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Sailboat Performance Testing Techniques

By Arvel Gentry

Boeing Commercial Airplane Company

Proceedings of the Eleventh AIAA Symposium on the Aero/Hydronautics of Sailing

September 12, 1981

Seattle, Washington

Abstract

December 1999

Testing methods, data reduction techniques, and data analysis programs used in the performance

testing of racing sailboats are reviewed. Instrumentation and recording equipment are also

discussed.

1. Introduction

2. Personal Experiences in Sailboat Testing

Sanding the bottom, checking and re-checking the sails

with the sailmaker, and studying the local wind and tide

conditions are standard practice for the serious racer. As

the boats get bigger, the stakes get higher, or the desire to

win increases; so does the search for other factors that

might improve performance. However, a still untapped

resource for many sailors is the knowledge of how his boat

should perform in given sailing conditions.

Sailboat performance testing has evolved rapidly over

the last ten years as a result of both intensive activity on the

12-meter boats, plus the advancement of electronic and

computer technology. My own personal experiences

illustrate this evolution.

My interest in boat performance started with attempts

to measure the effect of heel and bow-down trim on a 14

foot dinghy under very light wind conditions. Small pieces

of paper were dropped in the water and timed with a stop

watch to measure changes in boatspeed with trim changes.

Later, when I moved to a Cal 20 and had a knotmeter

available, I collected data to determine the optimum

downwind tacking angles (Reference 1).

Knowledge of boat performance can sometimes have

an important influence on your chances of winning. One

simple example is the knowledge of your boat's tacking

angle in different wind and sea conditions. In light winds,

the tacking angle will be quite large; on some boats, greater

than 100 degrees. In smooth water and higher winds, the

tacking angle may get down into the low 70's. Critical

tactical decisions such as tacking to the layline, close boat

crossing situations, and hitting the proper point on the

finishing line will be more accurate if you know your boat's

tacking angle characteristics.

In 1970 I found out that the 12-meter Intrepid was using

an onboard strip recorder to record boat performance. I

then set out to develop a similar device for my own boat. If

the 12-meter boats made use of new technologies in order

to increase their chances of winning, then why couldn't I?

My objectives were simple. I did not have the experience of

the people that I was racing against. I certainly didn't have

the resources like the 12-meter boats did, but maybe my

technical skills could help make up for my lack of sailing

experience. My approach was certainly small-time by

comparison to the 12's, but it is more typical of what most

sailors might be able to achieve.

Selection of sails for the next leg of a course requires

either good guesswork, or a good understanding of boat

performance and the apparent to true wind relationships.

Many boats carry the wrong sails or fail to reef at the

proper time because of a lack of knowledge of their boat's

performance characteristics.

All of these problems can be solved with experience.

However, proper boat testing can shorten this learning

period. The history of the America's Cup races shows a

continuous concern for achieving the best possible boat

performance. Performance testing with a pace boat and

the use of onboard electronic equipment and computers

have become standard practice on America's Cup boats, for

today's maxi-ocean racers, and to a lesser extent, for boats

on the grand prix racing circuit.

When I purchased Kittiwake , a Ranger 23, in 1972, I

equipped it with full sailing instruments. The first strip

recorder that I developed for use on this boat is shown in

Figure 1 and was described in detail in Reference 2 (all

References & Figures are at the end of the text). This device

recorded boatspeed as the primary signal, and periodically

switched to record a few seconds of wind speed. Wind

angle and heel angle data were recorded by hand or

written directly on the strip of paper as it came out of the

recorder. A sample strip of output is shown in Figure 2.

This paper presents a basic summary of the methods

used in modern sailboat testing. The information pre-

sented is based on the author's personal experience with

his own boats, plus what he has learned in sailing and

testing on other boats.

In 1974, Jim Kilroy asked me to build a recorder for his

new maxi-boat, Kialoa III , and to participate in the trials of

the boat off St. Petersburg, Florida. The requirement was to

develop a device that could be used on board during sea

C 1981 Arvel Gentry

Ranger 23 Newsletter

trials that would furnish an onboard record of perfor-

mance, and that would help to determine both average

sailing data as well as dynamic performance during

tacking. The device that I developed was more compli-

cated than that described in Reference 2, but still used a

strip recorder as the recording media. The Kialoa III

recorder is shown in Figure 3.

sets of onboard instrumentation. Even local racing boats

are sporting these expensive arrays of dials and digital

readouts. The basic instruments required for normal boat

testing are:

Boatspeed

Apparent wind speed

Apparent wind angle (0 to 360 degrees)

Compass

Heel angle

After the experience on board Kialoa III, I built a similar

recorder for my little Ranger 23, Kittiwake . New electronic

circuits were used, but all of the basic functions of the

Kialoa III recorder were retained. The electronic circuits

were designed by a friend, Alan Sewell. This recorder is

shown in use in Figures 4 and 5. It is described in more

detail in a later section.

