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- Joect NY 420 010.5, chnical Memorandum M-097

TEST OF ANCHORS FOR MOORINGS ANDGROUND TACKLE DESIGN IN MUD BOTTOM

15 December 1954

U. S.

naval

civil

engineering

research

and

evaluation

U ! "laboratory

port hueneme,

california

Iir~ 4.,

U.S. Naval Civil Engineering Research and Evaluation LaboratoryPort Hueneme, California

Project NY 420 010.5Technical Memorandum M-097

TEST OF ANCHORS FOR MOORINGS ANDGROUND TACKLE DESIGN IN MUD BOTTOM

15 December 1954

R.C. Towne and J.V. Stalcup

SUMMARY

The U.S. Naval Civil Engineering Research and . 'aluationLaboratory conducted tests in a mud bottom to determine the holdingpowvr of the BuDocks.designed steel, concrete-mushroom, and concretewedge-shaped anchors, and to compare the behavior and holding powerof these anchors with rhos. cf the present type of stockless anchors,with and without stabilizers.

Tests w-ere conducted on Navy stockless anchors, with and withoutstabilizers. Holding-Fower-to-anchor-weight r,tios in mud bottom averaged3.31 to 1 with stabilizers and 2.66 to 1 withou. ,fabilizers. It was con-cluded that stabilizrs should be installed on Navy stockless anchorsutilized in moorings in mud bottoms.

Addilional tests wore mode on BuDocks-designed anchors. Ofthese the 7500-lb concrete-steel anchor hod the largest holding-power-to-anchor-welght ratio, 2.92 to 1.

Tests were also mode on two new design Baldt mud anchors and ona Croseck anchor. The holding-power ratio, 6.62 to 1, of the 3170-lbanchor was greater than that of the Navy st, ckless anchors manufacturedby the Baldt Aichor Division.

Comparative holding-power tests were conducted on Lightweightanchors and Danforth anchors. Average holding-power ratios were 3.18to I and 5.87 to 1, respectivelv.

Two anchors using proposed new design criteria were designed andfabricated at the Lboratory. These provided an average holding-power-to-weight ratio of 10.1 to 1 and 10.0 to 1, respectively. Maximumholding power in mud of the anchor fluke anglo was determined to be50 degrees. It is recommended that a group or 'family' of mooringanchors be developed utilizing the design criteria obtained from theresults of the sand and mud bottom tests.

PREFACE

Thesm tests are a continuation of the anchor tests conducted insand1 during the period from 1948-1953. The sand test- -i.',ced aneffeclive means of stabilizing stockless anchors, establis.ied the flukeangle for obtaining a maximum holding power in sand, concurred w'hthe validity of the L3 low for holding power of an anchor, and provideda basis for the design criteria of an improved mooring anchor.

These tests wero madn in order to establish the holding powerand proper fluke angle of the present anchors in mud bottom and toverify the proposed anchor design criteria. The Cooperation and assist-ance furnished by the Son Francisco Nava. Shipyard, Hunters Pbint,made It possible to complete these tests with a minimum of delay andcost.

Ii

I

CONTENTS

pageINTRODUCTION .......................... I

ANCHOR TEST APPARATUS ................... I

SOIL SAMPLES ....... ..................... 2

ANCHOR CHAIN TESTS ...... ................. 2

ANCHt..( A' !t TEST INFORMATION ... ............ 3

NAVY STOCKLESS ANCHORS 3...........3NAVY STOCKLESS ANCHORS - FIXED FLUKE 4BUDOCKS DESIGN ANCHORS ............... 4BALDT DESIGN ANCHORS .................. 4LIGHTWEIGHT ANCHORS ................. 4DANFORTH ANCHORS ..... .............. 5FLUKE ANGLE TESTS ..... ............... 5PROPOSED MOORING ANCHOR DESIGN ........ 5CONCRETE ANCHORS .................... 6

DISCUSSION ....... ..................... 6

CONCLUSIONS ....... ................... 8

RECOMMENDATIONS ...... ................. 9

REFERENCES ........ ...................... 10

TABLES ......... ........................ 11

ILLUSTRATIONS ..... ................... ... 19

DISTR;SUTION LIST ..... .. .................. 59

LIBRARY CATALOG CARD ..................... 61

TABLES

pageI -Soil Analysis Data ..... .................. 112 .Anchor Stabilizcrs ..... ................ ... 133'- Holding Power Data of Navy Stockless Anchors Testod in

