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Editor’s note: This highly technical article has
been "scanned and converted", electronically, from the original printed text in
Sharkscan to this HTML copy. The process ended up with one out of context reference to
"wooden" and another to "mahogany" and there are no doubt other errors
which should be corrected. I proof read the final copy myself and was unable to
identify where the above words should have been placed. RUDDER In 1985 I designed and built a rudder, which we used on Geordie Moggridge's Shark "The Fool's Overture". From the point of view of control, the rudder exceeded all expectations. We almost never lost steerage, even on the toughest of spinnaker reaches. There was, however, a price to be paid for all of this control. During the summer of 1989 the lower gudgeon bolts fatigued. Shortly thereafter, we noticed that the lower pintle strap had a fatigue crack running three quarters of the way through. That fall, the rudder snapped. Quite frankly, when I drew up that rudder, I didn't pay a lot of attention to its strength. I just assumed that Shark rudders, built in accordance with the class specifications, had a reputation for Longevity so I wouldn't have to worry about it. Obviously I was wrong. As a result of that failure, Terry Moss' comments at the Canadian General Meeting and in light of the specifications being given a general overhaul, I have taken it upon myself to review section 6.5 (Rudder) of the class rules from a structural point of view. In the process of performing the calculations, I have concluded that although a structurally sound rudder can be made within the class rules, simply adhering to them is no guarantee of strength regardless of the materials used. In this article I would therefore like to share my findings with the Shark fleet and propose some additional specifications to the rudder rules so as to reduce failures in the future. Loads: One situation where the rudder is loaded is on a beam reach. The boat develops weather helm and the skipper compensates by pulling the tiller to windward. Consequently the rudder is moving through the water at an angle (the angle of attack) and generates a sideways force known as lift. (Figure 1) The lift balances the forces causing weather helm and the boat sails in a straight line. As the wind speed increases, weather helm increases and a greater angle of attack is required until at some wind speed, the maximum lift is achieved and the rudder starts to stall. Any increased weather helm will cause the boat to lose control and round up. A larger rudder can generate more lift and control the boat with greater weather helm than a boat with a smaller rudder. Although lift acts along the entire rudder blade, it can be visualized as a force acting at a certain depth, roughly midway down the rudder blade. If you multiply the lift force times the distance below some level, say the lower pintle strap, you can determine the bending moment on the rudder structure at that level. If the rudder lacks the strength at that level, it will break. A larger rudder blade will create a larger lift force, hence requiring a greater strength than would a smaller rudder. A deeper rudder, since it applies its force at a greater distance from the pintle straps, would also create a larger bending moment hence requiring greater strength than a shallower rudder. The strength is achieved by having enough width and thickness to withstand the bending moment caused by the rudder blade. In the case of a cored fibreglass rudder, one can also add extra layers in key areas. One can also use higher performance fabrics. Calculations: The $64,000 question when analyzing anything on a boat is "What are the actual loads?" I have exceeded 20 knots in a Shark but a rudder, which could withstand a full stall at those speeds, would sink the poor boat. For guidance, I therefore turned to the American Bureau of Shipping (ABS) Guide for Building and Classing Offshore Racing Yachts. Based upon the waterline length, displacement and rudder geometry for a given design, the ABS guide has calculations for determining the bending moment on the rudder. The guide also recommends safety factors and allowable strengths for various materials. For materials not covered by the ABS Guide I used values published in the "Boat Data Book". As it turns out, on a Shark the bending moment is calculated by assuming a fully stalled rudder at 6.74 kts. The pressure works out to be 9020 Newtons/square metre or 188 pounds per square foot. Your average rudder has to sustain a load in the neighbourhood of 2200 Newtons or 500 pounds. By the time you apply the recommended safety factor, the design Load becomes 5200 N or 1200lbs. Could you mount your rudder horizontally by the pintles and have your entire crew jump on the blade? Following are a have a few suggested additions to the current specifications. Limit the maximum allowable rudder depth: Currently a minimum rudder depth is specified, but no maximum. As mentioned above, an excessively deep rudder can overload the rudder structure. Also, a rudder, which is deeper than the keel (965mm below waterline, 985mm below the transom), is vulnerable to damage from hitting the ground. I therefore propose a maximum rudder depth of 95% of the vertical distance from the transom to the keel, specifically 935mm. Although this is considerably deeper than current design practice, it is consistent with earlier Shark rudder designs. By limiting the depth to 935mm, I have been able to reduce the lengthy calculations from the ABS guidelines to two simple charts for screening critical areas of the rudder.