Successful sailboat testing requires a thorough under-

standing of each of these gadgets.

2.1 Boatspeed

Boatspeed is usually measured by a sensor extending

through the hull. Present sensors fit into three classes:

1. Paddle Wheel

2. Propeller

3. Direct Force Measurement (strain gauge)

In late 1980, Jim Kilroy asked me to help out during the

sea trials of his newest Kialoa maxi-boat ( IV ). The trials were

again held out of St. Petersburg, Florida, but this time the

professionals provided the computer recording equip-

ment. The testing plan, organization, and engineering

support were provided by David Pedrick. The recording

and computing equipment were furnished by 12-meter

performance expert, Richard McCurdy. Most of the

equipment was the same as Pedrick and McCurdy had

used for the 12-meter, Clipper, in 1980 (where it was

identified as the Starship Nova system). My role on the

new Kialoa IV was as a performance testing engineer.

Figure 9 shows samples of these basic types of sensors.

The paddle wheel on the left is by Signet. The center

propeller sensor is the part of the B&G sensor that pro-

trudes outside of the hull. On the right is the direct force

measurement sensor by Telcor.

However, the basic problem with all this equipment is

the sensor location on the hull. The speed of the water past

the sensor location is not necessarily the true speed of the

boat. The water changes both speed and direction as it

flows past the hull. Sensor position error can be quite

significant, depending on the type and location of the

sensor, the size of the boat, and the sailing conditions.

The navigation station on Kialoa IV is shown in Figure

6. The CRT terminal at the left was used to control the

onboard Micro-Nova computer and to enter sail trim, sea

conditions, etc. The computer itself and the floppy disk

system is shown in Figure 7 (McCurdy is obviously a

hardware man!). Figure 8 shows the equipment used to

reduce and analyze the test data. This equipment was

located in a shoreside trailer and consisted of a Data

General Nova Mini-Computer, several terminals, and a

plotter. This shoreside computer equipment had also been

used for the 12-meter, Clipper . The floppy disk was not

used on Clipper since the data was sent to a shore receiver

by radio. The capability of this equipment is described in

more detail in Reference 3.

If the shape of the boat and internal structure permits,

the best position for the sensor is usually on the centerline

ahead of the keel. For the paddle wheel and strain gauge

type of sensors, this will give readings that do not change

from tack to tack (heeled readings may still be different

from upright values). It seems obvious that boat manufac-

turers should provide an appropriate centerline speedo

thru-hull location in with the design and construction of

the boat, but this is seldom done.

Sensors that use a small propeller or spinner, however,

still may read different between tacks, even when the

centerline location is used. This is caused by the fact that

the propeller rotates the same direction on both tacks. The

effect of the prop support and the weed guard may cause

the prop to spin faster on one tack than on the other.

The Micro-Nova computer and terminal were

removed from Kialoa IV after the sea trials. However,

McCurdy has since developed the necessary interfaces

between the Brookes & Gatehouse Hercules microcom-

puter system and an onboard Apple computer. The Apple

computer reads data from the Brookes & Gatehouse data

lines and the satellite navigation computer, and then

performs a variety of additional performance, tactical, and

navigational computations (4). The computer functions of

the B & G Hercules system are turned off, and the Apple

computer removed from the boat for races that do not

permit this type of equipment.

Large boats frequently use two sensors positioned on

either side of the hull in front of the keel. A gravity switch is

used to automatically select the lee-side sensor. Much time

and effort is required to obtain consistent readings

between tacks for this type of installation. Alignment of the

two sensors may have to be different in order to give

consistent readings on the two tacks. This may introduce

errors for the upright downwind sailing conditions.

Manual switching may be required to select the most

2. Electronic Boat Instrumentation

Almost all grand prix racing boats have rather complete

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reliable sensor signal, depending upon the sailing condi-

tion.

