Sailboats with auxiliary engines all have displacement hulls. Only a few rare examples, such as the MacGregor 36 (shown in the illustration of this article), are equipped with planing hulls. But does this common characteristic among sailboats give them identical features?No, not at all. They differ significantly in their appendages, particularly their keels.
The keel plays a crucial role in the sailboat's righting moment.
The ballast at the bottom of the keel counters the force of the wind applied to the sails. Capsizing occurs when the wind force on the rigging exceeds the righting force of the keel.
This righting force depends on two factors: the weight of the ballast and the lever arm acting on it, i.e., the distance between the center of gravity of the ballast and the transverse axis of rotation of the boat.
For this reason, racing sailboats are equipped with deep keels that provide a maximum lever arm to their lead ballast, a material nearly one and a half times heavier than steel.
Thus, the more the boat is designed to carry powerful sails with a tall mast, the greater the righting moment needs to be, and the heavier the ballast must be.
Sailboats are constantly subject to wind drift.
When sailing, they experience the force of the wind, which, depending on the angle to the wind, also generates more or less drift, which affects the boat's true heading. To limit this effect, sailboats have an underwater anti-drift plan. This consists of the rudder(s) and a centerboard, keel, or the hulls of a multihull.
All of these appendages "bite" into or push against the water, which generates a counteracting force, known as the anti-drift force.
Up until the late 1970s, all cruising sailboats had full keels. This keel, which is an integral part of the hull and not an added feature as in modern sailboats, gave sailboats significant longitudinal lift.
The full keel acted as a large underwater anti-drift plan that contributed greatly to the sailing of the boats. These boats could sail with the helm tied off, benefiting from this keel for their stability. Very stable, especially downwind, an angle at which modern sailboats tend to yaw, this keel had the disadvantage of poor maneuverability when moving in reverse, described as "unpredictable" by skippers of the time.
At the back of the full keel, the skeg housed the propeller and protected it from grounding. The full keel disappeared not because of its minor disadvantages, which could easily be compensated for today with thrusters, but due to the disappearance of the fine sterns where the curve of these keels would end.
Advantages:
Improved stability
Disadvantages:
The straight pins, or rather the straight pins, have succeeded the long pins. Several types are distinguished depending on the position of their ballast.
The first generation of straight pins is that of pins whose ballast is encapsulated. The composite keel is laminated with the shell acting as a receptacle for the ballast, located inside the shell. They are extremely solid skittles that require care and time to manufacture.
The Contessa 35 "Tessie" of Erik Anderaa, the Viking YouTuber who ventures towards Greenland in 10 Beaufort, is a sailboat with an encapsulated keel. Coincidence? Surely not! Unfortunately, not all fin keels are encapsulated. Bolted-on keels came afterward.
These are keels attached, with varying degrees of care, to most sailboats. These keels are generally made of cast iron, a monobloc cast that is coated, sealed, and painted to match the hull, and most importantly, bolted to it.
Depending on the shipyard, between six and fifteen threaded rods are used to attach the keel. The rods can be integrated into the keels or have a captive nut on the keel side, placed in a recess.
On the hull side, in both cases, the threaded rods are bolted into the bottom of the boat. At this point, there are two scenarios. The bottom of most modern sailboats is made from a monobloc composite molded by infusion. Its thickness is well below a centimeter.
A set of longitudinal and lateral stiffeners (stringers) is created individually and then laminated to the hull. The keel bolts pass through a kind of reinforcement cage designed to absorb the forces from the keel and transfer them to the hull.
The fastening of the keel bolts through the reinforcements laminated to the hull is the method used for small series composite sailboats.
Advantages: rigidity and the ability to visually inspect the keel where the bolts come through.
Another method involves using a composite hull base on which a lead bulb is bolted. This was (or used to be?) the method employed by Amel.
However, to simplify manufacturing, large-scale French shipyards came up with the brilliant idea of producing all the longitudinal (stringers) and lateral reinforcements (floors) as a single piece, then bonding it to the hull bottom.
The structural inner mold and the keel bolts of an Oceanis 50 represent this structural inner mold. The keel bolts pass through this piece for support. It's easy to build, but you completely lose visual and physical access to the keel from the interior. You can only see the heads of the keel bolts, not the corroded area between the inner mold and the hull.
Corroded? This is the lowest point of the boat, and there is often water there.
The bolts can corrode, which in extreme cases can lead to keel loss. This happened to a First 40.7, Cheeki Rafiki, which lost its keel during a storm that led to the disappearance of its entire crew.
Fin keels can feature ballast made of cast iron or lead to improve the righting moment. The ballasted keel can take various forms, such as L or T shapes, with the latter referred to as a torpedo keel bulb.
Nearly all sailboats today use a bolted-on keel. The highest-performing keels consist of a steel fin (the strut) and a lead bulb. Due to its higher density, lead enhances the righting moment.
Regarding density, in 1978, Tabarly used a keel made of depleted uranium, a material with a higher density than lead for specific reasons. The innovation sparked controversy at the time. The retractable keel belongs to the family of fin keels.
Advantages of the fin keel:
Disadvantages:
As the name suggests, the twin keel consists of two fins positioned off-center from the boat’s longitudinal axis. These short and wide keels allow the boat to rest flat on a sandy bottom for grounding. They are robust and make grounding easy without the need for props.
Twin-keel sailboats are particularly sturdy, especially older models like the Westerly with their encapsulated keels. They can be grounded without props on many types of surfaces and do not require cradles for winter storage.
However, older twin keels are often less efficient upwind compared to fin keel boats. Modern twin-keel boats like the Django or RM are much more efficient than their predecessors. The fins on the RM 1380 give it a draft of nearly 2 meters, and Django twin-keel boats have asymmetrical fins optimized for upwind sailing—nothing like the short fins of older Westerly boats.
Advantages:
Disadvantages:
