Validation of squat formulas for inland vessels
The CoVadem initiative aims at providing near real-time and expected water depth information of inland waterways in Europe, based on continuous water depth measurements by a large number of inland ships during normal operation. Currently, more than 80 ships are participating the CoVadem network and are equipped with the necessary instrumentation. Measured underkeel clearance, the ship draught and position data are transferred to a CoVadem cloud service. Actual water depth is calculated by summing the underkeel clearance with the ship draught at the start of the voyage and the squat (the combined dynamic midship sinkage and trim effects) both at the location of the depth transducer. In average operational conditions for inland ships, midships sinkage is expected to be approximately 10 cm and dynamic trim some centimetres over the ship length. In extreme conditions with relatively high ship speed through water and small underkeel clearance in a narrow waterway, midship sinkage can probably increase to approximately 40 cm with 10 cm trim over the ship length. For an accurate water depth calculation, the best squat formula available for inland ships is to be used.
This paper compares four squat formulas with model tests conducted at MARIN. Some squat formulas only provide a mean or maximum squat and no dynamic trim. However, most inland ships participating in CoVadem measure the underkeel clearance close to the bow, making the dynamic trim important. Therefore, only squat formulas are selected that also yield a value for the dynamic trim. Also, the formula should be valid for restricted waterways. The selected methods are Barrass (2006), Ankudinov (Briggs 2009), Tuck/Stocks (Stocks 2002) and Römisch (1989).