The biggest problem with wind speed sensors is the

location. The masthead is subjected to flow distortions and

speed errors due to the flow created by the sails. The height

of the sensor above the water must also be considered

when comparing data taken on different size boats

(because of the wind speed gradient with height).

The boatspeed sensors should be carefully calibrated

by sailing measured miles or by sailing close to a boat with

well calibrated instruments. Calibrations should be

performed at various heel angles on both tacks and in the

upright condition. Uncorrectable errors should be

recorded and the proper corrections applied to all mea-

sured boatspeed data.

2.3 Wind Direction

Several major problems plague boat wind direction

devices. The first is that most systems on boats that I have

been on are not aligned properly so that they read the

same on both tacks. Careful alignment at the masthead,

together with small electrical adjustments at the naviga-

tion table, should give consistent readings.

Sensors that use paddle wheel or prop rotation counter

circuits are usually quite stable once the instrument is

properly calibrated. With time, however, wear or damage

to the bearings can affect the readings. The electronic

circuits, themselves, can frequently be checked at the dock

by placing a 60 Hz signal near the sensor (such as a solder-

ing iron).

The next problem is that the wind direction sensor

measures what it sees (the wind direction at its location).

This may not be the correct apparent wind angle because

of flow distortion due to the sails (the upwash effect), and

because of heel angle. Means of correcting for these effects

will be covered later.

A direct force measurement sensor, such as that

manufactured by Telcor Instruments, is very sensitive at

low speeds and is not affected by local flow angles.

Calibration is accomplished just as with other sensors.

However, subsequent checks of the calibration can be

accomplished by simply hanging a small weight on the

retracted sensor tip and checking the reading. This can

even be accomplished underway.

Most wind direction sensors are integrated with the

wind speed device so that the rotating cups are located

under the wind vane. This means that the most practical

position for the unit is on a rod extending at an angle out in

front of the masthead. In this position it is subjected to

strong sail upwash effects. These effects may be corrected

for windward conditions, but are more difficult to account

for in the running and reaching conditions. On large boats

in broad reaching conditions, the removal of a staysail may

significantly affect the wind direction reading.

The type of boatspeed cockpit display depends upon

personal preference. An analog display can usually be

averaged by eye better than the digital display. The digital

display can give a more accurate instantaneous reading,

but since the readings are almost always changing,

average values are harder to read. Recording data manu-

ally requires some care. Either record an average reading,

or record many readings and determine the average

mathematically.

The last problem inherent in wind direction measure-

ments is the fact that the reading may be fluctuating quite a

bit. Average readings may be difficult to obtain. Most

systems have an adjustable electronic dampening control

to slow down the system response so that the readings are

not always jumping all over the place. This will cause

problems if you are studying dynamic maneuvers such as

tacks. It also means that attempts to sail by a VMG meter

may lead to bad results (since the VMG computations use

the apparent wind angle).

Some boats make use of automatic speed recording

devices. The type of sensor may influence the selection

and design of the recorder equipment. A digital circuit may

require a D/A converter if the data is to be recorded on an

analog device such as a strip recorder.

2.2 Wind Speed

2.4 Compass

There are several different types of wind speed sensors.

Figure 10 shows three examples. The rotating cup sensor is

most frequently used although it does have its problems.

They are nonlinear at the low speeds, the basic calibration

may be affected by heel angle, and they have bearings that

wear out.

Little new can be said about compasses, except that

they should be carefully adjusted before serious testing

starts. Any errors should be noted and corrections applied

to the readings before they are used in the data reduction

process. Keep magnetic objects such as pliers, screw

drivers, and portable radios away from the compasses

during testing just as you would during a race.

The wind speed sensor made by Telcor Instruments is a

solid state device with no moving parts, and avoids most of

these problems. The wind blowing past a thermistor tends

to cool the unit. The amount of current necessary to heat

the sensor back up to the balanced condition can be

measured and converted to wind speed. The thermistor

tip must be cleaned occasionally to remove spider webs

that degrade the sensitivity.

If you plan on using one of the more sophisticated

instrument and data recording systems, you will need a

compass with an electronic readout.

2.5 Heel Angle

Heel angle is an important parameter that is frequently

-3-

left off sailing data sheets. However, it is required if the

proper corrections are to be applied in the data reduction

process. Heel angle will usually have to be recorded by

hand from readings taken off of small bubble indicators.

