On 16-Jun-2005, Peter wrote:

boats that are 18' long overall will almost
always have waterline lengths greater than boats that are 14'

Fine, but we were comparing kayaks that were only a foot and a half or
so different in length. Of the 105 kayaks on the web page of Sea
Kayaker data, the average length is 5.2m (17 ft) with a standard deviation
of 41cm (16 in). 78% of the kayaks fall within one standard deviation of
the mean length. We're not talking about huge differences in length
typically, especially since the standard deviation is comparable to the
differences in LOA and LWL.

but it is very high (correlation coefficient is probably around 0.95).

Instead of pulling these numbers out of your ass, how about some facts?

Based on the data I posted on 18 kayaks (showing percent differences
in LWL and LOA), the actual correlation coefficient is 0.79. Not exactly
tight. In terms of performance, that is a significant difference. Thus
it is not reasonable to make sweeping statements that one can predict
performance based on LOA instead of LWL.

You guys are pulling out extreme examples based on hand-waving about
theories that few of you actually understand. I'm talking about
real kayaks in the real world. In the real world, we can't reduce
performance estimates on vague physical characteristics.

Mike

Michael Daly wrote:
On 16-Jun-2005, Peter wrote:


boats that are 18' long overall will almost
always have waterline lengths greater than boats that are 14'


Fine, but we were comparing kayaks that were only a foot and a half or
so different in length.

Your previous statement: "there is no correlation between overall length
and waterline length in kayaks" made no such distinction that it only
applied to some set of kayaks that all had about the same length, nor
was it limited to sea kayaks.

Of the 105 kayaks on the web page of Sea
Kayaker data, the average length is 5.2m (17 ft) with a standard deviation
of 41cm (16 in). 78% of the kayaks fall within one standard deviation of
the mean length. We're not talking about huge differences in length
typically, especially since the standard deviation is comparable to the
differences in LOA and LWL.


but it is very high (correlation coefficient is probably around 0.95).


Instead of pulling these numbers out of your ass, how about some facts?

Based on the data I posted on 18 kayaks (showing percent differences
in LWL and LOA), the actual correlation coefficient is 0.79.

Naturally the correlation coefficient will be less if you restrict the
kayaks under consideration to ones with fairly similar lengths (all but
one in the range from 16' to 19'). In a more complete list with play
boats, WW boats, surfskis, etc. also included the coefficient would be
much higher. Since your original statement just referred to the general
category "kayaks" my estimate was based on this broader selection.

However, a correlation coefficient of 0.79 is a far cry from your
original claim that there is "no correlation" which would imply a
correlation coefficient of 0. The numbers in this case are much closer
to perfect correlation than they are to no correlation.

In the reference to statistical terms I cited earlier, any correlation
coefficient of 0.5 or higher is regarded as "high" (0.1 - 0.3 is small,
0.3 - 0.5 is moderate) and greater than 0.7 is "very high."

Not exactly
tight.

Even taking your specified subset of kayaks, the correlation is "very
high" rather than your original statement that it is nonexistent.

On 16-Jun-2005, Peter wrote:

Your previous statement: "there is no correlation between overall length
and waterline length in kayaks"

From a perspective of useful information, that is still true. You can argue
semantics all you want, but sea kayak lengths (LOA and/or LWL) are all over
the place.

made no such distinction that it only
applied to some set of kayaks that all had about the same length, nor
was it limited to sea kayaks.

But for the fact that the discussion is about sea kayaks. I guess you
just forgot.

Naturally the correlation coefficient will be less if you restrict the
kayaks under consideration to ones with fairly similar lengths (all but
one in the range from 16' to 19'). In a more complete list with play
boats, WW boats, surfskis, etc. also included the coefficient would be
much higher.

WW boats? You're joking, right? They have even more variation in LOA
vs LWL.

I made no such restriction on lengths, I merely took the data that was
available and since we are discussing se kayaks, that's the data I used.

It still remains that overall length is not a useful indicator of
performance.

Mike

Michael Daly wrote:

On 16-Jun-2005, Peter wrote:


Your previous statement: "there is no correlation between overall length
and waterline length in kayaks"


From a perspective of useful information, that is still true.

