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J. H Curry.

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NAVY DEPARTMENT

DAVID TAYLOR MODEL BASIN

WASHINGTON, D. C.



DTMB -1947 (Nov.)



RESULTS OP EXPERIMENTS WITH MODELS OF HIGH SPEED
TOWING TARGETS INCLUDING ESTIMATES OP PULL-SCALE
TARGET DRAG AND CABLE TENSION



by



Jo Ho Curry and Jack Posner



w




lovember 1947



Report 595



5°l5



TABLE OP CONTENTS

page

ABSTRACT. ....................... 1

INTRODUCTION. .......... ...... 2

DESCRIPTION OP MODELS ...... . 3

TEST APPARATUS AND PROCEDURE 4

TEST RESULTS AND ANALYSIS OP DATA 7

DISCUSSION. 9

CONCLUSIONS ...................... 11

REFERENCES. ............... . 13

APPENDIX. 14




RESULTS OP EXPERIMENTS WITH MODELS OP HIGH SPEED TOWING TARGETS
INCLUDING ESTIMATES OFFULL-SCALE TARGET DRAG AND CABLE TENSION



ABSTRACT

Five models of high-speed towing targets, two flat-
bottom sled-types, two vee-bottom boat- types from Bureau of
Ships designs, and one vee-bottom toboggan- type David Taylor
Model Basin design, were tested at the David Taylor Model Basin
and in the Potomac River in an attempt to develop an improved
type towing target. The models were towed in the basin (a) by
a long cable attached to the dynamometer, and (b) under the
towing carriage with load increments applied to simulate various
lengths of cable „ Subsequent tests were made in the Potomac
River to observe the maneuverability of the models when towed
in waves. Motion pictures were taken during the latter tests
to show the action of the models in a straight run and in turns.
Estimates of full-scale target resistance and horizontal cable
tension* are given for various speeds and lengths of cable.

It was found that for a given displacement and
initial trim, a change in tow-point position, within the limits
covered by the tests, has little effect on target resistance
or directional stability. The resistances of the two boat-
type targets and of the TMB design target were appreciably
less when they were towed by a single cable in place of the
conventional bridle, but the effect on the directional stability
was small. When towed by the single cable the targets had a
tendency to be self righting, after deliberately being capsized.



•» The terms horizontal cable tension and horizontal target-
plus-cable drag are used synonymously in this report.



-3-

During the model basin tests for resistance, without
simulated cable load, the boat type targets exhibited undesirable
porpoising at speeds above 35 knots full scale. The porpoising
was of sufficient severity to make it inadvisable to operate the
targets at these speeds. The TMB design did not show any ten-
dency to porpoise, even when violently disturbed.

At high speeds and long cable lengths all of the
targets have practically the same horizontal cable tension, with
the exception of the revised G-60 sled-type target. For cable
lengths of 1500 and 2500 yards full scale and a horizontal cable
tension of 60,000 pounds, the estimated maximum speed attainable
for the respective cable lengths is practically the same for all
targets, with the possible exception of the sled- type target
mentioned above „

It is concluded, therefore, that the fundamental fac-
tors in the determination of an improved target sled are not
the conditions affecting target resistance or the target resis-
tance itself 9 but the cable drag, the target stability, and the
target maneuverability,,

INTRODUCTION

In April 1930 the Experimental Model Basin at the
Naval Gun Factory, Washington, D. Co, conducted experiments on
a series of models (1)* for the purpose of developing a



* Numbers in parentheses indicate references on page
of this report.



-3-

high-speed towing target for the War Department Coast Artillery.
A 25-foot by 12-foot rectangular timber sled was the result of
this investigation,.

Subsequently^, the demand for larger target screens
and higher target speeds led to further investigations and the
ultimate development of 30-., 40-, and 60-foot sleds (2)„ The
present 60-foot sled is reported to perform the best of the
series, however, it is not entirely satisfactory,, Because of
the high resistance of the target, the relative instability
when under tow, and the limited strength of the towing cable,
failures have occurred in service

In order to reduce the cable tension and develop an
improved planing-type target, the Bureau of Ships prepared plans
for three new designs of targets and requested (3) that the
Taylor Model Basin conduct tests on models of the new designs
and on a model of the revised 60-foot sled-type target to ob-
tain quantitative data.

In addition to the four models constructed from the
Bureau of Ships plans, a fifth model was developed at the
Taylor Model Basin,, '^he Bureau of Ships subsequently requested
(4) that maneuvering tests of the models be conducted in the
Potomac River to observe the effect of varying the fore and
aft position of the towpointo

DESCRIPTION OF MODELS

The TMB model numbers and the corresponding full-
scale design designations are given below;



-4-

Model Full-Scale

4024 Revised G-60 sled-type

4025 All-steel sled-type

4026 BuShlps design "A", boat-type

4027 BuShips design "B" , boat- type
4029 TMB design

Model and full-scale relationships are given in Table 1.
The model of the revised sled-type target was con-
structed of pine and the other models were made of balsa wood
in order to keep the total weights within the limits specified.
The models were all constructed to a scale of 1/15 full-size.
All models were four feet long* Simulated target screens
were installed on the models for the Potomac &iver tests „
Pitting room pictures of the models as tested are shown in Figure
21.

TEST APPARATUS AM) PROCEDURE

The test and analysis program was divided into four
parts o Part 1 of the program consisted of towing the models by
cable in the model basin to determine the comparative resistance
characteristics o A 1/32-inch diameter steel cable 400 feet long
was used for towing the models Each model was tested at two
initial displacements and at a series of initial trims, positions
of towpoint, and speeds, except the TMB design which was towed
with a single cable attachment in place of the conventional



-5-

bridle used for the other models and for one position of tow-
point only. For comparative purposes spot tests were made of
the TMB design with conventional bridle and of the BuShips design
"B" with single cable attachment. A still camera was set up at
a fixed position along the basin for taking pictures of the
models while under way.

Part 2 of the program consisted of tests in the model
basin to determine the resistance of the targets without cable
but with vertical load increments applied to simulate the effect
of the weight of various lengths of cable. For this purpose,
a special towing bracket was designed which permitted the appli-
cation of the simulated cable loads at the towpoint. The model
was free to trim and rise and the load increments were applied
through counterweights. Several increments were applied at
each speed to assure a range that was broad enough to cover
any full-scale cable loads anticipated. Resistance, trim, rise,
and wetted lengths were determined for each model at a series
of initial displacements, initial trims, and positions of tow-
point for several speeds ranging up to 35 knots full scale.
Additional tests, up to a full-scale speed of 60 knots, with
zero cable load, were made with the two boat-type and
the TMB design targets.

Part 3 of the program included tests in the Potomac
River off Indian Head, Maryland, to observe the maneuvering
characteristics of the models in turns and in waves at various



-6-

speeds 5 cable lengths, and towpoint positions For these tests the
models were fitted with target screens. The targets were
towed by a 1/16- inch diameter steel cable connected to the
models by a loop of wire with a breaking strength of about 75
pounds. AH turns were 180 degrees with the radius of turn
maintained as nearly equal to the length of cable as practicable.
All models were towed at the heavy displacement and at the trim
which gave minimum resistance in Part 1 of the test program.
The BuShips designs were towed both with conventional bridle
and with single cable attachment. The TMB design was towed
with single cable attachment only. When the Bureau of Ships
designs were towed with single cable attachment the point of
attachment was located midway between the extreme forward and
after locations used for the bridle attachment. Motion pictures
were taken of the models in action and a print of the film con-
stitutes a part of this report,,

Part 4 of the program consisted of expanding to full
scale the data gathered in Part 2 of the program and presenting
curves of estimated horizontal drag of the target including
simulated cable load and horizontal cable tension with varia-
tion of speed and length of cable.

It was not possible in all cases to obtain the trims
by the stern specified in Reference (1) without exceeding the
specified displacements. In such cases the models were trimmed
the maximum practicable with the available ballast.



_7-

TEST RESULTS AND ANALYSIS OP DATA

The variation of resistance with initial displacement,
initial trim, position of towpoint, and speed, as determined in
Part 1 of the program, is shown in Figures 1 to 5„ Still pictures
showing the corresponding running attitudes and spray formations
at a full-scale speed of approximately 35 knots, are included in
Figures 7 to 15» Figure 6 is a comparative plot of the resis-
tances of the five models as derived from Figures 1 to 5, at
the best trim for each model, that is, the trim for minimum
resistance o

The results of the simulated cable load tests, Part

2 of the program, are shown in Figures 16 to 20 at speeds cor-
responding to 10, 20, and 35 knots full scale „ The initial
points on the curves, indicated by circles, are the resistances
at zero cable load» It should be stated that the initial trims
noted correspond to the zero load condition, and were not reset
when the simulated cable loads were applied,, The curves shown
are for the extreme towpoint positions, the intermediate tow-
point positions being designated by symbols „

The maneuvering characteristics of the models, Part

3 of the program, are best illustrated by the motion pictures
taken during the tests „

The results of the estimated full-scale drag of the
targets including simulated cable load, and the estimated full-
scale cable tensions, Part 4 of the program, are presented in



-8-

Pigures 22 to 31„ It was assumed that minimum target drag at
high speeds would be of most interest, therefore, for each
target, only the model condition for each displacement which
gave minimum resistance at a full-scale speed of 30 knots was
used in estimating the full-scale target drag and cable tension
presented in Figures 22 to 31 „ The cable lengths were deter-
mined by computing 0, the angle of the cable with the horizontal
at the trailing end, from the relationship tan = ^d^l^etisWe*
and determining the corresponding ^abfe ( ieng tn°( full- scale) ratl °
at the various speeds from Figure 32 „ This figure was computed
by the method given in Reference (5)„ The full-scale target
drag was derived from the model data by the use of the Schoenhe rr
frictional resistance formulation. See the appendix for details
and a typical example of the computations

In connection with the possibility of self -propelled
targets at high speeds s the tests on the boat- type and the TMB-
design targets were extended to a full-scale speed of 60 knots .
The results of computations of full-scale target resistances
based on these tests are shown in Figures 33 and 34 „ For
speeds up to and including 30 knots full scale 9 the data were
taken from Figures 26 to 31 For full-scale speeds of 35 knots
and above, the computations were made using the model conditions
that gave minimum resistance at the various speeds,,



-9-

DISCUSSION

The resistance of the targets at a given speed may-
be changed by a change in towpoint position., initial trim, and
initial displacement. Parts 1 and 2 of the program show that
the effect of change of towpoint position on model resistance
is small. Therefore,, in Part 3, it was decided to test only
the extreme forward and after positions of the towpoints. In
the course of these tests., no appreciable difference could be
observed in the maneuvering characteristics of the models when
the towpoint was moved from the extreme forward position to
the extreme after position„

Variation in initial trim does have a noticeable
effect on the resistance and spray formation of the models .
All of the models have more advantageous resistance and spray
characteristics at the higher speeds when trimmed by the stern.
When trimmed by the bow s all of the models had undesirable and
in some cases dangerous spray formations" at low speeds,,

The models of the two sled- type targets, Model 4024
and Model 4025 s exhibited some slight tendency to yaw during
the maneuvering tests. Hard pounding in waves was also notice-
able,, The two boat-type targets heel the most in a cross wind.
As accurately as could be detected by eye, the boat-type sleds
towed equally well with bridle or with single cable attachment.
As had been noted on full-scale tows (2), during the maneuvering



-10-

tests the cable tended to rise on starboard turns and sink on
port turns due to the lay of the cable „ On port turns the
target tended to follow inside of the wake of the towing boat.

During the maneuvering tests the boat-type and the
TMB-design models were deliberately capsized when towed with
both bridle and single cable attachments „ With the single cable
attachment the models always' righted themselves., with some
damage to the target screens , whereas , with the bridle attach-
ment, more often than not, the wire loop broke before the models
were righted „ The estimated full-scale righting forces with
the single cable attachment did not exceed the strength of the
1-inch cable used in full-scale tows „ In Part 1 it was shown
that when towed with a single cable Model 4027 and Model 4029,
the only models so tested, had less resistance then when towed
with a bridle attachment, see Figure 6„

The full-scale estimates of horizontal target drag
with simulated cable load, Figures 22 to 31, show that the two
boat-type targets and the TMB design have substantially less
drag than the sled-type targets „' The same figures show that
at high speeds and with long cable lengths the horizontal cable
tension is virtually the same for all targets at both displace-
ments, with the exception of the revised G-60 sled„ A compari-
son of the estimated maximum speed attainable with full-scale
cable lengths of 1500 and 2500 yards and a full-scale cable
tension of 60,000 pounds is given in Table 2»



-11-

Figures 33 and 34 Indicate that at speeds above 35
knots the two boat- type targets have appreciably less horizontal
drag than the TMB deslgn„ However, at speeds above 35 knots
the TMB design was the only model with adequate stability. No
data are shown for the two boat- type targets at a speed of 60
knots full-scale because at this speed both models porpoised
violently and uncontrollably „ At speeds above 35 knots full
scale both boat-type models porpoised if disturbed, the ampli-
tude and period depending upon the severity of the initial dis-
turbance o In some cases slight ripples on the surface of the
water, were sufficient to start porpoising. The TMB design did
not porpoise at any speedy even when violently disturbed,,
During the towing tests in the basin with cable, Part 1 of the
program, Model 4024, the revised G-60 sled, was the only model
which porpoised, and then only when towed from the after tow
point with maximum trim by the stern and at a full-scale speed
corresponding to approximately 27 knots „ '

CONCLUSIONS

1. For a given displacement and initial trim, a change
in the position of the towpoint causes little change in the
resistance or directional stability of the targets.

2, Towing the TMB design and the BuShips design W B H tar-
gets by a single cable in place of the conventional bridle results
in an appreciable reduction in target resistance but does not



-12-

affect the directional stability. With the single cable attach-
ment the boat-type and the TMB-design targets have a tendency
to be self-righting when capsized.

3. At full-scale speeds up to 35 knots, the boat-type
and the TMB-design targets have towing and resistance charac-
teristics superior to the sled- type targets. At full-scale
speeds above 35 knots, the two boat-type targets have better
resistance characteristics than the TMB design but they are
susceptible to porpoising. The porpoising is of sufficient
severity to make it inadvisable to operate the boat- type tar-
gets at these speeds. The TMB design does not porpoise at any
speed, even when violently disturbed.

4. The estimated full-scale horizontal drag with simu-
lated cable load is less for the two boat-type and the TMB-
design targets than for the sled- type targets. With the excep-
tion of the revised G-60 sled-type target, all of the targets
have, at high speeds with long cable lengths and at both displace •
ments, practically equal horizontal cable tension. Similarly,
with the exception of the revised G-60 sled-type target, for
60,000 pounds horizontal cable tension and a given length of
cable, within the limits of the tests, the estimated maximum
speed attainable with either displacement is practically the

same for all of the targets. This leads to the conclusion
that the fundamental factors involved in the design of a tar-
get with improved towing characteristics are not the conditions



-13-

af fee ting target resistance or the target resistance itself,
but the cable drag, the target stability, and the target maneu-
verability,

REFERENCES

(1) "Experimental Investigation of High Speed Towing
Target Design," EMB Report 252, April 1930 „

(2) History of sled-type towing targets with enclosed
references prepared by BuShips, Structure and Form (443),

dated 24 April 1947.

(3) BuShips ltr QT- (3) (442-440) of 24 Feb 47 to TMB,
requesting construction and tests of models of towing targets.

(4) BuShips ltr QT- (3) (442-440) of 19 Jun 47 to TMB,
requesting maneuvering tests on models of towing targets.

(5) "On the Resistance of a Heavy Flexible Cable for
Towing a Surface Float Behind a Ship," by J. Go Thews and

L, Landweber, EMB Report 418, March 1936.



-14-

APPENDIX

Method of determining full-scale horizontal drag of
target with simulated cable load and full-scale horizontal drag
of target=plus-cable from model data.

Dt = (D sec * T) cos*C
where Dt - full-scale horizontal drag of target-plus-cable,
in pounds

D - full-scale horizontal drag of target with simu-
lated cable load, in pounds

- angle between cable and horizontal at trailing

end, in degrees Tan s A_

R m
T - cable tension, in pounds. T - PL

<X = angle between cable and horizontal at forward

. - w - \l w^ + 4
end, xn degrees. Cos* - ffe —

• s applied cable load, in pounds

R m - model resistance, in pounds

2

F : - tangential friction force in pound/feet = -—- „-

~r- ~ arbitrary constant (for all sizes of cable)

2

P, . . „ . Ibo sec,,
- mass density of water, TTZ

d = cable diameter, in feet
V = speed, in feet per second

L - cable length, in feet

W
w = g

W = weight per unit length of cable in salt water,

in pounds = 1.79

R = drag of unit length of cable perpendicular to

stream, in pounds - 50 P (for all sizes of cable)



-15-

Typical example of a computation.

Model 4029 - TMB-Design Target

., , -. . „ 0.54 fps- )_£5.3 fps, full scale)

Model speed, V =(3.87 knots)' 0-5. knots, full scale)

Model resistance, R m - 5.60 pounds

Wetted length = 3.00 feet

Wetted area = 5.65 feet 2

Applied cable load, S - 5.00 pounds

Mass density of sea water P /0 _ ^ n <->c 1°» sec?
— — Sg , '/d - 0.995 — gr 4 —

Mass density of basin water f/2~ o 968 ~^°* sec. 2
Cable diameter, d = 0.0833 feet

Basin water temperature - 70 degrees Fahrenheit
Sea water temperature = 50 degrees Fahrenheit

1) Determine D from model data by the Schoenherr
friction formulation: D - 18p00

2) Tan = |^°_ = 0.893; = 41.8 degrees

3) From Figure 32, £ = 2.16

4) L - 8320 feet

5) F - 1.70 pounds/foot

6) T - 14,100 pounds

7) R = 85.0 pounds/foot

8) w - 0.0210

9) cosoC- 0.990

10) sec = 1.34

11) D-t = (D sec * T) cos

= Ql8, 000) (1.34) + 14, 100 ] 0.990
= 37,800



TABLE 1
Model - Full-Scale Relationships



Item



Displacement



Speed



Towpo'int position
forward of stern



Initial trim



Cable length



Model


Pull Scale


21 7 pounds


75,000 pounds


28.9


100,000


36.2


125,000


43.4


150,000


1.29 knots


5 knots


2.58


10


3.87


15


5.16


20


6.45


25


7.75


30


9,04


35


15.49


60


17.5 inches


21.9 feet


19o2


24.0


20.5


25.6


22.4


28.0


24.0


30.0


25.6


32.0


28.8


36.0


33.6


42.0


0.29 inches by bow


0.36 feet by bow


1.68


2.10


3.35


4.19


0.50 inches by stern


0.63 feet by stern


1.01


1.26


1.26


1.58


1.34


1.68


1.76


2.20


2.26


2.83


300 feet


1500 yards


400


2000


500


2500



Linear ratio ship to model, X = 15
Speed ratio, X 1 / 2 = 3 * 87
Displacement ratio, 1.024 \ 3 = 3456



TABLE 2

Maximum Speeds Attainable with the Various Towing Targets
for a Horizontal Cable Tension of 60,000 Pounds





Cable Length -


1500 yards


Cable Length


= 2500 yards


Target


Di


Light
splacement


Heavy
Displacement


Light
Displacement


Heavy
Displacement


Revised G-60




33.0


knots


32.0


23.0


20.0


sled-type














AH steel




34.0




33.5


24.5


23.5


sled-type














BuShips

Design tt A w




34.0




34.0


24.5


24.5


BuShips

Design tt B"




34.0




33.5


24.5


24.0


TMB design




34 o 5




34.0


24.5


24.0



























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1 3 4

Online LibraryJ. H CurryResults of experiments with models of high speed targets including estimates of full-scale target drag and cable tension → online text (page 1 of 4)