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Are You at Optimum Trim?
"Arvel Gentry tells how to make your own
Sailboat Performance Recorder"
By Arvel Gentry
SAIL Magazine, March 1974
Ocean racing skippers are relying more than ever on wind velocity and direction indicators, and a speed
the many different types of marine instruments. Wind recorder that continually recorded boatspeed.
speed, boatspeed and wind direction indicators have Two years ago I wondered if I couldn't use this same
become almost indispensable on all-out racing machines,
approach as I started to equip my own 23' MORC boat.
not only for general navigational purposes, but to help the Excellent marine sensors and indicators were available,
skipper and crew keep the boat performing at its best. But but the most important part of the system, the speed
another important and new use for these instruments is to recorder, was not. I would have to build up my own. The
help the crew tune a boat and adjust the sails for best results of my efforts are shown in Figure 1; it is a device I
performance. call the Sailboat Performance Recorder.
However even with all this equipment, sailboat Now don't get me wrong, I'm not trying to sell
performance is still subject to many variables. Wind speed anything. You can do just as I did make up your own
and direction, sea conditions, sail trim, the helmsman's recorder using the available components I'm about to
concentration are all constantly changing. These describe. The cost? Only a little more than what you would
variations make it very difficult to gather good pay for one of your other marine instruments: well within
performance data at any instant of time. All the needles are a small boat budget. After two years of using the Sailboat
constantly moving and an "eyeball" averaging process is Performance Recorder, I'm firmly convinced that it's the
necessary to get a reading. most important piece of equipment on my boat. But, what
Now, this is not too bad if you are only interested in is it? How is it used? And, how can you make your own?
knowing if you are going about five knots. But, what if you The Sailboat Performance Recorder is an instrument
have changed helmsmen and you want to know if the new that records the boatspeed on a continuous strip of paper,
one is really sailing 1/10th of a knot better. Or you're trying and once every minute it automatically switches to record
to see if a change in sail trim, or maybe a sail change, is a short record of apparent wind speed. It gets its input
what is giving you that 1/10th of a knot instead of, perhaps, signals from the boat's conventional knotmeter and wind
an increase in wind speed. Ocean races are won and lost by speed instruments. Other important information such as
tenths of knots. apparent wind angle, sail trim conditions, sail
In the 1970 America's Cup campaign, Bill Ficker faced combinations, etc., are written by hand right on the strip of
these same problems on Intrepid. Without the use of a pace paper as it goes through the recording window or on the
boat as a trial horse, he and his crew relied upon careful viewing extension. The signal traces on this strip of grid
performance testing using an onboard instrumentation paper can then be examined to determine very accurately
system. This consisted of boatspeed indicators, apparent the average boat speed and windspeed for the sailing
Figure 1. Sailboat Performance Recorder mounted in portable box.
Figure 2. Typical output from
strip can be marked to show
when changes in trim took place.
conditions present. extension and the paper stayed within the recorder, it is
This check of the record can be made just after the not so useful in a real sailing application.
signals are recorded, or many hours later when a great At the end of a test day, I take the paper from the
number of records can be evaluated and compared. A recorder and study and compose the records. If a given
typical output strip from the recorder is shown in Figure 2. change in sail trim between two records gives an increase
Boatspeed is linear and can be read directly from the grid in average boatspeed (and the recorded wind speeds have
lines. However the wind speed is non-linear because of the not changed significantly), then I have found a way of
output signal from the Telcor wind instrument that I used. making the boat go faster. Each test condition is usually
Wind speed is read with a small calibration scale, and this repeated several times to verify the actual amount of speed
has turned out to be an advantage because it gives good improvement. The same type of procedure is used in
accuracy at the lower wind speeds where you need it. testing different sail combinations.
The Sailboat Performance Recorder consists of a The Sailboat Performance Recorder is also used in
commercially available strip recorder, control and filter actual racing conditions. All sail selection and trim changes
circuits to prepare the boat instrument signals for the are checked before and after they are made by reading the
recorder, a relay switching circuit to change automatically average boatspeed and wind speed from the strip recorder.
the recorded signal, and a housing box to protect the This is especially helpful on longer races when there are no
instrument as well as make it portable. The recorder I used other boats close by to compare with visually. On short day
was a 10 millivolt DC Rustrak recorder which is used in races I leave the strip recorder running during the entire
many non-boating electronic applications. The rest of the race. It is very interesting to look back on these records and
required electronics, the control, filter and switching see if the boat was sailed up to its full potential, and to see
circuits, are very simple and you can wire them up how much time was lost in bad spinnaker sets, etc.
yourself. Or, if you're not so inclined, you can have your A strip recorder makes it very easy to gather the data
local marine radio repairman do it for you. necessary to construct a complete speed polar for your
I use the Sailboat Performance Recorder in boat. Such a polar will help in making the proper sail
tuning/practice sessions and in actual races. The procedure selection for different wind conditions. The recorder is also
used in the testing sessions is simple and straightforward. useful in making a direct and accurate comparison of the
Once the boat is stabilized for a given set of sail and boat performance of different helmsmen. During very short
trim conditions, the recorder is turned on. It then races, the cassette magnetic tape recorder can be used to
automatically records the boatspeed on a continuous strip record supplementary data such as apparent wind angles,
of paper. Periodic short wind speed records are also made sail trim changes and crew positions.
by the automatic switching system. The main use of the recorder in all these applications is
At the end of the test run, I turn the recorder off, change to determine accurate average boatspeeds and wind
the sail settings, and repeat the procedure on the next test. speeds over a period of sailing time. This is very difficult to
Other significant information is written right on the strip do just by looking at the knotmeter and wind speed dials.
