Proceedings of the 15th ATTC, American Towing Tank Conference, ATTC'68, Ottawa, USA, June 25-28, 1968, Transactions, Volume 2

1

TRANSACTIONS

VOLUME II

24 JUU 1S78

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TRANSACTIONS

OF THE

FIFTEENTH MEETING OF THE

AMERICAN TOWING TANK CONFERENCE

VOLUME II

Held at:

Ship Laboratory,

National Research Council,

Ottawa,

CONTENTS

STATE OF THE ART REPORT

-INSTRUMENTATION, DATA PROCESSING AND ANALYSIS

LIST OF PAPERS

INSTRUMENTATION AND DATA PROCESSING

State-of-the-Art Report

for presentation to Fifteenth Meeting

of American Towing Tark Conference

National Research Council

Ottawa, Canada

INSTRUMENTATION AND DATA PROCESSING

FOREWORD

The Committee on Instrumentation and Data Processing

found at their first meeting in Stevens Institute of

Technology, that their task of producing a State-of-the-Art

Report will be hampered by following facts:

Towing tanks do not usually maintain on

their staffs one member who is familiar with

all

instrumentation and who is fitted to

disseminate technical information and

publicize new equipment.

Most tanks do not maintain manuals describing

their routine techniques and the instrumentation

used.

Instrumentation is usually described only in

appendices to the reports published by towing

tanks and then often in inadequate detail.

Towing tanks generally develop netN

instrumentation

in the process of carrying out sponsored research

and have neither the time nor money to

prepare

timely and adequate reports of new developments

in instrumentation.

e.

There exists neither a central repository

cf

information on instrumentation nor a mechanism

for maintaining continuity

of

interest in this

subject between Conferences.

At the same time the Committee set their

objective as "to survey the State-of-the-Art of Towing Tank

instrumentation and to present the current situation and

trends in a critical fashion".

Noticing that the Committee members represent a

fairly good sample of towing tanks on this continent,

regarding size and fields of interest, the Committee decided

to meet the first part of their objective by having each

member to prepare a survey of facilities and procedures

in the

institution of his affiliation.

To standardise the survey the main topics

and items

have been selected:

I.

Resistance Propulsion and Cavitation

Calm water measuring systems

Propulsion performance and wake measurement

Self-propulslon performance

Flow visualization

f.

Boundary layer measurements and turbulence

St

imulation

2.

Seakeeping

Model preparation

Wind and wave generators, beaches, wave

measurement and location of pickups

Towing methods including attachment of model

to carriage and methods of propulsion

Measurement of model motions, forces,

accelerations and pressures

Use of oscillators to determine hydrodynamic

coefficients

Moored systems in waves, representation of

mooring forces, and structural model

Flow visualisation

Motion picture technique, lighting and

photographic methods

3.

Steering and Maneuvering

Rotating arms

Oscillators

Step response technique

Free models and radio control

4.

Acoustics and Vibration

a.

Pressure pulses from propellers

Vibrations due to propellers

Hydroelastic effects

5.

Full Scale Instrumentation

Performance

Pressures

Stresses

Motions

Waves 6.

Model Making

Lines drawing and fairing

Choice of materials

Model cutting machines

Model finish

Model checking

The particular surveys are appended to the Report

which attempts to implement the second part of the objective

set up by the Committee.

J. H. Brandau

P.W. Brown (Recorder)

J. C. Gebhardt

D. Cospodnetic (Chairman)

T. Loukakis

O.J. Sibul

INSTRUMENTATION SYSTEMS IN TOWING TANKS

Characteristics of our time are complexity, growth and

charge.

Systems which we design must meet that challenge and

be complex for not to become narrow-minded, permit evolution

to avoid decay and foresee changes to prevent collapse.

An inspection of the Appendices of this report shows

that the towing tank community started moving in that direction.

However, the pattern of difficulties enumerated in the

Foreword of this report shows that systems planning couid still

be attributed to the pressures of specific jobs and no

tc a

development policy of towing tank instrumentation.

Some problems dropped out cf sight without

_{ever being}

solved.

For instance speed constancy of towing carriages

and its influence on calm water towing force.

_{This might be}

due to the fact that we left the mechanical balance and

a

soft spring where we could see the interplay and went

_{over to}

stiff electronic transducer and integrating voltmeter.

The latter seemed also to cure the problem of

vibrationalforces introduced by stiff transducers.

_Still,

since nobody wants to investigate the influence

_{of second}

order terms,

it would be wise,

if the bandwidth seperation

allows, to filter out the interference before

_integrating.

Another problem, and that is the

model environment

problem, might need our full attention

again.

It has been

shown (I) that some living matter

prospering in our terk is

able to add some non-newtonian

fluid to the

tankwater and

cause

friction drag reouction.

Perhaps we should try

again and monitor continuously

physical, chemical and

biological conditions in our tanks.

A permanent set-up

of temperature and turbulence meters

would take out the

cuess of when the

next run can start.

A periodical check with

a more sophisticated

standard model could supply a weekly, say,

table of correction coefficients.

It was actually the growing

importance of seakeeping

tests which gave an impetus to

applications of data acquisition

and data processing systems

in the tanks.

Much can be done in this

field by catalog engineering

except for the open ends of

the loop and that is presentation

of valid data to the

input and proper software to the output.

Again, it seems that our

assumptions are often in discord

with

reality.

How else do we explain sometimes extravagant

scatter

of results even in the case of so-called uni-directional,

regular head-seas?

Oblique seas in square tanks and associated

reflections point to the

necessity of havino

anti-reflection

beaches all around.

Even then, long-crested sea might not be

achieved.

Since it does not seem that we are acing to improve

wave generators very much it would be advisable to look for

means of screening and retairing the wave pattern the model

is just about to encounter and base our processing and analysis

on

it rather than on a stationary wave probe and assump+:on

of uniform conditions all over the tank.

It

is right that some

investigators use moving probes hut, even

if we disregard

errors introduced by testirc water in the case of surface

piercing probes or wave slope introduced errors jr the case

of sonic probes,

they yield only ore-dimensional scan.

Photogrammetry offers one solution, very time consuming when

processed in

a conventional way.

A modification if the

instrumentation using T.V. cameras and video-tape recorders

and using a

luminous transversal dotted trace preceding the

model

instead of IBM confetti as the target could lead to a

workable system.

After much work, of course.

Model motions measurement and processing do not present

problems any more.

For free models there are established

systems comprising remote control stable platform,

telemetry and either analog or digital tape recording at the

receiving end.

For those

who are not inclined tc double

integrate acceleration to get displacement there are still

problems of how to do it from shore.

Laser beam techniques

to establish references are being currently tried out

Model tracking, according to Appendices A to F, can be

done successfully in

different ways.

The thing is to choose

such a way which

would integrate with ways of

other seakeepinc

data acquisition into cne

system of data which can be readily

processed.

Another field where we embarked on long term projects

amassing vast amounts of raw data is ship motions

and stresses

due to waves.

