"Michael Daly" ) writes:
On 28-May-2004, (William R. Watt) wrote:


Sorry it's not an inverse relation.

??? The more you say about small craft hydrodynamics, the more I realize
how little you understand.

Skin friction increases with length, while wave-making resistance decreases
with length. Minimum total resistance is where the sum of the two is minimum.

Skin friction does not increase monotonically with length. It does
increase monotonically with the product of surface smoothness and wetted
surface area. Wetted surface area in a dead calm is a function of
displacement and hull shape but not a well-defined mathematical function,
and it is complicated by waves. There are quite a few indicators of hull
shape such as length-to-beam ratio, block coefficient, prismatic
coefficient, and girth.

Brian didn't provide any data.

He said 2% or so.

A percentage is not data. A percentage is a calculated number.

You have no such data that I've seen. Where do you have the data that indicates
how much resistance is due to scratches?


And where do you get that data, in your dreams?

"The Shape of the Canoe" by John Winters.

the hull resistance data on my website is from Winters' website. where he
got the data I do not know. at one time he was collaborating with a
university in Australia. the fellow at the university has posted in this
newsgroup. Its been a few years and I can't recall the university. the
"Winters" data is very general. I would like to see data specific to sea
kayaks. the wind and wave resistance data is from a book on sea kayak
cruising. it also appears to me to be pretty general.

you are not basing your arugment on data, but on your impressions.
I am basing my argument on actual data which I admit is pretty general.

I know what you are trying to express, that within a narrow range
there is a local optimum.
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On 30-May-2004, (William R. Watt) wrote:

Skin friction does not increase monotonically with length. It does
increase monotonically with the product of surface smoothness and wetted
surface area

If surface smoothness is constant, then skin friction increases monotonically
with wetted surface area. Why would you assume that surface smoothness is
variable among kayaks? For a constant displacement, wetted surface area
changes directly with length. Why complicate things?

Winters' website

His website has been gone for quite a while. I have no way to verify your
interpretations of what he wrote there.

I would like to see data specific to sea
kayaks. the wind and wave resistance data is from a book on sea kayak
cruising. it also appears to me to be pretty general.

In his book, he specifically states that there is no significant difference
in the behavior of canoe and kayak hulls. They come in the same range of
lengths and vary only in width. Most of the other parameters, such as block
and prismatic coefficients are in the same ballpark as well.

Most data is from smooth water conditions. There isn't much data on kayaks
in cruising conditions - that gets really complicated from the standpoint of
analysis, since kayaks operate in rough conditions where simple analysis
breaks down. However, it seems like most decent smooth water designs will
perform adequately in rough conditions, while a poor design in smooth
water is poor in rough. Most kayaks that perform well in smooth water but not
in rough suffer from other problems, such as weathercocking and tracking, not
so much from resistance to forward motion.

Mike

"Michael Daly" ) writes:

..For a constant displacement, wetted surface area
changes directly with length. Why complicate things?

because it's not true

In his book, he specifically states that there is no significant difference
in the behavior of canoe and kayak hulls. They come in the same range of
lengths and vary only in width. Most of the other parameters, such as block
and prismatic coefficients are in the same ballpark as well.

yes, compared to oil tankers, canoes and kayaks are in the same ballpark.
but compared to each other and relative to the low power source, canoes
and kayaks differ in performance. in my experience kayaks are faster than
canoes, their main attraction as far as I can tell. a lot of money goes
into advertising the differences among canoes and kayaks. I would like to
see more precise data. I suspect the subject has been studied by, say,
Soviet sports scientists, but they are not sharing their data.




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On 31-May-2004, (William R. Watt) wrote:

..For a constant displacement, wetted surface area
changes directly with length. Why complicate things?

because it's not true

I give up.

I suspect the subject has been studied by, say,
Soviet sports scientists, but they are not sharing their data.

There are lots of studies that have been published on racing canoes
and kayaks (Olympic class), but you'd probably question their relevance
to recreational canoes.

Mike

Winters' current website is www.greenval.com. The graph of resistance vs
speed is gone. He now has resistance vs speed-to-length ratio which is a
way to include length in the graph. From a quick look today it's still not
clear whether the data is for canoes or kayaks. I'll try to get back to
the library and copy some data points, also see if I can copy the
greatlakeskayaker data. It may be a while before I get around to redoing
the calculations with this data. In the meantime the best we have is a 7%
increase in effort due to hull scratches.




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On 31-May-2004, (William R. Watt) wrote:

also see if I can copy the greatlakeskayaker data.

That's my website and the data was taken from Sea Kayaker magazine
(Kaper results) or from:

http://www.unold.dk/paddling/articles/kayakvelocity.html

which appears to be from SK's Broze/Taylor results. Kaper is John
Winter's old resistance program and has a factor for plastic kayaks
among other things. It's now a public domain algorithm and John
told me he no longer uses it, since a commercial product (can't
remember the name) is more useful for him.

BTW, the following figure shows what I explained in a previous post
but which you claimed was not correct.

http://www.greenval.com/fig3_1.gif

Mike

I found what I was looking for at www.greenval.com/winters.html.
Writing about canoe design for frictional resistance John Winter says ...