Mud Bottom .... .. .................. ... 144 - Holding Power Data of Navy Stockless Anchors with

Fixed Flukes ...... .... .............. 155 - Holding Power Data of Steel Anchors Tested in Mud

Bottom ...... ............ ............ 166 - Holding Power Data of Steel Anchors Tested in Mud

Bottom ...... ... ...................... 177 - Holding Power Datu of Concrete Anchors Tested in Mud

Bottom ...... ... ...................... 18

ILLUSTRATIONS

figure page1 - Mud bottom test sitei n San Franciso Boy ... ....... vi2 - Mechanical analysis of soil at test site .......... ... 193 - Mechanical analysis of soil at test ,te ......... .... 204 - Sol analysis ..... ................... ... 215 - Soil analysis ..... ................... ... 226 - Soil analysis ..... ................... ... 237 - Soil analysis ....... ................... 248 - Soil analysis ..... ................... ... 259 - Soi analysis ..... ................... ... 26

10 - Unconfined compression test failure .......... .... 2711 - Direct shear test of soil ......... ...... ..... ... 2812- Direct shear test of toll ..... ............ ... 2913 - Direct shear test of soil .... .............. ... 3014 - Triaxial shear data on soil ... ............. ... 3115 - Triaxial shear test failure 32.............3216 - Strain gage used to mecmure break -out force of anchor. 3217 - Typical Navy stockless anchor ..... ........... 33

Iv

igu re N~ge18 - Typical stabilized Navy anchor. .. .. . .. ..... 3319 - Graph of test pulls on 10,000-lb Navy steel anchor

without stalilizers . .. .. .. . . . ....... **34

20 - Graph of tesr pulls on 10,000-lb Navy steel anchorwi th stab lllzIzers. r . .. .. .. .. .. .. .... 35

21 - Graph of test pulls on 6000-lb Navy stool anchor withfixed flukes . ........................ 36

22 - Graph of test pulls on 10,000-lb *Navy steel anchorwith fixod flukes.........................37

23 - Graph of test pulls on 20, 00-lb N-avy steel anchorwith fixed flukes * . . .i. .. .. .. .. .. .. 38

24 - Graph of test pullIs on 6000-lb Navy stael anchor withmovable flukes. .. ... .................... 39

25 - Graph of test pullIs on 10,000-lb Navy steel anchorwith movable flukes .. .. . .... . .. ..... 40

26 - Graph of test pulls on 20,000-lb Navy stool anchorwlth movable flukes. .. .. . .. .... . .... 41

27 - 7500-lb, concrete-steel anchor,..... . .. ..... 4228 - BuDocks 1430-lb straight-plate anchor . . .... 42

29 - BuDocks 1430-lb curved-plate anchor .. .. .. . .43

30 - Graph of test pulls ot, 7500-lb ctaicrete-steel anchor . . 4431 - Graph of teat pulls on 1430-lb straighi-plate anchor . . 4532 - Graph of test pulls on 1430-lb curved-plate anchor . . 4633 -31 70-lb Boldt anchor . .. .. . ..... .. .. 47?4 -3650-lb. Boldt anchor .. .. .. . .. .... . ... 473.5.- 3060-lb Croseck anchor .. .. . .. ... . .... 4836 - Typical Lightweight anchor.. .. . .... . .... 4837 - Graph of test pulls on 10,000-lb Lightweight anchor .. 4938 -2510-lb Danforth anchor .. .. .... . .. .... 5039 -2770-lb Danforth anchor .. .. . .... . .. .... 5040 -10,000-lb Danforth anchor .. .. . .... . .... 5141 -12, OO-lb Danforth ..nchor. .. .. .. .. .. ... 5142 - Graph of test pulls on 10,000-lb Danforth anchor . . 5243 -Graph of fluke angle tests. .. . ... . ..... 5344 -1620-lb mooring anchor. .. . ... . .. ..... 5445 -2900-b mooring anchor .. .. . .. . . .. . . . 5446 - Graph of test pulls on 2700-lb mooring anchor . . ... 5547 - 10,500-lb concrete wedge anchor .. .. . ...... 5648 - 10,500-lb concrete mushroom anchor. .. .. .. .... 5649 -2500-lb concrete mushroom anchors .. .. . .. .... 5750- Hlding power vs moment of luke area . .. .. .. 58

V

viC

INTRODUCTION

The Bureau of Yards and Docks ;niliated these tst as a meansof developing a stable mooring anchor for utilization in vessel mooringsend ground tackle for floating structures such as drydoclks, cranes andbarges. The Bureau is responsible for the design and constructior ofmorirg fa tle. I* ptert these veusael from the combined forces ofwaves, currents, and wind%. Tests we.. to be conducted in s nd, mud,and clay bottoms in order to provide sufficient knowledge of anchorreaction n these types of soil to foctuolly determine their holding powerand to permit development of onchor-design criteria.