This is no more ore-shot experiment judiciously

interpreted by the naval

architect but requires

thoroughly

planned coordination or

subsystems into one whole.

Unfortunately, as in tanks,

the main difficulty is

the

sea itself.

A workable system

(2) uses expendable wave

buoys

which telemeter vertical

acceleration to the ship

for recording.

That buoy uses an

intrinsic digital

accelerometers and sc

avoids communication

noise problems sending

information in pulse

rate coded -form.

There is still more to be done tc extend

the

reliable range beyond

6 nautical miles and

avoid occasional

screening effect of high

and steep waves.

Another school of

thought advocates

shipborne wave recorders

which are beset

with ship's wave system influence.

Experimental ways of

this influence out have been in%;estigated (3).

A special situation arises from the fact that it

is easy

to strain gage a ship to measure stresses and segment

a

model to measure bending moments in

waves.

Unless we learn

how to do the opposite we will have to corre;ate those two

quantities via the equality cf sea states, which is

the

weakest link in the chair.

Recently, much talk has been devoted to the application

of

computer systems in ship design (4).

_{Some of the ideas mentioned}

could Le applied in experimental environment.

_{This concerns}

especially the man-machine interaction, either

_{in real time}

when conducting experiments, or off-line when

_{preparing or}

processing data.

Examples for the latter applications:

computer graphics in

lines fairing and diagram plottirg,

preparation of tapes for numerically controlled

_model

manufacture (5), programming cf experimental

sequences,

wave generator (6) and carriage control tapes preparation.

In conclusion to this discussion

_{on systems we should}

pcint out that through A.T.T.C. the tank

_{community cn This}

continent is tied administratively into

_{one system.}

_{To work}

more effectively we need to establish closer compatibility

2tandards on the form of our experimental

_{procedures and format}

of our information acquisition.

_{This might appear a very}

laborious undertaking but it wouid certainly

_{prove to be a}

most rewarding one.

REFERENCES

Foyt, J.W.

"Detection of Algae and

Bacteriae in Town

Tanks", 15th A.T.T.C.,

Ottawa, June 19E8

Mathews, S.T. and

Kawerninski, L.I. "Main

Hull CirGer

St esses on S.S. Ontario

Fower", N.P.C.,

Mech.

Ens.

Division, Report ME-266,

Ott3wa,

October 19E7

Sellars, F.H. "A Note on

the Dynamic Calibration

of

Shipborne Wave Gages"

15th A.T.T.C.,

Ottawa, June 1968

Computer-Aided Ship

Design", Seminar at

University of

Michigan, Ann Arbor,

May 1968

Gospodnetic, D. "Numerically

Controlled Milling ot

Ship

Models" 15th A.T.T.C.,

Ottawa, June 19E8

Gagne, P.E. "Computer Program to calcuiate

the filter

Parameters for Special

Shapinc"

N.R.C.

Mech. Eng. Div.

Report MK-I8, June

19E6

APPENDIX

A

A Report to

The Committee on Instrumentation and Data Processing

of the American Towing Tank Conference

Ottawa, Canada, June 1968

Testing Facilities and Instrumentation

at the Naval Research and Development Center

Washington, D.C.

J.

H. Brandau

In view of diversified research interest and

variety of instruments and instrumentation systems it has

been decided to offer a survey of facilities available at

NSRDC summarily in a tabular form supported by a selection of

pertinent papers and reports.

Notes on past seakeeping practices (1961) at

NSRDC have been also added to stimulate discussion on form

and content of practical manuals on standard procedures.

Calm Water Measur-ing and TowMeasur-ing

Propulsor Perform-ance Measuring Propulsor Perform-ance Measuring (including cavitation) Self-Propulsion Performance Measuring Flow Visualiza-tion (including photography, etc.) Hydrofoil Perform-ance (including rudders) Measur-ing F. Measurements in

Boundary Layers & Turbulence, etc. Towing Basins Towing Basins Water Tunnels Towing Basins Towing Basins Water Tunnels Circulating Water Channel Towing Basins

Carriage Beam with Modular Force Gages

Carriage Propeller (Open-Water) Boat Transmission Dyna-mometers, Tunnel

Drive Shaft Strain Gage Dynamometers Either Transmission Type or Unsteady Force Dynamometers

Tufted Models, Use

of Dye Acid Trace Oil Flourescent Trace Carriage Arm Modular Force, Pressure Pickups Aerojet Corp-Six Component Hydro-foil Dynamometer

Towing Basin Pitot Tube Rakes

Water Tunnels Hot Wire

Wind Tunnels Hot Film

Type "A" - Go to IBM-7090 for Direct EHP

etc., Computations

Type "A" - IBM 7090 for Computation of Thrust, Torque, Efficiency

Type "B" - Go to Time Sharing Computer for Performance Computations

Type "A" - (Transmission)

Type "B" (Unsteady Forces)

IBM-7090 for Performance Computation

Photographic

Photographic

Photographic

Type "A" (Modular Force) - Use of Type "C"

Data Acquisition for Direct IBM-7090

Format

Type "B" (Strain Load Cells) - Directly to

Type "0" Data Acquisition

Type "A" (Pitot Heads Sampled on Wake

Survey System)

1-3.2

Modular Force Gage

Dynamometers & Rigs

Dynamometers & Rigs

Transmission Unsteady Test Rig Type "C" Data Logger Wake Survey Instrumentation Hot Wire Ref. 1 Ref. 2 Ref.3 Ref. 20 Ref. 2 Ref. 4 Ref. 5 Ref. 6 Ref. 7 Ref. B NSRDC (DTMB)

ATTC INSTRUMENTATION COMMITTEE REPORT

TANKERY INSTRUMENTATION SUMMARY

Type Test/ Measurement Test Facility Test Rig

Type Instrumentation Descriptive Reference

I. RESISTANCE, PROPULSION

II SEAKEEPING

Model Preparation Determination of Bending Moments

and Sheer Forces in Segmented

Models in Waves

Wave and Wind

Generation, Measurenent & Analysis Towing Methods Seakeeping Model Towing Measurements of Motions (slamming, accelerations, etc.) Hydrodynamic Coefficient Measurements for Prediction of Added Mass,

Added Drag, etc.