"A 5% decrese in wetted surface is worth bragging about, but a
single year's scratches and banging can easily double coefficient
of friction from 0.004 on a new fibreglass canoe to 0.008. This
more than offsets the designer's efforts. The cavalier attitude of
most canoeists towards their boats is evidence that a 50%
resistance increase is not often noticed if only because the onset
of its effect is so gradual."

Earlier I wrote in this online discussion that paddling in a group
would require extra effort to keep up with other members who were
in similar boats with smooth hulls. I only assumed a 10% increase
in frictional resistance. Winters implies a 50% increase is not
unusual. I used performance data from Winters' former website. All
Winters data applies to canoes (at one point he mentions a
"typical" 16 foot canoe) and is provided to illustrate the
principles he is writing about. Its not specific to any boat,
particularly not kayaks.

I was kicked off the computer at the public library after an hour,
but not before taking a look at the kayak data provided by Mike
Daly at http://www.greatlakeskayaker.ca/spee...anceGraphs.htm. I
found the graph very interesting. I've copied down the numbers and
would like to replace the resistance in pounds by the effort in
horsepower when I get a free moment. Of the 5 kayaks, the
Endurance 18 and the Arctic Hawk are equivalent and fastest. I
don't know if they are the same length. However the Nordkapp H20
and the Solstice GT are equivalent and second fastest even though
the Nordkapp is 2 ft longer than the Endurance (if I'm
interpreting the names correctly). Up to a speed of 4 knots all
four of these kayaks are equivalent. The two pairs only begin to
diverge at speeds over 4 knots. The remaining kayak, Sonoma, is
the slowest. Its length is unknown. There is an error in the data
for the Sonoma at the fastest speed, revealed by a sudden change
in its graph. The slowest boat is one for which John Winters
suppled the data and I'm sure it is for a canoe, not a kayak, as
all the Winters data I've seen is for canoes.

Even though the boats I currently paddle are only cheap home made
experimental plywood boats I'm careful not to treat them roughly
and get the hulls scratched and gouged. That is why I was so
disgusted to see the condition of the used rental boats offered at
a recent sale here.

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wait just a darned minute, are you saying the data you presented is not
test data but is calculated from dimensions using Winter's KAPER model?
that's not data. sorry, it doesn't count as data. it doesn't support your
case. I've used Winters' KAEPER model on one of my own boats for fun btu
it's nto measured data, just calculated numbers. I also calculate numbers
with two hull design programs but they are not the same as measurements
from actual in the water tests.

Michael Daly" ) writes:

That's my website and the data was taken from Sea Kayaker magazine
(Kaper results) or from:

http://www.unold.dk/paddling/articles/kayakvelocity.html

which appears to be from SK's Broze/Taylor results. Kaper is John
Winter's old resistance program and has a factor for plastic kayaks
among other things. It's now a public domain algorithm and John
told me he no longer uses it, since a commercial product (can't
remember the name) is more useful for him.

BTW, the following figure shows what I explained in a previous post
but which you claimed was not correct.

http://www.greenval.com/fig3_1.gif

I've seen it. I'm familiar with it. It does not.



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William R. Watt wrote:
in my experience kayaks are faster than
canoes, their main attraction as far as I can tell.

While this is certainly what attracts some kayakers, I'd say the main
attractions vs. canoes are the kayaks inherent seaworthiness and it's
ability to handle a broad rain of weather and water conditions with
aplomb. Kayaks make paddling in rough condition not only possible, but
fun. I don't see many canoeists paddling in 3'-4'+ seas or 15-20+ knot
winds (actually, I've never seen any), but it's a blast in a kayak. The
same boat can be used for a quiet, relaxing cruise around an estuary to
snap a few nature photos. Although I've seen photos of canoeists playing
in surf, I've never seen anyone do it, but we do it all the time in kayaks.

Of course, I'm talking about sea kayaks, rather than recreational or
whitewater boats. I'm also leaving out the class of boats like the
Kruger "canoes", which are canoes in name only and have more in common
with kayaks.

Brian Nystrom ) writes:

While this is certainly what attracts some kayakers, I'd say the main
attractions vs. canoes are the kayaks inherent seaworthiness and it's
ability to handle a broad rain of weather and water conditions with
aplomb.

I'd have to agree that the watertightness of a kayak is its second major
attraction, second only because all kayaks are faster than canoes while
not all kayaks are acquired for watertightness. It would be interesting to
know what proportion of kayak paddlers use spray skirts.

Of course, I'm talking about sea kayaks, rather than recreational or
whitewater boats. I'm also leaving out the class of boats like the
Kruger "canoes", which are canoes in name only and have more in common
with kayaks.

I'm of the opinion that if its paddled with a double bladed paddle, kayak
stlye, then it's a kayak. That includes undecked open "canoes" like the
Rushton Wee Lassie and excludes decked white water and sailing canoes.
It's the paddle, not the deck. One canoe club that TF Jones mentions has
that as a rule in their club races. You can't enter a canoe race with a
kayak paddle. If you want to use a kayak paddle you race with the kayaks.



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