These anchor tests, conducted by the U.S. Naval Civ:l Engineer-ing Research and Evaluation Laboratory, Port Hueneme, California,under Project NY 420 010.5 were mode in the mud bottorn of SanFrancisco Bay a the San Francisco Naval Shipyad, Hunters Point,California.

ANCHOR TEST APPARA1 US

The lost apoarutus for mud bottom lests consisted of two 5 by 12pontoon barges, used to carry the lest equipment, nd a 5 by 14 pontoonworp,,ng tug, used to drop and retrieve the anchors. The test equipmentwas composed of a 400,000-lb capcity electric dynamometer to measurethe holding power of the anchors and o mo.el BU-140 Skagit winch witha six-port line for dragging the ancho, The winch was spooled with2500 ft of I 3/8-in.-dcmeter wirm rope, and the wire rope was reevedthrough sheaves mounted on the two barges to form the six-pcrt line.One of the 5 by 12 pontoon barges was anchored with two 30,000-lbNavy stockless anchors and the other brge was attached to the testanchor with suitable lengths of anchor chain. Figure I shows a generalview of the barges at Hunt-rs Point during the tests. In this view thetest anchor is located beneath the buoy between the two forthet barges

and the buoy in the right foreground Inates one -! !Se "0.000-11-stockles& anchors used to hold the bar-es in p'.sition.

I

2

SOiL SAMPLES

Sanples of the mud were taker in the path of the anchor testpulls down to a depth of 27 ft %. tIizin- a 2-in.-diaonete, Porter sanplingdevice. A leboralory oisclysis of the soil was conducted by the TwelfthNaval District Public Works Office, Scn Bruno. Coliforni.. Testsperfoar~sd on the s=n.pies included a ma -nical c.-clysis (see Figures 2and 3), liquid and plcstkz limits, specific gravity unconfined coanpressiontests, and con-olidation data Gee Toble I and Figures 4 to 9). Uncon-fietd compression lasts wwd purformd t on the somnplis at their noturalw otr content. The rate of strain wos riainlard betwoen 1/2 per centto 1 1/2 per cent per minuts. The type of failure is shown in Figure 10.For the consolidation tests, thc specimens tokn in the ield were placed"n a fixed-ring consolidation device, seated firmly, and loaded inincrements a4 shown. Direct sheor tests were mode on the undisturbedspecimens, as taken with the Porter =mpaer, in a consolidation-quickcondition, at a constant displacement of .05-in. pat minute (seeFigures 11, 12, and 13). Shear tests were mod, on temples taken ata dpth of 22 ft only, becovie of the fluid nature of the material aboveth;1 d.pth. Tria iol shear data were obtained by conducting unconsolidated-urdrained tests on the =nples as taken with the Port - soapler (seeFigtre 14). The test lateral Fressure was applied instmnlaneously andthe specimen sheared quickly, using u stressometer of the proving-ringtype to register the shearing load. Volume changes were noted duringthe lasts. kiate of strain was equal to about I per cent per rninut (seeFigure 15).

The soil contai,-d approximately &C-per cent clay particles with6 2-per cent water content, grd the shear resistance was .41 ton persq ft.

ANCHOR CHAIN TESTS

Test pulls of the anchor chain caone were conducted to determinethe resistance of the chain dragging thro'gh the mud bottom. Theaverage holding power after 50-It drag of 450 ft, 360 ft, and 270 ftof 2 3/ 4 -in. anchor chain was 13.9, 11.1, and 7.6 kips, respectively,and for 180 ft of I 1/2-;n. anchor cnoir, 1.9 kips.

The proper anchor chain lengths for a 0-, 6-, or 12-deg chai.iangle at the anchor were obto;ned by the formula presented in NAV-DOCKS Mooring Guide. 2

3

ANICHOR AND TEST INFORMATION

The holding pov 'rs of the anschor-. were recorded at 5-ft intervalsfor a distance of 180 ft, thus providing data for plotting a continuouscurve of ancho' holding power. The ratio of the holding power toanchor weight in air, HP/wt, as used in this report, is token after theancho- has dragged a distance of 50 ft. Longer distance% of drag wiltproduce o larger holding power; however, a dist~nce of 50 ft 1,- beenselected cis the ma.xiaum allowable travel for moor!".s in confinedlocations.

