Yacht design
Sunday 11 July 2004.Types of hull.
We generally consider three types of hulls for monohulls. Displacement hulls.
Displacement hulls are generally used for boats with a high displacement to length ratio. This type of hull supports the weight of the boat using the Archimede’s principle i.e. the craft displaces an amount of water equal to it’s weight. The boat is thus supported by buoyancy force. Generally, round bottomed with a maximum displacement hull speed that is determined by waterline length. The longer the waterline, the faster the hull goes in its displacement mode. At its displacement speed, the whole hull remains in the water. This is the type of hull most often found on sailing boats.
Semi displacement hulls.
This type of hull is similar to the displacement hull, but is only found in racing boats, which are the only ones to have sufficient power. It generally has round bilges as well, but can end in chines at the stern. Since the power is higher, the yacht can be pushed through the water at more than their displacement hull speed. When this happens, the bow rises out of the water, and part of the force keeping the boat above water now comes from dynamic lift, similar to what happens for a surfboard.
Planing hulls.
Planing hulls are really only used in motor boats, as a lot of power is needed to go over the critical hull speed. The underwater hull section is now a deep-vee section, which gives a softer ride at speed, and by increasing the engine power sufficiently the boat can be propelled at speeds fast enough to plane over the surface of the water rather than push its way through it.
Types of rigs
Boats with a single mast and two sails. One of them is on the mast, the mainsail, and the other is on the forestay, the jib. This is the most common type of rig today, and they are used for boats of all sizes, from a four metres dinghy to a seventy metres superyacht. They have the advantage of being fairly simple to use and quite powerful, with good efficiency. The only drawback is that as you only have two sails, these have to comparatively bigger than on another type of rig, making them harder to handle, even more so for a small crew. This used to be a disadvantage for big boats but the advent of powered winches made it possible to use this type of rig on ever bigger boats.
This type of boat is very similar to a sloop, except that is has an extra sail between the mainsail and the jib, the staysail. It is hanked to the inner forestay, which runs from lower down on the mast than the forestay, to the deck. These rigs were often used on workboats, as they can be used without the jib if the wind increases. They also offer increased manoeuvrability, as the sails can be set to balance the boat accurately. Somentimes, the foot of the jib is cut quite high, as shown on the figure. It is then called a yankee.
This type of boat has two masts: The main mast, located forward of the center of the hull, and a smaller mizzen mast, located forward of the steering position, carrying a mizzen sail.On some boats, a flying staysail can be carried between the mizzen and the main mast when sailing off the wind.
This rig type is very similar to the ketch, the only difference being that the mizzen mast is located aft of the steering position, and is generally smaller. The mizzen is then only a stabilising sail, and does not offer much in tems of propulsive power, especially upwind.
Schooners are two masted boats, with the main mast aft and the smaller one forward. They come with a great variety of sail arrangements. When the two masts are of equal size, it is still called a schooner.
This is a boat with a single mast and a single sail. The most famous example is perhaps the Optimist.
There is a multitude of other rig types. the one shown here is a four mast barque. If there is a rig type that you know and would like to speak about, we would be very grateful if you could send us a short article about it.
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