Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014 COMPARISON BETWEEN FREEWHEELING AND LOCKED SAILBOAT PROPELLER
DRAG USING A COMPUTATIONAL FLUID DYNAMICS APPROACH (DOI No: 10.3940/rina.ijsct.2014.b2.160) R E Spall, Utah State University, USA SUMMARY
Computational fluid dynamics calculations were performed to predict the drag of locked and freewheeling 2- and 3- blade, 12 inch diameter, 6 and 10 inch constant pitch sailboat propellers. Drag levels and coefficients for both freewheeling and locked configurations are presented for flow velocities ranging from 1 m/s to 4 m/s. Results indicate that the drag for the freewheeling configurations were in all cases below those for the locked configurations at equivalent flow velocities. However, the relative magnitude of the decrease was found to be strongly dependent on the blade design.
1. INTRODUCTION
Most moderate to large sailboats utilize auxiliary inboard engines. Associated propellers are typically two- or three-blade;
configurations. An important
in fixed-blade, feathering, or folding consideration is the
parasitic drag caused by the propeller while sailing. Feathering and folding propellers impose less drag under sail than fixed-blade propellers but do have significant disadvantages such as increased cost and moving components which may corrode and/or jam.
Limited results concerning propeller drag exist in the literature, all of which are based on experiments. In particular, MacKenzie and Forrester [1] found that a locked, fixed-blade propeller produces up to 100% greater drag than a freewheeling propeller (particularly at higher speeds). The earlier work of Lurie and Taylor [2] suggests similar trends. Larsson and Eliasson [3] provide drag coefficients for a locked propeller that are a factor of
However, Kinney The
four greater than for a freewheeling propeller. [4] suggests that
results range from insignificant freewheeling
propellers result in greater drag, but without apparent justification. Klaka [5] also addresses the question by testing two propellers at speeds ranging from 3 to 7 knots.
drag
reduction to reduction levels up to 60% for rotating propellers over locked propellers. of torque was applied to
It is noted that a level the shaft for the rotating propellers to simulate resistance from a gear box.
In the work presented herein, the drag characteristics of fixed two- and three-blade sailboat propellers manufactured by Michigan Wheel Corporation were determined under locked and freewheeling conditions using computational fluid dynamics (CFD) techniques. In particular, domestic Sailer 2 and Sailer 3 models, each with a 12 inch (0.3048 m) diameter and 6 inch (0.1524 m) pitch, were evaluated. Blade section shapes were identical for both models. In addition, drag characteristics were also obtained for 2- and 3-blade, B-series propellers designed using the propeller design software PropCad [6]. These propellers were also 12 inch (0.3048 m) diameter. The 2-blade propeller was evaluated with pitches of both 6 inch (0.1524 m) and 10 inch (0.254 m). The 3-blade propeller was tested with a 6
©2014: The Royal Institution of Naval Architects
inch (0.1524 m) pitch only. The 2- and 3-blade, 6 inch (0.1524 m) propellers were of identical blade section shapes. The expanded area ratios (EAR) were 0.31 and 0.46 for each of the 2-blade and 3-blade propellers (Michigan Wheel and B-series), respectively. A significant difference between the Michigan Wheel and the B-Series propellers involved the leading edge. That is, the Michigan Wheel props have a flat (1/8 inch width) leading edge, whereas the B-Series design have a much sharper leading edge. For each propeller, the hub was 2.375 inches long and tapered from 1.75 inches diameter forward to 1.5625 inches aft.
Drag
results were obtained for approach velocities ranging from 1 m/s to 4 m/s.
2. NUMERICAL METHOD
The Reynolds-averaged Navier Stokes (RANS) equations were solved using the general purpose computational fluid dynamics code STAR-CCM+ [7]. The one-equation Spalart-Almaras model was applied for turbulence closure (c.f., Ref [8]). Second-order upwinding was used for the convective terms for all transport equations, while a second-order backward difference approximation was use for the time advance [8]. Pressure-velocity coupling was accomplished using the SIMPLE algorithm [8].
The freewheeling drag calculations were performed using a 6-degree-of-freedom (6-dof) solver available within STAR- CCM+. This is an unsteady approach in which the pressure and shear forces are integrated over the propeller to compute the resulting rotational motion.
An external moment
representing a torque on the propeller may also be imposed, although in this work the value was fixed to zero. We note however, that the imposition of an external moment may significantly increase the level of drag (c.f., Refs [9, 10]). Consequently, the drag results presented herein under freewheeling conditions represent the ideal; actual levels under sailing conditions are expected to increase due to an inherent torque imposed on the propeller shaft. The results for the locked propellers were obtained by solving the steady form of the RANS equations.
During the iterative solution procedure for the freewheeling calculations, normalized residuals for all B-81
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