3.1 Physical Components of Main Hull Resistance
3.1.4 Measurement of Model Total Resistance
3.1.4.2 High-Speed Craft and Sailing Vessels
High-speed craft and sailing yachts develop changes in running attitude when under way. Compared with a conventional displacement hull, this leads to a number of extra topics and measurements to be considered in the course of a resistance test. The changes and measurements required are reviewed in [3.29]. Because of the higher speeds involved, such craft may also be subject to shallow water effects and correc- tions may be required, as discussed in Chapters 5 and 6. The typical requirements for testing high-speed craft, compared with displacement hulls, are outlined as follows:
(i) Semi-displacement craft
High-speed semi-displacement craft develop changes in running trim and wetted sur- face area when under way. The semi-displacement craft is normally tested free to heave (vertical motion) and trim, and the heave/sinkage and trim are measured dur- ing the course of a test run. Running wetted surface area may be measured during a
T
w
x1
x2
Expected thrust line R
Tow force
Model free to trim and heave
Figure 3.11. Trim compensation for offset tow line.
run, for example, by noting the wave profile against a grid on the hull (or by a photo- graph) and applying the new girths (up to the wave profile) to the body plan. There are conflicting opinions as to whether static or running wetted area should be used in the analysis. Appendix B in [3.24] examines this problem in some detail and con- cludes that, for examination of the physics, the running wetted area should be used, whilst for practical powering purposes, the use of the static wetted area is satisfactory.
It can be noted that, for this reason, standard series test data for semi-displacement craft, such as those for the NPL Series and Series 64, are presented in terms of static wetted area.
Since the high-speed semi-displacement craft is sensitive to trim, the position and direction of the tow force has to be considered carefully. The tow force should be located at the longitudinal centre of gravity (LCG) and in the line of the expected thrust line, otherwise erroneous trim changes can occur. If, for practical reasons, the tow force is not in line with the required thrust line, then a compensating moment can be applied, shown schematically in Figure 3.11. If the tow line is offset from the thrust line by a distancex1, then a compensating moment (w×x2) can be applied, where (w
×x2)=(R×x1). This process leads to an effective shift in the LCG.wwill normally be part of the (movable) ballast in the model, and the lever x2 can be changed as necessary to allow for the change inRwith change in speed. Such corrections will also be applied as necessary to inclined shaft/thrust lines.
(ii) Planing craft
A planing craft will normally be run free to heave and trim. Such craft incur sig- nificant changes in trim with speed, and the position and direction of the tow line is important. Like the semi-displacement craft, compensating moments may have to be applied, Figure 3.11. A friction moment correction may also be applied to allow for the difference in friction coefficients between model and ship (model is too large). If RFmis the model frictional resistance corresponding toCFmandRFmsis the model frictional resistance corresponding toCFs, then, assuming the friction drag acts at half draught, a counterbalance moment can be used to counteract the force (RFm– RFms). This correction is analogous to the skin friction correction in the model self- propulsion experiment, Chapter 8.
Care has to be taken with the location of turbulence stimulation on planing craft, where the wetted length varies with speed. An alternative is to use struts or wires in the water upstream of the model [3.29].
Air resistance can be significant in high-speed model tests and corrections to the resistance data may be necessary. The actual air speed under the carriage should be measured with the model removed. Some tanks include the superstructure and then make suitable corrections based on airflow speed and suitable drag coefficients. The air resistance can also cause trimming moments, which should be corrected by an effective shift of LCG.
Planing craft incur significant changes in wetted surface area with change in speed. Accurate measurement of the running wetted area and estimation of the fric- tional resistance is fundamental to the data analysis and extrapolation process. Meth- ods of measuring the running wetted surface area include noting the position of the fore end of the wetted area on the centreline and at the side chines, using underwa- ter photography, or using a clear bottom on the model. An alternative is to apply the running draught and trim to the hydrostatic information on wetted area, although this approach tends not to be very accurate. The spray and spray root at the leading edge of the wetted area can lead to difficulties in differentiating between spray and the solid water in contact with the hull.
For high-speed craft, appendage drag normally represents a larger proportion of total resistance than for conventional displacement hulls. If high-speed craft are tested without appendages, then the estimated trim moments caused by the appendages should be compensated by an effective change in LCG.
Captive tests on planing craft have been employed. For fully captive tests, the model is fixed in heave and trim whilst, for partially captive tests, the model is tested free to heave over a range of fixed trims [3.29]. Heave and trim moment are mea- sured, together with lift and drag. Required values will be obtained through interpo- lation of the test data in the postanalysis process.
Renilson [3.30] provides a useful review of the problems associated with mea- suring the hydrodynamic performance of high-speed craft.
(iii) Sailing craft
A yacht model will normally be tested in a semi-captive arrangement, where it is free to heave and trim, but fixed in heel and yaw. A special dynamometer is required that is capable of measuring resistance, sideforce, heave, trim, roll moment and yaw moment. Measurement of the moments allows the centre of lateral resistance (CLR) to be determined. A typical test programme entails a matrix of tests covering a range of heel and yaw angles over a range of speeds. Small negative angles of heel and yaw will also be tested to check for any asymmetry. Typical test procedures, dynamome- ters and model requirements are described by Claughtonet al. [3.8].
In the case of a yacht, the position and line of the tow force is particularly impor- tant because the actual position, when under sail, is at the centre of effort of the sails. This leads to trim and heel moments and a downward component of force.
As the model tow fitting will be at or in the model, these moments and force will need to be compensated for during the model test. An alternative approach that has been used is to apply the model tow force at the estimated position of the cen- tre of effort of the sails, with the model set at a predetermined yaw angle. This is a more elegant approach, although it does require more complex model arrangements [3.8].
+V-P
+P-V +P-V
Figure 3.12. Pressure variations around a body.