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In this paper, seakeeping performance of vessels is comparatively investigated by means of improved 2.5D theoretical calculation, small-scale model towing tank test and large-scale free running model sea trial. This study also provides a scheme of ship vertical motion stabilization technique by using Semi-Submerged Bow (SSB) appendages. Firstly, an improved strip theory based 2.5D theoretical seakeeping algorithm, which considers the viscous flow effects attributed to bow appendages, is developed to estimate ship vertical motion responses in regular waves. Short-term predictions are also made on the basis of spectral analysis theory to predict ship motion responses in irregular waves. Then corresponding small-scale models are made and tested in a laboratory towing tank for both regular and irregular wave conditions to confirm the numerical results. The tank testing results between different ship schemes are also compared to experimentally investigate the effects of SSB and fin on hull vertical motion stabilization. Furthermore, large-scale model sea trials are conducted in coastal waves for a final validation of the ship seakeeping performance in short-crested sea waves. Finally, comparisons of the results by different testing approaches are systematically made and analyzed.

The novel testing technique for ship seakeeping behavior, which includes conducting large-scale model trial in realistic sea waves, has been given special attention by naval architects in recent years. Large-scale model measurement provides obvious advantages in the investigation of ship hydrodynamics in many aspects. However, there still exist some challenges that need addressing to improve this kind of testing technique. One of the great challenges is the reproduction of full-scale ocean waves when conducting downscaled model experiments in natural sea environment. This paper aims to explore the possibility of reproduction of full-scale wave states in coastal sea areas. Criteria, such as significant wave height, mean period, spectral shape and directional spreading, are involved in the wave similarity judgment. Various factors including season, climate, model scaling, coastal space and time are taken into account to determine suitable waves for model measurement. The wave selection principle and methodology proposed in this paper are demonstrated through a case study using coastal wave data of the Puerto Rico & Virgin Islands and the Gulf of Maine.

The development of large scale model measurements for ship hydrodynamic tests in natural environment in China is presented in this paper. Previous works of testing large scale monoblock models for seakeeping performance performed by Harbin Engineering University (HEU) are reviewed at first. To verify whether it is acceptable to perform tests near the shore instead of in deep-ocean, coastal waves were measured at different locations in the Bohai Sea and the Yellow Sea of China, and were compared with theoretical spectra. Then, a description of large scale model set-up, testing equipments, and experimental procedure regarding the segmented self-propelled model project is introduced and some typical results obtained from a recent sea trial are presented. Moreover, testing results by a corresponding small scale model in a laboratory basin as well as numerical results are compared with the large scale model experimental results to validate that the proposed testing method is reliable. Finally an overview of the ongoing large scale model laboratory plan and its future development directions is prospected.

The desire to benefit from economy of scale is one of the major driving forces behind the continuous growth in ship sizes. However, models of new large ships need to be thoroughly investigated to determine the carrier’s response in waves. In this work, experimental and numerical assessments of the motion and load response of a 550,000 DWT ore carrier are performed using prototype ships with softer stiffness, and towing tank tests are conducted using a segmented model with two schemes of softer stiffness. Numerical analyses are performed employing both rigid body and linear hydroelasticity theories using an in-house program and a comparison is then made between experimental and numerical results to establish the influence of stiffness on the ore carrier’s springing response. Results show that softer stiffness models can be used when studying the springing response of ships in waves.

The traditional laboratory models for the hydroelasticity and seakeeping performance of ships are tested in calm water and in uni-directional, artificially generated waves. A new alternative to the tank model measurement methodology is to conduct experiments using large-scale models in actual sea conditions. To implement the tests, a large-scale segmented self-propelling model and testing system were designed and assembled. A buoy wave meter was adopted to record the coastal waves that the model encountered during the tests. The analysis of the results of waves in sheltered waters by the spectral method shows good agreement with ISSC spectra. To investigate the difference between this new methodology and the traditional towing tank tests, a small-scale model, whose type and configuration are the same as those of the large-scale model ship, was used and tests were conducted in a towing tank. Comparison of the two experimental results shows that there is a remarkable difference in the response characteristics between the large-scale model at sea and the small-scale model in the tank. Numerical simulations of the responses of the ship under equivalent sea states were also carried out. The influence of directional spreading functions on the results was analyzed by a numerical approach. The classical model tests under long-crested waves in the towing tank over-estimate the motion and wave load responses; however, large-scale model tests carried out at sea are more reasonable for ship design and scientific research.

A towing tank experimental investigation was carried out using a segmented scaled model of a large ship prototype to study its behaviour when sailing in irregular head waves. The tests were carried out for typical sailing speeds and wave conditions that would be encountered by the large ship at sea. The vertical plane motion of heave, pitch, and the accelerations at bow and stern, and the vertical bending moment load responses of the ship were measured and analyzed. The significant amplitude values of motion and load response of the ship for the investigated wave conditions and speeds were determined by using spectral analysis. Slamming induced whipping loads were also investigated for the studied sailing conditions. In addition, the load response characteristics were evaluated in both time-domain and frequency-domain. Increasing the sea state had a greater effect on the motion and load responses of the large ship than increasing the sailing speed.

The need to benefit from economy of scale has driven the design of larger and larger ships with increasing tonnage. The hull structures of large ships are more flexible and the natural frequencies of the hull girder can even fall within the range of wave encounter frequencies resulting in the resonance phenomenon termed springing. A numerical linear hydroelastic investigation of the wave induced motion and load responses of a 400,000 dead weight tonnage (DWT) ore carrier is carried out in this work using an in-house program: Linear Elastic Compass Wave Loads Calculation System (WALCS-LE). The full load condition at different ship speeds, wave lengths and incident wave headings in regular waves is investigated. Long term predictions are also made. It is imperative to account for springing loads and the probable extreme motions and loads at the design stage of large ships to ensure their endurance and safety at sea.