Different types of keel
So let’s look at the different types of keels, their shape, performance, and individual merits. It’s important to remember that, no one shaped keel fits all and although preferences are sometimes subjective, choice often comes down to what type of sailing you have in mind; racing or weekend cruising and where you will be using the yacht; coastal or ocean sailing.
Full length keel
A full length keel is built in as an integral part and shape of the hull and extends from the bow all the way to the stern. This type of keel is usually found on pre-1970s designed yachts and uses length rather than depth to provide adequate lift and ballast for the hull. Their size and weight makes them the most robust of all the different types of keel with a low likely hood that the keel will fall off which is not always the case with a fin keel!
The rudder is often hinged to the aft end of the keel and hence the long keel provides protection to the rudder and propeller.in the event of running aground or if the boat hits any floating debris or heavy objects. Long keels also provide more stability when the boat is propped up on a hard surface or when moored alongside a harbour quay when the tide has receded
Yachts of this type such as the Nicholson 32 had a great reputation for seaworthiness, strength and stability when sailing off shore. They had a great reputation for tracking well and were resistant to broaching but needed decent winds to achieve some speed because of their weight. Sluggish performance in light winds makes them unattractive to many sailors now, plus when manoeuvring astern in harbours /marinas, full length keel boats are challenging to say the least.
In conclusion a full length keeled yacht has a displacement hull which allows the yacht to move through the sea, displacing water, rather than riding on top of it as in a yacht with a planning type hull. The dynamic stability of a displacement yacht makes for greater seaworthiness in strong winds and more comfortable cruising, when compared to fast/ planing yachts.
A fin keel is the most common type of keel on modern sailboats. Its name derives from the fact that it is shaped like a shark fin and has a variety of different design features based around the “fin” shape.
Fin keels have a variety and often confusing range of different names but a simple deep keel will be less than 50 % of the length of the boat. A fin keel unlike a full length keel is not part of the hull, but a separate component; a narrow and streamlined plate that is bolted to the bottom of the hull. Because of the relatively small amount of ballast provided by a fin keel they tend to have a deep draft that increases the righting force. The reduction in weight and reduced surface area of the keel reduces drag because it is more hydro dynamically efficient compared with a full length keel. This results in increased speed.
A fin keel makes the yacht highly manoeuvrable with a short turning radius enabling fast tacking and ease of handling in harbours and marinas especially when going astern.
The improved speed and performance and manoeuvrability permitted with a fin keel come at a price. In strong winds they require early reefing and are more prone to rolling and heeling due to strong winds and waves. In bad conditions the ease of manoeuvrability makes the boat directionally unstable.
Despite their improved performance, fin keels are in general considered inferior to full length keels when it comes to sea-keeping ability. As a result of the lighter design and less surface to water area, a narrow fin keel offers less resistance to rolling. This can result in sudden and strong heeling when a wave or wind gust hits the boat. While their ability to turn fast and easily is an advantage in marinas or when racing, it also makes these boats directionally unstable under harsh weather.
Another point to consider is that the rudder on fin keeled boats is usually a deep spade rudder and unlike a full length keel boat has no protection if the vessel hits under water floating objects or goes aground. There is also an issue with rudder failures associated with fin keels (where the keel is ripped off the hull). Since 1984 World Sailing (formerly ISAF) the world governing body for the sport of sailing, has investigated 72 cases of keel failure resulting in 24 deaths.
Fin Keel with Skeg Rudder
This is a variation on the common fin keel with a spade rudder. As already mentioned a fin keel in contrast to a full length keel offers no protection to the rudder. The solution to this is the introduction of a skeg, a robust sternward extension of the keel on which the rudder hinges. The skeg provides strong protection not only to the rudder but also the propeller.
Shoal Keel (shallow fin keel)
The shoal keel is virtually the same as a fin keel, only it’s more shallow(less deep) than a deep fin keel. Because it’s less deep it has the advantage of being able to navigate in shallow coastal waters, river estuaries or lakes. However, due to its smaller surface area, performance and stability in high winds will be less than a deep fin keel