The more sophisticated systems use an electronic heel

angle device. However, none of the presently available

microcomputer based boat systems include a heel angle

input.

is the multi-function display units that can be positioned

about the boat where they are needed (one boat is reported

to have twenty of these units). Figure 12 shows the cockpit

of Kialoa IV with its two sets of five readouts on either side

of the center hydraulic control panel. On board Kialoa IV ,

the Hercules 190 system produces data that is read by the

Apple computer, and the Apple computer, in turn, puts

output data back on the B&G data line for display on the

multi-function units. The Hercules System 190, itself,

contains 32 channels of data.

My own electronic heel angle system consists of an

instrumentation amplifier circuit with a weighted potenti-

ometer furnishing the heel angle signal. This unit is on the

right side of the photo in Figure 5.

Rochester Instruments makes a microcomputer based

boat instrumentation system that was used on Freedom in

the 1980 America's Cup. A photograph of the system is

shown in Figure 13. The system computes speed made

good (upwind or downwind), true wind speed, and true

wind direction off the bow. One nice feature is the output

port for a cassette tape recorder so that the basic sailing

parameters can be recorded automatically. Rochester

provides a service of converting the cassette tape to

printed output form.

2.6 Leeway Angle

There is presently no completely satisfactory leeway

angle measuring device. Various leeway angle measure-

ment techniques have been tried with varying success (the

local flow angles measured on the boat are not the same as

the true leeway angle). Sometimes, a line is towed behind

the boat and a large protractor used to record the angle

that the line makes with the boat centerline. However, this

system is difficult to use because of the normal small angle

changes in the boat heading as the wind and sea change.

Careful navigation from fixed sea markers can also be

used, but again, accurate results are difficult to obtain. In

the data reduction procedures used in this paper we will

use an empirical equation to account for leeway effects.

Signet also produces a microcomputer based boat

system. This system computes speed made good (VMG),

true wind speed and direction, and has a start timer.

The present boat microcomputer systems have only

limited capacity for the more complex computations. In

my opinion, several practical implementation problems

have not been solved. As stated previously, none of the

systems have a heel angle input, and none provide a

means for correcting for upwash. This makes their VMG

and true wind results suspect.

2.7 Navigation Instruments

Sophisticated modern navigation instruments may be

of some help in sailboat testing and their use should be

investigated if you have them on your boat. Satellite

navigation systems coupled with Omega systems have

been used to help detect water current variations that

would affect testing.

A display of relative boat performance would be

helpful (as compared to stored polars). The Hercules 190

system uses a built-in set of data that represents general

boat performance characteristics (using your input IOR

rating). The boat performance is compared with informa-

tion stored in the computer, and a performance percentage

number displayed. Data is provided for either windward

or reaching conditions.

2.8 Microcomputer Based Systems

The microcomputer chip is presently causing a revolu-

tion in the sailboat instrument business. Several manufac-

turers have systems that use microcomputer circuits. The

boatspeed, wind speed and direction, and compass

sensors send information to a central computer processor.

The information is then sent out to the cockpit display

instruments.

Ideally, the user should be able to determine his own

boat's performance, and to load it into an EPROM for use

by the boat microcomputer system. Another possibility

would be to have the data prepared by a home computer

(or by a service provided by the instrument manufacturer),

and then loaded into the boat microcomputer through a

cassette tape.

The new microcomputer based systems have tremen-

dous potential. However, I find the available systems still

lacking in some important areas. Most systems compute

what is called speed made good to windward (VMG).

Accurate computation of VMG requires that corrections be

applied for both the upwash at the masthead sensor and

for heel angle. None of the presently available microcom-

puter based systems have a heel angle input sensor. They

also do not provide a means for correcting for upwash on

different boats.

An onboard computer that is separate from the boat

instruments, such as the Apple computer on Kialoa IV ,

provides a powerful system to assist and supplement the

normal boat microcomputer instrument.

However, the use of something like the Apple com-

puter requires a number of difficult interfaces with the

boat's instrumentation, plus some sophisticated software

(4). And last, it would require a boat owner (plus probably a

navigator) who could understand and make maximum

The Brookes & Gatehouse Hercules 190 system is

shown in Figure 11. One of the best features of this system

-4-

use of such a system.

and that the output is not immediately available for use

within modern computers.

