THE PHYSICS OF DOCKING

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1 THE PHYSICS OF DOCKING INTRODUCTION Does your blood pressure begin to rise at the thought of docking the boat? No wonder boats are big, heavy, expensive toys. No one wants to break one. But to be honest, docking a boat is no more difficult than doing the Hokey Pokey. If you can put your right hand in, put your right hand out, give the wheel a spin, and look all about, I can teach you That s what it s all about! Once you understand the simple physics of the forces that work for and against the boat as it moves through the water, you can practice some simple exercises on your own boat to get a feel for how it responds to these forces. Before long you will face the challenges of bringing your boat to and from the dock with a minimum of excitement and dock rash. Physics is the science that deals with matter, energy, motion and force such as the forces that act upon boats. CENTER OF BALANCE The first concept I d like to discuss is the balance or pivot point. Dick and Jane playing on a teeter totter are a fine example of how a pivot point forms the center of balance between opposing forces. PITCH Anyone who has been on a boat knows the feeling of pitching on a teeter totter. The boat pitches on its balance point, fore and aft.

2 A wave picks up the bow Passes under the boat And picks up the stern A boat pitches up and down like a teeter totter every few seconds in a head sea. We also know what happens if Dick is a lot heavier than Jane. Dick s end of the teeter totter dominates the action. Jane gets tossed in the air. With the prop driving the boat forward, settling the stern into the water, and driving the bow into the waves, the bow gets pitched up and slams down, just like Jane. The forward cabin becomes the levitation cabin. Ideally the wheelhouse is near the pivot point of the boat to minimize the effects of pitching on the helmsman.

3 ROLL A boat also has a pivot point from side to side, centered above the keel. The boat rolls back and forth over this pivot point. A wave lifts the port side Passes underneath And lifts the starboard side Ideally the helm is located low to minimize motion. If it is too high, the roll is greater as the arc increases. See how tall Charlie gets tossed back and forth. The higher the helm station, the greater the motion.

4 SWING A third pivot point on a boat is the turning point. When a boat turns, it pivots on this point. When the rudder steers the stern one way, the bow rotates in the opposite direction. It s like walking through a house carry a 2x4. When you swing the forward end one way, the aft end swings the other. You might steer the bow through a doorway but if you aren t careful, the stern might smash Granny s china hutch. Ideally this pivot point lies directly below the wheelhouse so the boat turns around the helmsman. We will use this counter motion a lot when docking. PROPELLER FORCES The next set of forces to consider are created by the propeller. There are three kinds of forces created by the propeller THE PROPELLER AS A SCREW When a boat is moving forward, the prop acts like a screw, winding its way through the water with a minimum of disturbance, pulling the boat along. The leading edge cuts cleanly through the water. The pitch of the blades and the cup shape on the trailing edge winds its way efficiently through the water. THE PROPELLER AS PROP WASH When the prop first starts turning, because of the inertia of the heavy boat, the prop initially stays in the same place and pushes water away from it. We call this Prop Wash. Prop Wash flows evenly past the rudder if the rudder is centered. If the rudder is off center, the Prop Wash pushes against the exposed side. Think about what happens when you stick your hand out a car window while driving. Tip the leading edge of your hand up and the wind pushes your hand up. Tip the leading edge down and the wind pushes your hand down. The same thing happens when the rudder is off center. The Prop Wash hits the exposed face of the rudder and pushes the stern sideways. As the stern moves one way, the boat pivots so the bow moves the opposite direction.

5 If the boat is stopped or moving slowly and the rudder is put hard over before the prop begins to turn, the Prop Wash will push hard against the rudder. The stern will pivot significantly before the prop overcomes the inertia and begins to move the boat forward. In Neutral with helm hard to port In Forward, Prop Wash pushes stern to starboard EXERCISE #1 PROP WASH Next time you are out in quiet waters with plenty of sea room, stop the boat completely. Put the helm over hard. Line up the bow on some distant point. Put the boat in forward gear for one second, then back to neutral. Observe how the boat pivots Do this several times, then put the helm over the other way and repeat the exercise. On a well balanced vessel, the Prop Wash will rotate the stern equally in either direction. THE PROPELLER AS A PADDLEWHEEL A prop acts very differently in reverse gear. The pitch on the shaft and cup of the blades make a screw very efficient when moving forward. But the same design makes it inefficient in reverse. Before it can dig into the water to begin moving the boat backwards, it will paddlewheel sideways. This is called Prop Walk. Prop Walk works in only one direction opposite the way the prop turns in forward gear. Prop Walk behaves differently on different boats depending upon several factors including the size of the prop, the pitch of the prop, and the size of the rudder. Also if the shaft is close to parallel with the water surface, Prop Walk is less effective than when the shaft is at a great angle Let s take a moment to review the direction a prop turns. Looking at a propeller from behind the boat, if it turns clockwise, it is called a right handed screw. If it turns counter clockwise, it is called a left handed screw.

