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Sailboat Propeller Drag ARTICLE IN PRESS Ocean Engineering 35 (2008) 28–40 www.elsevier.com/locate/oceaneng Sailboat propeller drag P.M. MacKenzieÃ, M.A. Forrester Department of Mechanical Engineering, University of Strathclyde, Glasgow G1 1XJ, Scotland, UK Received 15 March 2007; accepted 1 July 2007 Available online 10 July 2007 Abstract All but the smallest classes of modern keelboats are fitted with inboard engines and consequently, when making way under sail, the craft experience parasitic drag due to trailing propellers and associated appendages. The variety of screw configurations used on sailing boats includes fixed-blade, feathering, and folding set-ups, with blades numbering two or three. Although the magnitude of the resultant drag is thought to have a significant influence on sailing performance, the published literature having regard to this problem is sparse. Here, the aim was to evaluate the drag effect of fixed-blade propellers of types commonly used on sailing craft. The results of towing tank tests on full-scale propellers are presented for the locked shaft condition; these are presented along with reconfigured data from the few previously published sources. For the case in which the propeller is allowed to rotate, tests were conducted on a typical screw with a range of braking torques being applied. It was hypothesised that the performance coefficients of the Wageningen B-Screw Series could be used to characterise adequately the types of screw of interest and that these could be extrapolated to enable prediction of the drag of a freewheeling propeller; an assessment of this formed part of the investigation. r 2007 Elsevier Ltd. All rights reserved. Keywords: Drag; Experiment; Propeller; Sailboat; Towing tank 1. Introduction racing, are fitted with conventional fixed geometry two- or three-bladed propellers. There are several reasons for this. For several decades, virtually all sailing yachts over First of all, low-drag deployable propellers are mechani- about 8 m overall length (LOA) have been equipped with cally complex and consequently have a high initial price — inboard engines driving through either fixed propeller up to 10 times that of the fixed-blade equivalent. Having shafts or, increasingly so, the more compact ‘‘saildrive’’ moving parts, and operating in an exposed and corrosive units. Some examples of common configurations are environment, they are prone to wear and damage. Some depicted in Fig. 1. Even race-oriented designs are, in designs of folding propellers in particular, in which the general, equipped with quite substantial auxiliary power blades are deployed by centrifugal force when the prop- units, despite the penalty in sailing performance caused by shaft spins, have blade geometries which are compromised the additional weight of the machinery and fuel, and the by the impositions of the deployment function such that hydrodynamic drag arising from the extraneous appen- their hydrodynamic efficiencies are relatively poor driving dages such as propellers and shafts. ahead, and are significantly worse for astern propulsion An obvious way to achieve a reduction in hydrodynamic and manoeuvring, when compared to the fixed-blade losses is to fit a propeller with folding or feathering blades. alternatives. Finally, there appears to be no consensus in Nevertheless, a cursory survey of any winter lay-up yard the sailing community, nor in racing handicap administra- will confirm that the majority of yachts used exclusively for tion, nor in the sailboat design field, as to the actual gains cruising, and indeed a small proportion of those used for to be had in sailing performance, in an objective and quantifiable sense, when the low drag configuration is ÃCorresponding author. Tel.: +44 141 548 2045; fax: +44 141 552 5105. employed in preference to fixed-blade propellers (some E-mail address: [email protected] (P.M. MacKenzie). measure of the range of views may be gleaned from, for 0029-8018/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.oceaneng.2007.07.004 ARTICLE IN PRESS P.M. MacKenzie, M.A. Forrester / Ocean Engineering 35 (2008) 28–40 29 Fig. 1. A variety of propulsion configurations commonly found on sailing boats. Clockwise from top left: fixed three-blade in aperture behind long keel; fixed three-blade on exposed shaft well behind fin keel; feathering three-blade on shaft, fin keel; folding two-blade, deployed; folding two-blade, folded; folding three-blade, folded, on sail-drive strut behind high aspect-ratio keel. example: Henderson 1983; Johnson 1997; Skene 1938; and condition may simply be that of a flat disc, bluff body.) Warren 1972). Adding to this confusion is the myth of Part of the present work would be directed at assessing the common currency among many yachtsmen that the veracity of this convenient approach. practice of locking a fixed-blade propeller to prevent Lurie and Taylor (1995) reported the