Skipper’s Tip – Boat Handling Under Power (Part I)

– By Tradewinds instructor Don Gilzean

I think the single most embarrassing time I ever experienced (yes, I have had a few) on a sailboat was turning a 27’ boat sideways in a slip at Angel Island.  I was brand new to sailing and managed to get out of that one without damage to anything but my pride.  To this day, I am thankful the owner of the very nice, very new, very big boat next to me, was on board his boat, knew how to fend off, and did not have any heart problems.   The experience had one major positive effect; it encouraged me to gain a better understanding of how and why a boat works under power.

This is the first of two or three Skipper’s Tips on the topic of Boat Handling Under Power.  In them, you will find a lot of theory; however, I promise that if you hang in, there will be some practical exercises along the way.  I also promise that if you take the time to do the exercises your will learn an unbelievable amount about docking and motoring, if you take the time to do them, and keep very  good notes about the results.

So, to get started, there are a multitude of forces acting upon a boat.  In these tips, we are going to look at six of them, and how to use them to improve your docking and motoring skills.  The six are Wind, Current, Momentum, Inertia, Current Discharge (prop wash) and Unequal Blade Thrust (prop walk).  Part I looks at the first four, leaving “the propeller” as a separate topic.

Wind & Current

By the time you graduate from BKB, hopefully you have a pretty good idea of what wind and current are going to do to your boat.  Assuming there is nothing else preventing it, you will pretty much move with the current.  The wind blows you to leeward with the bow moving faster to leeward than the stern.  How much faster?  Every boat is different.  Try this sometime.

With 10 or 15 knots of wind coming from the SW, tie up on the leeward side of the D Dock pump out station.  Instead of motoring off of the dock, cast of your dock lines, and let the boat drift away.  How fast are you moving?  How fast is the boat turning down wind?  Now, ask yourself how you could stop that turn from happening.  Here are a couple of hints … a line on the bow will control how fast and how far the bow moves … or some “discharge current” (a bit later in the tip) might work.  At what point do you have enough speed to give you rudder control?

Now head directly into that same wind at 1 knot of boat speed.  Shift into neutral and coast to a stop.  Use the rudder as little as possible, but, keep the boat headed directly into the wind.  How long did it take to stop?  How about at 2 knots of boat speed?  Turn the boat around and head directly downwind.  Hint … you will not stop until the wind dies, or you run into something.

Those two exercises should give you a pretty good idea of how your boat will react to just wind and current, and how wind and current can be used to overcome our next topic, momentum and inertia.

Momentum & Inertia

Yes, there is a difference, but not so much you need to worry too much about it.  At this point, just know … momentum is mechanics … inertia is physics … momentum is motion, inertia is a resistance to change requiring energy (power) to overcome.  In the previous exercises, we looked at using wind to overcome the inertia of a boat resting against a dock and moving into the wind.  Instead of looking at momentum and inertia separately, let’s look at their net results.  If your boat is at a stop it is going to stay stopped until something makes it move (could be the wind as above), or it could be the application of power through the propeller.  A boat that is moving will stay moving until energy (drag, friction, wind resistance, propeller) brings it to a stop.  A boat moving in a straight line wants to keep going in a straight line even though you want it to turn.  These next two exercises work best with no wind or current, and should have something to judge distance by.  Like the Marina Bay practice buoys.

Get the boat moving forward at a nice controlled speed (1 or 2 knots) shift into reverse and apply a controlled amount of power.  Measure the time and distance it took to stop.  Now, do the same thing in reverse.  Use the same speed and stopping power.  Compare the two.  In most cases, stopping while backing requires substantially less time and distance.

For the next exercise, get forward motion with good steerage way … like the speed you would have in a fairway approaching a slip only in safe water.  Make a hard turn to starboard; marking the point you put the rudder over.  Stop your turn when you have made 90 degrees.  For example, start on a heading of 000, and turn to 090.  Mark your point.   Chances are you will have traveled further along the 000 course line (technically known as advance) than along the 090 course line (known as transfer).  Momentum and inertia keep you going in the initial direction of travel while the boat is turning.  Repeat the exercise turning to port, and at various speeds (e.g. 1 knots, 2 knots, & 3 knots).  Try to use the same amount of rudder each time.

These two exercises can literally keep you busy for hours.  Not that it’s sooo much fun, there are just so many variations to be done … such as direction of turn, speed, amount of rudder, and wind speed.  What is the benefit of doing it?  Have you ever watched someone approaching a slip turn the wheel once and set it, and the boat magically turns into the slip without touching the sides, sliding to a stop in an upwind slip with little or no reverse power?  You can only do that if you know the correct speed to result in exactly the amount of advance needed for the transfer required to get into the slip.

Once you have these four exercises down in safe water, start to apply the lessons in closer quarters maneuvering … such as docking in a slip.  Good luck and we will see you next time for a discussion of current discharge and unequal blade thrust.

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