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CHAPTER 4 MODIFICATION OF HULL AND APPENDAGES
4.1 INTRODUCTION Now that the scaled up model of the slender yacht has been analysed and compared to a wider hull form, some of the appendages need to be modified to reasonably or practical values. This will make it possible to see if the slender hull form can be used and will also enable a possible choice of racing class to be chosen, which will suit the slender hull best. Areas that will be looked at will be:
Once the slender yacht has been modified it will be possible to access its viability for use in racing.
4.2 KEEL LENGTH As was stated the vessel was scaled up to match the size of the Open 50 class of racing yachts. Since these yachts use keels which are quite long, it was decided to take the keel length from the ones used by these yachts. The current keel length on the scaled up model is 5.4m, this is very long but not considerably different to those used on the Open 60 and Open 50 yachts with the longest measuring 4.6m. Since the super slender yachts rely on the length of the keel and the ballast ratio to provide all the stability, a long keel is necessary so a value of 4.6m would be reasonable. The length of the keel is restricted by structural strength as some of the yachts in the 1996-97 Vendee Globe lost there keels, with the most famous being the loss of the keel on Exide Challenger sailed by Tony Bullimore which left him trapped inside the hull for four days. Part of the reason for this could be due to the way these yachts are developing side force. The recent trend seems to be the use of very thin keels with the addition of lee boards to reduce the leeway. This would reduce the strength of the keels and make them more likely to suffer from failure. This problem may effect slender hulled vessels less as the deep rocker helps to reduce leeway, so a slightly wider keel could be used without inducing larger amounts of drag from lee boards. The advance in material technology have also helped the lengths of the keels to grow, and it may soon be common to find Open 60s sailing will keels the same scale length as those used on models.
Another innovation which can be seen on some of the yachts is that of the canting keel, where the keel swings from side to side depending on the tack, to increase the GZ value shown in Figure 4.1. This allows the yacht to carry more sail area and also reduces the length of the keel required for the same GZ value. The reliability of these devices does not seem to be a problem as they have been used in both the past two Vendee Globe races and none of them have failed. The mechanism used for canting the keel is a simple single or twin hydraulic ram arrangement. Obviously the structure around this area would have to be designed to take the higher load that this system may impose. This device would allow the slender yacht to carry more sail area but as was stated in the motion section, a higher GZ value is not always best.
Figure 4.1, The Canting Keel System
With the reduction in the length of the keel there will also be a reduction in the ballast ratio. Currently the ballast ratio is at 66%, this would probably reduce to around 60% for a Open 50 design. This all leads to a increase in the KG value, which will effect the GZ value. The new KG value would probably be around -0.75 to -0.5m, this would give the yacht a maximum GZ value of 1.9 to 1.7m if the results are compared with the stability tests which were carried out on the slender hull form. These KG values give the GZ curves which are shown in Figure 4.1. As can be seen the yacht will still have a vanishing angle of 180° in both situations, and therefore this reduces the chance of the yacht becoming inverted and staying inverted. It must be remembered that this graph does not give the full story, as the damping from the rig of the vessel will also determine how easily the yacht is righted.
Figure 4.2
4.3 SAIL AREA The sail area used on the scaled up models was 68.37mē for upwind and downwind sailing as no spinnaker was used. Looking at some of the statistics for the current Open 60 and Open 50 class of racing yachts, upwind sail areas of 170mē is being used on the Open 50 yachts, while sail areas of 260mē - 350mē are being used on the Open 60s. Looking at the rules governing these two classes the Open 60 rule states that the total amount of overhang on the vessel must be no more than 1.0m. Some of the yachts such as Aquitaine Innovations have the main boom filling this overhang, and by positioning the mast around mid-ships gives the boom a length of around 10m, and having a mast height of around 25m, the total sail area can be massive. The down wind sail areas are also quite large on these yachts. The Open 50s may carry a sail area of around 320mē while the Open 60s can carry sail areas up to 650mē. These sail areas make the sail area on the scaled up test models seem insignificant.