6 A Left Handed Prop turns Counter-Clockwise A Right Handed Prop turns Clockwise It is important to know which way your prop turns because it will Prop Walk in the opposite direction. A right hand prop will Prop Walk to the left and a left hand prop will Prop Walk to the right. PAE makes many models with an offset cabin. These boats are designed to normally dock to starboard. They mostly have left hand props, meaning that they Prop Walk to starboard. This helps when you are docking on the starboard side. You can pivot a boat in a tighter radius in the direction that Prop Walk works for you instead of against you. Prop Wash and Prop Walk are the keys to maneuvering a boat in tight quarters. EXERCISE #2 PROP WALK Next time you are out in quiet waters with plenty of sea room, stop the boat completely. Line up the bow on some distant point. Put the boat in reverse gear for two or three seconds, then back to neutral. Observe how the boat pivots and which direction. The direction the boat pivots depends on whether you have a right hand or left hand prop. PIVOTING IN PLACE Now let s combine the forces of Prop Wash with Prop Walk. From a dead stop, we put the helm over hard in the direction of our prop to starboard for a right handed prop and to port for a left handed prop. Shift into forward gear for one or two seconds. Do not throttle up at all. The Prop Wash will kick your stern over. With the stern swinging slightly but before the boat

7 actually starts moving forward, shift into neutral for a few seconds. Inertia will continue to swing the boat in a circle. Shift into reverse for two or three seconds.. The Prop Walk will paddlewheel the stern further in the same direction. Because the prop is more efficient in forward than reverse, you will stay in reverse a little longer than in forward in order to stop all forward motion. As you repeat this sequence, the boat will pivot in a circle. Prop Wash Prop Walk Prop Wash Prop Walk Prop Wash Prop Walk EXERCISE #3 THE RHYTHM OF SHIFTING GEARS Hold your pen or pencil vertically against the table. Imagine that it is the gear shift on your boat. Remember that when you shift from neutral into forward or reverse, it clicks into a little détente that lets you know you are in gear. Shift into forward by moving the top of the shifter forward. Don t dawdle make it a quick, firm movement to the détente but no further. After one or two seconds, shift back to neutral. Again, imagine that you will feel it click against the neutral détente. Pull the stick back to shift into reverse. It will click into a détente. After two or three seconds, shift back to neutral. Now sing the Hokey Pokey song. Put your right hand in, put your right hand out, put your right hand in reverse and look all about. Put it back in neutral and do it all again. That s what it s all about! This will help establish the pace at which you will shift in and out of gear when performing a pivot turn. Try this a few times and imagine that your boat is responding to your commands.

8 EXERCISE #4 PIVOTING THE BOAT IN PLACE Next time you are out in quiet waters with plenty of sea room, stop the boat completely. Put the helm hard over to the right if you have a right hand prop; to the left if your prop is left handed. Line up the bow on some distant point. Put the boat in forward gear for one or two seconds, then back to neutral for one or two seconds. Observe how the boat rotates with little or no forward motion. Put the boat in reverse gear for two or three seconds, then back to neutral for one or two seconds. See how the boat stops moving forward but continues to rotate. Repeat several times until you have made a complete 360 degree pivot. Look over the side to observe the wake to see the diameter of your turning radius. It shouldn t be much more than a boat length. Try the same exercise in the other direction. Note how the pivot goes slower with a larger radius when working against the Prop Walk. Once you are confident that the boat will pivot in a tight radius, you can practice in more confined circumstances. USING THE THRUSTER(S) You will notice that so far we have ignored thrusters. Not that there is anything wrong with using your thrusters I certainly do. But if conditions overpower your thrusters or they fail, it is good to know how to use good old fashioned seamanship to maneuver your boat. We can use the thrusters to improve the performance of a pivot turn. If you have only a bow thruster, give the bow a push in the direction you want to pivot to break the inertia before engaging the transmission. If you have a stern thruster as well, engage both thrusters at the same time in opposite directions. Engaging one or both thrusters before going into gear will make the boat pivot more quickly. BACKING THE BOAT Many larger boats prefer to dock stern-in to a slip with fingers rather than docking bow in. It is often easier to get on and off the boat from the stern. It also makes your departure easier. It is best to learn to back the boat out in open flat water. The goal is to back in a straight line. There are three ways to control the boat in reverse. STEERING BACKWARDS USING THE RUDDER ONLY The most obvious way to steer the boat in reverse is using the rudder. Simply point the rudder in the direction you wish to go and the stern will follow. However, as you will recall from our previous discussion, the rudder is most effective when there is Prop Wash pushing the stern around. In reverse, the rudder is ahead of the prop so any Prop Wash moves away from the rudder, not past it. The boat will only respond to the rudder

