Basic stability theory
This is based on two concepts. The first is the ‘centre of gravity’ or CG, which is the effective centre of the weight of all the elements comprising the boat and the point through which the total weight effectively acts. It does not change unless weight is changed or moved. Then there is the centre of buoyancy’ or CB, which is the geometric centre of the immersed part of the hull of the boat, and through which the buoyancy force effectively acts. It is continually moving as the boat heels or trims. A boat is stable if, as the boat moves, the CB generates a moment sufficient to return the boat to the upright.
Figure 1 shows that when the boat is upright, the CG is directly above the CB, and so it will remain stationary. If however the boat is heeled by some external influence (eg: wind or waves) the CB moves to one side, thus generating a restoring moment about the CG – see Figure 2. The further the CB moves for a given heel angle, the greater the tendency to return to the upright. The size of the ‘righting moment’ is the weight of the boat multiplied by the distance GZ as shown in Figure 2, which is sometimes known as the ‘righting lever’.
If weights are moved to one side, so that the CG is no longer on the centreline, the boat will adopt a steady angle of heel so that the CG and CB are once again in the same vertical line – see Figure 3.
One can also see from Figure 2 that the size of the righting moment crucially depends on the CG height. Raising the CG inevitably reduces the stability. Conversely lowering the CG improves the stability. But if this is achieved by adding ballast rather than lowering existing weights, it reduces freeboard and may cause down-flooding through openings at lesser angles of heel.
It is clear, therefore, that the stability of a particular boat is dependent on both the hull shape and the amount and position of its component weights. Changes to either will also change the stability characteristics. For this reason the basic design of the boat dictates its stability properties. The hull shape determines the way the CB will move. The layout, which determines the position of most of the heavy components, limits to a large degree the extent to which the overall CG can be controlled. The stability can then only be adjusted by carefully locating the remaining components or adding ballast.
The value of the righting moment varies with the angle of heel, and is normally plotted on a graph as shown in Figure 4. This curve is often used to define minimum static stability properties for boats that may encounter substantial waves.
Note that it is not only the shape of the hull below the gunwale that is significant. The stability at large angles of heel, which governs behaviour in the event of a near-inversion, is greatly affected by the deckhouse and superstructure design.
Boats don’t sink unless they get water inside them! If they have swamped flotation even then they will not do so. The Stability Standard addresses this by setting the following requirements:
◦ minimum height from the waterline to any potential down-flooding aperture, measured when the boat is upright;
◦ minimum heel angle before potential down-flooding openings become submerged;
◦ the size, watertightness and draining ability of all recesses including cockpits;
◦ the watertightness of all closures such as windows, hatches and doors;
◦ the integrity of through-hull fittings.
Traditionally the term ‘freeboard’ is the height from the waterline to the deck, but in stability and buoyancy terms it is the height to potential down-flooding apertures and openings that is important. ISO 12217 uses the more specific term ‘down-flooding height’ rather than freeboard, to avoid confusion.
This maybe necessary for one of two reasons:
◦ showing that a swamped boat will support the required weight;
◦ demonstrating theoretically that a boat will float after being completely inverted or swamped.
The first approach is used for smaller boats where a practical test is most convenient. The most difficult aspect is finding a suitable depth of water in which to conduct the test. If the boat should accidentally sink in deep water, it creates problems!
The second approach requires that the weights of all the components built into the boat are known, together with the density of the material of which they are made. This enables the buoyant volume of each component to be calculated. If the total buoyant volume has a sufficient margin over the loaded weight the boat will float even if swamped or holed.
Clearly stability is not a parameter that can be directly measured such as the length. Neither is there one measure of all aspects of the stability of a boat.
We can measure the amount of stability when the boat is nearly upright by a practical test. To get the stability at large angles of heel (such as might occur in waves) not only practical tests are involve, but also calculations are involved — nowadays most conveniently done by computer.