The vortdcol farce required to brook the test anchor% loose fromthe mud bottom ai the "-d of iianch test pull was measured by means ofa strain gMe mnounrv!d on the warping-tug winch line (see Figure 16).The depth of water w-3s approximately 30 ft at the test site.

NAVY STOC3KLESS ANCHORS. Mt. present Navy stockless anchori4 designed tor shipboard operetion and has been adopted for mooring

uewithout mdfcto.The 1500, f3000* 6000; 10,000', with01

otstabilizers at a 0-deg choii angle only. Figure 17 show% atypical Navy stockless anchor. '.ubsequenlly each individual anchorwas equipped with a suitable stc,i..etr (see Table 2) arid was retestedat 0-, 6-, and 12-deg chain angles. The stoal-plate stabilizers wiredesigned by the Bureau of Yards o..J Docks and hod been tested pro-vm--xrly in sand bottomn. 1 Six test pulls %ve-. made on each anchor ateach chain angle. lligure 18 shows a typt. itoablized anchor, andFigures 19 and 20 are graphs of the test Ful!s . -he 10,000-lb anchor,with and without stabilizers.

Table 3 contains the holding power of each znch r with andwithout stabilizers, the fluke angle, the average volding rxowars, minimumholding power which occurred duringj the six test puml:, the Woding.power-to-crnchor -weight ratio, depth of burial into 1'%c mjd, and theaverage vertical force r#%quired to break the anchor, loose ftrim theI mud bottom. The largest average HP/vs ratio at 50 ft was 5.60 itz I(or the 1500-lb orchor. The average HP/vs ratio at 50 ft for all ooanchors was 3.31 to 1. It was observed during the initial tests on ,-stockless anchors that the holding powers were not uniform whichindicated that the flukes were not opening properly. A stuay wasmade to determine this effect upon the holding powers of the anchors.

NAVY STOCKLESS ANCHORS - FIXED FLUKE. Three of theNavy stocloes anchors ware selected for thoe s ;n order to stud) theeffect of the fluke cngle upon the unifosrmity and amount of the ha'dingpower. The 'fluke angle,' as us,d in this report, is the angle subtendedbetween the shank and the flukes, wh.4 en the flukes are rotated to ex-tremeo open position.

The anchiors used were the =00-, 10,000-, and 20,000-tb Navyanchors with stabilizvrs. The flukes were fixed at open position and.he anchors were each pulled six times at 0-dog chair, angle. Thuholding power ratios found 4uring the previous tests were increased from2.85 ta 1, 2.42 to 1, and 2.21 to 1, up to 3.53 to 1, 4.88 to I and4.32 to 1, respectively. In adc'ttion, the hoding powers wcro morauniform (see Table 4). Figures 21 to 26 are graph% of tha lost Fulls onlthe anchors with and without fixed flukes.

8UDC)CKS DESIGN ANCHORS. Three steel onchoirs, a 7500-lbconcrete-stual.' a 143fl-lb straight -plato, and a 1430-lb curved-plate,were fabricated at the Laborato'y for test (.ec Figures 27, 28, and 29,.The 7500-lb concrete-steel anchor was tested at 0-. 6-, and 12-dogchoa angles while the remaining two anchors were ,.ullai:1 at a 0-dogchain angl only. Results of tLese tests are shown int Table 5. Theaverage HP/wt ratio was 2.92 to I for the 7500-lb concrete-steelanchor ond 2.23 to 1, 2.44 to 1, !or the straight-plate and cu-ed-plate anchors, respctivel/. Figures 30, 31, and 32 ore graphs of thetest pulls for the three anchori.

BALDT DESIGN ANCHOI(S. The Anchor, Chain and Forge Divisionof the Boston Metals Company, Chester, Pe-sytvania, furnished threeanchors fo. tit, ua 3170-lb Baldt, a 365G-t. Boldt and a 3060-lbCrosect (see Figures 33, 34, and 35). These anchors were -'ulled ait0-dog chain angles only. Initial tests an the 3170-lb Baldt anchorindicated that the flukes were not opening in every test; therefore, thechain length was shorletied to provide an initial lift on 'ho anchor shank,in effect opening the flukes. The iiffect of this procedure was toincrease the HP/wt ratio from 2.66 to 1 to 6.62 to 1 . Results of thetests are contained in Table 5.

LIGHTWEIGHT AW-HORS. A 500-lb and a 1000-ib Ughtweightanchor were tested at 0-, 6-, and 12-dog chain angles for compara-tive purposes (see Figure 36). In addition, tests3 were made on 2000-,3000-, 4000-, andA 10,000-lb anchors for 6eo Bureau of Ships. Theres.ults are included in Table 5. The largest HP/wt ratio was 3.90 to

5

1 for the 3000-lb anchor and the average H?/wt ratio was 1.18 to 1for all the Lightweight archors. Figure 37 is a graph of the test pullson the 10,000-lb anchor.

DANFORTH ANCHORS. The Danforth anchors are commercialanchors patented by Me. R.S. Donforth of Berkeley, California, andwere loaned 'o the Laboratory for comparative test purposes. Theanchors tested weighed 2510, 2770, 4000, 10,000, and 12,000 lb.The flukes of the anchors used in these tests were fabric',ted frc-, steelplot* rather than cost or forged, , ih ; e exception of !he 4000- and10,000-lb anchors oo Figures 38, Z9, 40, and 41),

The anchors were pulled at 0-, 6-, and 12 dog chain angleswith the exception of the 165-lb anchor which was pulled at 0-degchain angle only. Test results are contained in Table 6. The largestHP/wt ratio was 9.92 to I F-., ,i. 2770-lb anchor and the average HP/wtratio for all the Danforth anchors was 5.87 to 1. Figure 42 is a graphof the test pulls on the 10,000-lb anchor.

FLUKE-ANGLE TESTS. Those tests were conducted in order toestablish the fluke angle which would prov 'e the maximum holdingpower for any anchor in mud bottom. The 2770-lb Donforth anchorwas utilized in these tests because its construction permitted the flukeangle to be readily varied to large angles as it wes anticipated that alarger fluke angle would be established for m.ud bottom than was foundfor sand bottom.

The anchor was tested at a 0-deg chain angle with fluke anglesof 45, 50, 55, 60, 70, and 80 deg. Fiure 43 is a graph of the testresults. The maximum holding paw- m0s found to occur at a flukeangle of 50 dog. The HP/wt ratio at this fluke angle was 20.5 to I.This ratio is excessive (.)r mud bottoms as explained in the DiscussionSection.

PROPOSED MOORING ANCHOR DESIGN. The Laboratoryfabricated two anchors, :lesigned on the basis of the test results of themud and sand b,t,.m te.ts. The first onchcr, weighing 1620 lb wasfabricated from steel plate, with movabl, flukes opening to a 60-degfluke angle, round stock stabilizers, r.nd a large-area tripping plateattached to the flukes (see Figure 44). This anchor was pulled at 0-,and 6-deg chain angles and tht HP/wt ratio was 8.51 to 1 and ?.69to I, respectively. The fluke angle was changed to 5. deg and re-tested at a 0-deg chain angle and the HP/wt ratio was 10.1 to 1.

6

ne tuces of the 2900-1b anchor were fabricated with a doublethickness of plate in order to increase the weight at this point and tendto drop the flukes into the mud upon inlial setting. Thz shank was abox section formed from plate to . ake the shank lighter :o as to tendto raise it upon initial setting in the mud. The fluke angle was 50 deg(see Figure 45). The anchor was pulled at a 0-dog chain angle onlyand the HP/wt ratio was 10.0 to I. Figure 46 is a graph of the testresults on the 2900-lb anchor. Results of the tests on both anchors arecontainod In Table 6.

CONCRETE ANCHORS. The concrete anchors, built in accord-ance will, the Bureau of Yards and Docks instructions, consisted of one10,500-lb wedge tye (Figurn 47), one 10,500-lb mushroom type(Figuse 48), and four 2500-lb mushroom type (Figure 49).

The two 10,500-lb anchors and the four 2500-1b anchors were oiltested4 at 0-, and 6-dog chain angles irmnediately after setting, andat 0-deg chain angle after setting 24 hours and after setting in the mud14 days. The four 2500-ib anchors were pulled in tandem, close-coupled.

Results of the tests are contain'ed in Table 7. The 10,500-lbwedge and mushroom anchors each hod a HP/wt r-tio of I. 18 to 1 aftersetting 14 days as composed4 to a HP/wt ratio of .88 to I when pulledimmediately after setting.

