless you actually want the stern to move left (as in a fairway rotation).<\/p>\n
Here\u2019s part two of the earlier exercise.\u00a0 Center the boat in the slip with the rudder centered.\u00a0 Place the boat into reverse at idle.\u00a0 The stern will move to port.\u00a0 Turn the rudder to port.\u00a0 Not much happens.\u00a0 Turn the wheel to starboard.\u00a0 Again, not much happens.\u00a0 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.<\/p>\n
It\u2019s time to get out of the slip and into sheltered open water to put it all together.<\/p>\n
Do four \u201cfairway rotations\u201d<\/p>\n
Here are a few tips.\u00a0 While turning clockwise, prop walk is a major contributor.\u00a0 Balance it and prop wash for the most effect.\u00a0 While turning counter clockwise, prop walk will stop your turn.\u00a0 Eliminate as much of it as possible, and use prop wash as the major contributor.\u00a0 You can\u2019t do a fairway rotation while moving backwards.\u00a0 Bring the boat to a stop with a centered rudder.\u00a0 Then turn the rudder in the direction you wish to rotate, using prop wash to start the process moving.<\/p>\n
Docking with prop walk<\/b>.\u00a0 Find a dock where you can safely conduct this next maneuver.\u00a0 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.\u00a0 Approach the dock at a 45 to 90 degree angle, slowly but with steerage way.\u00a0 Stop the bow of the boat within two or three feet of the dock with the port side bow cleat near a dock cleat.\u00a0 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.\u00a0 Make sure the dock line is NOT over any lifelines or pulpit rails.\u00a0 DO NOT TRY TO HOLD THE BOW DOCKLINE by hand.\u00a0 You can\u2019t. \u00a0Secure it in a way it can be easily adjusted, and keeps hands and fingers clear!!!\u00a0 After the bow line is secure, place the boat into reverse and slowly take up slack in the line.\u00a0 Once the slack is removed, the stern will begin to walk slowly toward the dock.\u00a0 Use only enough engine speed to keep the stern moving.\u00a0 When close enough, get a stern line onto the dock, and secure the boat.\u00a0 This maneuver works great in tight spaces with a boat in front of and a boat behind your docking location.<\/p>\n
Departing a dock with prop walk<\/b>.\u00a0 Tie up starboard side to the dock.\u00a0 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).\u00a0 Center the rudder.\u00a0 Release the stern line.\u00a0 Engage reverse at idle, and allow the stern to prop walk away from the dock.\u00a0 When clear, release the bow line and back away.\u00a0 This is a great way to get away from the windward side of a dock with limited space fore and aft.<\/p>\n","protected":false},"excerpt":{"rendered":"
Hopefully, by now you have done the boat exercises in Part 1.\u00a0 If so, you know the \u201cadvance\u201d and \u201ctransfer\u201d associated with your favorite boat. You may have noticed that advance and transfer is different while turning to starboard than … Continue reading