Coefficients

Determined in Air and Water

(Stationary & Underway)

Moored System Measurements in

Seas

Towing Basins Carriages II & V Maneuvering Bridge Carriage Basins

Towing Tank Carriages

Towing Basins Carriage II Maneuvering Bridge Carriage

Basins Motion Study Girder

Fitting Room Basin & Maneuvering Basin

Full-Scale

Model Support Rigs, Motion Study

Girder

100,000 lb and

200,000 Strain

Dynamometers

Type "B" (Record on Magnetic Tape, A-0 Conversion and Go to IBM-7090 for

Computation

Programmed Wave Signal Sequence, Tape Input to Electrohydraulic Control of Pneumatic Wavemaker,

Measurement of Wave Height by Sonic Probe In Most Cases, Models are Self-Propelled,

Motions - Type "B" (potentiometery) gyros

and recently additional force balance, Accelerameters (Donner Servo

Reposition-ing Type)

Forces - Type "B" Strain Gage Accelero-meters or Force Balance, Donner

Accelerometers, Recording or Graphic or Magnetic Tape

Type "B" Type "B"

Type "B" - Load Cell Mooring Cable Links. Data is Recorded on Mosley Type "X" and "V" Recorder or Varian Servo-Drive Recorder

This type of Measure-ment is in DevelopMeasure-ment

Stage

No Reference

Towing Basin Segmented Model Maneuvering

Basin

Segmented Model Ref. 10

Tests,

Past Seakeeping Ref. 11 Procedures (1961)

Wavemaking Ref. 12

Wavemaker Program Ref. 3

and Control

Simulation Ref. 13

Past Seakeepina Ref. 11

Procedures (1961)

Past Seakeeping Ref. 11

Procedures (1961)

Ref. 11 Ref. 10

D. Maneuvering

Characteristics

IV. ACOUSTICS &

VIBRA-TIONS (casued by

hydrodynamic forces)

A. Pressure pulses

from Propellers

"J" Basin Free Models

(Carriage I) Surface

Maneuvering

Basin

Submarine

Models Propeller in Waves

Water Tunnels Tunnel Drive System

Large Scale 1/4-Scale Model

Structural Nuclear Submarine

-Vibrations & Structural Scaled

Acoustics - Model

NSRDC Field

Station

-Lake Pondera,

Bayview, Idaho

Type "A" - Force & Moment Gages, Integrating

Digital Voltmeters, Digital Printer

Type "A" - Force & Moment Gages, Integrating

Digital Voltmers, Digital Printout

Combination of Type "A" - Force Gages and Type "B" Potentiometer Displacement Gages -Recorded on Strip Chart & Magnetic Tape Photographic - Timed Light Source Technique

for Positioning Recording, Rudder Angle and Propeller Control Manual Servo System. Radio Controlled Free Running Models

Experiments Conducted

Hydrophone (crystal) - 0-100 kc DC amplifica-tion into a General Radio Analyzer

Type "B" - also Self-Generating Velocity

Pickups, Hydrophones all Magnetic Tape Recording

1-3. 4 II. DETERMINTION OF STABILITY

AND MANEUVERING CHARACTERISTICS

A.

Stability

& Man- Rotating Arms Towing Strut Rig euvering

Character-istics

(Direc-tional Stability)

Stability & Man- Carriage II

Oscillators

(in-euvering Character- cluding Planar

istics (Pitch, Heave,

Motion Mechanism) Yaw, etc.)

Stability & Man- Carriages I Step Response

euvering Character- and II Techniques

istics

Description of Ref. 1 Force Measuring Technique Description of Ref. 14 Digital Data

Logger of Their Type

Force Pulse Testing Ref. 15

of Ship Models

Research & Facilit- Ref. 16

B. _{Vibrations Due Tunnels}

24-Inch Water _{Type "B" - Solid State Shaft Mounted}

Ref. 4 to Propellers _{Basins Tunnel}

Dyna-Amplifiers - Sliprings

mometer, Various _{Type "B" - Solid State Shaft Mounted}

Ref. 4

Model Unsteady _{Amplifiers - Sliprings, Records up to}

Force Dynamometers _{Six Components of Propeller Induced}

Vibrations and Analyzes for Amplitude and Phase on IBM-7090

C. _Hydroelastic

Basins - Support Rig, Strain _{Type "B" - DC Excitation & Balance, Magnetic}

Description of Ref. 17

Effects _{Carriage V}

Gage Flexures Tape

Pertinent

Measure-Water Tunnel _{Tunnel Drive -}

Pro-Type "B" - DC Excitation & Balance, Magnetic _ments

peller Blade Foil Tape

"Singing Propellers"

Strain Gage

NSRDC Hydro Lab

Report (in prep.)

FULL-SCALE

CORRELA-TION

A. Performance _Ocean

Ship-Shaft Torque, _{Type "A" - DC Amplifier - Magnetic Tape}

Description of Torque, Ref. 18

Powering Studies _{Measured Mile Thrust & RPM Meters}

Recording

Thrust (Hydraulic Maneuvering

Studies Cell), RPM

Instru-ments Open Ocean Continuous Ship _{Shore based Theodolite Manual Trackers with}

Free Route & Tactical Course

Positioning Measurements

Synchronized Potentiometer Angle Positioners,

Shipboard Heading Gyro - Radio Coordination,

Ship Position May also be Obtained from Radio Direction Finding

B. Pressure (Various _{Open Ocean Ships Hull (Static}

Strain Gage Pressure Transducers Hull _{Several Model-Full-Scale}

Sea States)

Pressure Calibra- _{Mounted - Type "B" Excitation and Control}

Correlations Have been

Con-tion Only) _{Magnetic Tape Recording}

ducted by NSRDC

C. Stress (Various

Sea States)

Open Ocean _{Large Number of}

Strain Gages

3 KH Carriage, Balance & Control _System,

Recording on Light Sensitive Oscillograph

Description of Full- Ref. 19

Scale Stress

Measure-Mounted on Hull _{or Magnetic Tape}

ments Structure

D. _{Motions (Various Open Ocean}

Ship Mounted "Donner" Type Accelerometer _{DC Amplifiers}

-No Reference available

Sea States)

Stabile Platform _{Recording on Sanborn Strip (for on-spot)}

or

Magnetic Tape - Processing by Seadac or

E. Waves

Towed Bodies

(Minesweeping,

etc.) (Various

Sea States)

VI. INSTRUMENTATION FOR

MODEL PREPARATION (making) Lines DWGS, & Fairing Materials Model Cutting Machines Finish of Models Checking Models & Prototypes Open Ocean Towing Rigs. Development in Towirg Tank Ocean Prooftesting

Tucker Sea State

Meter

Splashnik Sea State

Buoy

Carriage II & V

LVDT Pressures & Accelerometers (Hull

Mounted Sensors) Signal Conditioning

and Recording Instrumentation Topside

Buoy Contains Accelerometers which Modulate an Adjacent F.M. Transmitter

Type "B" - DC Excitation & Balance with Data Recorded on Strip Chart & Magnetic Tape

Type "E"

Tucker Instruction

Manual

DTMB Report (Wave Motion

Telemetry Buoy)

Description of Ref. 20

Towed Body

Measurement Instru-mentation

No Production Work Done with Numerical Control in Model Design and Preparation. It is Planned to Incorporate

NSRDC BASIC TANKERY INSTRUMENTATION SYSTEMS:

Type "A" Differential reluctance gages with NSRDC control units (AC

excitation, demodulator and balance circuits). A-D conversion

in Integrating digital voltmeter and printed on digital printer

with paper punch tape.