Choosing a sail area is not as simple as choosing it on a model. On a full size yacht you have the ability to reef the sails while sailing if the yacht is becoming over powered. However on the model tests it was found that a slightly reduced sail area of around 9% seemed to help the slender test model by reducing the pitching moment, and reducing the heel angle. This should also apply to full size yachts and therefore on the scaled up model a sail area of around 160mē which is 9% of the minimum sail area previously used which was 170mē. Down wind sail area is harder to choose, as the models do not carry spinnakers except in the Americas Cup model class, which do not use slender hull forms, it is therefore hard to determine how a slender yacht would perform carrying a spinnaker. Once again a slightly reduced area would probably be a safe choice at around 150mē.
4.4 TYPES OF RIG 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.
Figure 4.3, Rig types
4.4.1 V.P.P run 4 The best way to establish which rig is better is to test the performance of the yacht with each one. Since the facilities and time are not available to test each rig the next best option is to use the V.P.P which was used to establish the performance of the yacht in section 3.2. The hull used in the tests was the same as before but the keel length was reduced down to 4.6m as decided in section 4.2. The percentage area split for each sail was taken from existing designs as the optimum percentages are not known. The size of the spinnaker was chosen by using the V.P.P to optimise the dimensions. Initially a spinnaker area of 150mē was chosen but it appears that to obtain the best performance the spinnaker area should be 166mē. As was mentioned the "scetch" rig is a variant of the ketch and also since the V.P.P did not allow for this sail configuration it was not tested. The sloop rig used had the same total area as the ketch rig.
The results proved very interesting indeed. The maximum speed of the yacht had now increased to 16.21 knots in 30 knots of wind sailing at 150° to the wind with the ketch rig configuration, while the sloop rig gave a maximum speed of 16.65 knots at 120° to the wind. The difference in times over one nautical mile were plotted giving the results shown in Table 4.1. The results show that the ketch is better in the higher wind speeds when sailing at angles up to 150°. The ketch does lose out as the wind speed decreases with the sloop rig giving superior performance in 4 knots of wind. The reasons for these performances are quite clear. As would be expected the ketch rig performs better in stronger wind speeds as it causes the yacht to heel less. While the sloop is better in light winds when heel angle is not causing a problem but rig efficiency is critical. Deciding which of the two rigs is the best is hard but one reason for choosing the sloop is found in Table 4.1. The time differences give a representation of the performance of the yachts. The ketch rig finishes first by only a few seconds, while when the sloop rig finishes first the greatest time difference was 161 seconds. Looking at all the time differences when the sloop rig finishes first the time differences are a lot larger than when the ketch finishes first.
The choice between the two rig types must also be considered from a point of handling, and also the types of environmental conditions that the yacht is likely to encounter. The yacht has been compared to the Open 50 class of offshore racing yacht which is single handed and also used for the transatlantic race and the Vendee Globe round the world race. The fact that the yacht is single handed indicates the need for ease of sail handling as mentioned before, so from this point of view the ketch is better. Looking at the environmental aspects the global races do operate under extreme weather conditions in some parts of the race, while other parts of the race experience very low wind speeds.
As in most situations the answer is not a simple one. From a safety and ease of handling view the ketch would be best, but from a performance or all out speed point of view the sloop may be better. It must also be remembered that the sailor must also make the choice of what rig he/she prefers, and it will be this decision that decide the final outcome.
All results are shown in Appendix 2
Table 4.1
4.5 OVERALL LENGTH OF THE VESSEL AND HULL SHAPES The length of the hull at present was scaled up to the Open 50 class length of 50 ft or 15.24m. The beam of the yacht at the deck edge is 1.64m which is considerably small. As was mentioned in the section dealing with the accommodation this causes numerous problems when the designer has to design the interior. One possible way to overcome this problem is to flare the hull sides to increase the beam at the deck edge as shown in Figure 4.4. This has been experimented with by Bruce Farr on some of the 1997-98 Whitbread 60 round the world yachts, which have decided to reduce their beam at the waterline. The purpose on these vessels was to provide greater moments generated by the water ballast tanks. This would also increase the internal volume but may cause problems in waves as it is not clear how the vessel would react, and whether the flared hull sides may increase the chance of capsize with the deck edges dipping into the wave face, as described in section 3.4.4. The flared hull section does have another advantage as it increases the local shroud base of the yacht. This would reduce the loading on the mast and the hull, and could reduce the amount of internal structure required and therefore reduce the overall weight.
