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post #6 of Old 10-20-2004
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Ketch/Yawl Handling

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As was found during the model tests and also the V.P.P runs, the heel angle of the vessel is critical to the performance. Since in section 4.2 it has been specified that a reduction in the keel length is necessary, if the same rig with the same sail area is carried on the new yacht she will heel to a greater angle as the righting moment is reduced. In section 4.3 the decision was made to increase the sail area to 160mē from 68.37mē. This will also increase the heeling moment depending on the location or height of the centre of effort. If the same rig i.e. a sloop is used on the yacht the sails will have to be reefed earlier than the wider vessels as this was discovered in V.P.P run 1. If however the centre of effort of the sails could be lowered is some way the heel angle would be reduced. This can be done by either reefing the sails or changing the rig. By changing the rig to either a ketch or a "scetch" rig as shown in Figure 4.2, the centre of effort of the sails is lowered considerably. The disadvantage with these rigs are that they are less efficient than the sloop, but the efficiency of a reefed sloop must also be considered. When a sail is reefed the efficiency is reduced and therefore the driving force is reduced. It is hard to establish whether a ketch is less efficient than a reefed sloop of the same area without carrying out wind tunnel tests, which are outside the scope of this project.

Apart from performance of the rig and yacht as a whole, the usability or ease of handling of the rig must also be considered. As was decided the yacht is being compared to the Open 50 class and since they are single handed, the ease of handling of the rig is critical. The larger the individual sail areas the harder it is to handle and set the sail. This is where the advantage of the ketch or "scetch" rig is found. Since the sail area is split over two masts the individual sail areas are reduced and therefore it is easier to handle the rig. The disadvantages come in the form of reduced performance and increased weight as two masts are now required, and the number of winches and equipment to handle the sails are also increased. This increase in weight may effect the performance of the yacht, but it must be remembered that the ease at which the person can deal with the sails will also effect both performance and safety. For the reasons of ease of use and increased safety it would seem more advantageous to use a ketch or "scetch" rig. From a structural point of view there is a possible disadvantage with the ketch and "scetch" rig due to the position of the masts. The sloop rig would have the mast positioned around amidships while the ketch and "scetch" would have them further forward and near the aft of the yacht. The beam at the point of the mast will effect the size of the shroud base, and therefore the loading on the mast and the hull at that point. Since the mast of the sloop rig is positioned at amidships the beam is at its widest and therefore the shroud base is larger. The ketch and sloop masts are positioned away from amidships and therefore the shroud bases would be smaller. However it must be remembered that the sloop rig is carrying all its sail area on one mast and therefore the loads will be higher around the mast, while the ketch and "scetch" split the sail area and therefore the loads on each mast are lower. The point that is being made is that the structural design of the yacht needs to be reconsidered if a ketch or "scetch" rig is used.

The decision of choice of rig between a ketch or a "scetch" is difficult as it is hard to determine which is best from a performance point of view. The ketch rig is easy to set from a balance point of view, but will result is a higher centre of effort than the "scetch" rig of identical sail area. The "scetch" rig however is harder to set when it comes to balance as the helm of the yacht can be effected quite considerably. On the plus side the "scetch" rig does lower the centre of effort and will reduce the heel angle. Since the handling of the sails is important it would seem that the ketch rig would be more advantageous. A simple spreadsheet was set up to calculate the height of the centre of effort of the different rigs. It was found that by using an upwind area of 160mē, and by varying the percentage area given to the mizzen, the centre of effort could be lowered by around 1.5m when using a ketch rig compared to a sloop of the same area. Obviously the optimum percentage area for the mizzen is unknown, the only way to obtain the value would be to do full size and model tests, but due to time and cost restrictions this is not possible in this project, but could be a subject of a further study in this area. The result does however illustrate the advantage that the ketch rig provides with respect to heeling. It would therefore seem that this type of rig would be a good choice for the slender yacht as it reduces the heel angle and also makes sail handling easier.

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