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    Re: Question on currents and waves
    From: Jeremy C
    Date: 2009 Dec 16, 20:31 EST
    I have seen dramatic changes in wave shape when in and approaching a current.  The effects are especially noticeable when the current is running in an opposite direction as the wind and both are significant.  Of course the relative velocity of each affects the wave shape.  How this observation can be utilized in navigation is much more complicated.  The Gulf Stream near Florida and the English Channel are good examples of this.
     
    Set and drift are a combination of both wind and current, so it is difficult to quantify in a meaningful way the effects of either one separately.  The resultant vector of the wind leeway and the set and drift of current is what we can determine on board the ship.  We can use other environmental factors to try and determine which has the greater affect, but it is certainly not an exact science as practiced at sea.
     
    Since we can measure the wind, we could attempt to see what the effects of this alone are on the ship, and then deduce the effect of current by the differences between the calculated set and drift, and the measured set and drift, but that would be time consuming to say the least and would greatly depend on the specific ship and it's specific speed and heading.  For example, my ship is affected much more by a beam wind at slow speed than one on the stern due to it's construction.  This kind of analysis is not practiced at sea in my experience.
     
    The general rule of thumb is that you need 30 knots of wind to equal the effect of 1 knot of current on a large ship.  In ship handling of large vessels, we are much more concerned with current until the winds start blowing at near gale force, at which point we will factor the effects of wind in greater detail.  This isn't to say that wind is totally neglected, it's just saying that we are typically more concerned with current than wind unless there is a blow.  In a small vessel with very little draft, this rule doesn't apply.
     
    As a matter of course when I am at sea, I observe the current by the surface wave forms to know that I am entering it and am ready to respond if and when the current causes me to yaw, however I don't use the specific wave form to attempt to quantify my response.  The amount of rudder and/or throttle required for correction is based more on instinct and how the ship is moving while actually in the current and not by analyzing the waves and attempting to determine the speed and direction of the current in that particular patch of water.
     
    Jeremy
     
    In a message dated 12/16/2009 6:13:28 P.M. Eastern Standard Time, apacherunner---.com writes:

    I'm trying to track down some information on the detection of currents in the open ocean.    

    I have three sources that discuss the following phenomenon - seasoned navigators are able to tell the set of the current by looking at the shape of waves - they tend to steepen up in the direction of the motion of the current.   So far, I have three sources 1.) A short mention of this in Joshua Slocum's Sailing Alone Around the World, 2.) First hand interview with a Micronesian sailor in The Last Navigator, and 3.) Discussion and interviews by David Lewis in We the Navigators.

    The issue that I'm having some difficulty in understanding how wave shapes could be affected by current in the open ocean.   If everything is in a moving frame of reference together, I wouldn't expect any effect on the shape of waves.   The only explanation I could come up with is that there's a stationary layer not too far under that is creating a kind of drag effect which would cause the waves to steepen up.     

    So - anyone have any information on this phenomenon, or sources that make conjectures about it?   Any other anecdotal experiences with this?

    Thanks!

    John Huth

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