Estimation and analysis of the uncertainty introduced by using a numerical model for the investigation and study of any type of flow problem have become common industry practice. Through understanding and evaluation of the uncertainty introduced by a numerical model, the accuracy and applicability of the model itself are evaluated. In this paper, the numerical uncertainty of a CFD-methodology developed to analyse the hydrodynamic performance of a collective and cyclic pitch propeller (CCPP) is estimated and analysed. The CCPP is a novel propulsion and manoeuvring concept for autonomous underwater vehicles, aimed to generate both propulsion and manoeuvring forces through advanced control of the propeller's blade pitch. The numerical uncertainty is established for three performance parameters, the generated propulsive force, the side-force magnitude, and the side-force orientation, by conducting a grid and time-step refinement study over three operational conditions. Additionally, the influence of the oscillatory uncertainty, introduced by the periodic nature of the problem, is investigated although shown to have a minimal effect when properly monitored. Based on a least-squares regression analysis of the refined simulation results, the numerical uncertainty is proven to be dominated by the introduced discretisation errors. In the case of the propulsive and side-force magnitude, the total uncertainty is dictated by the time discretisation uncertainty under bollard pull conditions, while the total uncertainty of the captive cases is mainly a result of the spatial discretisation uncertainty. The total uncertainty in the side-force orientation is observed to be primarily a consequence of the time discretisation uncertainty for all simulated cases. Overall, the

Propulsion and maneuvering of autonomous underwater vehicles require a combination of effective and efficient operation at both high and low speeds. The collective and cyclic pitch propeller (CCPP) is a novel system designed to provide the required operational flexibility through control of the propeller's blade pitch. Collective pitch control governs the forward generated thrust, whereas cyclic pitch control governs the generated maneuvering force(s)/side-force(s). In this article, a numerical analysis into the CCPP's hydrodynamic performance at bollard pull is set-up, reducing the complex three-dimensional flow problem to a two-dimensional problem. Through a force break-down model, the CCPP's hydrodynamic performance is related and matched to the operation of a pitching hydrofoil. Analysis of the two-dimensional numerical results can thereby provide insights into the performance of the three-dimensional CCPP. First, the performance of the pitching hydrofoils is investigated as such, relating the generated lift, drag, and moment to the occurrence of dynamic stall. Next, the methodology's applicability and limitations are discussed by comparing the numerical results with recent experiment CCPP work to allow the model to be used for a numerical evaluation of the CCPP's performance. Under the evaluated conditions, testing a range of collective and cyclic pitch angles under bollard pull, the side-force generation by the CCPP is shown to be highly dependent on the generated drag force at higher collective pitch angles. At low pitch angles, the side-force generation is controlled by the lift produced over the pitching blades, and efficient but not highly effective. As the collective pitch is increased, the generated drag affects both the effectiveness of the side-force and the side-force efficiency, defined by the large resulting side-force orientation. At larger collective pitch angles, the lift forces are overtaken by the drag generation, resulting in effective but inefficient side-force generation. Autonomous underwater vehicles (AUVs) have become a widely used and researched tool for underwater exploration and reconnaissance (Alam et al. 2014). AUVs distinguish themselves from other unmanned underwater vehicles in their ability to complete a pre-determined mission autonomously over large distances and long time periods, i.e., without the need for regular human interaction. The diversity in industry applications for AUVs has resulted in a wide range of AUV shapes and designs (Button et al. 2009). Applications include different areas such as underwater pipe-line inspection in the oil and gas industry, sample collection for marine biology research, and military surveillance missions (Chyba 2009). One key requirement of any AUV design, as a result of their specific mission profile and inherent functionality, is the combination of efficient long-endurance travelling capabilities with effective maneuverability at low speeds (Wernli 2000). Traditional maneuvering systems using control surfaces lose their efficiency at low speeds and lowspeed maneuvering aids such as side-or podded-thrusters reduce the long-endurance travelling efficiency. A novel propulsion and maneuvering system, aimed at providing both efficient long-endurance propulsion and effective maneuvering at all speeds, is the collective and cyclic pitch propeller (CCPP).

In prawn-trawling operations, otter boards provide the horizontal force required to maintain net openings, and are typically low aspect ratio (∼0.5) flat plates operating on the seabed at high angles of attack (AOA; 35–40°). Such characteristics cause otter boards to account for up to 30% of the total trawling resistance, including that from the vessel. A recent innovation is the batwing otter board, which is designed to spread trawls with substantially less towing resistance and benthic impacts. A key design feature is the use of a sail, instead of a flat plate, as the hydrodynamic foil. The superior drag and benthic performance of the batwing is achieved by (i) successful operation at an AOA of ∼20° and (ii) having the heavy sea floor contact shoe in line with the direction of tow. This study investigated the hydrodynamic characteristics of a generic sail by varying its twist and camber, to identify optimal settings for maximum spreading efficiency and stability. Loads in six degrees of freedom were measured at AOAs between 0 and 40° in a flume tank at a constant flow velocity, and with five combinations of twist and camber. The results showed that for the studied sail, the design AOA (20°) provides a suitable compromise between greater efficiency (occurring at lower AOAs) and greater effectiveness (occurring at higher AOAs). At optimum settings (20°, medium camber and twist), a lift-to-drag ratio >3 was achieved, which is ∼3 times more than that of contemporary prawn-trawling otter boards. Such a result implies relative drag reductions of 10–20% for trawling systems, depending on the rig configuration.