9 when the boat has enough way on to have some flow over the rudder. Moving fast is rarely the goal when learning to dock. Because of inertia, a heavy boat will be slow to respond to the rudder in reverse. And once the boat begins to turn, inertia will tend to make it oversteer. Using the rudder alone is the least effective way to steer in reverse. EXERCISE #5 STEERING BACKWARDS USING THE RUDDER On a calm day with plenty of sea room, stop the boat Center your helm Line your bow up with a distant reference point Shift into reverse. Observe whether your bow stays aligned with your reference point. When the boat gets off your line, put the helm over in the direction you wish to go. See if you can steer in a straight line. STEERING BACKWARDS USING PROP WASH A better way to steer in reverse is to use Prop Wash. This means periodically shifting briefly into forward to straighten up your line. Remember your pivot point. If the stern is too far to one side, you must move the bow the opposite direction to straighten up. Usually this means spinning the wheel in the opposite direction, then applying a small force in forward gear to move the stern where you want it. Once you are lined up again, go into reverse again and resume backing up. EXERCISE #6 STEERING BACKWARDS USING PROP WASH On a calm day with plenty of sea room, stop the boat Line your bow up with a distant reference point Shift into reverse. Observe whether your bow stays aligned with your reference point. When the boat gets off your line, put the helm over and go forward briefly to pivot the stern into alignment using Prop Wash. Then reverse again. With practice, you can back in nearly a straight line, steering by kicking your stern around. USING THE BOW THRUSTER If you have a bow thruster, steering in reverse is much easier. Rather than steering with the stern, you can steer with the bow. The bow thruster acts like a rudder to pivot the boat, moving the bow in the opposite direction from what you want the stern to do. Again, remember your pivot point. If the stern starts moving in the wrong direction, simply thrust the bow in the same direction and the stern will swing in the desired direction.

10 EXERCISE #7 STEERING BACKWARDS USING THE BOW THRUSTER Once you master backing using the rudder, try using the bow thruster like a rudder to steer the boat from the bow. Line your bow up with a distant reference point Shift into reverse. Observe whether your bow stays aligned with your reference point. When the boat gets off your line, use your bow thruster to push the bow in the opposite direction. This will pivot the boat into alignment. Try other maneuvers. Can you back the boat in a circle? Later use these techniques to back into a slip. Again, make sure you first practice on calm days with no current. And put out plenty of fenders A hint good headsets make communication between the helm and the deck crew much easier, with a minimum of shouting. Ideally the deck crew simply provides information, not instruction. For example, the deck crew might say, You are 3 from the finger pier and 7 from the dock. This lets the helm know what s/he needs to do to bring the boat home. DOCKING THE BOAT Let s bring all these forces into practical use as we dock the boat. The first few times you practice docking, do so on calm days where there is little or no current. Do not try to get the boat close to the dock. Plan to stop parallel to the dock about 10 away. Once you are comfortable maneuvering to that distance, ease your way closer next time. Approach the dock slowly at about a 30 to 45 degree angle with the rudder straight. Shift to neutral well in advance and coast in. Because the bow is pointed, you can get closer to the dock than appears comfortable at first before making your next move. Just before you touch the dock and still in neutral, spin the wheel to turn the bow away from the dock. (Hint: Use the joy stick and the wheel simultaneously to move the wheel quickly.) Shift briefly into forward and back to neutral. Prop Wash will swing the stern toward the dock. Practice this maneuver a few times from a distance to get a feel for how close to get before you spin the wheel, how slowly the boat should be moving, and how much force it takes to end up parallel to the dock, close enough for your crew to step ashore. Assuming you are docking on the side where Prop Walk will assist you, you will end this maneuver by going into reverse to suck the stern up against the dock.