Type "B" _{Strain gage and potentiometer type transducers with DC excitation}

signal conditioners. DC amplifiers with output recorded on

magnetic tape, strip chart (on-spot look only) or A-D conversion

with digital printout and paper punch tape.

Type "C" _{Strain gage and potentionmeter type transducer with DC signal}

conditioners and amplifiers. On-line digital data acquisition

with Digital Data Acquisition System (DIDAS).

Type "E" _{Strain gage and potentiometer transducer with FM-FM direct wire}

and FM-AM telemetry. Recording done on direct write and magnetic

REFERENCES

Gertler, Morton, Report 2523, "The DTMB Planar-Motion-Mechanism System",

July 1967.

Schoenherr, K. E. and Brownell, W. F., Report 1660, "The High Speed

Basin and Instrumentation at the David Taylor Model Basin", November 1962.

Brownell, W. F., Report 1690, "Two New Hydromechanics Research Facilities

at the David Taylor Model Basin", December 1962.

Brandau, John H., DTMB Report 2350, "Static and Dynamic Calibration of

Propeller Model Fluctuating Force Balances", March 1967.

Loving, D. L. and Katzoff, S., RASA Memo 3-17-59L, "Fluorescent Oil Film

Technique-Boundary Layer Flow", March 1959.

Ficken, N. L. Jr. and Dobay, G. F., Report 1676, "Experimental

Determin-ation of the Forces on Supercavitating Hydrofoils with Internal

Ventil-ation", January 1963.

Luistro, James A., DTMB Report 1630, "A Multichannel Digital Data

Acquisition System", September 1964.

Hunt, R., Hydro Lab Tech Note 92, "NSRDC Wake Survey Instrumentation",

1968.

Purdue University

Delleur, J. W. et al,/Hydro Lab Tech Report 13, "Hotwire Physics-Turbulence Measurement in Liquids, February 1966;

Wachnik, F. G. and Schwartz, F., DTMB Report 1743, "Maneuvering Tests

of a Segmented Ship Model", July 1963.

Notes on DTMB Model Scale Seakeeping Instrumentation and Procedures,

1961.

Brownell, W. F. et al, DTMB Report 1054, "A 51 Foot Pneumatic Wave

Maker and Wave Absorber", August 1956.

Zarnick, E., Pritchett, C., Woolaver, D., Hydro Tech Note 80,

"Sim-ulation of Sea Spectra in the Naval Ship Research and Development Center Carriage II Test Facility", June 1967.

Goodman, A., Technical Manual 818-1, "Description and Operation of

Planar Motion Mechanism Instrumentation System, Hydronautics

Incor-porated, February 1968.

-Smith, W. E. and Cummins, W. E., Progress Report, 5th Symposium on

Naval Hydrodynamics, " Force Pulse Testing of Ship Models-Progress

Report", September 1964.

Research Facilities at the David Taylor Model Basin DTMB 1913, October 1964. A Monograph DTMB 2557, "Hydroelasticity"

Pinlott, R. and Dawson , DTMB 1127, "Manual on the DTMB Magnetic

Micrometer Torsionmeter, 1956.

DTMB 2610

Singleton, R. J., DTMB 2001, "Instrumentation for Towed Bodies".

Brownell, W. F. and Miller, M. L., Report 1856, "Hydromechanics Cavitation

Research Facilities and Techniques in use at the David Taylor Model

Basin", May 1964.

PAST SEAKEEPtNG PRACTICES

N.S.R.D.C.

(D.T.M.B.) 1961

Section 1. Models.

DTMB.seaworthiness models are generally made out of sugar pine, Honduras

Mahogany or fiberglas-reinforced-cloth-polyester-resin. Pine is used for models where the weight of the model is of no consequence and tests

are limited and of routine nature. Mahogany is frequently used for segmented models, models that are extensively tested in water, and .

light weight models. Fiberglas is used in the construction of light

weight models, models of unusual form, and models in which the

material enhances desirable structural properties of the hull.

Tank I - The usual length of models tested is 5.0 to 6.5 feet.

Tank II - Models range from 10 to 20 feet in length.

Tank III - A single 15 foot model is being constructed for tests in

this basin. It is intended to make future models 10 to 16

feet in length depending on the type of the best and models

displacement.

Tank I The weight of the model hull, depending on length and

material, is 15 to 21 lbs.

Tank II The usual weight of the model hull is 65 to 500 lbs.

Tank III The weight of the mahogany hull that is being built is

274 lbs.

(i) Resistance experiments in waves are usually conducted in

Tank I, and in most cases the model is tested without appendages.

Designation for the purposes of this paper only. See section 6 where the principal dimensions of the tanks are given.

Tank I - Propulsion experiments have not been conducted. Tank II - All appendages are usually fitted to the model

during propulsion experiments.

Tenk III - All appendages are being fitted to the model for

propulsion experiments.

Tank I - Most of the appendages are usually excluded from

the model in the measurement of pitch and heave,

as these are the only motions measured in this

basin.

Tank II - All appendages are usually fitted to the model for

the motion experiments.

Tank III - All appendages are being fitted on the model for

the experiment in progress.

e) (i) The vertical center of gravity is determined from the calculation of the location of the transverse metacenter

of the model and an experimental determination of the GM

from an inclining experiment in water (see 1-0.

The Iodation of the longitudinal center of gravity is

determined from the calculations of the longitudinal

position of the center of buoyancy and ballasting of the

model to the appropriate water line.

The longitudinal radius of gyration of the ballasted model is determined by one of the following three methods; bifilar

pendulum, two springs or

a

compound pendulum. In the

bifilar pendulum method the model is supported in the upright position a small distance above the v.c.g. ( to insure

stability) and at the far ends of the model by two long (about

model length) cables. The model is oscillated about its

vertical axis, and it is assumed that the radii of gyration

about vertical and transverse axes are approximately

the

same.

In the

spring method of determination of the longitudinal

radius of gyration about transverse axis the model is supported

1-3 :12

distance I should be appreciably different from the radius

of gyration of the model. The model is oscillated to heave

and then pitch. The ratio of the periods of oscillation is

equal to the ratio of / to the radius of gyration.

Com-pound pendulum method as described by Reiss is also used.

In this method the model in upright position is suspended

at two different points above the v.c.g. From measurements

of the periods and placement of additional known weights on

the deck of the model the v.c.g. and radius of gyration are

calculated.

(iv)

Tne

transverse radius of gyration of a

ballasted model is

determined by the natural roll period, after the vertical

center of gravity is appropriately adjusted.

TMB is currently constructing an inertia apparatus that

will enable measurement of the moments of inertia about all three axes of the model as well as the space center of gravity

of the model in essentially one operation. In the method considered the model will be suspended within the hull on

a knife edge that can be rotated to be parallel with the

longitudinal or transverse axes. The knife edge will be

located some known distance above the intended position of the vertical center of gravity or at the center of gravity

itself. Moments of inertia about the longitudinal

and

trans-verse axes will be determined from the free oscillations of the model or forced oscillation, if a spring is used to

pro-vide a restoring force, about an appropriate axis. Moment

of inertia about the vertical axis will be determined from

an oscillation of the model about that axis on a supporting low friction bearing with the restoring force provided by

a spring. Vertical center of gravity of the model will be

found by inclining the model in an air, with a known moment.