DISCUSSION

The fixed-fluke type of anchor has on ..Jvontoge over movabl:,fluke anchors when operating in mud bottom as no tripping device isrequired for the flukes. However, because of the fixed position of theflukes, it is necessary to lower the anchor to the bottom with theflukes pointed down, instead of simply dropping the anchor overboard.Lowering the anchor requires odditioncl equipment such as a bargecrone or warping tug that mny r t ntways be available.

It was apparent during the tests on the Navy steel anchors thatthe flukes were not opening properly in every tett as the maximum andminimum holding powers varied considerably for the six tes.t ' ulls. Theeffect of the stabilizers on the amount of cnchor holding power couldnot be determined accurately due to this inability of the Navy otockless

I

7

anchors to dig into the mu on every pull; however, it appeared likelyfrom the data that the increase in holding power due to the addition ofstabilizers was approximately 25 per cent. The stobilizer area amountedto an average of 60 per -ent of the fluke area for each anchor. Thereappeared to Ie on insufficient area in the fluke tripping plates to tiltthe flukes and start them into the mud. By shortening the chcin lengthand thus providing c,% initial lift on the shank, the flukes tended tobury more consistently and by changing the chain angle to 6 deg the n-tial holding power actually in creased in some instances. lowev r,the final holding power then would decrease due to the shorter chainlength and the flukes were still not successfully trapped in every test.This was apparent when the fluke angles of the anchors were fixed inopen position and the holding powers were increased and wore nvireuniform (refer to Figures 21, 22, and 23).

The anchor flukes are forced upward due to the veitical reactionof the mud against the bottom area of the flukes as twy settled throughthe soft mud. Therefore a tripping plate with sufficient area to over-come this mud reaction against the flukes must be provided or theanchor will skid along with the flukes in a raised position.

Because of the decrease in amount of shear resistance in the mudbottom as conpared with sand bottom, less force is required to bury (heanchor and, therefore, the fluke angle m-/ be larger. This wns re-flected in the fluke angle tests which indicated a fluke angle of 50 degwould provide the largest holding power compared to a 35-deg flukeangle for a sand bottom. The fluke cngle may bo varied from 50 dogto 35 dog simply by fabricating the anchor with the larger angle andinserting a wadge between the shank and the stop to reduce the angle.

' The soil at the test site was termud 'mud' because of the large

water ccntent in the silty clay material that produced o low shear value.lowever, the mechanical composition of the soil showed 60 per centclay content and this indicated that 3t a certain dep.h, approximately22 ft, the material would be firmer and would result in much largerholding powers comparable to those of a 'clay' bottvi. This wasapparent in the tests with two Laboratory anchors, the 2770-lb Danforthanchor and the 3170-1'b Boldt anchor. The design of these anchorsenabled them to bury themselves to a c.onsiderable depth, as much as24 ft for the 2900-lb Laboratory anchor and, therefore, the resultingholding powers are not to be compared in ihe strict sense with '.aud'bottom holding powers. However, the ability of anchors of this design

8

to penetrate %off mud layers crd to bury into firmer -jnderlying strataare additional advantages as te holding power is dependent upon themoment of the projected fluke area plus the stabilizer area about theground surface. Figure 50 is r qraph of the test retults showing thisrelationship.

CONCLUSIONS

The following conclusions are based on resolts of tests conductedin mud and sand bottoms. They do not app!) to the anchor% tested herenor to similar anchors under dissimilar bottoms such as marl or rock.

The Navy tockless anchor may be prevented from rotating byaddition of the BuDocks-designed steel-plate stabilizer welded in apot.tion normal to the flukes. This stabilizer will provide a more uniformholding power for the anchor and will increase the holding power inmud bottoms slightly and approximately 10 per cent in sand. Stabilizersare requited in mud after the anchors have buried sufficiently to en-counter a soil reaction that produces a rotational torque on the onchor.

Changing the fluke angle from 35 deg to 50 aeg increaos.d theholding power approximately 100 per cant.

The ratio of holding power to weight of the 7500-lb con:rate-steel anchor, 2.92 to I, is comparable to the ratio for the stabilizedNavy stool anchors in mud, 3.31 to 1. These :imilar ratios are due tothe small fluke angle, 28 deg, on the 7500-lb anchor. The fixedposition of the shank on the 7500-lb anchor would be a marked dis-advantage if thn anchor was to be utilized both mud and sand bottoms.

he break-out forces for thu Laboratory and Danforth anchorswere larger than for the stockless anchors because of the additicnaldepth of burial. The break-out force tended to vary directly with thefinal holding powers.

