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8/14/2019 Underkeel Clearance - Capt. John Hamill
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UNDERKEEL CLEARANCE
SHIP SQUAT & TIDES
BEST NAVIGATION PRACTICES
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DEFINITION OF SQUAT
REDUCTION OF UNDERKEEL CLEARANCE (UKC)
CAUSED BY A SHIPS MOVEMENT THROUGH
THE WATER.
(IT IS NOTAN INCREASE IN DRAFT. RATHER, IT IS
THE BODILY SINKAGE OF A VESSEL WHICH
PLACES IT CLOSER TO THE SEA BOTTOM.)
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STATIC DRAFT + SQUAT
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SQUAT FACTS
C >.7 (bulker) squats by bow. C < .7 squatsby stern
Squat varies directlywith breadth (doublingbreadth doubles squat)
Squat varies by the square of speed (doubling
speed quadruples squat) Squat can be doubledwhen meeting another
ship
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SQUAT FORMULAE
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BEST GENERAL FORMULA
V squared/100= SmetersSpeed=6KTS
36/100=.36 meter squat
Speed=12kts
144/100= 1.44 meter squatDoubling the speed quadruples the squat
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HULL TYPES
BLOCK COEFFICIENT (C )
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LARGE BLOCK HULL
C > .7
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FINER HULL
C < .7
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VARIOUS SQUAT FORMULAE YIELD
SIMILAR CONCLUSIONS
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SQUAT PREDICTIONS
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TIDAL CONSIDERATIONS
ZERO TIDE
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TIDE +4 FEET
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TIDE +8 FEET
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TIDAL CONSIDERATIONSUSING TIDE TO GOOD ADVANTAGE
NOAAS PORTS TOOL ONLINE
LOCAL KNOWLEDGE OF PILOTS
CREDIBLE PREDICTIONS OF METIORILOGICALFACTORS ON TIDES (NOAAS FORECAST
PROGRAM)
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NOAAS OPERATIONAL FORECAST
SYSTEM
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OPERATIONAL FORECAST SYSTEMSCREEN SHOT 7/24/11
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NOAAS PHYSICAL OCEANOGRAPHIC
REAL-TIME SYSTEM (PORTS)SCREEN SHOT 7/24/11
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NOAAS SEARCHABLE STATIONS
www.tidesandcurrents.noaa.gov
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PORTS COMPOSIT PAGE
CHESAPEAKE CITY, MD
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PORTS AIRGAP OBSERVATIONSC&D CANAL
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Association of Maryland Pilots
H FLAG
PILOT ON BOARD