NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Slip
From: Doug Royer
Date: 2003 Apr 24, 10:39 -0700
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.