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Couples and Forces

I 've added this chapter because it seems that many people do not understand this.
To my opinion because it is common practice to add, subtract force vectors, speed vectors, all indicated with arrows, and one works with some strange none existing pivot points.
I want to show that it is much easier if one keeps it simple.
Further I noticed that some well sold sailing books do it wrong.


A force is a tendency to move. (that is a difference to the speed or the direction of movement)
A force can be applied by pulling or pushing.
A force spreaded over a surface is called pressure (or lower pressure) A force has a direction. I indicate the direction with an arrow.
Every force has an opposite force. Newton law: Action force = Reaction force.

Example: When I move a table I need a force equal to the friction of the table with the floor.
Other example: When I kick a ball the kicking force is the force needed to accelerate the ball.

The trick is to keep is simple. There are more forces. If I would use them all it would be a mess.
For the example with the ball: A Friction force slowing the ball down, The gravity force, the force due to the higher pressure inside the ball, the force on the ball because the earth spins,etc.
If one considers all these forces when kicking the ball it might be very well possible one hits the wrong goal because on mixed up just one force.
For this reason it is common practice not to draw all forces, I try not to draw them all anyway.

One can add and subtract forces with each other, but it is not simple for someone who did not have much physics.
I feel using your farmers instinct is sufficient.

  • Pulling gently with two man a table has the same result as the pulling of one person full power.
  • Two man faced opposite pushing a table has the same result as nobody pushing.
  • If two people are pushing a table under an angle forward, the table will go straight.
    If two people are pulling the table at a corner, it is the same as one big person pulling the table in the middle.
    In other words:
  • Forces in the same direction can easily be added.
  • Forces in exactly the opposite direction can be easily subtracted.
    The combined force of two forces in a different direction is in a direction between them.
  • Two Forces in the same direction but a bit spaced apart can be replaced by a big force in between them.

But watch out, a direction of force does not always predict the direction of movement:
  • If one pushes against a brick wall, there will be a force, but no movement.
  • If one pushes a toy car under an angle from behind it will still go straight.
  • If one pushes really hard to a truck it will barely move.
  • If one pulls a sled one needs to keep pulling hard to keep going fast.

Couples and Moments

A couple is the tendency to rotate.
It consists of two forces, a force and its opposite force, with distance between them.
The bigger the forces and the bigger the distance between them the bigger the couple.
In other words: Couple = Force X Distance.
Please note that the distance is the shortest distance between the working lines of the forces.
measuring the distance between the arrowheads is in general of no use.

Now we are going to apply this to a sailboat.
If one wants to demonstrate how steering with the sail works the following might help:

  • draw the force perpendicular to the sail at approx 1/3 from the front of the mast
  • Do not start with drawing two sails, or add the forces of the sails into one big one.
  • Always draw the force from the underwatership to the keel in the opposite direction of the sailforce.

At above picture of a sailboat sailing with the wind from the side one can see a small couple left, thus making the ship turn towards the wind.
The ship will sail almost completely ahead, although the sailforce is more directed to the side.
This because the boat is provided with a keel (or centreboard) that is hard to move sideways and is easily to move ahead.
If you do not understand that above boat is going ahead (and will comes up a little) instead of sideways imagine pushing a car at an angle from behind(steering straight).
The car will go straight.
The centreboard (or keel) has the same characteristic as a wheel. easy to go ahead, hard to move leeway.

This is important if you want to understand how a sailboat can go upwind.

Of course this comparison is not perfect, The road will not move, but the water will move giving some sideway slip to the keel.
That sideway slip is the leeway you are making.

When sailing the boat close hauled the boat is making a lot of leeway, because the keel needs to develop a lot of lift in order to balance the sail force
But now continuing with how to steer with your sails.

Steering with the sails.

Below a sketch of a boat with only a mainsail:

  • with the sail sheeted out to far, (only the aft part of the sail catches wind, and thus the sailforce moves aft.
  • with the sail sheeted correct.
  • with the sail sheeted in too far, and stalling (the aft part of the sail is not helping because the flow is separated there, thus the sailforce moves forward)

And this is corresponding with reality (if one does not change the heeling, and does not bend the mast).
Now a boat at different courses with the correct sail sheeting.
You will see that the boat is neutral at courses close to the wind, and with the wind from the aft tends come up.
This is also corresponding with reality.
A rule of the thumb here is "The more the mainsail is sheeted out the more the boat tends to come up." (no heeling and no mastbending)

Now let us take a look at the jib.

you will see that the jib has the tendency to steer the boat away from the wind.
Only with the wind from aft the jib has the tendency to steer upwind
In practice many sailors sheet their jib too far in with the wind from aft, resulting that even here the jib helps bearing away from the wind.

The often heard rule of the thumb "the farther the jib is sheeted in, the more the boat will bear away" is right.
This rule often overrules the "The more the mainsail is sheeted out the more the boat tends to come up." rule, because when one lets go the main a little, the sailforce of the mains becomes less, and is overruled by the sailforce of the jib.
In other words: if one lets go the main, the effect of the jib is left.

Some people think that making the jib bigger will result in a ship steering naturally away from the wind.
Often the reverse is true, because the extra area aft of the jib will have the tendency to make the boat come up.
A big genoa is probably balanced somewhere with the wind from the side.

Steering using heel.

Below you will find a topview of a sailboat.
On the left she heels towards the wind, on the right she heels away from the wind.
The sheet is locked.

Clearly shown is that the left boat tends to bear away, and the right boat tends to come up.
This because the sailforce is moved out if the heeling increases.
Nothing more to explain here.

Than I have some comments on some books about sailing:
"If the sailforce is in the side view inline with the lateral force the boat is balanced" THIS IS WRONG, see the picture below.

One can see clearly in the top view a well balanced boat, while from the sideview this can not be concluded.
Please never use sideviews to explain steering with sails.

Extra resistance due to excessive rudder use.

If one does not touch the rudder it does not have a rudder force.
If one steers than the rudder deflects the water and a steering force is generated.
If one steers a littlebit this steering force is more or less directed to the side, if you steer a lot the steering force is directed also aft, slowing the boat down.

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