The calculated longitudinal and transverse position of the center of buoyancy can be corrected by the longitudinal and

2_{Reiss, Howard R., "A Procedure to Impart Specified} Dynamical Properties to Ship Models," David Taylor Model Basin Report 986 (Mar 1956).

transverse adjustment of the pivot point in the initially

ballasted model in water to the desired water line. The

weight of the pivot plate that will move with the model

will be about 15 lbs. The periods in the inertia c.g.

experiment will be measured with an electronic counter. The

angle, since the accuracy of measuring it contributes

significantly to the error in calculation', of the v.c.g.,

will be measured to within a couple of minutes with a

mirror and a reflected light beam projected on a scale.

In the present experiments the period is measured with

a stop watch over at least 60 cycles.

The transverse metacentric height is determined by an inclining

experiment in water. In the experiment an instrument is )1used that

contains a pendulum to measure the tangent of the inclining angle

and a permanent set of weights with equally spaced pegs to measure

the moment. The location of the c.g. of this instrument is known and

maintained constant during the experiment. The small correction to

the v.c.g. due to the instrument placed on the deck is accounted for

in the GM calculations. The inclining moment is plotted versus the

tangent of the inclining angle and the slope at zero angle is used in the evaluation of the metacentric height.

The natural rolling period in calm water is obtained from a free

oscillation of the model, about longitudinal axis. The model is initially

inclined some known angle and then released. The natural pitching and

heaving periods in calm water are obtained from a free oscillation of

the model as for the natural roll period, or from a forced oscillation

by hand. _{The periods are determined from a recorded sample of the}

motion on paper tape.

In the seaworthiness experiments turbulence is not stimulated on on the models.

Section 2. Motion measurements in regular waves

In Tank I a gravity dynamometer is used to tow the models.

In Tank II only once the model was towed in waves. Tank II models

are usually self propelled and are only guided during the test.

In Tank III it is planned to measure motions on self propelled guided

models.

Tank I With the gravity towing system used in this tank, the sway and

yaw motions are restrained. The model is towed by a

line stretJi

between two pulleys about 140 feet apart. The tension in the line

(about 8.0 lb) prevents sway and yaw. The system also has some influence

upon surge and heave.

Tank II In most of the tiasts conducted in this basin the sway, yaw

and frequently roll are positively restrained. The motion apparatus

has also some influence upon surge. Free model tests in all six degrees of freedom have also been conducted in waves.

Tank III The new Model Motion Measuring Apparatus that will be used in

the Seakeeping Facility allows all six degrees of freedom.

However,

due to its mass the apparatus has some effect upon surge, sway and yaw

motions. The added inertia due to this instrument,

on a 15 foot model

is 37. of the model mass in surge and sway and 370 of the model's moment

of inertia, in yaw. Model Motion Measuring

Apparatus also imposes a

force on the model in surge and sway. This force is a function of

displacement of the c.g. from the center point on the carriage, and

does not exceed 1.5 lbs. This equipment is new and further work is

being done to reduce the inertial and static effects upon model motions.

In Tank I additional weights are used to

accelerate the model to the

mean speed in waves. The model is decelerated manually.

In Tank II special restraining ropes are installed allowing

the self

propelled model about 6 ft of longitudinal motion. Although thaimodel

is accelerated and to some extent

decelerated by the propeller,

the ropes provide additional acceleration and deceleration.

Tank III No permanent system is designed as yet for this facility.

The tranducers (gyros, potentiometers, accelerometers etc.) used at

the David Taylor Model Basin impose very negligab1e-1 amount ot inertia

or force, at the frequency tested, on the model motions.

In Tank I the pitching motion is measured with a displacement gyro:,

about the horizontal axis in space. In Tanks II and III the pitching

motion is measured with a potentiometer about a horizontal axis in space.

0

In Tank I the heaving motion is measured with an accelerometer and by the nature of this instrument the measurement is made along the vertical

axis of the model, moving with the model. No correction is made for

the signal due to change in orientation in the gravitational field of

the model, . In Tank II and III the heaving motion is measured

with a potentiometer along the vertical direction in space.

In Tank I the rolling motion is measured with a displacement gyro,

about a horizontal axis in space.

In Tank II and III the rolling motion is measured with potentiometers,

about the longitudinal model axis.

In Tank I the average model speed is measured by recording the elapsed

time during which the model Moves a known distance. Time is measured

with an electronic counter. The distance is measured by the number

/of revolutions made by one of the pulleys of the gravity towing system.

In Tank II the model speed is matched to the carriage speed. The

carriage speed is measured over a period ot time by a magnetic pickup

and a gear. Electronic countenswith a time gate are used to record

the speed. Relative position of the model to the carriage is also

recorded, so that any change in model speed with reference to carriage

can be accounted for.

In Tank III attempt is being made to measure the speed of the

carriage and the speed of the model relative to the carriage. The

Low frequency phenomena are amplified and recorded on a 150 Sanborn

Amplifier Recorder. Higher frequency signals are amplified on 350

Sanborn Amplifier and recorded on magnetic tape, or by Consolidated

Engineering Corporation's amplifier with.recording on the string

oscilloqaph.

In general the natural frequency of the transducer is at least by factor of two higher than the highest frequency recorded. The damping

of the transducer is also considered. The frequency response of the

150

Or

350 Sanborn is about 240 cps for the amplifier

and about 50

cps for the recorder. The frequency response of the tape is,at the

speed of 1-7/8 ips,332 cps and at higher speeds proportionaLly:

higher. The CEC amplifier has the frequency response

of 3000 cps and

the oscOlograph, depending on the galvanometer, up to 3000 cps.

In general the double amplitudes of the motions are measured from the

record for about 10 cycles and the average amplitude is calculated. The analysis of the wave heights and motions corresponds to the same

time interval. Currently work is being done on automatic data

reduction. In the system of automatic data reduction the motions are

on magnetic tape. The tape is then electronically digitized

(14

channels at the same time) and amplitudes, periods and phase angles

between the desiredchannels are calculated on the IBM 7090.

1) The phase angle between wave height and

the motions is presently being

analyzed on some tests. Generally however, the phase angle between

pitch and heave only is calculated. Plans are being made to measure

the phase angle between the waves and the motions in the future

experi-ments. Phase angles are read from the Sanborn

records of the wave

height and model motions. Currently work is being done on automatic

phase angle analysis as explained in the above section.

m) In addition to the measurement of model motions vertical and

transverse

accelerations are measured at various points along the hull.

Wetness

of the model is observed. Slamming pressures,

accelerations and strains

are also measured.