paper with a pencil, and this usually includes the compass Most important of all, the strip recorder provides a
heading, relative wind direction and any sail setting data permanent record for later study under more relaxed non-
(distance off spreader, genoa car position, halyard tension, sailing conditions.
etc.). Sometimes this information is written on a separate My recording instrument itself is a Model No. 2194
test log sheet or is recorded on a cassette magnetic recorder Rustrak Recorder (range 0 to 10 millivolts) with a 12volt DC
for subsequent correlation with the strip recorder, wind unregulated drive motor (10 rpm). The actual paper chart
speed, and boatspeed traces. speed is determined by the drive motor rpm and the
The strip recorder has a viewing extension so that about interchangeable gear train selected. Rustrak gear train # 15
10 minutes of the strip record is in view before the paper and the 10-rpm motor gives a paper speed of 75" per hour.
feeds back into the recorder take-up spool. I consider this After trying several different paper speeds, this speed
an important feature. If the recorder did not have this seems to be best for most applications. With the 75"/hr
paper speed, one roll of paper will last 10 hours. The other the circuits are all electrolytics and are used to help smooth
Rustrak item needed is the Model 223 Chart Viewing out and filter the signals to minimize the trace wiggles. The
Extension. The total cost of the Rustrak recorder and different amounts of filtration and the different character
accessories is approximately $250 (Rustrak Instrument Div. of the boatspeed and wind speed traces makes it very easy
Gulton Industries, Manchester, New Hampshire 03103). to tell which is which.
The only other item is the chart paper itself. A number Once the input signals are ready for the strip recorder,
of different recorder chart paper styles are available from you need a means of automatically switching the signals,
Rustrak. I use style WB chart paper which gives a full scale for the recorder is only able to record one signal at a time.
reading of 7.5 knots with grid lines at 1/10 knot intervals. The circuit for this is shown in Figure 5. To activate the
Once you have the strip recorder and your knotmeter switching circuit, I use the flange on the Rustrak recorder
and wind speed instruments you'll have to build circuits to take-up spool itself. I added a small contact point insulated
hook the recorder and instruments together. This consists from the recorder frame and that rides on the spool flange.
of circuits to fine-tune the recorded signals so that they The circuit is broken whenever the contact point is lifted by
match the marine instruments, and to dampen the a piece of tape on the flange. Splicing tape used for
recorded traces to aid in giving a better average boatspeed magnetic recording tape seems best for this.
and wind speed readings. The boatspeed circuit is shown The width and number of tapes on the spool flange
in Figure 3. The variable resistance at R2 is the control used control how often the switch is made between boatspeed
to adjust the recorded signal trace so that it matches exactly and wind speed. Two 1/4" pieces of tape give about 12
the knotmeter instrument reading. The value of resistor R1 seconds of wind speed every minute. The transistorized
will depend upon which knotmeter you use. I used a Telcor switching circuit shown in Figure 5 controls the relay that
knotmeter with the R1 value shown in Figure 3. actually switches the signals being sent to the Rustrak
The wind speed connection circuit is shown in Figure 4. recorder.
Again, the Telcor wind speed indicator was used and R3 To drive the recorder and switching relay circuit you
had a fixed value of 3.3K. However with other brands, I need a 12-volt battery source. Normally, the recorder can
would recommend that R3 be variable as shown. Consult be powered by the boat's 12-volt battery. However, with
your own instrument manufacturer for the proper values the Telcor windspeed instrument, the Rustrak recorder
to use for resistors R1 and R3 in these circuits, or calculate had to be driven by a separate 12-volt source because of the
them yourself using the equation given in the Rustrak +6-volt reference voltage used by Telcor. Because of this,
owner's manual. and the desire to make the unit completely portable, I
These resistors must step the marine instrument included a Rustrak rechargable battery pack (model 154A-
signals down so as not to exceed the maximum 10 millivolt 12-400) though a regular 12-volt dry cell can be used just as
signal tolerated by the Rustrak recorder. The capacitors in well.
The final task is to build a box to hold and protect the
recorder and control circuits from the marine
environment. I attached a small hatch board to the box so
that it could be conveniently positioned in the
companionway. Since the unit is portable I can take the
whole box home after a race to remove the paper and
recharge the battery.
Figure 3. Boatspeed circuit.
The top of the box is a hinged piece of Plexiglas to keep
out spray. A manual switch is added at the battery source
to turn the recorder on and off. I also added a double throw
switch so that I could record boatspeed only, wind speed
only, or have the signals switched on automatically. Figure
6 shows all the unit components; the battery is in the
bottom of the box. Then the two circuit boxes go in on top
and they are followed by the Rustrak recorder. The battery
and circuit boxes lie underneath the chart-viewing
Figure 4. Wind speed circuit.
If making your own recorder, connecting, and
switching circuits is too much work for you, contact Signet
Scientific Company, 129 E. Tojunga Ave., Burbank,
California 91503. I believe they now are selling strip
recorders for use with their instruments at about $500 per
single channel device. I am told that some of the new large
ocean-racing boats are ordering as many as four and five of
these recorders to record everything from wind speed,
Figure 5. Switching circuit.
Figure 6. Detail of Sailboat Performance Recorder
boatspeed, heading, rudder angle; probably even the
heartbeat of the helmsman! However, if you are on a small
boat budget, you can make your own Sailboat
Performance Recorder as I did, and still have virtually the
same equipment as the big boys!