Yachting Monthly - April 2016

(Elle) #1

PRACTICAL SEAMANSHIP


PHOTOS: COLIN WORK


GRAPHICS: MAXINE HEATH

There are two common types of fog: advection fog, caused
by warm moist air blowing over cold water, and radiation fog,
caused by air that is cooled by the land, reaches its dew point and
condenses as fog before drifting down to the sea.
Advection fog usually occurs earlier in the season when water
temperatures are lower, and southwesterly winds bring in warm,
moist air. Advection fog hugs the sea in winds of up to Force 4,
above which it lifts to become a blanket of low cloud. As the
causes won’t change either until there is a change of air mass
such as a cold front, advection fog can last a while.
The good news is that radiation fog forms over land, so it clears
as you head out to sea. Also, it is associated with high pressure so
the sun normally burns it off, giving a pleasant afternoon.

Warm,
moist air

ADVECTION FOG

RADIATION FOG Heat

Which kind of fog?


The water is warm, moist
air condenses when its
temperature is lowered
to the air’s dew point by
cooler water, creating a
low bank of patchy fog

Land loses heat quickly
at night, via radiation.
The cold ground cools
the air to its dew point
and a shallow layer of fog
forms, which then slides
downhill to the coast

With the plotter below, I used an iPad to check our position while
motoring down the shallower Boat Channel, free of shipping surprises

An eerie shadow
emerges from the fog,
and it turns out to be
the tall ship Kaskelot,
with a lookout posted
on the bowsprit

APRIL 2016 http://www.yachtingmonthly.com 33

iPad on deck – not a wet weather
option but it kept me on deck and
I think was safer than being below.
Radiation fog can be incredibly
thick but it is often patchy
so if you are on deck when it
lifts slightly you can pick up
valuable clues to your position,
such as buoys, and choose the
right moment to cross the main
channel: the kind of clues you
would miss staring at instruments
below. A really important
instrument visible from the helm
position is the depth sounder.
This is critical in Poole, which has
shallow mud banks outside each
of its many channels and we were
leaving on an ebb tide.

A ghost in the mist
We found our way out of Poole
steering courses from buoy
to buoy, using the small boat
channel to avoid the ships and
crossing over the channel when
the visibility improved slightly. We
listened carefully for foghorns.
It was particularly thick when
we reached the entrance of the
harbour, where a large chain ferry
can be diffi cult to negotiate even
in good conditions. I was pretty
anxious that it might set off and
clatter its way towards us so
quickly that we would be unable
to avoid the chain, so I cautiously
kept in the middle of the channel
and listened out for it, unsure
which side it was.
At that moment there was a

foghorn ahead. A large vessel
was coming into Poole. As soon
as we were past the ferry we
steered quickly over to starboard,
the thinking being that, if you
encounter anything moving, a
starboard turn is generally the
least hazardous option. And
it was, because out of the fog
appeared the bowsprit of the
tall ship Kaskelot, bearing a
gesticulating crewman. She was a
stunning and slightly eerie sight.
Once outside the harbour
the small boat channel gave
no problems in the improving
visibility and we enjoyed the rest
of the trip in fi ne sunshine. We
arrived at Portland marina in good
time and it seemed unbelievable
that the fi rst two miles had been
such hard work! W
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