Section 3. Bending moments and shear force experiments in regular waves

Bending moments are measured by means of strain-gages mounted on the

horizontal surface of a flexure beam interconnecting the model

segments.

Bending moments have been measured at seven stations. Out of 20 stations

the model was segmented at stations 3, 5, 7, 9, 11, 13, and 15. In

two other models the bending moments were measured at station 5 (to

obtain loading on the bow).

In each part of the model the weight, longitudinal center of gravity

and longitudinal moment of inertia of the segment were scaled down.

The natural frequency of the ship was scaled in the model.

The lateral bending moments were me4sure# in different models at

stations 3 and 5. The measurements were made by means of strain gages

mounted on the vertical surface of the flexure beamis.

The vertical shear forces were measured at seven stations along the

model length. The measurements were made at Stations 3, 5, 7, 9, 11,

13, and 15. Vertical and horizontal shear forces were also measured

on two other models at station 5. The shear forces are measured by

means of taking a difference between two bending moment measuring

half bridges, located a known distance apart.

Section 4. Resistance experiments in regular waves

Resistance of the model in waves is measured in Tank I only. The

gravity dynamometer is used in measuwing the resistance. The model

constraintsin Tank I are explained in sections 2-a, b, and c.

b) The drag of the model in waves is measured with a gravity type

Ordinarily, the EHP of the model in waves is not calculated.

The resistance experiments in waves were conducted on 5 to 6 foot

models without appendages.

Section 5. Self-propulsion experiments in regular waves

The propeller thrust can be measured with a differential transformer

thrust dynamometer.

The propeller torque can be measured with a differential transformer

torque dynamometer.

The propeller rpm is measured with a magnetic pulse generator recordd

on an electronic counter over a period of one to twelve seconds.

During the self-propulsion tests the propeller loading depends on the

propeller diameter, number of propellers, stern hull form, resistance

in waves and model speed, in rneral however, the loading is

moderately

to heavily loaded.

lAk

₄

1?)

k 1.

f) The self-propulsion experiments in Tank II are conducted at

essentially constant mean speed of the model. The models are tested

in all wave conditions at predetermined discrete mean speeds

covering

the range from zero to the depAgned ship speed. Propulsion motor voltage is adjusted so that drift between model and carriage is a

minimum.

In Tank III tests at constant rpm, thrust or power will also

be conducted in the near future.

The frequency response of the transducer is about 100 cps.

The

frequency response of the recording instrumentation is 3000 cps.

The method of towing the self-prepelled model and the constraints

are the same as explained in section 2-a, b and c for Tanks II and III.

The ship speed loss in waves is estimated at_constant resistance

or constant power (R x v).

The same model motions are measured as in Section 2.

Section 6. Tank and wavemaker equipment

a) At the David Taylor Model Basin the following basins have generally used wave making equipment.

b) The cross section of the basins is rectangular.

c) The wave-makers used are of the pneumatic type.3

d) The wave length and wave heights generated by the wave makers are approximately as follows:

Tank Wave Lengths Wave Heights

2 ft. to 13 ft. 0 to 7 in.

II 5 ft.Aoo. 50 ft. 0 to 24 in

III 3 ft. to 55 ft. 0 to 24

in.

e) The wave period is measured with an electronic counter with the accuracy

of + 0.001 of a second. The wave trace is recorded with capacitance

or sonic4 type transducer. In Tank I the transducer is stationary

in the basin and located about 20 ft from the wave maker. In Tank II

and III the transducer is located on the traveling carriage about

3Brownell, W. F., "A Rotating Arm and Maneuvering Basin," David Taylor Model

Basin Report 1053, July 1956.

4Straub, Lorenz, G. and Killen, John, M., "The Sonic Surface-Wave Transducer," St. Anthony Falls Hydraulic Laboratory. _{Memorandum M-74, Feb 1959.}

Tank Length Breadth Depth

I 140 ft. 10.0 ft. ' 5-5 ft. II 1800 ft. 52.0 ft. 20.0 ft.

10 ft forward of the model.

In Tank I a large number of layers of brass screens are held at an

angle to, and piercing, the surface. In Tank II and III1concrete

grids of 2.0 in. x 2.5 in. horizontal bars are used as wave absorbers.3

The wave absorbers are inclined 12 degrees to the surface and extend

7 ft below the surface.

g) The usual time between experiments is about:

Tank Time

20 min.

II 1 hOur

III 15 - 30 min.

h) No other devices are used to quiet the water surface

after a wave test.

i) The side wall wave suppressors are out of water and

above the waves

while the waves are generated.

j) In Tank I no informatio is available on the

variation of wave height

along the length of the basin.

In Tank II the variations of wave height along the basin length is recorded during each experiment.

In Tank III, as part of the initial calibration waves are being

measured at three positions from the short bank of wavemakers.

k) Tank I has no carriage.

Tank II the carriage speed is from 0 to about 18 knots.

Tank III the carriage speed is from 0 to 15 knots.

Saction 7.

Oblique regular wave tests

The model can be tested at any continuously variable angle between

0 and 180 degrees.

(00-450

90_135om, 1800-225°, 270°-305°)

The model will be self-propelled and automatically steered in

oblique waves.

The carriage speed will be controlled by the model.

No special equipment is designed as yet to control the model during

acceleration and deceleration:: period.

Fail-safe system,

is

incorporated to prevent damage to the apparatus.

In the first tests

it is also planned to use several safety-lines that may be helpful

in gaining initial experience.

The model sway, yaw and roll will also be measured in obiique waves

No restraikas or limits are imposed on the motions in the newly

designed apparatus for the six-degree of freedom tests.

The test results are transmitted by means of appropriate cables from

the model and the Model Motion Measuring Apparatus to the recording

instruments oil,the carriage.

Section 8.

Irregular wave tests

No attempt is being made to precisely duplicate scaled down

ocean

spectra.

_{In the generation of irregular waves however, effort is}

being made to produce uniform energy distribution and Noltaann

spectra.

Presently the irregular waves are produced by varying randomly the

valve frequency, generating Variable frequency

waves.

Although the

irregular waves are generated at a constant

_{blower rpm, the waves}

vary in height.

_{Work is also being done on the installation of}

hydraulic actuator drives that can be programmed to vary the valve

-frequency and amplitude. The programming of actuators should be a

more versatile system.

Tests conducted indicate, that the irregular wave patternsgenerated

are very closely reproducible

All six,motions as well as any other desirable information can be

measured on the model in the irregular wave tests.

The motions are recorded essentially in an analog form and then cont.

verted to FM on a magnetic tape and monitored on a Sanborn recorder. Spectral analysis of the waves and motions can readily be made from

the tape. The tape can also be digitized for further mathematical

calculation on the IBM 7090.

0

Routine spectral analyses of the wave height and the motion are

carried out.

g) The wave and motion spectral analyses are

obtained by the filter-square

analog method on the DTMB SEADAC.8 The amplitude response operators

can only be calculated by hand from the wave and motion spectra.

