Chapter 13        Flow visualisation for the conventional Bermuda rig

 

Chapter 12 showed the way that the use of the single sailing winch limits the possible relationships between the settings of the fore sail and the main sail of a model yacht. The Hele- Shaw rig can be used to get some idea whether that relationship is reasonably acceptable.

 

We already have the flow pattern for the rig when it is beating because it will be the same as that for a swing rig. Now we have to choose some intermediate positions to look at in the Hele-Shaw rig. There is a constraint on the travel of the main sail when its boom reaches and touches the mast shroud. This is usually at about 90° and must be one position to view. Taking the fully sheeted-in position to be along the centre line gives a total angular travel of the boom of 90° and then the obvious intermediate positions are 22.5°, 45° and 67.5°.

 

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Figure 13-1

Figure 13-1 shows a typical set up for 45°. In these pictures from the Hele-Shaw rig the horizontal black line is always the apparent wind and the slanting line represents the centreline of the deck. In every picture the main sail makes the same angle of 32.5° to the apparent wind. Then the centre line is set at 22.5°, 45°, 67.5° and 90° to the main sail boom. As we have used 32.5° for the angle of attack of the main sail the centre line progressively makes angles of 55°, 77.5°, 95° and 122.5° to the apparent wind. In each case the fore sail is positioned as if it is turning on its swivel and set at the angle determined by the analysis in Chapter 12. These angles can be taken off the Graph 12-11. They are 37°, 54°, 72°, and 90°. The full flow pattern is shown in each case with a close-up alongside it.

 

The resulting pictures (regrettably still with the unwanted image) are given as Pictures 13-2a to 13-2h. It seems to me that these flow patterns show that the flow over the sails is as good as one might expect to achieve. The flow over the main sail hardly changes at all and the flow over the fore sail changes nicely from beating to reaching in a progressive way without any difficulty. Given that these sails have to work in an unsteady wind and the fact that the sails will be twisted any refinement on the most simple sheeting system is unlikely to produce any discernible improvement.