The anchors having relatively long thin flukes and least restric-tion to burial, such as tlke Laboratory, new Boldt, Croseck, Danforthand Lightweight anchors, produced the larger HP/wt ratios.

Fixing the flukes of the steel Navy anchors in an open positionIncreases the holding power but requires the cnchor to be initiallyset in an upright position.

9

Design criteria for a mooring anchor operating in a mud bottomas determined from these tests wojld be as follow%:

a. Lightw,),ght, fabricated from steel plate.b. T-vo flukes which can rotate to a 50-dog angle from

the shank. Fluke area of the anchor to be dependentupon the required holding power. Length and widthof the flukes to be proportioned to produce the maximummoment or !.olding power.

c. A fluke tripping plate of aufficient arma and slope toovercome the resistance of the mud on the lower sideof the flukes upon initial setting.

d. Cbstruction to anchor burial to be restricted to aminimum.

e. Adequate-size stabilizers to prevent rotation of theanchor.

RECOMMENDATIONS

It Is recommended that stabilizers be Ided to Navy stocklessanchor% which are to be vsed in moorings or ground tackle in a mudbottom.

Because of the size of the stabilizers and the added shippingcuboge involved, the stabilizers should bt stored and shipped asseparate items from the anchors.

It is recommended that a group -r 'fanily' of low-cost, light-weight mooring anchors be develope. ro cover the entire range ofrequired holding powers.

Design criteria for these mooring anchors should be based on theresults of the sand and mud bottom tests.

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REFERENCES

I. iWAVCERELAB Technical Memorandum M-066, Test of Anchorsfor Moorings and Ground Tackle Design, by R.C. Towns,10 June 1953.

2. Bureau of Yards and Docks Technical Publ;cation NAVDOCKSTP-PW-2 MOORING GUIDE, Volume 1, 1 March 1954.

3. NAVCERELAB Technical Note N-195, Tests of BuShips Anchorsin Mud and Sand Bottoms, by R.C. Towne o.-d J.V. Stalcup,5 August 1954.

4. Bureau of Yards and Docks Addendum to Testing Procedure,Suggested Testing Procedure for Dotermining Safe Holding Powerof Concrete Anchors - Wedge and Mushroom Types.

TABLE I. Soil Analksis Data

Hole Somple Elev. Unit weight MoiSLcontent Unconfined compression1,. No. depth (dry lb/cu ft) (% dry weight) (ton/sq 't)

1 4 72 99.6 collopsed under own wt2 5 73 -4.7 r.ollopsed under own wt3 10 77 47.1 :ollopsed under own wt4 1' 74 73.7 .07325 15.5 75 68.8 .04886 16 75 69.0 .07327 20 75 76.1 .04888 20.5 75 82.6 .07939 21 75 70.9 .0976

10 24 75 71.1 .097511 24.5 76 69.6 .121912 25 76 74.6 .1077

2 1 5 72 105.6 collopsed under own wt2 9 72 129.3 .02443 9.5 74 81.8 .03174 10 74 75 -) .03665 14 73 77.2 .04276 14.5 74 81.5 .03907 15 75 78.3 .04278 19.5 74 81.5 .03679 20 75 70.0 .0402

10 24.5 75 71.3 .039011 25 75 67.7 .0975

3 1 21.5 73 103.0 collpsed under own wt2 22 74 90.9 collapsed under own wt3 22.5 74 102.8 collapsed under own wt4 23 75 88.7 collopsed under own wt5 23.5 76 77.9 .02446 24 74 91.1 col lapsed7 24.5 75 81.8 .02938 25 73 78.1 .02209 25.5 74 79.4 .0242

10 26 75 82.5 .0348

12

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I _________

13

TABLE 2. Anchor Stabilizers

Anchor WI Stabilizers(ib) length width thickness

Navy stockless 1500 21 6 1/2Navy slockless 3000 3) 13 1/2Navy stockless 1000 42 16 1/2Navy stockles$ 100)0 3! 19 3/4Navy stockless 20000 45 21 1Navy stocklets 30000 50 23 1

Danforth 2510 51.5 4 1/2 roundDanforth 2770 51.5 4 1/2 roundDanforth 4000 56 5 roundDanforth 10000 62.5 5 1/2 roundDanforth 12000 R2.0 10 round