results of their rotation results in less drag than would allowing it to tests of a comprehensive range of sailboat propellers freewheel. The present work seeks to address some of the covering virtually all popular configurations. Their more important aspects of these lacunae. study was principally directed towards assessing the performance of each screw as a propulsion device but 2. Existing hypotheses they also produced some measurements of the drag characteristics for the locked shaft condition (with Whilst, with obvious justification, the subject of ship regard to parasitic drag, this being the least desirable propeller performance and efficiency has had a great deal of all). In the concluding part of their work, they went of research effort devoted to it, the topic of parasitic on to allude to the possibility of being able to propeller drag appears rarely in the literature. extrapolate propeller performance curves to predict the Larsson and Eliasson (1994) proposed that the following parasitic drag force for a notional speed of screw relationship be used to estimate propeller resistance, RP,in rotation, but stopped short of assessing the validity of a boat design context: this. One of the aims of our investigation was to develop 2 this proposal further. RP ¼ 0:5 rV CD AP (1) and that the drag coefficients are: 3. Proposed approaches C ¼ 1.2 for fixed blades, shaft locked. D The main thrust of this work, therefore, was to esta- C ¼ 0.3 for fixed blades, shaft free to rotate, zero D blish the veracity of the existing hypotheses sum- braking torque. marised above, and where appropriate, to assess possible C ¼ 0.06 for a two-blade folding propeller. D refinements or alternatives. The investigation can thus AP is the projected frontal area of the propeller, V is the be divided conveniently into two elements: consideration boat speed and r is the density of water. (They do not of the drag produced by freewheeling propellers and directly identify the source of these CD values, and one evaluation of the drag characteristics of propellers with assumes that the coefficient given for the locked shaft locked shafts. ARTICLE IN PRESS 30 P.M. MacKenzie, M.A. Forrester / Ocean Engineering 35 (2008) 28–40 4. Investigation — freewheeling condition 4.2. Frictional torque 4.1. Performance characteristics of propellers A key part of our proposition is that, in cases where a vessel is making way under sail and its propeller is allowed The proposal being investigated in this part of the work to freewheel, the rotational frictional force on the propeller is built around the following three forward-performance shaft, the negative torque, may be assumed to be constant characteristics of propellers: for a given installation, regardless of, within realistic practical limits, the shaft speed of rotation. In practice, (i) Advance ratio: of course, factors such as stern gland tightness, gearbox oil viscosity (largely a function of temperature), bearing V J ¼ A , (2) efficiency and wear, and so on, will influence the level of D Á n friction. Nevertheless, for virtually any boat among the where VA is the velocity of advance of the propeller, D types described in the introduction, the torque required to is the propeller diameter, and n is the rotational speed. rotate the shaft should be readily measurable in situ using (ii) Torque coefficient: the most basic of apparatus; and obviously, account may Q be taken of the spectrum of frictional conditions for a given KQ ¼ (3) rD5n2 set-up if so required. The main point here is that, given a particular set of shaft friction conditions, then for all where Q is the torque on the propeller shaft. rotational speeds likely to be encountered in practice, the (iii) Thrust coefficient: frictional torque, Q, can be assumed to be constant T throughout the speed range. In this respect, we diverge KT ¼ , (4) rD4n2 from Lurie and Taylor who suggested that an estimated average speed of rotation, n, be used in the analysis. (Their where T is the thrust on the propeller. thinking was that by fixing n, one would be able to obtain J for any value of velocity, and thus the relevant values of KT In ship design practice, the relationship between these could be extracted from the extrapolated performance three parameters is used routinely to characterise indivi- chart. Hence, it would be a simple step to calculate the dual designs of propeller (as propulsion devices) and this is resultant negative thrust using Eq. (4), and thereby usually depicted in the form shown schematically in Fig. 2. generate a chart of drag vs. velocity. Attractive though The curve for efficiency of propulsion, Z, is also generally this would be, it is erroneous, as we will show, since a very presented as shown along with these data but for our wide band of rotational speed is observed in practice.) purpose, namely the prediction of propeller drag under sail, it is of less significance.
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