Presented by Douglas Cochrane The Physics of Docking Presented by Douglas Cochrane

Such as the forces that act upon boats. Physics is the science that deals with matter, energy, motion, and force. Such as the forces that act upon boats. Physics sounds like a big word. I mean, we have to get clear to high school before we study it. But the physics that I am going to talk about are things that we learned in kindergarten. This isn’t rocket science. If you can do the hokey pokey, you can dock your boat.

Here are three of the forces that affect boats when docking: Pivot or Balance Points Prop Wash Prop Walk

The Pivot Point The balance point between two opposing forces

The Pivot Point Here is an example of a pivot point. Dick and Jane are opposing forces balanced on a pivot point.

The Pivot Point

The Pivot Point Dick goes up and Jane goes down.

The Pivot Point

The Pivot Point

The Pivot Point

The Pivot Point

The Pivot Point

The Pivot Point

The Pivot Point

The Pivot Point

The Pivot Point

Boats have three pivot points For example, a boat PITCHES on a pivot point

Pitching The Fore and Aft pivot point is at the waterline near the center of the length.

Pitching A wave picks up the bow

Pitching Crosses beneath

Pitching And picks up the stern

Pitching

Pitching

Pitching

Pitching

Pitching

Pitching

What if one side dominates the Pivot Point?

The Dominating Force Dick is heavier than Jane.

The Dominating Force As long as he plays fair, they go up and down evenly

The Dominating Force

The Dominating Force

The Dominating Force

The Dominating Force

The Dominating Force

The Dominating Force

The Dominating Force But if he jumps up and comes down hard…

The Dominating Force

OOPS! Jane goes flying!

A boat ROLLS on a Pivot Point

Roll

Roll A wave lifts one side

Roll Passes below

Roll And picks up the other side

Roll

Roll

Roll

Roll

Roll

Roll

Roll

Roll

Being close to the Pivot Point reduces the motion

Low Center of Gravity Little Sally sits on the center of the teeter totter

Low Center of Gravity She barely moves as the teeter totter rocks back and forth

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity A boat with a low center of gravity is much the same

Low Center of Gravity The helm moves through a narrow arc as the boat rolls.

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

Low Center of Gravity

The further from the Pivot Point, the greater the forces

High Center of Gravity Tall Charley has to work hard to stay aboard

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity The same is true of boats with a high A/B ratio

High Center of Gravity The poor helmsman sweeps through a wide arc as the boat rolls.

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity

High Center of Gravity This is pretty uncomfortable

High Center of Gravity

High Center of Gravity

High Center of Gravity

Thirdly, a boat TURNS on a Pivot Point

This pivot point is on the centerline fore and aft and near the center lengthwise.

Stern to Port, Bow to Starboard When the bow turns one way, the stern turns the other

This is like walking through a house with a 2x4 on your shoulder This is like walking through a house with a 2x4 on your shoulder. You have to be careful if you steer the “bow” through a doorway or the “stern” may crash into Granny’s china hutch.

Pivot Points – Recap

Pivot Points – Recap A boat PITCHES on a Pivot Point 2) A boat ROLLS on a Pivot Point 3) A boat TURNS on a Pivot Point So there are three forces that act upon the shape of the hull

Physics of the Propeller Now let’s talk about the forces developed by the propeller

Physics of the Propeller #1 The Prop as a Screw

A prop is designed to screw its way through the water

The Leading Cutting Edge The Cupped Edge The leading edge cuts cleanly through the water. The pitch on each blade shoves the water along. The cupped following edge increases the grip on the water. Pitched Blades

The Pitch and Cup of the blades screw their way through the water DEMO THE SMALL ELECTRIC FAN

Physics of the Propeller #1 The Prop as a Screw #2 The Prop as a Water Pusher

Boats are heavy with lots of inertia Initially a Prop stays in one place and pushes water away from it If we held our small fan in one place, instead of moving itself through the air, it would move the air through the fan.

Prop Wash Boats are heavy with lots of inertia Initially a Prop stays in one place and pushes water away from it This is called Prop Wash

If the rudder is off center, Prop Wash pushes it (and the stern) sideways Remember what it feels like to stick your hand out the window of a car as it moves? If you hold your hand level, the wind just passes by. But if you tip the leading edge up, the wind forces your arm up. If you tip your hand down, the wind forces your arm down. Prop Wash does the same thing with the rudder, forcing the stern to the side.

If the rudder is off center, Prop Wash pushes it (and the stern) sideways. What happens when the stern moves one way? The bow turns the other. The boat begins to pivot on its turning point.

Stopped in Neutral with helm hard over to port Here is an example. The boat is stopped and the helm is turned hard to port.

In Forward, Prop Wash pushes stern to starboard As soon as we shift into Forward, Prop Wash pushes the stern to starboard and the bow pivots to port and the bow pivots in the opposite direction

Physics of the Propeller #1 The Prop as a Screw #2 The Prop as Prop Wash Prop Wash works in either direction Prop Wash works either direction, depending upon the rudder location

Physics of the Propeller #1 The Prop as a Screw #2 The Prop as Prop Wash #3 The Prop as a Paddlewheel

A Prop is much less efficient in Reverse A prop is designed to work well in Forward, smoothly cutting into the water and screwing its way forward. But the same design means that it is inefficient in Reverse. Instead of a sharp leading edge, the cupped edge is now the leading edge. It takes a while for a prop to begin moving the boat in reverse.

