501 Heeling Dynamics

In this Module we will discuss the dynamics of Heeling.  It's a subject that has been brought up by one of our viewers and as there is much debate about the verious effects of Heeling, I thought it would interesting to take a look at some of the issues involved.
 
We will first look at the ideal situation where the sailboat is upright and moving forward.  See Sailing 201 for a review of the basics of wind power.
 
 
We would want to actually examine the Sail Surface Area and the Keel Surface area but an analysis of the verticle dimensions will yeald roughly simular results.
 
The other factor that we will ignore is the drag caused by hull contact with water.  We will assume that it is the same whether Heeled or Upright.
 
We will use a Sail Height = 25 ft. and a Keel Length = 4.5 ft.
 
 
Let's look at the ratio of Keel Length:Sail Height in our Ideal situation.
 
Keel Ideal   =  4.5ft    = 0.18
Sail Ideal        25 ft
 
 
Now lets look at the ratio in the Heeled Condition
 
Keel Heeled   =  3.7 ft    = 0.18
Sail Heeled        20.5 ft
 
As expected they are the same.  What this means is that we should not be slipping sideways because we are heeled over, however, what actually causes a boat to Heel is an increase in wind velosity.
If we assume the wind velosity remains constant we can calculate the 
efficiency of the surfaces.
 
Let's look at the surfaces presented to the wind and the water.
 
We have reduced the amount of sail surface area catching the wind by a factor of:
 
Sail Heeled   =  20.5 ft    = .82 or 82% of Sail presented to the wind.
 Sail Ideal          25 ft    
                                                                                 
 
 And we have reduced the amount of Keel surface area resistance by a factor of:
 
Keel Heeled   =  3.7 ft    = .82 or 82% of Keel exposed to the    
Keel Ideal          4.5 ft           water resistance.
 
Now let's assume that the wind force has picked up causing us to heel over.  Let's further assume that the wind force is enought to make our sailboat act as though it still had the same amount of sail surface area presented as in the ideal conditions however the resistance presented by the water must remain the same for the heeled over condition.  We will look at this new ratio:
 
Keel Heeled   =  3.7 ft    = 0.148
Sail Ideal          25 ft
 
Compared with the ratio:
 
Keel Ideal   =  4.5ft    = 0.18
Sail Ideal        25 ft
 
We can see a decrease in the ratio.  This would indicate that sideways slipping will be taking place.  Sideways slipping is called Lee Way.
 
Things That Affect Forward Movement
 
The wind velosity to sail area ratio.
 
Wind Spill slows a vessel down.
 
Sideways Slipping, Lee Way, slows a vessel down.
 
Degree of Heeling affects the efficency of the Keel as well as changing the Hull drag through the water.
 
Surface area of the Rudder adds to water resistance and would decrease Lee Way.
 
Hull Chine type:  Round vs Soft vs Hard Chines.  Generally the harder the chine the more surface area of hull that is in the water during a Heel and there for there would be more drag on the boat, however, a hard chine has more resistance to Heeling and one could argue that since it keeps the vessel more upright you might gain velosity.
 
And then there is that darn issue of Hull Speed throwing a monkey wrench in the works.  See 401 Sailing for a discussion of Hull Speed.
 
Conclusions
                                                                                                                                                                    
 As you can see, Sail dynamics are complicated and the resultant velosities associated with them are complex as well.  And Sailors, well, we love to argue...so maybe...this discussion belongs in the Rum Barrel instead of the classroom!!!
 
 
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