International Journal of Small Craft Technology
AN EXPERIMENTAL STUDY OF THE HYDRODYNAMICS OF A YACHT WITH A CANTING KEEL AND FORWARD RUDDER. F S Maes and M J Downie, University of Newcastle upon Tyne, UK SUMMARY
There is a growing trend to add a forward rudder to a canting keel configuration to offset the loss of hydrodynamic side force from the keel in its canted position. The configuration has the added advantage that the forward rudder allows the yacht to turn about a vertical axis and be sailed with virtually zero leeway. This paper presents an experimental investigation of the effects of an increased canting keel angle on resistance, side force and vertical lift carried out in calm water in a towing tank. On the basis of the model test results the flow interaction between the forward rudder and the canting keel strut is investigated, and the drag and side force characteristics of a yacht with two turning rudders are presented and analysed.
NOMENCLATURE Aw
Aws CK
waterplane area at design waterline wetted surface area canting keel
CK_angle angle of the canting keel Fn
Froude number
FR forward rudder L lift force LW leeway LWL SF
Tcb Tck V
VF β
φ
1.
waterline length of canoe body side force
draft of the canoe body draft of the bulb tip forward speed
vertical lift force leeway angle
heeling angle
density of water in the tank INTRODUCTION
The canting keel was introduced in 1991. Today it is installed on most new-built offshore racing yachts and is even beginning to make an entry to the cruising yacht market. The concept uses a swinging mechanism to bring a leaden bulb to the windward side of the yacht which increases its righting moment. The increased transverse stability can then be used to enhance the performance of the boat by increasing the sail area or reducing the ballast weight. However, a sailing yacht keel is required not only to provide a righting moment but
also a hydrodynamic side force balancing the
aerodynamic side force generated by the sails. It is in this respect that the canting keel displays a significant drawback. By swinging the keel sideways, its lateral area is reduced which leads to a reduced side force.
In the early days of canting keel design, sailing at a higher leeway or drift angle was considered sufficient
compensation for the increase in loss of side force. Today more advanced appendage systems are being installed that are designed to decouple the production of side force from the generation of a righting moment, but very little can be found on the topic in the open literature. One such appendage configuration is the canting keel with forward rudder, otherwise referred to as CBTF™ technology.
One of the main benefits of the turning forward rudder is the fact that the side force produced is independent of the leeway angle. The optimal lift distribution on the two rudders can be determined using the biplane theory for a sailing yacht by Claughton et al. [1]. In the present study, tests were undertaken in a towing tank to determine whether sailing with the optimal rudder loading at several
leeway angles had a significant
benefit on the resistance and side force production of the yacht.
In 2004, Pavon et al. [2] conducted a brief study on the hydrodynamics of the CBTF™ configuration. They investigated the effects of the canting keel angle on the yacht’s resistance and the drag reduction caused by the turning forward rudder, but a number of aspects of the subject remain unaddressed.
The starting point of the present study also involved model tests carried out in a towing tank with a view to exploring the influence of the canting keel angle on resistance, vertical lift and side force. Its purpose, in addition to obtaining this basic data, was to address a number of issues concerning the
hydrodynamic
performance of canting keels that have not been dealt with elsewhere in the public domain.
The canting keel contributes to all aspects of yacht resistance. As the keel angle increases, the interaction with the free surface, and hence the wave resistance,
©2007: Royal Institution of Naval Architects
B-39
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