1/ Measure the rudder blade width in millimetres (mm) at the transom Level (Width A on Fig 2) 2/ Measure the rudder maximum thickness in mm at the transom Level. 3/ Calculate the rudder area in square metres (m2). 4/ On chart 1 find a curve which represents the width of the rudder, for example the third curve from A the bottom represents a width A of 230mm 5/ Follow that curve until it crosses the measured rudder thickness. 6/ Draw a horizontal line across to the left-hand axis of the graph to read the maximum allowable area. For example, a 230mm wide rudder, which is 40mm thick, would be allowed a maximum blade area of 0.21 m2 (2.26 sq. feet). If you designed a Mahogany Shark rudder made to the minimum allowable dimensions of 178mm wide (dimension A on Figure 1) by 35mm (T) thick at the transom level, it can not be made small enough (according to existing specifications) to survive. Screen the rudder stock using Chart 2: The lower pintle level should be screened since it sees the highest bending moment on the entire rudder. Although it is not currently regulated, it should become so in the future. At the lower pintle follow a similar series of steps as above: 1/ Measure the rudder stock width in mm at the tower pintle level (Width B on Fig 2). It is assumed that the rudder cross section is rectangular at this level. At the time the specifications are written procedures for dealing with non-rectangular cross sections will have to be written. 2/ Measure the rudder thickness in mm at the lower pintle level. 3/ Calculate the rudder area in m2 (same value as step 3 above) 4/ On chart 2 find a curve, which represents the width of the rudder stock, for example the fourth curve from the bottom represents a width B of 180mm. 5/ Follow that curve until it crosses the measured rudder thickness. For example, a 180mm wide rudder, which is 46mm thick, would be allowed a maximum blade area of 0.24 m2 (2.58 sq. feet). Provided the shape of the rudder is changed gradually and with no notches, the rest of the rudder should have adequate strength. Wood Construction: The current wording of using "Hardwood" should be redefined. By definition, hardwood comes from deciduous trees and softwood comes from conifers. Balsa wood is technically a hardwood yet it would be unsuitable for a rudder. This rule should be amended to specify South or Central American Mahogany. Fibreglass Construction: The variety of reinforcements and construction techniques makes the writing of specifications for 'fibreglass' construction very difficult. My first suggestion is that the same rules governing the width and thickness of wood be applied to 'fibreglass' and that it is up to the builder to ensure that the rudder is as strong as its equivalent. If you don't have the background to make this assurance or you are not prepared to destruction test a sample rudder, stick to wooden construction. My second suggestion is that if 'fibreglass' is being used as a protective sheathing, it is to be used only over a hardwood core. In order to compare various materials, I calculated the strength of a mahogany plank. For the identical width and thickness and for several 'fibreglass' fabrics, I calculated beams of equal strength to the mahogany plank. The relative weights and deflections are as follows:
If you use conventional Mat, Woven Roving or Cloth (similar properties to Woven), you should expect your rudder to be 30% to 40% heavier than mahogany. Your laminate will also be 30% to 40% of the rudder thickness. By feathering the skins in the low load areas, you can achieve slightly better results. If you can get your hands on higher performance Bi-Axial and Uni-directional/Mat fabrics, you can achieve weight savings over mahogany albeit with a more flexible rudder. The third recommendation I would make with regards to
'fibreglass' construction is that the basic rudder in standard measurement trim but minus
corrector weights shall weigh no less than 6.8 Kg. Although this Carbon Fibres: The conventional interpretation of skin and
core in composite construction is that the skin provides the bending strength and the core
acts as a lightweight spacer which transfers the shear stresses. In making this decision, Shark sailors should bear in mind that carbon fibres are significantly more expensive than glass and that the use of carbon fibres is an all or nothing situation. Carbon fibres are so stiff that you either have enough to take up the entire load or you have none. Anything in between and the carbon will snap. You can't just throw a few dollars worth of fibres into the structure. Core: Although a Lightweight material is typically used for the core, it is not just along for the ride. The core an important and complex function. It has to be stiff enough to maintain a constant distance between the rudder shells when loaded. It has to be rigid enough in shear that the shells co operate with each other and it has to be strong enough in shear so that it doesn't fail. Materials such as Core-cell, End-grain, Balsa, Airex, Klege-cell and Divinicell are specifically made to fulfill these functions. Solid 'fibreglass' some polyester putties, plywood and micro-balloon putties can also be used, especially in high stress areas such as the pintles. Air, Styrofoam and polyurethane foam on the other hand are not suitable except where the Loads are very small. I suggest that the wording "Except where high strength localized reinforcements are used, only core materials intended for use in 'fibreglass' or sandwich construction are permitted for use as a core." be added to section 6.5 a. So how did the old two-piece Fool's rudder fit into these curves? At the transom level, it was 91% of the necessary strength and at the lower of the pintle it was 78% Will following these recommendations guarantee that your rudder won't fail? No. Through a lack of maintenance your rudder could develop rot. It is possible that the ABS guidelines underestimate the speed of a Shark. The pintle straps may be concentrating the load badly or you may have weakened the rudder by adding holes when replacing pintles. These recommendations will guide you to making a much stronger rudder than by following the current specifications. In closing I would like to encourage a dialogue amongst
Shark sailors. After some reasonable period of time I hope we can put these issues to a
vote and make the appropriate amendments to the rules. At a minimum Richard Hinterhoeller ISCA Specifications Officer If you have any comments, I can be reached at (905) 336-7490
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