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

2. 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.

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

4. The Pivot Point
The balance point between two opposing forces

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

6. The Pivot Point

7. The Pivot Point
Dick goes up and Jane goes down.

8. The Pivot Point

9. The Pivot Point

10. The Pivot Point

11. The Pivot Point

12. The Pivot Point

13. The Pivot Point

14. The Pivot Point

15. The Pivot Point

16. The Pivot Point

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

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

20. Pitching
A wave picks up the bow

21. Pitching
Crosses beneath

22. Pitching
And picks up the stern

23. Pitching

24. Pitching

25. Pitching

26. Pitching

27. Pitching

28. Pitching

30. What if one side dominates the Pivot Point?

31. The Dominating Force
Dick is heavier than Jane.

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

33. The Dominating Force

34. The Dominating Force

35. The Dominating Force

36. The Dominating Force

37. The Dominating Force

38. The Dominating Force

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

40. The Dominating Force

41. OOPS!
Jane goes flying!

42. A boat ROLLS on a
Pivot Point

43. Roll

44. Roll
A wave lifts one side

45. Roll
Passes below

46. Roll
And picks up the other side

47. Roll

48. Roll

49. Roll

50. Roll

51. Roll

52. Roll

53. Roll

54. Roll

56. Being close to the Pivot Point reduces the motion

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

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

59. Low Center of Gravity

60. Low Center of Gravity

61. Low Center of Gravity

62. Low Center of Gravity

63. Low Center of Gravity

64. Low Center of Gravity

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

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

68. Low Center of Gravity

69. Low Center of Gravity

70. Low Center of Gravity

71. Low Center of Gravity

72. Low Center of Gravity

73. Low Center of Gravity

74. Low Center of Gravity

75. Low Center of Gravity

76. Low Center of Gravity

78. The further from the Pivot Point, the greater the forces

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

80. High Center of Gravity

81. High Center of Gravity

82. High Center of Gravity

83. High Center of Gravity

84. High Center of Gravity

85. High Center of Gravity

86. High Center of Gravity

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

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

90. High Center of Gravity

91. High Center of Gravity

92. High Center of Gravity

93. High Center of Gravity

94. High Center of Gravity

95. High Center of Gravity

96. High Center of Gravity

97. High Center of Gravity
This is pretty uncomfortable

98. High Center of Gravity

99. High Center of Gravity

100. High Center of Gravity

102. Thirdly, a boat TURNS on a Pivot Point

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

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

108. 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.

117. Pivot Points – Recap

118. 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

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

120. Physics of the Propeller
#1 The Prop as a Screw

121. A prop is designed to screw its way through the water

122. 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

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

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

125. 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.

126. 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

127. 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.

128. 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.

129. 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.

130. 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

131. 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

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

133. 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.

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

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

136. 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

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

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

139. 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

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

141. 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.

142. 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.

143. 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.

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

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

146. 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.

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

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

149. 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”

150. Pivoting the Boat

151. Pivoting the Boat

152. Pivoting the Boat

153. Pivoting the Boat

154. Pivoting the Boat

155. Pivoting the Boat

156. Pivoting the Boat

157. Pivoting the Boat

158. Pivoting the Boat

159. Pivoting the Boat

161. 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.

162. 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.

163. Departing a
Tight Fairway
We shift into forward.

164. 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.

165. Departing a
Tight Fairway

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

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

169. Docking on a Side Tie

170. Docking on a Side Tie

171. Docking on a Side Tie

172. Docking on a Side Tie

173. Docking on a Side Tie

174. Docking on a Side Tie

175. Docking on a Side Tie

176. Docking on a Side Tie

177. Docking on a Side Tie

179. Docking on a Tight Side Tie

180. Docking on a Tight Side Tie

181. Docking on a Tight Side Tie

182. Docking on a Tight Side Tie

183. Docking on a Tight Side Tie

184. Docking on a Tight Side Tie

186. Leaving a Tight Side Tie

187. Leaving a Tight Side Tie

188. Leaving a Tight Side Tie

189. Leaving a Tight Side Tie

190. Leaving a Tight Side Tie

191. Leaving a Tight Side Tie

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

193. 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

194. 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

195. Thank You
Thank You!