If a very careful ‘inclining experiment’ is conducted, then the CG position can be calculated, after which the stability at a whole range of heel angles and weights can be calculated. This process requires specialist knowledge, and for many boatyards the most sensible approach may be to involve a consultant naval architect. One needs to know with as much accuracy as possible:
◦ the weight of the boat, by direct weighing or calculation from the drafts measured forward and aft;
◦ the size of the heeling weights and the distance through which they are moved;
◦ the hull shape — from the lines plan;
◦ the precise heel angle for a known weight shifted through a known transverse distance.
Usually a series of angles are measured for a series of heeling moments applied to both port and starboard. The heel angle is traditionally measured by a pendulum, with the plumb bob submerged in a tank of oil or water to dampen any movement. However, a water tube can be used, in which the upright legs of the tube are as widely spaced as possible. A constriction can be introduced to help damp out fluctuations in the water level. An electronic level or inclinometer is also a useful solution.
Tremendous care is needed if an inclining experiment is to yield accurate results. The only boats for which an inclining experiment is not practical are very small boats, which are too sensitive to the wind and small wavelets, and catamarans, because their initial stability is so great in relation to the CG height.
Thursday, February 22, 2007
Thursday, February 15, 2007
The only way is up (part 1 of 2) An article posted on European Boat Builder online concerning boat stability (referencing the use of clinometers or inclinometers)
Andrew Blyth and Tom Nighy review the RCD’s stability and buoyancy requirements, which they both helped to draft as part of an ISO Small Craft Committee working group.
Ever since man first started building boats it will have been obvious that there is a lot more to successful design than merely staying afloat. Staying the right way up and keeping more water outside the boat than in have always been fundamentals.
However, the need to prove that a boat has adequate stability for its intended use to the satisfaction of an inspecting body is something relatively new to this industry. While adequate ‘rules’ and ‘standards’ for various types or classes of craft have existed for many years, it took the arrival of the EU’s Recreational Craft Directive to force a comprehensive standard.
The task of establishing a Stability Standard within the scope of the directive and its design categories fell to an ISO Small Craft Committee working group made up of delegates from 13 countries, including most EU and EEA countries, the
The Stability Standard
In 2002 the Stability Standard was published as an International and a European Standard (Norme) No 12217. It has three parts:
· Part 1. Non-sailing boats of hull length greater than or equal to 6m
· Part 2. Sailing boats of hull length greater than or equal to 6m
· Part 3. Boats of hull length less than 6m
The RCD Essential Safety Requirements state that ‘The craft shall have sufficient stability and freeboard considering its design category… and the manufacturer’s maximum recommended load’, and ‘The craft shall be constructed to ensure that it has buoyancy characteristics appropriate to its design category… and the manufacturer’s maximum recommended load.’
The RCD introduced Design Categories to ensure that craft have acceptable minimum stability, freeboard and buoyancy to cope with the environmental conditions for which the boat is designed.
· Design Category A ‘Ocean’. Designed for extended voyages where conditions may exceed Force 8 (Beaufort Scale) winds and significant wave heights of 4m and above, and vessels are largely self-sufficient.
· Design Category B ‘Offshore’. Designed for offshore voyages where conditions up to, and including, Force 8 winds and significant wave heights up to, and including, 4m may be experienced.
· Design Category C ‘Inshore’. Designed for voyages in coastal waters, large bays, estuaries, lakes and rivers where conditions up to, and including, Force 6 winds and significant wave heights up to, and including, 2m may be experienced.
· Design Category D ‘Sheltered Waters’. Designed for voyages on small lakes, rivers and canals, where conditions up to, and including, Force 4 winds and significant wave heights up to, and including, 0.5m may be experienced. (Note: In the planned amendment to the RCD the 0.5m wave height in Category D will be changed from ‘significant’ to ‘maximum’, which is a reduction of 47 per cent. The Stability Standard anticipates this change and uses the lower height, which is already being used by builders.)
