Swim/Surf Fin Static Thrust Measurements
Date: Sun, 05 Mar 2000 15:42:52 -0800
From: sdbchguy <sdbchguyNOsdSPAM@hotmail.com.invalid>
Newsgroups: alt.surfing
Subject: Re: Swim Fins (& kneeboarding)

Reference Table for "Swim/Surf Fin Static Thrust Measurements."

sdbchguy's reply to an alt.surfing post for more information on how the
relative thrust of the fins were measured and evaluated:

Thanks for the kind words.

This all started out during a period when the surf wasn't that
big, and I was getting especially frustrated with longboarders
sitting outside of where I'm able to catch waves on my
kneeboard--and not letting a single one pass. So I was wondering
how much power might be required to achieve a speed equal to, or
exceeding that of a typical longboarder on a good paddling board
(i.e. ~ 10', not too much rocker, and not too thin).

My first idea was to take my kneeboard to a wave tank/channel,
then attach it with a line and spring scale to the tow cart. Then
I would begin paddle at about the same energy level as when I'm
trying to catch a wave. At the same time, the cart would tow me
at approximately double my normal unassisted speed--and we'd
measure the line pull required to achieve that increase.
Unfortunately, it turned out that I could paddle (unassisted)
faster than the top speed of the tow cart--so that approach
clearly wasn't going to work.

In order to get at least some idea of what power might be
required, I next decided to just measure the static thrust I
could develop, then assume that I could achieve the same thrust
when underway (an untested assumption). In that case, if I also
assumed that the horsepower required was proportional to the cube
of the speed (i.e. drag force as the square), I could calc. how
much additional external power would be required to double my

So I originally intended to only make these measurements with the
Duckfeet that I normally use when kneeboarding. But a friend of
mine was curious about how some fins that he had purchased would
compare with the Duckfeet--so we decided to test them as well.
That quickly lead to an idea to compare all the various fins we
collectively owned.

The measurements were carried out by my friend and I in Agua
Hediondia lagoon one morning back in October, 1994 (when the surf
wasn't particularly good). The procedure was: One of us was the
'swimmer'; the other read the spring scale and recorded the
thrust values. The swimmer would be about 20' from the beach
(deep enough so that one wouldn't contact the bottom while
swimming), with a rope harness around his waist. A 1/4" poly line
ran from that harness to the shore (nearly horizontally when
under tension), where it was attached to one end of a linear
spring scale. The other end of the scale was attached to a pole
that was (already) in place on the beach.

BTW, we started out with a shoulder harness, but immediately
discovered that there was enough of an offset between the thrust
line from the fins and the restaining line that at peak thrust
the swimmer's body could rotate up a little, making an angle with
respect to the surface of the water. Once that rotation started,
the moment arm was increased, and the situation rapidly got
worse. The waist attachment seemed to cure that problem.

The swimmer would put on a pair of fins, wade out into the water
until the slack was taken out of the line, settle down into the
water, and then try to swim away at a constant level of effort
from the pole. As best I recall, the swimmer tried to choose an
effort that he felt he could sustain for around 20-30 seconds.
During that period, the shore observer would watch the scale and
estimate the average pull (there are some fluctuations of +/- a
pound, or so, with each stroke of the swimmer's legs). After a
good (i.e. 'steady') reading was obtained, the scale reader would
note the value, then shout "OK". Then the swimmer would briefly
kick just as hard as he could, while the shore person read the
peak pull (ignoring, if necessary, an occasional inital anomalous
surge with the first hard kick, or two as any slight slack/dip
came out of the line).

The swimmer would then come back to the beach and rest a bit
while changing to another pair of fins. That was also when we
explained what we were doing to a few people who interrupted
their early morning walks to try and figure out what this
crazy pair of guys were doing (clearly the swimmer wasn't going
to be able to get anywhere). When the swimmer felt he was rested,
the procedure would be repeated.

After the first set of measurements were complete, the
measurements were repeated for a subset of the fins (indicated by
the multiple measurements in the table)--but now in the reverse
order. The purpose was to determine if order of the measurements
affected the relative results (due to fatigue)--as well as
provide an estimate of the repeatability of the measurements.
When the second series of tests was complete, a third measurement
was made for one of the fins (again, to determine if swimmer
fatigue was a significant factor).

Following that set of measurements, we repeated the measurements
for one pair of Duckfeet, the Redleys, and the Hydrofins. But now
the swimmer used more rapid, but shallower strokes (flutter
kicking) than he had in the previous runs (power kicking). The
Redleys and HydroFins (as we suspected) worked better with this
technique (the Duckfeet did not, but apparently we didn't save
the values).

Following that, we did the measurements with the booties and
feet. For a long time I had suspected that kicking efficiency was
less with booties than with feet--and it was nice to see that
impression confirmed.

Then as an after-thought, we also decided to try the webbed
gloves to see how their thrust compared with that of the fins.
Since I always wear the gloves (although I don't always paddle
with them), I didn't think about measuring the thrust with just
the hands. Clearly an oversight...sorry.

Sometime in the future I'd like to repeat the experiment, but
with a much more comprehensive and representative array of fins
from various mfgrs, and of various styles/types (perhaps I can
get one or more surf shops to lend me various pairs of fins for a
couple of hours sometime). I'd also like to make the measurements
using an in-line load cell, with a signal conditioner and an A/D
(connected to a portable computer), to collect time-series of the
line pull (instead of watching a spring scale). That would make
it easier to get representative average and peak values, as well
as assist in determining if the kicking was truly at essentially
a constant level of effort.


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