No, it clearly was never true. Even taking the subset of kayaks you
chose, you calculated a correlation coefficient of 0.79 indicating a
very high level of correlation. If all kayak types were included the
correlation would be even higher.

You can argue
semantics all you want, but sea kayak lengths (LOA and/or LWL) are all over
the place.


made no such distinction that it only
applied to some set of kayaks that all had about the same length, nor
was it limited to sea kayaks.


But for the fact that the discussion is about sea kayaks. I guess you
just forgot.


Naturally the correlation coefficient will be less if you restrict the
kayaks under consideration to ones with fairly similar lengths (all but
one in the range from 16' to 19'). In a more complete list with play
boats, WW boats, surfskis, etc. also included the coefficient would be
much higher.


WW boats? You're joking, right? They have even more variation in LOA
vs LWL.

Not joking at all. In a compilation of all kayaks, the play boats and
WW boats will have short LOA and LWL figures, the surfskis will have
long LOA and LWL figures, and sea kayaks will come in in between. The
overall correlation coefficient between LOA and LWL will be very high.

I made no such restriction on lengths, I merely took the data that was
available and since we are discussing se kayaks, that's the data I used.

It still remains that overall length is not a useful indicator of
performance.

I have two sea kayaks. One has an overall length of 11' 8" and the
other has an overall length of 17' 6". I bet you can tell already which
one has a higher top speed - and you'd be right. Seems to be a pretty
useful indicator.

In the particular case of the two kayaks considered by the OP, their
lengths only differed by about 2' but the hull shapes appear to be quite
similar with no obvious difference in overhang. Therefore it's highly
likely that the Biscyne which is longer overall will also have a longer
waterline length.

On 16-Jun-2005, Peter wrote:

No, it clearly was never true. Even taking the subset of kayaks you
chose, you calculated a correlation coefficient of 0.79 indicating a
very high level of correlation.

Not high enough and nowhere near the level you claimed without any
proof.

The _fact_ is that at that level, the differences in overall length
between two kayaks are comparable to the differences in overall
length and waterline length in one kayak. Clearly a much higher
level of correlation is required than 0.79. In this case, the
mathematic definition of correlation has to take a back seat
to the more pragmatic need to produce information that is of
some value.


If all kayak types were included the
correlation would be even higher.

Your claim - how about something resembling proof? Your last
guess of 0.95 was based on nothing.

In the particular case of the two kayaks considered by the OP, their
lengths only differed by about 2' but the hull shapes appear to be quite
similar with no obvious difference in overhang. Therefore it's highly
likely that the Biscyne which is longer overall will also have a longer
waterline length.

Even if it does have a longer waterline length, that still does not
guarantee that the speed is higher. Hydrodynamics trumps simple
geometric parameters.

How about offering something of value instead of simply trying
to not-pick? Like offering some data that actually backs up you
ludicrous claim that what I am saying is false.

Mike

Michael Daly wrote:
Like offering some data that actually backs up you
ludicrous claim that what I am saying is false.

You already provided it yourself. After first making the claim that
there was "no correlation" between LOA and LWL, you later provided data
indicating that the correlation was 0.79 which clearly showed your
initial statement to be false. QED.

On 17-Jun-2005, Peter wrote:

You already provided it yourself. After first making the claim that
there was "no correlation" between LOA and LWL, you later provided data
indicating that the correlation was 0.79 which clearly showed your
initial statement to be false. QED

I've already addressed that - the correlation is not sufficient to
allow for prediction of performance. You are ignoring that _fact_.

As a further indicator of the relevance of LOA as an indicator of
performance, let's look at the correlation between the lengths
and the drag for the kayaks already presented.

Correlation coefficient, LOA vs Drag: -0.35
Correlation coefficient, LWL vs Drag: -0.69

Clearly, an intelligent person would not use LOA as an indicator
of performance. This further shows that the correlation between
LOA and LWL is insufficiently high. It also shows that other
factors beyond just length dictate drag, otherwise the coefficient
for LWL vs drag would be higher.

For cranky ol' rick, I'll get to other factors later.

Mike