Simrad EK80 - Page 53

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Propellernoise
Thissourceisoftenthemainsourceofnoiseathighervesselspeeds.Variablepitch
propellersorfastmovingpropellersusuallymakemorenoisethanxedpropellersorslow
movingpropellers.
Thisnoiseisusuallywater-borne.Insomecases,however,shaftvibrationsorvibrationsin
thehullnearthepropellermaybestructure-bornetothetransducer.Ifapropellerbladeis
damaged,thismayincreasethenoiseconsiderably.
Propellercavitationisaseveresourceofnoise."Singing"propellersmightbeasourceof
noise,whichinterferesatdiscretefrequencies.Insomecasesstaticdischargefromthe
rotatingpropellershaftmaybequitedisturbing.
Cavitation
Cavitationusuallyoccursmorewillingly
inairlledwaterandtheoccurrenceis
dependentonthehydrostaticpressure.
Cavitationisaseveresourceofnoise.
Thenoiseismadewhenthevoids
implode.Cavitationnoiseoftenoccursat
thepropellerandnearextrudingobjects
athigherspeeds.
Contributionstotheownoisemay
alsobecausedbyairbubbleshittingthe
transducerface,orbythesplashcaused
bythewavessetupbythevessel.In
somecasesaresonantphenomenonissetupinaholenearthehull.Thissoundwillhavea
discretefrequency,whileallotherownoisewillhaveawidefrequencyspectrum.
(ImagefromU.S.Navyinthepublicdomain.)
Flownoise
Everyobjectthatmovesthroughwaterwilldisturbthemedium,anditwillcausefrictionin
thewater.Thefrictionzoneiscalledtheowboundarylayer.Theowinthisboundary
layermaybelaminarorturbulent.
•Thelaminarowisanicelyordered,parallelmovementofthewater.
•Theturbulentowisadisorderlyowpattern,fullofeddies.
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