there is some question regarding boat speed ( optimum hull speed ) and
boat length. this was initiated by a simple question with what I would
have thought would have been a generalized but simple reply.
i have a decent understanding of the theory and practical dynamics
involved.
i will wade in with a pretty simple set of dynamics when I get a chance
..
this should not run a slight to anyone or any kayak as they all have a
purpose, a clientele, and a place.
semantics and sniping at individuals will only deminish what should be
a civil comparison of notes.
for speed and efficiency i think the Inuit had it about right.
go at it .
fiona stirling.

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?

On 21-Jun-2005, "Fiona Stirling" wrote:

Does this seem wrong to anyone

Not bad, non-technical description. A more technical description
from an expert is at:

http://www.greenval.com/shape_part1.html

Mike

Thanks... gives me something to think about. I like the conceptual
approach, as it gives me something to visualize as I paddle along.

Great website! Informative and also very readable..

That's a pretty good synopsis, but you forgot a couple of things, the
effect of wavemaking drag (and the attendant "theoretical hull speed")
and the effect of cross section shap.

While we can argue whether the standard calculation (1.34 x square root
of the waterline length) applies to kayaks, there is no debate that
wavemaking drag becomes the major source of drag on all kayaks at some
speed. The shorter the boat, the the lower the speed at which it becomes
an important factor. With short recreational boats, this can become a
factor at speeds as low as 3 MPH.

As for cross section shape, a cylindrical hull has the least surface
area for a given amount of displacement and therefore the lowest surface
drag. A square cross section hull has the most surface area and drag.
Most kayaks are somewhere in-between these extremes.

you are right brian wave making drag was not mentioned because the
basic design described minimizes that. it remains an important issue
that was dealt with 6 or 8,000 years ago by the uinuit.
again on cross section shape you are exactly correct but perhaps not
realy right. a cylindrical hull is the fastest. a pattent was done on a
yacht in about 1963 titled archuform design. that silly little boat was
finished and raced very successfully. the problem with the cylinder in
a kayak is stability, most boats ( kayaks around here ) have a shallow
v design that allows for longditudinal stability, ( tracking )
Wave making issues are no stranger to a kayak, if you want an example
just paddle over a shallow that is about 6 inches deaper than your
draght, if you paddle in at a steady 3 or 4 knots it will slow you down
to the point you are wondering which friend just grabbed your boat.
that is where the wave action is most apparent.
standard calculation???? it is a little thin as so many variables in
design kick in before you can rely on a simple number swap.


brian i have not been on the news group for some time. i was in the uk.
if you remember my friend from our o.t . conversations ( iraq ) she and
hed daughter and son are in england hosted by a church. the little girl
is quite beautifull now and they will be living there unless i can
convince her to come to canada..

Michael Daly wrote:
On 21-Jun-2005, "Fiona Stirling" wrote:

Does this seem wrong to anyone

Not bad, non-technical description. A more technical description
from an expert is at:

http://www.greenval.com/shape_part1.html

Mike
great site. some details i would like to talk to the author about but
it seems quite accurate.
i have a bsc. but not in naval archutecture ( or spelling ) i have been
around boat designers and builders up untill i came to canada a couple
of years ago.
one small detail about things the author thought , kayaks do plane,
well partially anyway. have you ever surfed? how about 10 knots next to
a much shorter white water boat. i will grant you when i surf it is
normally not that far and fast but i did 18 km in my capella and kept
it up for a good distance.
i try to surf when i can with a gps on deck, it is fun.
thank you michael.
fiona

"Fiona Stirling" wrote in
ups.com:



Michael Daly wrote:
On 21-Jun-2005, "Fiona Stirling" wrote:

Does this seem wrong to anyone

Not bad, non-technical description. A more technical description
from an expert is at:

http://www.greenval.com/shape_part1.html

Mike
great site. some details i would like to talk to the author about but
it seems quite accurate.

You might try going to the Paddlewise mailing list (www.paddlewise.net for
more info). John, and at least a couple of other boat designers are
regulars there. John is an excellent writer but sometimes when he and the
two others start discussing design the general many in the general audience
will have their eyes glaze over.

Fiona Stirling wrote:
you are right brian wave making drag was not mentioned because the
basic design described minimizes that. it remains an important issue
that was dealt with 6 or 8,000 years ago by the uinuit.

I'm not quite sure what you mean by that. If you mean that it's less of
an issue for long, narrow boats, I agree.

again on cross section shape you are exactly correct but perhaps not
realy right. a cylindrical hull is the fastest. a pattent was done on a
yacht in about 1963 titled archuform design. that silly little boat was
finished and raced very successfully. the problem with the cylinder in
a kayak is stability,

If taken to an extreme, such as in racing boats, it can be a problem.
However, if you review the following article...

http://www.guillemot-kayaks.com/Desi...tyArticle.html

....you'll see that it's entirely possible to have cylindrical hull kayak
that's quite stable.

most boats ( kayaks around here ) have a shallow
v design that allows for longditudinal stability, ( tracking )

Sure.

Wave making issues are no stranger to a kayak, if you want an example
just paddle over a shallow that is about 6 inches deaper than your
draght, if you paddle in at a steady 3 or 4 knots it will slow you down
to the point you are wondering which friend just grabbed your boat.
that is where the wave action is most apparent.

That and when paddling at top speed. Jump in a Pintail sometime and try
pushing it beyond 4 knots or so on open water and you'll see/feel the
effects of wavemaking drag, bigtime.

standard calculation???? it is a little thin as so many variables in
design kick in before you can rely on a simple number swap.

True.

brian i have not been on the news group for some time. i was in the uk.
if you remember my friend from our o.t . conversations ( iraq ) she and
hed daughter and son are in england hosted by a church. the little girl
is quite beautifull now and they will be living there unless i can
convince her to come to canada..

I vaguely remember the conversation. I'm glad that your friend is happy,
wherever she's living.