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US POWERBOATING   Course to Steer Calculator
 

This calculator is designed to calculate the Course to Steer 
when a boat is sailing across, against or with a current.   Not all courses are possible and some are impractical since the boat will not make significant headway.
  
The boat's speed as read on the GPS is a combination of both the current and boat power, as read on the boat's tachometer or knotmeter (the instrument on a boat which measures distance and is used to compute speed is called a "log" ) is reflective of the boat's power (it's speed in relationship to the water).  This latter value is used in field #2 of the calculator.

The effects of current must be accounted for in order to maintain the desired heading.  If a boat is traveling 6 knots in relationship to the water and is sailing against a 3 knot current, 45 degrees off the bow, the current will push the boat 28.7 degrees off the desired course (course to make good) with a speed of advance of 4.4 knots.    Go To:  Current Deviation Calculator.  To maintain the proper heading one will have to sail into the current 20.7 degrees resulting in a predicted speed of advance of 3.5 knots. (See the below calculator)
    

 
Course to Steer Calculator
               
1.  Enter the Speed in knots of the current
  

2.  Enter the speed in knots of the boat in
relationship to the water as determined
by the boat's knotmeter: 

3:  Enter the of the direction of current
(angle CM) to the Course to Make Good (Destination being 0 degrees -
Origin is 180 deg)


  
 

Where CM > =  90 degrees.
( Sailing With the Current )
The Power Vector Must Be => X

P = 180 - CM
CM1 = 180 - ( 90 + Power )
x = sine ( CM ) *  Current
CM2 = arc cosine ( x / Power )
CM = CM1 + CM2

Course to Steer (CS) =
180 - ( CM2 + 90)

Speed Advanced =
(p*sine(CS2)) + (c*sine(CS1))
  

    
Where CM <  90 degrees.
( Sailing Into the Current )
The Power Vector Must Be >
Current Vector

x = sine ( CM ) *  Current

Course to Steer (CS) =
arc sine ( x / Power )

CM = arc cosine ( x / Power )
CM1 = arc cosine ( x / Current )

Speed Advanced = 
(sine(CM)*p)-(sine(CM1)*c)
  


The values determined from this calculator should be used for educational purposes only.  This calculator should not be used in determining the suitability of purchase, charter or passage on a boat.  Instead the manufacturer should be contacted to obtain the boat's characteristics.  The results of this calculator should not be relied upon for navigational purposes.  US SAILING and its members assume no liability for the information presented on this website.  The user is responsible to check the validity of all information.
 


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