Re: [Paddlewise] Reading currents

Black Coffee 2002"<black-coffee2002_at_karavshin.org> wrote:

> I looked at the Hydrophic Department charts
> Singapore publishes of the area and they don't indicate any currents on the
> chart, but perhaps they're marginal enough here for most vessels to neither
> notice nor care.  From my readings of Fundamentals of Kayak Navigation
> (David Burch) it seems that relation between current flow and the tidal
> pattern are not as obvious as you'd think.

> Any good advice on seeing/reading tides and currents beside from just
> knowing the tidal schedule?  In my case, I have a relevant tide station 
> very
> near my paddling area:
> http://www.mobilegeographics.com:81/locations/5729.html

> I was there in the early/mid afternoon, which based on this chart for today
> (one day later) it seems that is one of the steepest changes on the tidal
> chart, so I presume I was experiencing strong tidal flow.

BC,

You are on the right track.  Indeed, Burch identifies a couple of oddball 
current/tide relationships, but almost all the time the tide follows the 
current ... meaning flooding precedes a high tide, and ebbing precedes a low 
tide, more or less, unless there is a river estuary involved.

The problem is that the "flood" current moves from a tide station which has a 
higher tide to one with a lower tide level, as a first approximation.  So, in 
order to guess at the current in an open passage (as contrasted with a 
passage that has only one end open to the sea) from tide information, you 
need __two__ tide stations.  If you don't have that in your area, then about 
all you can do is establish empirically which direction the flood usually 
runs, and then key it to the high tide at the one station you have.  You can 
even construct your own rough empirical relationship between the difference 
between these two stations and the current velocity, if you can measure the 
velocity at times when you know the difference (see hint below).

A couple caveats:

1. The layout of the passage where you experienced this current, so the 
geography (and bathymetry) will also play a role.

2. In passages where the maximum currents are less than a knot, there is 
often no note on the chart, because a current this weak is of little 
consequence to a freighter or larger powered vessel.  But, a beam current of 
one knot will set a paddler of hull speed 3 knots about one sea mile every 
hour ... a pretty large error if you are crossing a 3 sea mile passage at 
right angles to the current.

And one hint:  if you read Burch carefully, he gives you some angle 
measurements you can make with your deck compass to get a quantitative 
indication of the current magnitude.  Yes, you do need a deck compass!

--
Dave Kruger
Astoria, OR 
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