Squat

This article is about the sinking of a ship. For sports discipline, see squats . For information on taking possession of vacant living space, see Squatting . For one type of rail fault in railroads, see rail fault #Squat . Sunk is a redirect here; the valley in Styria see Sunk (Hohentauern) .

Squat ( English squat 'squat) is a term used in navigation and driving dynamic vertical drop referred to a vessel on the actual draft addition ( Absunk or sunk ), with simultaneous trimming . The trimming forwards or aft depends on the block coefficient .

Driving dynamics decrease with simultaneous trimming (squat) (above graphic: trimming aft, lower graphic: trimming front)

definition

The sinking can be observed in all moving ships, especially when navigating a river or canal, and is dependent on the cross-section of the ship, the speed, the waterway cross-section and the traffic situation, i.e. H. meeting or overtaking other ships. The lower the water level, the bigger the squat. While the ship is moving, the lateral and depth limitation creates a return flow from the bow to the stern of the ship. This backflow causes the water level to drop and thus also the ship. Under extreme conditions, the sinking can be so severe that the ship hits the ground and the hull or the propulsion system are damaged.

Decrease in the water level over time (time axis runs to the right) at a point fixed on the bank when a ship (forwards) passes by (to the right or to the left). Alternative view: stationary water sinking with a ship - in front of, along and after the ship moving to the left. The secondary stern wave oscillates, however, so it is not stationary.

In rivers there is inside of curves to silting . However, since the current and thus the resistance are lowest here, barges usually use the inner curve when traveling uphill . If the ship now comes too close to the silting, there is a dynamic increase in draft, which can be recognized by the higher stern wave that moves forward. In this case you have to reduce the speed immediately, otherwise there will be ground contact due to insufficient float water. Floating water describes the safety distance between the bottom of the fairway and the bottom of the ship. The ship's sinking plus the floating water result in the freedom of the keel.

Squat, achieved by driving at high speed, can be used to drive under critically low clearance heights under bridges.

Calculation approaches

Calculation approach according to Tuck

Ernest O. Tuck determined the following calculation for the drop and the trim with the help of the slender body theory :

${\displaystyle s=c_{s}{\frac {\nabla }{L_{pp}^{2}}}{\frac {F_{h}^{2}}{\sqrt {1-F_{h}^{2}}}}}$

With

• ${\displaystyle s}$ = Decrease
• ${\displaystyle c_{s}}$ = Decrease coefficient
• ${\displaystyle \nabla }$ = volume of the ship sunk in water in m³
• ${\displaystyle L_{pp}}$ = Length of the ship (p / p)
• ${\displaystyle F}$= Froude depth number

${\displaystyle \tau =c_{\tau }{\frac {\nabla }{L_{pp}^{2}}}{\frac {F_{h}^{2}}{\sqrt {1-F_{h}^{2}}}}}$

With

• ${\displaystyle \tau }$ = Trim
• ${\displaystyle c_{\tau }}$ = Trimming coefficient

The sinking coefficient and the trim angle coefficient are complex expressions of characteristics of the respective ship. ${\displaystyle c_{s}}$ ${\displaystyle c_{\tau }}$

Dand's calculation approach

Ian W. Dand found the following equation for the dip and trim, considering these as vertical force and moment

${\displaystyle s={\frac {\int d(x)B(x)dx}{\int B(x)dx}}}$

With

• ${\displaystyle s}$ = Decrease
• ${\displaystyle B(x)}$ = Width of the ship measured at the waterline at point x

${\displaystyle \tau ={\frac {\int xd(x)B(x)dx}{\int xB(x)dx}}}$

With

• ${\displaystyle \tau }$ = Trim

Calculation approach according to Führer and Roman

In the calculation approach by M. Führer and K. Römisch, they first developed an equation for the squat at critical speed with the help of a model. The critical speed is the limit from which the water displaced by the ship is no longer completely drained backwards against the direction of travel due to the narrowed drainage cross-section. For the squat at critical speed we get:

${\displaystyle s_{b,{\text{krit}}}=0{,}2\left({\frac {10C_{B}B}{L_{pp}}}\right)^{2}T}$

With

• ${\displaystyle s_{b,{\text{krit}}}}$ = Squat at critical speed and bow trim
• ${\displaystyle C_{B}}$= Block coefficient
• ${\displaystyle T}$= Draft

${\displaystyle s_{s,{\text{krit}}}=0{,}2T}$

With

• ${\displaystyle s_{b,{\text{krit}}}}$ = Squat at critical speed and aft trim

Others

The faster the ride over the ground, the greater the drop. The cruise ship Queen Elizabeth 2 therefore ran aground on August 7, 1992 on a rock near Cuttyhunk Island ( Elizabeth Islands ). The ship's command did not take sufficient account of the subsidence and the available map contained imprecise information on the water depth.

See also

• Shallow water resistance

Individual evidence

1. ↑ A. Hartling; J. Reinking: Nature measurement of the squat , HANSA, 1999 - No. 8
2. ↑ EO Tuck: Shallows water flows past slender bodies , Journal of Fluid Mechanics, Vol. 26, part 1, 1966, p. 81-95
3. ↑ IW Dand: Full form ships in shallow water: Some methods for the prediction of squat in subcritical flows , Teddington, National Laboratory, Rep. No. 160, 1872
4. ↑ M. Führer, K. Römisch: Effects of modern ship traffic on inland and ocean-waterways and their structure , Section 1 Inland Navigation, 24th International Navigation Congress, Leningrad 1977, PIANC Brussels
5. ↑ Nick Perugini: Grounding of the Queen Elizabeth 2 (response) ; June 26, 2009, In: Hydro International, July / August 2009, Volume 13, number 6.

Web links

• European Development Center for Inland and Coastal Shipping : Comparison of the fairway conditions of the Straubing-Vilshofen section of the Danube with selected waterway sections in the discharge year 2003 (PDF; 1.9 MB)
• Briggs, MJ (2006): Ship Squat Predictions for Ship / Tow Simulator , US Army Corps of Engineers (PDF; 211 kB)

literature

• S. Schuster: Investigation of flow and resistance conditions when ships travel on restricted water , Yearbook of the Shipbuilding Society , 52nd volume, 1954
• W. Führer; K. Römisch: Effects of modern ship traffic on inland and ocean waterways and their structures ”, 24th International Maritime Congress, Leningrad, pp. 1–3
• K. Römisch: Achievable speed and diving of a ship on flat water - a contribution to the optimal dimensioning of inland and sea waterways , Mitteilungen der FAS, H. 24, 1969
• K. Uliczka / B. Kondziella: Dynamic driving behavior of extremely large container ships under shallow water conditions , bulletin of the Federal Institute for Hydraulic Engineering No. 86 (2003), p. 83 ( PDF )