Despite the labels of Ocean, Offshore and Coastal, it is the wind and sea conditions that matter most. Large waves and strong winds can occur close to shore. For example, the
Therefore, ISO 12217 provides graduated stability requirements according to the Design Category taking account of the following hazards: Boats used in Categories A and B may encounter ‘steep breaking waves’ with a height greater than their beam. Such waves have the capability to invert the boat, they can also induce heavy rolling that can cause progressive flooding when openings become submerged.
For many boats the ‘angle of heel’ induced when all the people on board crowd to one side may be a critical feature. This may either result in excessive heel with the risk of people losing their footing, or the risk of flooding due to openings becoming submerged. Sailing boats particularly must be designed and equipped to resist the ‘capsizing effect of the wind’. And this can be significant for some non-sailing boats too. For small open boats the ‘risk of swamping’ may be countered by having sufficient freeboard relative to the sea state, or by installing sufficient flotation material to ensure that the boat will float and support the crew when swamped.
Manufacturer’s Maximum Load
The stability standard is also used to determine the maximum safe load (meaning people and any carry-on items) for a given boat in a specific Design Category. The maximum safe load must be displayed on the builder’s plate to meet the regulation.
Floating — as Archimedes discovered 2,200 years ago, any floating body will displace a volume of water equal to its own weight. If, as is the case with all surface craft, the total volume of the boat exceeds the volume needed to support its weight, it will have a reserve of buoyancy. Thus the shape, which defines the hull volume, and the weight are both intimately involved. Changing either of these changes the buoyancy (and stability) properties of the boat. If water is kept out of the boat , either by sufficient freeboard or a watertight deck, the buoyancy derived from the volume of the hull will cause a boat to float even if it is built of heavy materials such as metals.
A boat is said to possess ‘swamped flotation’ if the materials from which it is built have sufficient volume in relation to the total weight enabling it to float when completely filled with water. Many small open boats incorporate sufficient low-density buoyancy materials or dedicated buoyancy compartments (air tanks) to enable the boat to support itself and the crew when swamped.
Stability — a boat is said to be stable if, when disturbed from its initial position, it has a natural tendency to return to that position. The greater the forces that may attempt to capsize the boat, the stronger that needs to be. So an appropriate minimum amount of stability depends on the hazards that a particular boat is likely to experience. For example, an ocean-going boat needs different stability properties to a vessel of similar size for use on a sheltered river or lake.
For Part 2 click here
Monday, February 12, 2007
Tuesday, February 06, 2007
Sound Reinforcement Products for Line Array Positioning: SLS Debuts RLA/4 (LS7500) Full-Range, Bi-Amped True Line Source Array Module
Monday, February 05, 2007
With Sony putting so much time and money into their PlayStation 3, it seems that it’s falling short in sales, giving Sony a $1.65 Billion (US dollar) loss, and possible looking up to a $2 Billion (US dollar) loss according to IGN.com, as people just aren’t buying these things as Sony hoped. I don’t blame people for not buying them as they are rather pricey. A few weeks ago, I was at a Wal-Mart and was asking if there were any Wii’s available, an employee told me they are gone in literally minutes when they come in, then I looked over on the shelf and saw a few PS3’s just sitting there all lonely by themselves. At the time I was quite surprised, but it seems that this is the trend for Sony’s new $500-$600 console.
I’ve also noticed that the PS3 has tried to make the system more interactive with a controller (SIXAXIS) having a tilt sensor, where the player can tilt the controller, having a tilting affect in the game. An example of this would be a game involving a flying plane, where the player would want to cut left, they just tilt the controller to the left. I personally think Sony tried to put this feature in their system after they saw Nintendo reveal there innovative and very impressive motion sensor controllers. With Sony making attempts to become more interactive, it hasn’t seemed to help it’s slumping sales, especially against Nintendo’s Wii and Microsoft’s XBOX 360.