11 Approach at degrees Go to neutral and drift in Put the helm hard over Prop Wash to pivot in place Reverse to stop, Prop Walk If you have thrusters, you can use their force to fine tune the position on the dock. Initially the tendency will be to stop the boat about 5 from the dock, then use the thrusters and prop walk to close the gap. After some practice you will need the thrusters less and less, primarily to shove the boat tight as your crew gets the lines snugged up. CRABBING THE BOAT If you have a bow thruster, you can crab the boat sideways. Set the rudder to turn the bow away from the dock, which will pivot the stern toward the dock. As you put the transmission in forward, simultaneously engage your bow thruster to push the bow toward the dock. The boat will move sideways toward the dock. As when pivoting the boat, do not throttle up. The goal isn t to move the boat forward very much, if any, but to use the Prop Wash and Prop Walk to move the stern sideways whilst the bow thruster moves the bow in the same direction. If you have bow and stern thrusters, you can crab even better. Since bow thrusters are often more powerful than the stern thruster, using the Prop Wash and Prop Walk forces to augment the stern thruster to make crabbing even more effective. EXERCISE # 8 CRABBING THE BOAT On a calm day, stop near an empty end tie or side tie dock Put the helm hard over away from the dock Use the bow thruster to push the bow toward the dock for about four seconds At the same time, shift into forward gear for a second or two. Observe how the boat crabs forward. Practice several thrusts to learn how much thruster force vs. Prop Wash it takes to keep the boat parallel to the dock.

12 Avoid too much movement forward and aft someday you may have to crab into a small space between two vessels. If you are moving forward too much, shift to reverse for a few seconds and use Prop Walk to stop the boat and move the stern closer to the dock. If you have a stern thruster too, try crabbing using both thrusters to push the boat sideways. Combine Prop Wash and Prop Walk to enhance the movement. LEAVING THE DOCK BACKED IN TO A SLIP If you have backed into a slip with finger piers, departing is simply a matter of easing forward until you clear the pilings, then performing a pivot turn without moving forward or backwards more than a couple of feet until the boat is facing down the fairway. If you have practiced pivot turns in open water, you will have a good feeling for how much power it takes to swing the boat in its own length. SIDE TIE A close side tie requires a different technique. Put the helm over hard toward the dock. Shift into forward, using Prop Wash to rotate the bow toward the dock, which pivots the stern away from the dock. You can rotate further than you d expect, thanks to the shape of the bow. Before the bow touches the dock, reverse and use Prop Walk to continue the rotation as the boat backs up. Repeat this sequence another time or two until the stern is well clear of the boat behind. Then back away until you are well clear. Pivot the boat and go on your way.

13 USING A SPRING LINE If you are really boxed in, use of a spring line to prevent the boat from going forward at all will help. Attach a long line to the bow cleat, through the hawsepipe, and around a cleat on the dock near the midpoint of the boat. Return the bitter end to the bow and SNUB IT TIGHT! As described above, turn the helm to drive the bow into the dock. When you shift into forward, the spring line prevents the boat from moving forward at all so all the force goes to moving the stern sideways. If the stern has not cleared the vessel behind, the deckhand can tighten the spring line and cleats it again. The helmsman powers forward again. The spring line keeps the boat from moving forward so the stern rotates further away from the dock. Once the boat pivots enough to clear the vessel behind, the helmsman moves the boat forward slightly to ease the strain on the spring line. The deckhand releases the bitter end of the spring line and hauls it around the cleat and onto the boat. Leaving an end tie is typically the easiest of all. You can use a combination of the rudder to move the stern away from the dock or the thrusters to crab off until you can move forward. Remember that the boat will pivot on its balance point. If you steer the bow away from the dock too soon, the stern may swing in and clip the dock or a piling. Learning how big your tail is comes with practice. Initially, slower is better bumps not crunches! Fenders, not benders. PREPARATIONS FOR DOCKING Before we close, let me provide a quick checklist for things to do when preparing to dock your boat.

14 1) If you have been running for more than a couple of hours, exercise the main engine by running for 10 minutes at wide open throttle. This is good for the engine and good for your peace of mind, knowing the trusty engine will perform if needed. 2) Visit the bathroom. Take pressure off before things get exciting. 3) Have a bite to eat. Make sure that you don t have a blood sugar slump in the midst of an adrenalin rush. 4) Stop the boat and make sure you can shift into reverse. If there is a problem it will be easier to deal with out in open water than in tight quarters. 5) Test the thrusters. 6) Have your crew mount fenders and lines on both sides of the boat. Sometimes circumstances in a marina require a quick change of plans. Having the option of going bow in instead of stern in is a good thing. 7) Approach slowly. Often the difference between a challenge and a crisis is to stop in place and figure out what to do. CONCLUSION Now that you understand the physics of the forces that make a boat move in the water, you can practice using those forces to dock calmly and gracefully. Practice these exercises in calm water until you have a good feel for how your boat responds to your commands. You will gradually become more and more confident of your abilities. Respectfully submitted, Douglas Cochrane M/V Orion, N57-25