2.9 Automatic Data Recorders

2.9.2 Electronic Recording

Performance data can always be recorded by hand onto

data forms. However, this means of gathering data

depends upon the judgment and diligence of the person

writing down the numbers. Automatic recorders are a

more reliable means of recording the number data but

have their own problems which must be solved. The

primary one is the recording of data that is not available by

electronic means.

The electronic recording and processing of sailboat

performance was used extensively in the 1980 America's

Cup season. David Pedrick and Richard McCurdy devel-

oped a rather sophisticated system for Clipper in an

attempt to shorten the learning and boat tuning time (3).

Much of the equipment from Clipper was used during

the sea trials of Jim Kilroy's new Kialoa IV in 1981. For Kialoa

IV , McCurdy had to develop interfaces, incorporate the

onboard floppy disk system, and develop new shoreside

computer software. As a result, this sophisticated equip-

ment was not ready for the first part of the sea trials.

During this period it was necessary to record data by hand

and to do all of the data reduction on an HP-41C program-

mable calculator (that was my job).

Data is useless if you do not know what was happening

on the boat when the data was taken. This should include

such data as the sail configuration, all the sail trim parame-

ters (genoa car location, outhaul, halyard tension, etc.),

backstay pressure, running backstay pressures, babystay

pressures, helmsman, sea conditions, etc.

2.9.1 The Strip Recorder

During the Kialoa IV sea trials, performance polars were

updated daily as new data was gathered. The performance

testing on Kialoa IV was probably the most complete and

sophisticated yet applied on a racing yacht (including the

12-meters).

The strip recorder provides one means of recording

both the electronically generated data and the other

information mentioned above. Notes can be made right on

the strip of paper as it comes out of the machine. The

recorder developed by the author for use on Kialoa III , and

on his own boat, Kittiwake, was a rather sophisticated

instrument for its time.

3. Testing Techniques

The most important time for serious testing is right

after the boat is completed and before the first race. Most

owners want to get the most out of their boat as soon as

possible, and careful testing can aid significantly in

accomplishing this. However, the performance testing

must not interfere with other equally important aspects

such as crew training, general boat familiarization, sail

inventory checks, and rig tuning.

This recorder as it was used on Kittiwake is shown in

Figures 4 and 5. The recorder was kept below during races

but used in the cockpit during other testing periods.

During short races, an audio cassette recorder was started

with the strip recorder and recorded all of sail trim,

tacking, and tactical information. A typical record from

this recorder is shown in Figure 14.

On the Kialoa III and Kittiwake recorders, a total of six

data signals could be input to the control unit. Only two

signals could be recorded at a time, but a combination of

automatic and manual switching permitted the effective

recording of six parameters on a single strip of recorder

paper. Boatspeed (VS) was the primary signal on the lower

channel. The two secondary parameters on the lower

channel were the apparent wind speed and a spare

channel (used for the Brookes & Gatehouse Horatio

computer output on Kialoa III ).

Initial sea trial testing provides the first indications as to

how the boat will perform under various conditions. Data

gathered during this period should be considered as being

preliminary since significant improvements in perfor-

mance will usually be obtained during actual racing

conditions. These initial tests, however, will usually

provide a chance to obtain general trends that will be

useful in correlating the data obtained during racing

conditions.

Accurate performance polars require hundreds of data

points. If possible, the data gathering process should

continue throughout the racing life of the boat. This will

provide an excellent baseline for comparison if modifica-

tions are subsequently made to the boat.

Any signal could be recorded full-time, or the primary

signal and one of the selected secondary signals could be

automatically alternated. The upper channel had two

primary signals that were selected by a switch on the

control unit. These were the apparent wind angle scaled

from 0 to 180 degrees and the closehauled wind angle. The

signal from an electronic heel angle indicator was the

single secondary signal for the upper channel. This

recorder was used on board Kittiwake for all of its races and

practice sessions for over two years.

It is important that the maximum amount of data be

gathered for the boat sailing in smooth water conditions.

This usually gives the maximum performance characteris-

tics for the boat. If you know what the boat should be able

to do under ideal smooth water conditions, you are better

able to judge how the boat should be sailed as the sea

conditions deteriorate. Rough sea conditions degrade the

performance of the boat. Eventually you will want to

The disadvantages of the strip recorder are that it needs

someone to write all of the notes on the recorder paper,

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