The

co- and quadrature spectra at present can only be calculated on

the

IBM 7090 from the digitized data of the waves and motions.

Work is

being done to conduct that analysis automatically from the

magnetic

Tape.

6Marks, Wilbur and Strausser, P. E., "SEADAC - The Taylor

Model Basin Seakeeping

Data Analysis Center," David Taylor Model Basin Report

1353, July 1960.

NOTE:

Recently in

the

140 foot basin, (Tank I) a towing carriage has been

instiled to replace the old gravity dynamometer. At present a subcarriage

system to measure the model pitch, heave and surge are in design stage. In

this syst4m tha model will be towed. The towing force will be applied

through a magnetic clutch and can be adjusted while the model is underway.

Additional

weights to tow the model can also be used, if required. Sway,

APPENDIX B

A Report to

The Committee on Instrumentation and Data Processing

of the American Towing Tank Conference

Ottawa, Canada, June 1968

Testing Facilities and Instrumentation

at the Stevens Institute of Technology

Hoboken, N.

J.

W. Brown

1. Resistance, Propulsion and Cavitation

Ia. Calm water measuring systems

The Davidson Laboratory's force and motion measuring equipment is

based on the linear variable differential transformer. For steady state measurement DC/DC LVDT's made by Schaevitz of Camden, New Jersey, are used.

These units require 24 v. DC excitation and have a maximum output of 5 v.

for the maximum deflection of

0.050

inches. They contain solid state electronics, are highly stable and cheap, costing $100 each.

For force measurement these transducers are built into force

balances made by DL (Davidson Laboratory). These balances connect two

heavy parallel plates by four spring flexures and are made of 7075-T6

aluminum. All balances used at DL are made in house and range from drag

balances measuring up to 10 lb to three component balances measuring up

to 1000 lb on to six component balances measuring about 100 lb. The

sig-nals are fed to integrating digital voltmeters capable of counting up and down, typical integrating periods being of the order of 10 seconds.

lb. Propulsion

DL uses a propeller boat for open water measurement incorporating

a thrust-torque dynamometer built by DL and using the transducers described

in la. Propellers are driven by synchronous motors excited by a rotating

transformer.

Id. Flow visualization

Some success at low speed has been had with flow visualization by

means of hydrolysis, the resulting bubble streams being viewed and

photo-graphed through a periscope.

If. Boundary layer measurement' and turbulence stimulation

Hot wire and hot film equipment is in use but only limited

experi-ence with hot film has been had. The hot wire is extensively and

success-fully used in wind tunnel work.

For ship forms turbulence is stimulated by towing a strut ahead of

the model. On sail boat sand strips on the stem are applied. Hama strips

2. Seakeeping

Models

Dynamic wood models are used. In the case of aircraft tests balsa

wood covered with thin glass cloth gives a very rugged light model.

Wind and wave generators, etc.

The DL wave maker consists of a plane-faced wedge plunger of fixed

stroke which can be driven at variable or constant frequency. Slatted end

beaches sloping 100 are used (with small side beaches for calm water tests)

and a wave suppressor is automatically run up and down the tank between

runs. Resistance type wave wires with a linearizing circuit are

employed,

excited by carried amplifiers at 2400 cps. These wires are located at a

number of stations down the tank and are also towed with the model.

Towing methods

A light model carrying carriage is towed by a falling weight drive

from the main carriage attached to the drive cable. The light carriage

and model are completely free-to-surge (friction is about 0.01

lb) and

the speed of the main carriage is servoed to follow the surge carriage.

For oblique sea work a rail is run at an angle to the waves in a

75 ft x 75 ft tank. In this case the model is free in all six degrees,

the surge and sway motion again being followed by servoed main carriages.

This equipment also incorporates an automatic steering engine to keep

the

ship on course.

Measurement of motions

All six degrees of motion are sensed by either linear or rotary

AC LVDT's signals being fed to Sanborn 350 carrier amplifiers.

Recordings

are made on Sanborn pen recorders at low frequency

and on ultra-violet

light beam strip recorders at high frequency (up to 1000

cps). If spectral

analysis is to be carried out signals are

simultaneously recorded on a

14 channel analogue magnetic tape recorder.

Recordings are usually made

at 7.5 ips and played into a digitizer at 15/16 ips.

The digitizer can

sample at up to 30 times per second and produces a

perforated paper tape

which subsequently serves as input to an IBM 360 computer.

The combination

of tape speed and scan rate gives considerable latitude of real-time

scan rate. However the system is cumbersome and DL is currently obtaining

a digital magnetic tape recorder.

Pressure measurement are rarely made but accelerations are picked

up with miniature 50 g strain gage accelerometers. These have proved

very fragile and are being replaced by DC servo accelerometers.

2h. Motion picture technique

16 mm motion picture cameras are used. Movies are taken in slow

motion so that when projected at 16 pps the time scale of the motion is

expanded to full-scale. Both framing cameras and prism cameras have been

tried and though they are more costly the framing cameras are much

pre-ferred.

3. Steering and Maneuvering

3a, Rotating Arm

DL uses a rotating arm exclusively to obtain stability derivatives.

3d. Free models

Free models with limited radio control are used for determining

turning circles and performing maneuvering experiments.

6.

Model Making.

Lines drawings are prepared by hand and sent to the model maker.

The models used at DL are up to 10 feet in length, made of sugar pine and finished with five coats of varnish rubbed down

to a dull finish.

A few

models have been made of fibre glass which is suitable when multiple models

are needed but is otherwise heavy and expensive.

In the course of his work the model maker produces section template

in hardboard and these are later checked against the lines drawing and the

model.

DL is investigating the use of the computer to produce lines

APPENDIX C

A Report

to

The Committee on Instrumentation and Data Processing

of the American Towing Tank Conference

Ottawa, Canada, June 1968

Testing Facilities and Instrumentation

at The University of Michigan Ship

Hydrodynamics Laboratory

Ann Arbor, Michigan

Resistance, Propulsion and Cavitation

A. Calm Water Resistance Measuring Systems

For several years the Ship Hydrodynamics Laboratory at The University of Michigan has employed load cells manufactured by The Daytronic Corporation as transducers for measuring

steady state and very low frequency forces. Each transducer

consists of a linear differential transformer which senses

the small deformation of a diaphragm spring system. Many

cells, with ranges from 0-5 lbs. to 0-1000 lbs., are

avail-able. The load cells, which are very rugged and insensitive

to transverse loads, are excited by Sanborn carrier preampli-7iers, and the output of the amplifiers is recorded on one of

several instruments depending on the requirements of the test.

When only one or two forces is being measured simultane-ously and steady state conditions prevail, the output of the amplifier is filtered by a passive, two stage, L-R-C filter

and recorded on a Moseley liquid ink X-Y recorder. _{When several}

time dependent forces are being measured simultaneously _and

must be correlated with time, an Ampex SP 300 seven channel FM magnetic tape recorder is used to record and store the data

that has been used with some success

digitizes analog data

re-corded on magnetic tape and converts the data to a digital

magnetic tape in a form easily analyzed by a digital computer.

