NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Question on currents and waves
From: Jeremy C
Date: 2009 Dec 16, 20:31 EST
--
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To , email NavList+@fer3.com
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@gmail.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--
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To , email NavList+@fer3.com
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To , email NavList+@fer3.com