Figure 4.4, Flared hull sides
The logical step would be to increase the size of the yacht and scale her to the size of an Open 60, which would give a length of 18.29m and a beam of 1.92m. This beam is better than that on the Open 50 but it is still small. There is no saying that this yacht could not be built but it would still be extreme, and the internal volume would not be considerably larger. As was stated this slender hull with low internal volume was better suited to single handed sailing, as one person does not require the same amount of space as a full crew. The maximum length that can be used for single handed sailing is 18.29m as this is law due to safety reasons.
If more crew were to be carried one possible class of racing yacht could be the yachts that enter "THE RACE" the Jules Verne round the world race. The design rules for the vessels that enter this race are simpler than the Open 50 or Open 60 class. The rule simply says that any vessel can enter, there are no limits on length or sail area, so the yachts that enter this race tend to be on the very large side. Both multihulls and monohulls can enter, and the only limitations are on the winches used and a qualifying race. Proposals have already been put forwards on the types of yacht that may be used for this race. Both multihulls and monohulls are entering but one of the most extreme designs put forward is that designed by Philippe Briand, the yacht is a monohull and is a massive 50m in length while her beam is 6m. This gives her a length to beam ratio of 1:8.3. Her specified draught is 7m which indicates a keel of around 6m in length, and indications are that this keel will also use the canting system to increase stability. The vessel will displace around 25 tonnes and will carry 575mē of sail upwind, and 1175mē downwind on a raked sloop rig. Internal volume on a vessel of these dimensions is no problem even if a full crew is carried.
If the test model was scaled to this length the beam would be 5.24m. This beam would provide adequate internal volume without needing to flare the hull sides. There is also the possibility that the beam could be reduced even further to help reduce resistance. How a vessel with an even higher length to beam ratio would perform is unknown, and it is likely that a completely new set of problems would occur. Already a yacht designed for this race the TAG Heuer which measured in at 43m has suffered from structural failure due to first damage due to slamming, and then massive delamination of the hull causing the yacht to be scrapped. It is structural design that is going to be the cause of most of the problems in these massive yachts, as not many yachts of this size have been built, and therefore very little is known about the loads that are imposed on the hull from the sea.
4.6 APPLICATIONS As discussed in section 4.5 the size of the vessel is the main limitation on the application of the yacht. If the length of the yacht is around that of 15m the beam at the deck edge is very small. This effects the interior design and it was stated that this would best suit single handed sailing. This lack of internal space does restrict the yacht to racing only as a cruising yacht requires a lot more comfort below deck. If the yacht was to be used for racing it would fare better in classes which are quite open and have few restrictions. If however the yacht was rated to one of the rules such as the IMS or CHS, it is not absolutely clear what type of rating would be obtained. As has been mentioned in previous sections standard resistance prediction techniques are not applicable to this type of yacht, and therefore some of the rating rules may come into difficulty producing a fair rating. The lower stability of the design would help in a modern rating system as high stability is generally punished, while the longer length and low beam would undoubtedly be a disadvantage as anything relating to speed is mostly punished. Even if the yacht did slip through the rating rules net, it is highly likely that the vessel would be penalised if it showed good performance and dominated most races.
Another option that was discussed in section 4.5 was that of increasing the overall length of the yacht to around 40-50m. This allows for extra room inside the yacht and allows a larger crew to be carried. With a yacht of around 50m the beam would be 5.24m. This would allow the vessel to be used for cruising as the internal volume is very large. The yacht designer Philippe Briand has already designed a cruising yacht called the Mari Cha III at 45m in length with a displacement of 120 tonnes. It is obvious that some modifications would have to be made to the current hull form if it was to be used for one of these yachts. Modifications in the displacement would have to take place, as would modifications to the rig. It is interesting to note that the Mari Cha III uses a ketch rig which was also mentioned for use on the super slender Open 50 design in section 4.4. The increased safety due to the better motions and lower capsize risk would also make this yacht better for cruising.
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