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Stresses on a Sailboat Hull
The safety of a boat primarily depends on its hull. Safety of a hull has to be seen from the three aspects of its strength, water tightness, and stability. Safety construction rules of the administration and classification rules for construction provide the necessary guidelines.
The survey has to be carried out on the hull strength, stability, and water tightness of the sail boats at regular intervals. The survey includes the overall inspection, close up survey, thickness measurement, and the inspection of corrosion protecting devices. The survey is not completed until all problem conditions are rectified and the sailboat is seaworthy.
The hull of the sailboat must be strong enough to withstand the following stresses:
- Stresses in the bottom and side structures due to the hydrostatic pressure of the sea water around the ship.
- Stresses in the tank boundaries – bottom, side, and tank top along with the fittings such as manhole covers, due to hydrostatic pressure of the liquid inside the tank.
- Stresses due to bending moments- Longitudinal tensile and compressive stresses occur due to hogging or sagging in still waters due to the uneven loading, and at sea when the buoyancy distribution keeps changing with the movement of the waves. Bending also gives rise to shear stresses in the vertical and lateral direction.
- Stresses due to racking – Transverse stresses occur when racking is caused by abeam seas. When the waves are in an angular direction, racking of the forward and aft parts occurs in opposite directions. This causes torsion of the hull girder.
- Stresses in the side plating due to painting when the bow and the stern alternatively get submerged and rise above the water level as the boat rides the head seas, this is accompanied by pounding which means hammering of the bow or stern as they re-enter the water.
- Local stresses in the deck plating and tank tops due to effects of machinery mounted in the area.
- Local stresses in side plating due to push by tugs and when coming alongside a berth, or double banking with the boat.
- Local stresses in the bottom plating while the dry docking in way of supporting blocks or when the boat rests on the sea bed during low tide or when grounded.
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New and Future Sailboat Hull Designs
1. Mono-Hull Vessels
Many high speed small mono-hulls have had hard chines. Rounding bilge forms at higher speeds have had stability problems. Hard chine forms with greater beam and reduced length give improved performance in the calm water, but experience high vertical accelerations in a seaway.
Their ride can be improved by using a higher dead rise angle leading to a deep “vee” form. Current practices favor round bilges for lower power demands at cruising speed and sea kindness, with the adaptation of hard chines for Froude numbers above unity for better stability. One advantage of round bilges for sea keeping is that it can be fitted more readily with bilge keels to reduce rolling.
2. Multi-Hulled Vessels
These include sailing catamarans, trimarans, off-shore rigs, diving support vessels, and ferries. Catamarans are not new as two twin-hulled paddle steamers of 90m were built in the 1870 for cross channel service.
They were liked by passengers for their sea keeping qualities, but were overtaken fairly soon by other developments. Their greater wetted hulls can have reduced resistance at high speed, sometimes assisted by interference effect between two the two hulls. A hull separation of about 1.25 times the beam of each hull is reasonable in a catamaran and maneuverability is good.
High transverse stability and relatively short length means the sea keeping is not always good. This has been improved in the wave piercing catamarans developed to reduce the pitching and in the SWATH (Small Waterplane Area Twin Hull) design where the water plane area is very much reduced and a larger part of the displaced water volume is well below the water line. The longitudinal motions can be reduced by using fins or stabilizers.
As a development of twin hull vessels and many designs studies indicated many advantage with no significant disadvantages, comparisons of mono-hull with multi-hull craft are difficult. Designs of each type should be strictly optimized to meet the stated requirements.
3. High Speed Craft
These may have civil or military application, so they are considered before going on to consider specific warship types. A number of hull configuration and propulsion system are discussed; each is designed to overcome problems with other types or to confer some desired advantage. Thus catamarans avoid the loss of stability at high speed suffered by a round bilge mono-hull. They also provide a large deck area for passengers. Hydrofoil craft benefit from reduced resistance by lifting the main hull clear of the water.
The effect of waves on performance is minimized in the SWATH concept. Some craft are designed to reduce wash so that they can operate at higher speeds in restricted waters. Although most applications of these concepts have been initially to smaller craft, some are now employed in the medium size boats, especially for high speed ferry services.
In commercial applications they can be applicable with any of the special characteristics as mentioned above in the deciding factor in the adaptation of a particular hull form. By doing the changes in the hull forms we ultimately try to increase the “Transport Efficiency Factor” which is the ratio between the product of pay load and speed to the total installed power.
So we can use any one of the above new technologies in building the future sailboats keeping the “Transport Efficiency” factor in mind.