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    Slip
    From: Doug Royer
    Date: 2003 Apr 24, 10:39 -0700

    After reading Mr. Allen's and Mr. Huxtable's experiances I wrote the
    awareness lecture.We all have experiances that we learned from that happened
    on the water.Boating is a deadly serious endevere.As these 2 stories can
    attest to it can turn to crap real fast out there.Awareness of conditions or
    phenomenon are keys to survival.Life on a boat is dangerous at best and can
    be downright deadly at its worst.Life and limb depends on an officer's
    arareness and decisions(I include pleasure boat skippers in this catagory).
    The exam question wasn't posed as a question but as an example.I went
    through life fat,dumb and happy untill I was made aware of these
    phenomenon.All the questions on these exams are there for a purpose,at the
    least to let the examiner know you understand the concept.In this instance
    slip is important for calculateing fuel consumption and if not taken into
    consideration you won't be where you think you are.
    I missed this question on the exam that is why I remember it.You chose the
    most suitable answer to the question on these exams.That said timed mile
    markers are in statute miles.The answers: A. 351.7 mi., B. 378.1 mi., C.
    404.6 mi., D419.3 mi.The most suitable answer is C.
    The shaft revs. are counted on these vessels for nav. purposes but not as
    you think.The data is fed into the ship's inertial navigation system.
    Here are some definitions of terms and formuli taken from "The Merchant
    Marine Officer's Handbook"
    Cavitation: Excessive propeller tip velocity is the main cause of the
    creation of cavities or voids in the water,creating the phenomenon known as
    cavitation.Other contributing factors are air - foil section prop
    blades(sections with even curature are more desirable),insufficiant tip
    clearance(12% of the diameter is acceptable),and a disturbed flow of water
    to the prop.Distubances are easily created by posts not faired,struts too
    close.Clearance ahead of a prop should be 20% of its diameter.In order to
    delay cavitation,it is desirable to use sections with even curature rather
    than sections with uneven curvature(airfoil sections),and wide blades.
    Temperature phenomenon: As water temp drops,its density increases.A prop
    suitable for warm water operations being used in colder water has its rpm's
    drop below the designed level.Based on a 70*F temp,reduction of the prop
    diameter approximately 1% for each 10*F drop in temp is desirable.
    Slip: Slip can have a pos. or neg. quantity.There are 2 kinds of
    slip.Apperant slip is the differance between the observed advance and that
    calculated by pitch and rpm's.Apperant slip is the most important of the
    two.True slip is usually greater than apparent slip,due to the wake moving
    with the vessel.The wake that affects slip is a body of water surrounding
    the prop and moving along with the vessel.The amount of wake is determined
    by the amount of friction produced by the hull moving through the water.
    Slip is determined by speed.So each time a vessel changes speed slip must be
    recalculated.
    Ts = Theoretical speed
    Obs = Observed or actual speed
    Vessel's max. speed = (rpm of prop x pitch in ft.)/1215.22
    Max. speed with slip factored = [((rpm of prop x pitch in ft. )/1215.22) x
    ((100 + - slip)/100)]
    Ts = [(pitch in ft. x eng. rpm)/(reduct. ratio x 88)]
    % slip = [(Ts - Obs)/Ts] x 100
    Most vessels have preobserved speed cards in the bridge to help with
    calculations.These calculations are really only approximations of the
    phenomenon but taken into effect will help provide a more accurate DR or EP.
    
    
    

       
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