The resistance of lapped butt joints of ships shell plating to motion through water

UNIVERSITY OF LIVERPOOL.

THE HARRISON HUGHES ENGINEERING

LABORATORIES.

ISSUED FROM

THE DEPARTMENTS OF NAVAL ARCHITECTURE AND

MARINE ENGINEERING.

The Resistance of Lapped Butt Joints of Ships'

Shell Plating to Motion Through Water,

BY

T. B. ABELL, M.Eng., R.C.N.C. (Retd.), M.I.N.A.

AND

J. H. LAMBLE, B.Eng., A.M.I.N.A.

(1851 Ehbiton Scholar in Nava! Architecture, 1929-193!).

LIVERPOOL

THE RESISTANCE OF' LAPPED BUTT JOINTS OF SHIPS' SHELL

PLATING TO MOTION THROUGH WATER,

by

T. B. ABELL, M.Eng., R.C.N.C. (Retd.), M.I.N.A. (Professor of Naval Architecture),

and

J. H. LAMBLE, B.Eng., A.M.I.N.A.

(1851 Exhibition Scholar in Naval Architecture, 1929-1931).

CONTENTS

1.-OBJECTS OF THE EXPERIMENTS.

2.-NATURE OF FLOW IN THE NEIGHBOURHOOD OF THEJOÎNT.

3.-MEASUREMENT OF THE RESISTANCE TO MOTION. 4.-MEASUREMENT OF VELOCITY.

5.-CONDITIONS OF THE EXPERIMENTS.

6.-RESULTS OF THE EXPERIMENTS.

THE RESISTANCE OF LAPPED BUT1 JOINTS OF SHIPS' SHELL

PLATING TO MOTION THROUGH WATER.

§1.Objects of the experiments

The experiments described in this pamphlet were conducted with the object of examining the suggestions recently revived that an appreciable reduction of the total surface resistance of a ship would be secured by lapping one plate of a strake of the shell plating inside that of the plate immediately

abaft it, so that the butt end of the outer plate exposed to the water faces

forward instead of aft according to the general practice in this _{country. The} idea underlying these suggestions is based on the general theory of

"stream-lining " submerged bodies, by which a marked decrease in _{resistance is}

experienced when the after end is made finer than the fore end of the_bodies. This theory is correct, but it has a limited application and probably requires that the general surface of the body must be "fair " and must have no sharp edges placed transversely to the direction of motion. _{This limitation of the} theory has undoubtedly been overlooked in its application

_{to the subject}

The experiments have been directed to

An examination of the nature of the flow past lapped _{butt joints with} the butt end facing (a) forward, (b) aft

The measurement of the resistances of the joint as ordinarily constructed

and of various simple modifications of it.

§2.Nature of the flow in the neighbourhood of the

_joint

The reproductions of photographs given in _{Plate I. show the flow past a} wedge piece representing the exposed lap joint. _{The wedge was placed in} a channel between two pieces of plate glass along which water bearing_small air bubbles was passing at a steady speed.* These photographs, whilst not

representing accurately the conditions in the ship. show _{the characteristic}

differences in the flow past the wedge as it presents its _{butt end or its taper} end to the flow, and, moreover, since the wedge piece is placed against one side of the channel, the water on meeting the wedge_{has already experienced} the frictional drag of the side of the channel much as the wedge in the ship

does that of the surface in front of it.

Fig. i shows. the water moving from the right. _{As the water reaches the} sharp edge of the butt end it has the same direction as the face of the wedge. and since it cannot follow in quickly behind the butt end there is a marked

diminution of pressure in the rear of the wedge. with the consequent formation

of eddies indicated by the large dark circular blobs which are pictures of the air bubbles drawn in from the stream when small, and entering a region of reduced pressure they expand and often coalesce forming large _{bubbles. The}

water behind the butt end is not dead, the constant interchange _{of water}

The apparatus was designed and made, and the photographs were taken by Mr. J. ()kill. M.Eng., Lecturer in the Department oT Mechanical Engineering, The _{University, Liverpool,}

Plate I.

Fig.

1.Direction of flow of waterright to left.

F'ig.

2--Direction of flow of waterright to

Fig. 3.Direction of flow of

waterright to left.

between the stream and this space being clearly exhibited in the channel

though it does not show in the photographs.

particular attention must be given is that the general direction of the stream

suffers no marked change at the exposed after edge of the butt end. Fig. 2 shows the water moving from the right, the butt end thus facing

the stream. Here the water is thrown off quite abruptly at the outer edge of

the face of the butt end.

and these persist in a very marked degree throughout and beyond the length

of the face of the wedge. The butt end disturbs the motion for a considerable distance, measured transversely, from the side of the channel, and the streams

on the forward side of the butt end are much more steeply inclined to the

channel than in Fig. 1.

Figs. 3 and 4 correspond respectively to Figs. i and 2. They show the

effect, in a general kind of way, of chamfering the edge of the plate at an

angle of 45 degrees, leaving the face of the butt end one-half of its original

thickness. The effect of the chamfer upon the flow seems very much more

marked when the butt end faces up-stream than when it faces down-stream, in keeping with the general characteristics of stream-line " forms with full

"entrance" and fine "run."

eddying of a marked character on the face of the wedge in Fig. 4, it was by no means absent, and was clearly visible in the apparatus.

§ 3.Measurement of resistance.

Particulars of the aparatus.Models of the wedge-shaped projections of a lapped butt beyond the general surface of the shell plating were cast in

an aluminium-copper alloy and mounted one on each side of a brass plate

48 inches long, 77 inches wide and inch thick. Three sets of wedge pieces

were made. Two sets represented, full size, the double-riveted lap of a plate

inch thick (one set being 3 inches wide ; the second 5 inches wide, measured

parallel to the lapped butt joint). The third set represented a width of 5 inches of a quadruple-riveted lap of a 1-inch plate. The projecting rivet points were not represented the exposed surfaces of the wedge pieces were milled quite smooth but were not polished.

The supporting or foundation plate was " stream-lined " on all its edges, and was attached (see Figs. 5 and 6) by means of two slender swords, having stream-line sections, to a swinging frame mounted over a channel of uniform

rectangular cross-section, through which water flowed at various uniform

rates. The plate was suspended with its plane vertical and with its upper

edge 88 inches below the free surface of the channel when the water was

stationary.

The swinging framet was suspended by four vertical steel tubes about 6

feet long attached to the frame and to a fixed platform above by thin cantilever

springs of strip steel, whose planes stood across the channel. Its movement

in the vertical plane in the direction of the stream was controlled by two

link rods standing in the horizontal plane, also attached to the franie and the

fixed platform by cantilever springs. The resistance experienced by the immersed plate and by any fittings mounted upon it caused the frame to

deflect. Provided the deflection is not large it is proportional to the resistance

experienced. The relation of deflection to resistance was obtained by direct calibration the force producing deflection was provided by a sliding weight

tTrans. Inst. Nay. Architecte, Vol. LXXII., 1930; p. 304.

88 77 135' i w

- 6-.

Fig. 5.Transverse sectional elevation of channel and swinging frame

showing the frame, f, the plate, p, the swords, SW, the wedge pieces

W, the shrouds, sii, the Pitot tube, Pt.

p

Fig. 6.Longitudinal elevation of swinging f rame showing the frame, f,

the plate, p, the swords, SW, the wedge piece, W, the butt end, ¿, and the shrouds, sii.

w SW n s K' ¿

w

5

with vernier index on the horizontal arm of a bell-crank balance the deflection of a finely engraved scale attached to the swinging frame was read by means of a microscope with a scale eye-piece. When the deflection was caused by the resistance of the plate it could be reduced by the pull of the dyxiamometer

or balance arm, on which known weights could be placed. The actual

deflection of the frame from its mean position could in this way be kept very

small. The niovement of the frame was steadied by means of a dash-pot,

which made the niotion of the frame and the objects mounted upon it immersed

in the channel just dead-beat when the water was stationary.

The -inch wedges were first tested with their upper and lower sides also exposed to the stream. Then shroud plates (see page 14) of roughly elliptical

outline with stream-lined edges were attached to each side of each wedge

piece, Fig. 7 (b). The 1-inch wedges were tested only with their shroud plates,

which were rectangular in outline, Fig. 7 (c). In order to keep these shroud

plates in their own plane they were pinned to the vertical foundation plate

and attached to the sides of the wedge pieces.

The wedges were fixed so that the butt end occupied the same fore-and-aft position on the plate whether the butt end or the taper end faced up stream.

This device ensured that the source of greatest disturbance was situated at

the same position in the frictional wake belt of the foundation plate.

a) I.) u (C) Sh a Í

Eig 7.(a) Modes of shaping the butt ends;

plan of shrouds, s h, for the i-inch wedges, W, relative to the

plate, p

plama of shrouds, s h, for the 1-inch wedges, W, relative to the plate, p.

To ascertain what reduction in resistance would be realised by shaping the

butt ends the edges were chamfered at an angle of 45 degrees to half the

thickness of the plate. This could be readily effected in the shipyard at very little cost and would leave ample material at the caulking edge in the 1-inch

plate, though, perhaps, not quite sufficient in the -inch plate. The ends

of the 1-inch wedges were also shaped to an arc of a circle of 175 inches radius

tangential to the surface of the wedge and passing through the half-thickness of the plate, see Fig. 7 (a).

As a further experiment the resistance of the wedge pieces alone in open water was measured. For this purpose a wood wedge was made up of two

butt lap projecting wedge pieces for -inch plating, placed back to back.

To divide the streams when the butt ends faced the stream a thin brass strip was fitted between the wedges. The strip projected 1 inches beyond the butt ends. The whole wedge was carried by a stiff bar sliding in a tubular

socket mounted vertically on the swinging frame. The width, i.e., the height, of the wedge was 20 inches. It was tested at various depths of immersion;

the immersion of the bottom varied from il i to l8 6 inches. This wedge

was tested only with the normal type of butt ends. The position of the

foundation plate referred to above, and the immersion of the wood wedge in relation to the streams, is shown in Fig. 8.

si

ID

r r

Fig. 8.Curve of velocity, y y y, at the central vertical plane through the plate, showing also the vertical position of the plate, p, and the

range of immersion of the wood wedges, w w (see page 16), relative to the surface. s I is the still water level, rl is the running level.

¿4,

V W

§4.Measurement of velocity.

The distribution of velocity over the channel had already been very carefully

and completely mapped out by means of a Pitot tube. The curve showing

the variation of velocity at the vertical plane in which the plate was placed is shown in Fig. 8, and in the horizontal plane, through the middle of depth of the plate, in Fig. 9.

The average speed of the stream could be varied by resistances in the field and armature circuits of a shunt-wound motor driving the pump for creating the stream. The speed of the stream approaching the plate was measured

by a Pitot tube of the standard pressure-head and total-head type, placed

6 inches below the surface and abreast, and 18 inches from the plane of the

plate, see Fig. 5. In this position there was no interference between the

plate and the Pitot tube. The Pitot tube did not measure the actual velocity

of the stream at the plate, but by calibration it was found that the velocity

reëorded by the tube was I 028 times that of the velocity at the plate's

V I V

Fig. 9.Curve of velocity, r y V, at a horizontal plane through the middle of depth of the plate.

§5.Conditions of the experiments.

The nature of the flow in the stream is entirely "turbulent" over the range of speed at which the measurements were made.

In a ship, the whole of the projecting laps lie within the frictional belt and are not, therefore, subject to the full velocity of the ship relative to still water. In these experiments the width of the frictional belt of the bare plate at the position of the butt end of the wedge pieces was about 1 inch, so that, whilst the butt end did not extend through the belt, a large portion of it was subjected

to practically the full velocity of the stream. The general character and

intensity of the wake at the butt end is shown in Fig. 10, in which the ordinate

represents the velocity of the water relative to the plate. The resistances measured in these experiments are therefore not directly applicable to a ship

all that can be deduced with certainty from the results is the relative importance

of the placing of the butt ends, whether facing ahead or astern, and of the

methods of shaping the end of the shell plates.

Proceedings Royal Society, A, Vol. 130, 1S30: pp. 90-97.

The conditions here have been changed since the calibration given in the Proc. Roy. Soc., which indicated equal velocities at these positions.

11

T

Baker's experiments on the Snaefellt showed that, at a point on the ship's

plating 196 feet abaft the bow, the width of the frictional belt was about 2 feet, and that quite close to the shell plating the velocity of the water,

V

V

OE" 04 06 ds .0"

DISTAAICE OLIT FROM PLATE

Fig. 10.Curve of distribution of velocity, y y y, in the frictional belt at the position of the butt end. The ordinate, V, is the velocity of the

stream relative to the plate.

reckoned relative to the ship, was 05, and at 1 inch from the shell o-7 times the ship's velocity relative to still water. The average velocity corresponding

to the square root of the mean square of the speed of the inner 1-inch width

of the belt is 0.635 that of the ship.

In these experiments the same average velocity as just defined is 0907

that of the strea in. To reduce the results to ship figures it would be necessary to multiply the resistances of the wedge pieces by a factor having a value of

the order of magnitude of () 0-5 for the 1-inch lap butt.

factor cannot be regarded as exact and, as stated above, the results here given can only be regarded as qualitative, and moreover only represent the order of merit of the different modes of fitting and shaping the lapped butt ends. With regard to the experiments with the double wedge, the nature of the flow at both ends, the upper and lower, of the immersed portion will nc doubt

change slightly as the actual depth immersed varies. The wave-making

resistance, which forms an appreciable portion of the total where the butt

end faces the stream, will increase slightly with depth of immersion, The

nature of the flow round the bottom of the wedge will change very little,

if at all, over the range of immersion for which measurements were made. The temperature of the water, which was fresh water, during the whole of the experiments remained approximately constant at 63 degrees Fahrenheit, so that no correction for its variation was necessary.

§6.Results of the experiments.

The results of the experiments were first plotted in the form of total resistance

to a base of velocity head, since these were the quantities actually measured. After fairing these plottings the curves of resistance were converted to curves

of R/V2 to a base of V, where R is the resistance in pounds and V is the velocity

of the centre of the stream at the plate in feet per second.

The results are shown in the form of R to a base of velocity head for the i-inch wedges of the smaller width in order to indicate the consistency of the measurements when their magnitude was least. These are given in Fig. 11

the individual observations are shown by the spots adjacent to the curves.

25...

DIRECT/ON OF FLOW

-

13

0 5 10 IS 20 25

VELOCITY HEAD IN MM OF WATER

Fig. 11.Curves of total resistance for plate, swords and finch wedges, 3

inches wide.

X is the curve of resistance for the plate and swords only.

Y ,, ,, ,, swords only.

Fig. 11 also gives a key plan to the lettering of the curves appropriate to the different models tested. Plain letters refer to the foundation plate and its

supporting swords together with the wedge pieces only. The letters with

suffix 's " indicate the presence of the shrouds in addition. These are shown

in Figs. 5, 6 and 7. For example, in Fig. 11, " A " refers to the combined

resistances of the plate, swords and wedges, "As" to the resistances of the

plate, swords, wedges and shrouds. In all the diagrams the curves labelled X

refer to the plate and swords combined Y to the swords only, i.e., without the plate. The difference in ordinate value between the curves X and A

04 Oi n, A 13 X

Figs. 12, 13 and 14 show R/V2 to a base of velocity in feet per second for

the assembly of plate, swords, wedges and shrouds for the i-inch wedges, 3 inches wide and 5 inches wide, and for the 1-inch wedges, 5 inches wide

respectively.

.75 fr25 5 fr73 2O 2 25 V YEC

Fig. 12.Curves of

V in feet per second.

The differences from the curves for the plate and swords are not shown

in a separate diagram because it is not practicable to eliminate the effect of the shrouds. Moreover, to have shown the differences separately in this way

might prove misleading, for it might be assumed, rather hastily, that these

differences were directly applicable to the ship conditions, whereas, in fact,

they are not.

The diagrams are now self-explanatory, but it may be worth while to point

out one or two features of a general character that may be drawn from the

experiments

(1) The effect of shrouding the wedges, which is shown only for the i-inch wedges, is to increase the total resistance by an amount appreciably greater

than that due to the skin friction of the shroud plates. This is probably due to the motion past the wedges being converted from flow in three

dimen--

-u

Q.-L

Th

Fig. 13.Curves of

for Finch wedges, 5

inches wide.

F'ig. 14.Curves of

for 1-inch wedges, 5

sions, flow taking place in the vertical direction at the upper and lower sides

of the wedges, as well as in the horizontal planes, to a condition appròximating to flow in two dimensions, where flow is in the horizontal plane only.

(2) The curves of R/V2 for the shrouded wedges are in all cases approximately

parallel to the curve for the plate and swords alone, thereby indicating a

difference due to the wedges varying as the square of the speed. Without the shrouds the differences seem to vary at a rate rather less than the square.

07 06 (j) 05 04 (j) (1)03 -J 02 0I CURVES /to7 4 FLOW CURVES 8ro/4

4-WA 4

_A-____rv

4f14

VELOCITY HEAIJ IN MM 0F WATER

Fig. 15.Curves of resistance for double wedge.

Fig. 15 shows the results for the double-wood wedge plotted in the form of total resistance in pounds to a base of velocity head in millimetres for a series

of depths of immersion of the wedge. The total resistance includes the

wave-making 4sistance, which probably varies very little indeed over the range of immerion tried. The least immersion, 111 inches, was roughly twice

the length of the wedge measured in the direction of flow, the greatest

immersion was three times the length. Observations of the wave profile

were made for a series of speeds at two immersions, hut as far as the eye could detect there was no change in the profile with the change in immersion. There

are, therefore, prima facie grounds for saying that the actual differences of

the resistances for different immersions enable an approximate estimate

of the wedge's resistance per unit depth of immersion under completely sub-merged conditions to he made.

FPS /86 '7.3 /6/ /48 /36 /23 I/I K Q /86 (13 /6/ /48 /36 123 1/.! 9 /0

I

The upper group of curves give the resistance when the butt end faces the stream, the lower group when the taper end faces the stream.

Individual observations are shown on the diagram for the 11 1-inch

arise from unsteadiness of the eddies or perhaps from interference of the how and stern systems of waves.

The latter can hardly be important

because any interference that occurs is between a very large bow-wave

system caused by the butt end, and a very small stern-wave system caused

at the taper end.

observations without giving special value to any individual observations.

The resistance, whether the butt end is advancing or trailing, appears to vary as the square of the velocity. The resistance per unit width in the former condition is about twice that in the latter.

Fig. 16 shows specimens of the wave profiles. There is a small change of

water level of the surface in the channel with change of speed, but this is

very small, being that corresponding to the increase in the hydraulic resistance of the channel. The still-water level shown is that corresponding to zero

velocity. This is also the datum for measuring the immersion of the wedge.

STILL

'

Fig. 16.Wave profiles of double wedge butt end facing the stream taper end facing the stream.

17

F PS 20

FLOW

Fig. 17, shows a photograph of the wave profile for a speed of 117 feet per

minute when the butt end faces the stream

Fig. 17.Wave profile of double wedge at a speed of 117 feet per minute.

Direction of flowleft to right.

§7.Sunimarv of results.

Comparative results for each series of experiments with the shrouded

wedges are given in the following table for a speed of 120 feet per minute

:-A, B, C. D, E F

i-inch laps, 3 inches wide 1 -641 1 Ø

-i-inch laps, 5 inches wide 1735 1-0 1-146 0-933

._

-1-inch laps, 5 inches wide 1-60 1-0 1-224 0-980 0-866 0-973

The differences between the resistances of the plate with the swords, and

the resistance of the plate. swords, shrouds and the wedges with the butt

ends aft, are assumed to he unity for each size of wedge. This corresponds

to the normal arrangement of butt laps. The values given in the table are not strictly comparable because each figure contains a fraction which is

sensibly constant throughout each series of models. The differences from unity in each series may, however, he said to be roughly comparable. For example,

the effect of chamfering the butt end when aft is represent by (BD), viz.,

O-067 or 6-7 per cent. for the .-inch wedge and 0-02 for the 1-inch wedge; the effect of chamfering the butt end when forward is represented by (AC),

viz., 0-589 and 0-376 for the -inch and 1-inch wedges respectively; the

effect of partial stream-lining the butt end of the 1-inch wedge is represented

by (BF) with the butt end aft, viz., 0-027, and by (AE) with the

A comparison between the plain butt end placed aft and the stream-lined

butt end placed forward is obtained from (B8E)=0-134, showing an