squat - suny maritime · pdf filethe squat effect is the hydrodynamic phenomenon by which a...
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Squat
Prof. Charles J. Munsch SUNY Maritime College
October 15, 2014
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The squat effect is the hydrodynamic phenomenon by which a vessel moving quickly through shallow water creates an area of lowered pressure that causes the ship to be closer to the seabed than would otherwise be expected. This phenomenon is caused when water that should normally flow under the hull encounters resistance due to the close proximity of the hull to the seabed. This causes the water to move faster, creating a low-pressure area with lowered water level surface (See Bernoulli's principle). This squat effect results from a combination of (vertical) sinkage and a change of trim that may cause the vessel to dip towards the stern or towards the bow.[1] Squat effect is approximately proportional to the square of the speed of the ship. Thus, by reducing speed by half, the squat effect is reduced by a factor of four.[2] Squat effect is usually felt more when the depth/draft ratio is less than four[2] or when sailing close to a bank. It can lead to unexpected groundings and handling difficulties.
Amazingly accurate
From Wikipedia
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Bernoulli’s Principle
p + ρgh + ½ ρ v2 = constant static pressure gravity pressure hydrodynamic (velocity pressure)
P is pressure ρ is mass density
g is acceleration due to gravity v is ships speed (ft/s or m/s)
Vessels push water in front of her bow. Water flows back at sides and under the vessel to fill the void of water
displaced by the ship, barge or tug. Water forced to move faster.
IF ONE PRESSURE GOES UP OTHERS GO DOWN
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IF ONE PRESSURE GOES UP OTHERS GO DOWN
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ALL Vessels Squat! In open water Squat Negligible
Open channel
Minimum Squat
SQUAT = CB x V2/100 (meters)
V in knots Dr. C. B. Barras
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ALL Vessels Squat!
In restricted water! Maximum Squat
SQUAT = 2CB x V2/100 (meters)
V in knots Dr. Barras ( both rough es1mates)
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ALL Vessels Squat! A more involved and more accurate formula
Still an estimate! Maximum Squat
SQUAT = (CB x s2/3 x V2.08)/30
(meters)
V in knots
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Vessel Parameters B = Beam; T= Draft
CB= Block Coefficient; Cw = Waterline Coefficient Typical for barges 0.90 - 0.95 0.98 – 1.0
Channel Parameters h = water depth; W= Channel Width
hi = (4.96 +52.68(1-Cw)2 x T Wi = (7.7=45(1-Cw)2 x B
hi – critical depth Wi – critical width
UKC – Under Keel Clearance
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Channel Parameters Confined
As = Ship Area Ac = Channel Area S = Blockage As/Ac
s = S/(1+S)
SQUAT = (CB x s2/3 x V2.08)/30
(meters)
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Vessel Parameters for example
B = 30 m; T= 10m CB= 0.95; Cw = 0.99
h = 11 - 14m
W = 200m and 100m hi = 49.7m
Wi = 231.1m UKC = 1 – 4 m
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0.0000
0.5000
1.0000
1.5000
2.0000
0 2 4 6 8 10 12 14 16
Calculated
Squ
at
Vessel Speed
Open channel Squat calcula2on
Open simple ukc cw 1 ukc cw 2 ukc cw3 ukc cw 4
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0.0000
0.5000
1.0000
1.5000
2.0000
2.5000
3.0000
3.5000
4.0000
0 2 4 6 8 10 12 14 16
Calculated
Squ
at
Vessel Speed
Confined Channel Squat Calcula2on (W=200)
Confined simple ukc cw 1 ukc cw 2 ukc cw3 ukc cw 4
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0.0000
0.5000
1.0000
1.5000
2.0000
2.5000
3.0000
3.5000
4.0000
0 2 4 6 8 10 12 14 16
Calculated
Squ
at
Vessel Speed
Confined Channel Squat Calcula2on (W = 100)
Confined simple ukc cw 1 ukc cw 2 ukc cw3 ukc cw 4
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Reference; Ship Stability for Masters and Mates
Chapter 31 Barras and Derrett BH
http://www.ship-squat.com/index.html