At present up to seven channels can be digitized simultaneously

at the rate of approximately 90 samples of each channel per

second, with a conversion accuracy of one part in 2042.

Al-though this system is far from ideal since the analog tape

must be digitized "off-line," it nonetheless enables

the

Lab-oratory to handle large amounts of recorded data in a relatively

short time when the need arises.

The Laboratory is currently using, and/or developing

var-ious dynamometers which employ the above mentioned

transducers

as sensing elements for standard tests

involving force

meas-urement. When ship models are

tested, a simple bellcrank

trans-lates the horizontal towing force into a vertical

force which

is sensed by the load cell. This arrangement

permits easy

cal-ibration and zero suppression by means of calibrated weights

suspended from the bellcrank.

A swinging beam dynamometer is

used when the models being

tested must be restrained in all degrees of

freedom except

surge. Again, a load cell senses the

towing force. The

dyna-mometer is calibrated with hanging weights

which act on the

beam through a bellcrank.

A new

dynamometer for measuring drag,

side force, and yaw

moment on sailing boat models is in the final stages

of

opment and is scheduled for initial use late this summer. It,

too, uses three load cells as sensing elements and is designed to be substantial enough to tow models that are larger than those commonly used at Stevens Institute and The Massachusetts

Institute of Technology.

The Laboratory is just beginning to evaluate a new, digital data acquisition system for use in measuring calm water

resis-tance. The analog output signal from the carrier amplifier

is converted to a frequency proportional to the resistance, and this frequency is counted by a digital counter for a prescribed

time interval. The resulting count is proportional to the

resistance and can be made to read in pounds if the time

in-terval and amplifier gain are selected appropriately.

Simul-aneously, the carriage speed is measured digitally by means of a rotating slotted disc transducer linked to the carriage

wheels.

At the end of the run the true average carriage speed and the true average resistance during the same ten second

time interval are obtained. Aside from the technical advantages

of such a system over the analog recording methods used pre-viously, it has been found that technicians, either consciously or unconsciously, are apt to do a more careful job of calibra-tion and recording when they are watching numbers flashing in front of them instead of watching a pen draw on a piece of paper. The Dymec voltage to frequency converter now being used is

However, it cannot distinguish betwee- a positive and

negative

input signal. This feature of the instrument

makes it necessary

to apply a bias voltage to the input in order to insure that

the input signal never changes sign.

This makes the output

a little difficult to interpret, but does not otherwise

inter-fere with the system's operation. This drawback may be avoided

by using a true integrating digital

voltmeter but the cost

consequently increases by about a factor of four.

B. Propulsor Performance Measuring Systems

The Michigan Laboratory uses a propeller boat

with a Kempf

and Remmers strain gage torque and thrust

transducer to

meas-ure open water propeller performance.

The Sanborn carrier

amplifiers mentioned previously are well suited for

exciting

the strain gage bridges in this dynamometer.

R.P.M. is

measured

with a magnetic proximity pickup

and recorded on a digital

counter.

Essentially the same instrumentation

package is used when

self-propulsion tests are carried out on ship models.

The

Kempf and Remmers dynamometer and the proximity

pickup are

mounted in the model together with a propulsion motor.

A

second strain gage type torque and thrust dynamometer, similar

in design to the German

instrument

but built locally by Lebow

Associates, Inc. of Detroit, Michigan, is used for

self-pro-pulsion tests on twin screw models.

In the past few years much work has been done to

develop

a system for measuring the velocities in the wake of models

easily and accurately. A five hole pilot tube system

utiliz-ing Sanborn 268B differential pressure transducers was discussed

at the 1964 ATTC [2]. It was used successfully to measure

the wake components of one hull of the Project Mohole vehicle,

but because of the fragile nature of the pressure transducers

and the inherently low frequency response (', 5 Hz) it was deemed

necessary to develop a different system for current

applica-tions. The new system employs a probe consisting

of three

mutually perpendicular cylindrical hot film probes. The films

are quartz coated to insure stability and to eliminate

elec-trolysis when used in water. Three constant temperature

ane-mometers (Thermosystems, Inc. Model 1033) are used to heat the

films. The hot film has several important advantages over the

pitot tube as a velocity sensor: First, the frequency response

of the hot film is several orders of magnitude greater than

the conventional pitot tube; second, the hot film can

econom-ically be built much smaller than the pitot tube; third, and

most

important for

work in the towing tank, is the fact that

the hot film is most sensitive at very low velocities, whereas

the pitot tube is least sensitive at low velocities. This

new system is now in the initial stages of calibration.

II. Seakeeping

A.

Wave Generation

calibra-ting a plunger type wave-maker in its 365' foot model basin.

Towing Methods in Waves

A new dynamometer, in the final stages of its development,

will enable us to tow relatively large models in waves with

a constant towing force [3]. Heave, pitch, and surge relative

to the main carriage are measured with potentiometers. The

dynamometer consists of a subcarriage suspended from the main carriage on rails.

The model is restrained beneath the subcarriage by a load

cell which measures the towing force on the model. This load

cell is mounted rigidly to the heave rod in such a way that it is insensitive to any transverse loads which may occur

be-tween the model and the heave rod. The towing force on the

model, measured by the load cell, is compared with the desired

constant towing force. If the measured force is greater than

desired, a servo motor moves the subcarriage backwards with

respect to the main carriage, lowering the measured force.

Conversely, if the measured force is less than desired, the

servo motor moves the subcarriage forward, increasing the

tow-ing force. Thus, during a run down the tank, the subcarriage

will be oscillating about a mean position with respect to the

main carriage while towing the model with a constant towing'

force.

Pressure Measurements and Photography

A-recent series of tests has shown that the Laboratory

has the capability to measure hydrodynamic loads on models in severe seas and to photograph the model's behavior in very

slow motion. An array of flush mounted piezo-electric

pres-sure transducers in conjunction with charge amplifiers was used to measure the impact pressures on the deck house of a

model running in severe head seas. The pressures were recorded

on a seven channel visicorder equipped with high frequency

galvanometers. A high speed Fast-air movie camera recorded

at 100 frames per second the wave action on the hull and

super-structure. Very interesting flow details were visable when

the film was played back at 16 frames per second.

III. Steering and Maneuvering

E. Free Models

Small, free running, radio controled models can be tested In the Laboratory's new 60' x 100' x 6' maneuvering basin. The system currently in use consists of two time-multiplexed channels of radio control operating on the Citizens' band. Each channel operates a continuous motion linear servo motor. One motor controls the speed and direction of the propulsion motor while the other controls the rudder angle.

Tu:70 _{channels of FM-FM telemetry operating on the 88-108 mHz}

band are employed to transmit rudder angle and RPM back to the

operator. Each channel consists of a miniature, battery powered,