Squat - a briefing for mariners
Transcript of Squat - a briefing for mariners
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SQUATand
U.K.C.a briefing for mariners
prepared by: Marc Van de Velde
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SPEED FWD.
00 knots
“STATIC” UKC
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SPEED FWD.
15 knots
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SPEED FWD.
15 knots
SPEED FWD.
13 knots
“DYNAMIC” UKC or
“NETT” UKC
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SIGNS OF SHALLOW WATER 1. Changing wave pattern around ship
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SIGNS OF SHALLOW WATER 1. Changing wave pattern around ship 2. Vibrations
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SIGNS OF SHALLOW WATER 1. Changing wave pattern around ship 2. Vibrations 3. Ship slows down
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SIGNS OF SHALLOW WATER 1. Changing wave pattern around ship 2. Vibrations 3. Ship slows down 4. Trim changes
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SPEED FWD.
13 knotsTrim aft: with squat, ship trims more aft
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SPEED FWD.
13 knotsEven keel: with squat, ship trims forward
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SPEED FWD.
13 knotsTrim foward: with squat, ship trims more forward
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SIGNS OF SQUAT 1. Changing wave pattern around ship 2. Vibrations 3. Ship slows down 4. Trim changes 5. Loss of manoeuvrability, steerage,….
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SIGNS OF SQUAT 1. Changing wave pattern around ship 2. Vibrations 3. Ship slows down 4. Trim changes 5. Loss of manoeuvrability, steerage,…. 6. Turning circle diameter x2, … x3…
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SQUAT FORMULAE
in focus
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V = ship speed (knots)Cb= block coefficientS2= velocity return factor = S/(1-S)S= blockage factor = As/AcAs= midships cross sectional area (m2)Ac= cannel cross sectional area (m2)
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S2= velocity return factor = S/(1-S)S= blockage factor = As/AwAs= midships cross sectional area (m2)AW= channel cross sectional area (m2)
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Cb= block coefficient
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Cb (this ship) > Cb (this ship)
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V = ship speed (knots)
This is : ship’s speed through the water !Example:
SpeedOverGround = 10 knots + 4 knots countercurrent knots = speed 14 knots
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Example: Gerardus Mercator
for V= 15 knots -> squat = 1.8 m for V= 10 knots -> squat = 0.8 m
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Empirical formulae
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Situations
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Situations
Formula Dr. Barass
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Situations
Formulae Dr. Barass
Or better:
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Situations
Formula Dr. Barass
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U.K.C.
Static UKC (with ship’s speed =0)Nett UKC (with sailing ship)
Under Keel Clearance
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Heel
New draft = ½ beam sin(list) + old draft. Cos(list)
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What is a safe NETT UKC ?
According textbooks on navigation:
1.1 x to 1.25 x largest draught
Example: 12m draught,
safe UKC = 1.2 to 3 meter
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What is a safe NETT UKC ?According PIANC
PIANCwww.pianc.org
-
Muddy bottom 0.3 m (*)
Sandy bottom 0.5
Rocky bottom 1m
(*) muddy bottom: in some cases a negative UKC is possible !
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1. Exact draught measurement ?
2. Online real-time tidal heights onboard ?
3. Heel measurement ?
4. Estimate of ship’s motion in waves ?
5. Exact survey of shallow route ! ! !
What does it takes ?
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Lowest value: 13.3 m
Highest value: 12.5 m
Mean value for his area: 12.9 m
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FIRST EXAMPLE Gerardus Mercator in Palm II, 2006
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Design max. depth 12.4 m
Max. draught 12 m
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SECOND EXAMPLE Cristobal Colon in Cuxhaven, 2010
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Max. draught 15+ m
Channel depth 14.6 m
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Why all this fuzz ?
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An “error” in draught of 50cm…
50 cm less draught x 45t/cm = 2250 ton=
1000 m3 sand per trip less=
7% of daily production (!)
(example Gerardus Mercator)
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… or …a much higher risk of
grounding….
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References http://www.ship-squat.com/