any boat put in the water will displace water. the water displaced will
about equal the gross weight of the boat. In my case that would be 215
lbs. That means I am going to have to move 215 lbs of water aside as i
travel.
that is weight of me, the kayak, paddle, safety equipment , etc.
The trick is to move as little water as slowly as possible and move it
a minimum distance. As the water returns behind the boat that as well
should be done as smoothly and slowly as possible.
the cross section of the centre of the boat dictates how far you move
the water, capella for example is 21 inches but some seaward options
have that down to 19 inches.
here is where length comes in. if a boat is 19 feet long and has a beam
of 19 inches like the seaward might it will retain the same
displacement as a much shorter kayak .
the shorter kayak, we will use a smaller sea kayak of 15 feet and a 25
inch beam. The cross section is wider so the water being moved out of
the way has farther to move. our shorter boat may draw more water ( it
may be deaper ) so it pushes more water down as well as to the sides .
this water is moved much faster as the boat keeps that speed. the water
has to move to accomodate the 24 inch beam and it only has 15 feet to
get there and back.
The displacement remains the same for both boats.
The longer boat moves 215 lbs of water a much shorter distance and it
takes more time to do it. normally this makes a longer boat inherantly
faster. ( Length applied being water line in this case.)
To atain the greatest speed the boat must move the water out of its way
slowly ( longer boats can do this ) in a uniform way to the widest part
of the boat where the water should be lead gently following the line of
the boat to the stern causing as little drag as possible. any undue
variations on a gentle curve will cost effort due to drag.
a smooth surface on the hull of the boat that has little or no flex
will allow the water to flow around it quickly with a minimum of drag.
The boat should be as thin and draw as little water as possible.
a kayak unfortunately has to accomodate a human bum. i can squeeze into
a 19 inch wide kayak and make it move rather well.
as we rarely paddle on glass like water an upswept bow as found on most
kayaks is an advantage in waves . if a boat powers through a wave the
cross section presentation now will include more of the hull, perhaps
the deck and in last nights paddel the paddler as well. that is why
most boats have an overhang on the bow and that beautifull shear line.
speed is retained in rougher conditions.
the surface of the boat will have some effect on how it moves. A hull
that causes little or no friction will help it move a little faster.
Ocean racers will scrub and polish the bottom of sail boats and should
something attach during a race they will work at getting it off,
plastic boats with fuzzies will prove a little slower than say a fibre
glass boat with a glossy polish.
The boats weight will effect speed as it will reduce the displacement.
the original inuit kayaks and todays greenland boats seem to have the
recipe , the hull design took a curve that was as near as possible to a
streight line from the bow to the cockpit then from the cockpit to the
stern. very smooth.
all this assumes the same effort is being applied to move the boat
forward or that effort is metered and applied to the equation..
This is the truth as I see it , there are anomolies like surfing with a
hull that planes but for the most part this is to my mind a rational
description of kayaks and hull speed.
fiona stirling.
Does this seem wrong to anyone?