T&T: Boating Mystery....SOLVED!

[John Baker]

I did not read all of the posts regarding this thread so forgive me if someone offered this answer.  I will also apologize for my lack of perfectly nautical proper terms.
   
  We all know if the current is in our "face" we will suffer a groundspeed loss just like we all know if it is from astern we will be treated to a nice little boost in groundspeed.  You gotta think of what happens when it is on the beam.  Let's assume a "thru-the-water speed of 7kts.  If you are planning a trip and there is a direct cross current between point A and point B, do you think your groundspeed is going to equal your thru-the-water speed.  NO is the proper answer.  Some energy(velocity would be the proper word here) will be "wasted" just trying to counter the current.  So your groundspeed may be 6.x knots because of the cross current.  This could even happen with a current that is aft of the beam if it is a strong one....making you think that you have been "robbed" of a favorable current.  You still need energy to counter the current....energy that will not be applied to forward progress.
   
  An ampified example is lets assume the same 7kt boat going across a river with a 5 kt current.  We also have to assume our goal would be a landing zone directly across the river so therefore we would have to mainatain a certain COG.  This boat would be doing a LOT less than 7kts SOG if the proper HEADING is steered to maintain the proper COURSE because much energy is lost just maintaining the course.
   
  Seeing how the current can hamper our progress  on 3 out of 4 points of sail, I'd say the "75% rule" is pretty darn close.
   
  Make sense???
   
  Trawler on,
  John
   
  PS...I say this after years of watching this same phenomenon on aircraft with pretty decent computers to tell you how much is head/tail wind compnent and how much is crosswind component.  A strong direct crosswind will cost you of groundspeed.

 
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