Kongsberg EA440 - Page 27

392616/B
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causedbythepropellers.Propellercavitationisaseveresourceofnoise.Thecavitation
startswhenthewaterowsinthesamedirectionasthepropellerblades.Thisiswhere
thepropellerbladesmovedownwards.
Insomecasesaresonantphenomenonissetupinaholenearthehull.Thissoundwill
haveadiscretefrequency,whileallotherownoisewillhaveawidefrequencyspectrum.
(ImagefromU.S.Navyinthepublicdomain.)
Flownoise
Theupperwaterlayersoftheseacontainamyriadofsmallairbubblescreatedbythe
breakingwaves.Whenthehullmovesthroughwateritwillcauseadisturbance,andthis
willgeneratefriction.Thefrictionzoneiscalledtheowboundarylayer.Theowin
thisboundarylayermaybelaminarorturbulent.
•Thelaminarowisanicelyordered,parallelmovementofthewater.
•Theturbulentowisadisorderlyowpattern,fullofeddies.
ATurbulentow
BLaminarow
CAirbubbles
Airbubblesabsorbandreectthesoundenergy,andtheymayinworstcasesblockthe
soundtransmissionaltogether.
Theboundarylayerincreasesinthicknesswhenitbecomesturbulent.Theboundary
layeristhinintheforwardpartofthevesselhull,andincreasesasitmovesaft.The
thicknessdependsonshipsspeedandontheroughnessofthehull.Allobjectssticking
outfromthehull,ordentsinthehull,willdisturbtheowandwillincreasethethickness
oftheboundarylayer.Whentheowspeedishigh,theturbulencecanbeviolentenough
todestroytheintegrityofthewater.Smallvoidsorcavitiesinthewaterwilloccurand
thisiscalledcavitation.
Rattlenoise
Rattlenoisemaybecausedbylooseobjectsinthevicinityofthetransducer,likexing
bolts.Therattlemayalsocomefromlooseobjectsinsidethehull.
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