Keels

Sail boats that have balast weight hung beneath the bottom come in all types of sizes, shapes and styles. The keel is there to ensure that they cannot capsize, but of course there is no 100% guarantee (for very extreme situation).
A significant sub-category of the keelboat constists of smaller open regatta boats, many of which have become classic designs, used by the legendary Dragon, Etchells, Soling (for 3-man crew).
The traditional full-length keel has been almost totally superseded by variations on the fin keel. This removes the after part of a full length keel, leaving the ballasted equivalent of a dinghi daggerboard to provide the necessary righting moment and resistanceto side slip and is likely to offer the best performance in all its forms.
The more performance-oriented the yacht, the deeper and narrower the fin profile is likely to be since hydrodynamic force is proportional to the area of the keel. However, a deeper, narrower fin may prevent the yacht from sailing and mooring close inshore. Drying out on thebottom when the tide goes out is particularly difficult, and the boat is likely to be more sensitive under sail. Most cruising yachts will therefore compromise with a shallow fin keel, which provides a high level of stability combined with a long, straight bottom for the yacht to rest on (and very flat at that), This type of keel is frequently used with a skeg (mini keel at the back of the yacht that helps balance the rudder).
Bulb and fin keels combine a slim, narrow keel with a large lump of ballast fixed to the bottom. This may appear to be a good solution for maximizing the righting moment of the keel due to having al the ballast on the tip, but the bulb offers poor performance compared to a tapered fin when it comes to cutting through water with minimumum resistence for maximum speed. A bulb and fin profile is sometimes used for lifting keels, which retract to the hull. The fin lifts into a housing inside the boat and the bulb lies flush under the hull to minimize draught.
The disadvantage is the complex mechanical lifting system and a keel case that takes up considerable room inside the boat. Apart from being able to lift the weight of the keel inside the main corpus of the boat, the design must ensure that the keel stays totally rigid and inmovable down while sailing. Bilge keels provide a clever solution for yachts that have to dry out frequently on a mooring or at anchor in tidal waters, and require shoal draught. The identical twin keels protrude from the bilges and allow the boat to sit upright rather than rest on its side when aground. Due to much greater hydrodynamic resistance the racing performance of a bilge keeler is much inferior to a yacht with a single fin keel, but cruising performance ism ore than adequate with only a small percentage is lost in terms of absolute speed and pointing ability. The delaying in speed is due to the much greater wetted surface area. However, the bilge keeler provides unsurpassed stability when drying out in tidal location, with its two stubby keels resting on the bottom and the ability to sail or motor into shallow anchorages.
The ballasted keel of a keelboat ensures that it cannot capsize in a conventional sense like a dinghi boat. The more the boat heels (that means sways leftwards or rightwards), the more effective the righting moment of the keel becomes, until there comes a stage, where no more heeling can take place. But in extreme cases the keel boat can be knocked flat on the water by the combined effect of wind and waves. Fortunately, this happens rarely. Problems with filling with water have beein solved by self-draining cock-pits and better buoyancy distribution. Even short-handed yachts that get knocked down while racing through the Southern Ocean tend to remain afloat even when they lose their keels and turn turtle (upside-down).