Laboratory 1620 36 3 1/2 roundLaboratory 2900 48 3 1/2 round

Concrete-steel 7500 30 12 I

Boldt 3170 18 29 3Baldt 365G 15 3 round

Croseck 3060 24 4 1/12 round

BuDocks 'straight' 1430 18 9 1BuDocks 'curved' 1430 24 9 1

Lightweight 500 23 2 roundLightweight 1000 29 2 1/2 roundLightweight 2000 36 3 roundLightweight 3000 46 3 1/2 roundLghtweight 4000 46 4 1/4 roundL'ghtwelght 1000 56 5 1/ round

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Figure 16. Strain goge used to measure break-out force of anchors

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Figure 28. BuDocks 1430-lb straight-plate anchor

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Figure 34. 3650-1b Baldt anchor

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Figure 36. Typical Lightweight anchor

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Rigure 39. 2510-lb Danforth anchor

Figure 40. 10,000-lb Danforth anchor

Figure 41. 12, 000-lb Danforth anchor

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Figure 47. 10,500-lb concrvte wedge anf- hr

Figure 48. 10,500-lb con''ete mushroom an~chor

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59

DISTRIBUT:ON LIST

No. ofcopies

I Navy Secr. vy, Research and Davelopment Bocrd1 Chief of Novol OperationsI Chief, Office of Novw, ResearchI Chief, Bureau of Ships, Research Division

)1 Chief, Bureau of Yards an Docks (D-400)I Co.mander Naval Forces, For EostI Commander Naval Forces, Ph;lippines1 Commanding Officer and Director, David W. Taylor

Modei Basin, Arodlfnom cs Laboratory1 Co-manding Ofifcer, U.S. Navol Amnph;b;oui Base

A CB No. ICommanaing Officcr, U.S. Naval Amphib;ow. BoseACB No. 2Conimnding Officer ad virector, U.S. Naval Engineer •ing Experiment Static.nCommanding Officer, U.S. Navy Supply Corps SchoolTest and Development Dept

2 Director, Picific Div;sion, Bu .cu -t Yards and Docks1 Director, Atlantic Division, Bureau of Yards and -rocksI Yards and Docks Supply Officer, CBC, Port Hueneme

U.S. Naval School, CEC Officers School, Port Hluen e1 Commandant, ist Noval District, Attn: Dist Civil Engr

Commoaont, 3rd Ncvo'! );strict, Attn: Dist Civil Engr1 Commandant, 4th Noval District, Attn: Dist Civil Engr

Commandant, 5th Naval District, Attn: Dist Civil Engr1 Commandant, 6th Nc -,I District, Attn: Dist Civil Engr1 Commandant, 8th Ncval District, Attn: Dist Civil Engr1 Commandant, 91h .level District, Attn: Dist Civil EngrI Commandant, 10th Naval District, Attn: Dist Civil EngrI Commandant, I1th Naval District, Attn: Dist Civil EngrI Commandant, 12th Nav ', District, Attn: Dist Civil Engr

I Commandant, I ' Nvel District, Attn: Dist Civil EngrI Commandant, 14th Naval District, Attn: Dist Civil Ery;rS Con.mandant, 15th Naval District, Attn: Dist Civil Engr1 Commandant, 17th Naval District, Attn: Dist Civil EngrI Commandant, Potomac River Naval Command, Attn: Dist

Civil Fngr

-- L

60

cop.ies

1 o Commandant, Severn River Naval Comano. d, An- DistC:l EngrDistrict Public orks , :i.e, lIt Naval District

Distr*ct Pkli, V,', kcs Mffica, 3rd Noval DistrictI D'str~t Publi. Works Office, 4th Naval District1 District Public Works Office. 5th Naval District

I istrict Public Vvorks Office, 6th Naval Dist--t1 District Pub ic Works Office, 8th Navnl District1 Dt;I.j % .:, Works Office, n-h Naval DistrictI District Public W, orls Office, 10th Naval District1 District Public Works Office. 11th No.,:! D,'tr;-t

District Public Works Office, 12th Naval Distr:ctDistrict Public Works Officc, 13th Naval 0,-trrtDistrict Public Works Office, 14th Navaol District

I District Public Works Otflice, 15th Naval Dst,'ct1 Disyiict Public Works Office, 17th Naval District

District Public Works Office, Potomac River Naval CommandI District Public Works Office, Severr, River Naval Command1 D;recnr, Waterways Experiment Stot-on

Director, Division of Plans and Potscias, 1- alq.ortcrsU.S. Marine Corps

1. *

-------- --- - ------

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