The Magazine for Those Working in Design, Construction, and Repair
Total Page：16
File Type：pdf, Size：1020Kb
The magazine for those working in design, construction, and repair NUMBER 162 INCLINING EXPERIMENTS AUGUST/SEPTEMBER NEW IMOCA 60 RULES 2016 SANITATION SYSTEMS $5.95 U.S. A BERTRAM REFIT PRACTICAL SOLUTIONS Best of Inclinations A designer looks at the importance of quantifying vessel stability, along with a variety of on-the-water inclining experiments to determine or to verify it. Text and photographs by Butch Dalrymple-Smith oat stability is important. We may its significance goes beyond that. of an existing vessel. While simple in Bregard it as simply another statu- Just how to quantify and measure concept, it can be complex in prac- tory requirement among many other stability has been central to the tice, but it is a practical measure to things required for certification by refi nement of hull design and perfor- confi rm that a boat will perform as the European Recreational Craft mance for centuries now. Inclining the designer’s theoretical stability cal- Directive (RCD) or International tests are an on-the-water method of culations suggest. Marine Organisation (IMO) rules, but determining the actual stability profi le Let’s start with some stability basics. Above—During the inclining experiment, temporary panels protect the deck from possible damage as a crew shifts dollies laden with lead pigs from one side of the boat to the other in a set routine; then, changes to the heel angle will be measured to reveal the vessel’s stability and enable calculation of the vertical center of gravity. If test weights are on wheeled dollies, as shown here, the wheels must be locked during testing so the weights can’t roll across the deck as the boat heels. 24 PROFESSIONAL B OATBUILDER Stability Primer required to heel it over. The regula- Measuring the longitudinal center Stability varies in proportion to the tory requirements even take into of gravity is straightforward. We put length and to the cube of the beam account the combined effect of a strong load cells under each leg of the cradle, on the waterline. Thus, when you wind and a big wave arriving at just and if we know how much to deduct double the length you double the the wrong moment, or what happens for the cradle, we can calculate the righting moment, but if you double if everyone on board rushes over to weight and fore-and-aft position of the beam, the righting moment one side, which is presumably most the center of gravity. This is important increases eightfold. A rough-and-ready relevant on whale-watching boats. for determining where to paint the approximation for a sailing boat is: Despite the importance of stability, waterline. Measur ing the vertical cen- the righting moment per degree of the published specifi cations of most ter of gravity—essential for calculat- heel is equal to half the length times yachts either ignore stability informa- ing stability—is a harder task. The the cube of the waterline beam. (This tion or put it in the small print far variables in play are: weight, meta- rule of thumb works only for calcula- below the generator capacity or the center, stability, and position of the tions in meters.) number of toilets. Perhaps one reason center of gravity. If we know any three On a sailing boat, stability is the for this is because stability changes of these, we can calculate the fourth. single most important factor deter- so dramatically with loading. Thus, With the boat floating, start by mining the required strength in the too many conditions need to be measuring the draft (or freeboard) rig. Ultimately, and somewhat sur- attached to any stability or center-of- forward and aft. Once this has been prisingly, sail area has little to do gravity data for it to be conveniently transferred to a lines plan, we can with it, because in strong winds meaningful. work out the displacement, a for- yachts simply set less sail. What really Thanks to Pierre Bouguer, who dis- merly tedious task that’s now easily drives the strength requirement is the covered or fi rst described the meta- accomplished on a computer with righting moment the hull can exert center in 1746, designers can calculate the right software. And if we know the against the heeling force the rig is the theoretical stability of a design generating. Everything stems from this quite easily. The metacenter is a func- factor—the power required from the tion of the boat’s shape and can be winches, the structure to support calculated from the lines plan (see the those winches, the size of the cord- sidebar next page ). But it is only one age, and the weight of sailcloth nec- part of the stability equation. Equally essary for the sails. necessary is knowledge of the dis- Even on powerboats, stability is placement and vertical position of the crucial. On small ones you need to boat’s center of gravity. To that end, it know how many people can be safely is theoretically possible for a naval carried on board without any risk of architect to total up all the weights capsize if they all decide to sit on the going into a boat, perform a vertical same side. On larger powerboats moment calculation, and come up with the stability determines how large the a precise fi gure of a design’s future stabilizers must be. It sounds counter- weight and center of gravity. intuitive, but the less stable the boat, But it’s not so simple. I remember the less hard the stabilizers have to sending the plans of two identical work. This is true whether they are 28m (91.9 ' ) aluminum yachts to two fi ns, gyroscopes, pendulums, or bilge different builders and fi nding after- keels. The designer of a stabilized ward a 0.5m (1.6 ' ) difference in the motor vessel is therefore caught positions of the two centers of grav- between the need to reduce stability ity. On another occasion, I congratu- to help the stabilizers provide a more lated myself on my precise calculation comfortable motion and the obliga- of a yacht hull’s weight being within a tion of providing the minimum stabil- couple kilograms of the measured ity required by the authorities, who weight the builder reported. My pride sensibly imagine the worst-case situa- evaporated when I found that the tion of encountering bad weather reported weight did not actually include when the stabilizers are out of action. the deck, while my calculation did. The RCD defines, among other Experience has taught me that irre- things, the stability requirements for spective of the material, it is virtually If there are no draft marks, you must boats between 2.5m and 24m (8.2 ' impossible to accurately predict the nd a datum, such as a deck edge, that and 78.7 ' ). For a larger, ocean going weight and center of gravity of a also appears on the plans. An easier way boat, the IMO defi nes what could be boat’s hull, with the exception of to measure is to oat a piece of dark- considered a reasonable stability production boats after the fi rst few colored plywood below the measurement based on its static righting moment at have been completed. So, if it cannot point, and bounce a laser range nder various heel angles and the energy be predicted, it must be measured. signal off the wood. AUGUST/S EPTEMBER 2016 25 SOLUTIONS: Stability Testing Far left— To calculate the actual weight of the vessel, ! nd its displacement and the density of the local seawater when the freeboard measurements were conducted. Here, a seawater sample in a bucket is tested with a hydrometer to measure its speci! c gravity. Left— A ! rst step in an incline test is to measure the draft or freeboard forward and aft, and transfer those ! gures to a lines plan to calculate displacement. density of water the boat is fl oating degrees the boat has inclined. This is in, we can calculate the boat’s actual known as an inclining experiment, weight. We also get the longitudinal which makes it sound like a cross center of gravity and the metacenter between the scientific method and at that fl otation. black magic. The next step, if we want to calcu- late the vertical center of gravity, is Stability for Ratings Rules to physically measure stability. The In my early experience, the chief conventional method is to measure demand for incline experiments was the initial sideways trim of the boat, for sailboat race ratings calculations. and then apply a known heeling My fi rst exposure to the art of inclin- moment and measure how many ing was in the 1960s, when the Metacenter Explained etacenter is the theoretical Left—The stability of a buoyant vessel Mpoint at which a vertical line M is similar to that of a person in a passing through the center of buoy- rocking chair, where G is the center of ancy when the boat is upright inter- θ gravity, B the center of buoyancy, and sects the vertical line through a new M the metacenter. Below—The center of buoyancy created when G righting moment (GZ times the weight) the boat is heeled in the water. It Z stops the boat, or chair, from falling explains how a boat whose center over and returns them to upright. of gravity is higher than its center of buoyancy can stay upright. It sounds like balancing a pencil The all-important GM, the distance B 1 on the tip of your fi nger until you B between the center of gravity and realize that a rocking chair is much the metacenter, is a fi gure commonly the same.