A Prop is much less efficient in Reverse Until it overcomes the inertia of the boat, it will paddlewheel sideways

A Prop is much less efficient in Reverse Until it overcomes the inertia of the boat, it will paddlewheel sideways

A Prop is much less efficient in Reverse Until it overcomes the inertia of the boat, it will paddlewheel sideways This is called Prop Walk

In Forward, Prop Wash Pushes Stern to Starboard Let’s look at our previous example again. Prop Wash begins the rotation.

In Reverse, Prop Walk Carries Us Further Prop Walk continues to keep the boat pivoting

Physics of the Prop #1 Prop Wash #2 Prop Walk Works in either direction #2 Prop Walk Works in one direction only While Prop Wash works either direction, Prop Walk works only in one direction

Physics of the Prop #1 Prop Wash #2 Prop Walk Why? Works in either direction #2 Prop Walk Works in one direction only Why?

Turns Counter Clockwise Left Hand Prop Turns Counter Clockwise Right Hand Prop Turns Clockwise Let’s take a moment to talk about prop direction. A Left Handed Prop turns counter clockwise, when view from behind. A Right Handed Prop turns clockwise.

Right Hand Prop Walks Left Left Hand Prop Walks Right Right Hand Prop Walks Left But when a Left Handed Prop is reversed, it will paddlewheel to the right. And a Right Handed Prop will paddlewheel to the left. Many Nordhavns are built with an offset house and no side deck on the port side. It is much easier to dock these boats to starboard. Therefore, most Nordhavns have a left handed prop. This makes it easy to walk the stern starboard to the dock. Some boats, like our 57, have a very level shaft. They won’t prop walk as well as a boat that has more pitch to its shaft.

will pivot the boat in a tight radius Prop Wash + Prop Walk will pivot the boat in a tight radius Let’s look at how we can use these forces to pivot a boat in its own length.

Pivoting the Boat Here is our boat, stopped in neutral with the helm hard to port

Pivoting the Boat We put it in Forward and the Prop Wash pushes the stern to starboard. The bow rotates to port.

Pivoting the Boat Before the boat begins moving forward very much, we shift into reverse. This stops any forward motion and the Prop Walk continues to carry the stern to starboard.

Pivoting the Boat Before the boat begins moving backwards, we shift into forward. Prop Wash continues the rotation.

Pivoting the Boat Back to reverse to keep the boat from moving forward whilst continuing the rotation.

Pivoting the Boat Now let’s do a little exercise. Hold your pencil upright on the desk and pretend it is your shifter. A shifter has a little détente, a dimple, that let’s you feel when you are in neutral. There is another détente for when you shift into forward and another for reverse. Some shifter also include the throttle. In this case, if you push the shifter forward past the détente, it throttles up the engine. But we don’t want to do this now, as we want to keep the forward and reverse motion to a minimum as we rotate the boat into position. So practice shifting into forward for a second or two until the boat just begins to move. Then shift back into neutral until the machinery comes to a stop. Then shift into reverse. You will stay in reverse longer than when you were in forward since the prop is less efficient in reverse. Maybe 3 or 4 seconds vs. 1 or 2. Shift back and forth a couple of times to get the muscle memory established. As you do so, sing the Hokey Pokey: “Put your right hand in, Put your right hand out”

Pivoting the Boat

Pivoting the Boat

Pivoting the Boat

Pivoting the Boat

Pivoting the Boat

Pivoting the Boat

Pivoting the Boat

Pivoting the Boat

Pivoting the Boat

Pivoting the Boat

NOTE: A boat will pivot more effectively in one direction than the other. In one direction, the Prop Walk works in the direction of rotation. In the opposite direction, it opposes the rotation.

Departing a Tight Fairway Let’s look at a real life example how you might use a pivot. Here we are docked in a tight slip. The fairway is narrow so we barely have a boat length between us and the dock on the other side. There is a big boat moored next to us. What do we do? First we take a deep breath and screw up our courage. We center the helm so we don’t bang into the dock on one side or the big boat on the other.

Departing a Tight Fairway We shift into forward.

Departing a Tight Fairway As soon as our deckhand shouts, “Clear”, we spin the wheel to port and shift into forward. The boat begins to pivot.

Departing a Tight Fairway

Departing a Tight Fairway Before we hit the dock on the other side, we shift into reverse. Prop Walk carries the pivot further.

Departing a Tight Fairway Before you know it, we are facing down the fairway and on our way.

Docking on a Side Tie

Docking on a Side Tie

Docking on a Side Tie

Docking on a Side Tie

Docking on a Side Tie

Docking on a Side Tie

Docking on a Side Tie

Docking on a Side Tie

Docking on a Side Tie

Docking on a Tight Side Tie

Docking on a Tight Side Tie

Docking on a Tight Side Tie

Docking on a Tight Side Tie

Docking on a Tight Side Tie

Docking on a Tight Side Tie

Leaving a Tight Side Tie

Leaving a Tight Side Tie

Leaving a Tight Side Tie

Leaving a Tight Side Tie

Leaving a Tight Side Tie

Leaving a Tight Side Tie

Leaving a REALLY Tight Side Tie (using a Spring Line)

To Recap We can rotate the boat around its Pivot Point Use Prop Wash to push the Stern in the rotational direction desired Use Prop Walk to assist the Rotation when possible

Next Steps: On a calm day with no traffic… Experiment with Prop Wash and Prop Walk Practice pivoting the boat in place Practice backing the boat in a straight line Try crabbing the boat Practice docking and departing the dock

Thank You Thank You!