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Keel types and their uses - Introduction

By Stuart Wilson

Boat Keel Introduction

The shape, performance and design of keels has continued to evolve driven by technology and new modern materials. In the 1970’s onward the biggest driver in this evolution was the realisation amongst designers that the majority of people buying yachts were for the most part holiday/recreational and weekend coastal sailors who rarely ventured offshore and therefore was it really necessary to design yachts that could withstand the harshest deep sea weather conditions.

With this in mind sailing yacht hull and keel design saw the advent of the Cruiser-Racer sailboat. This generic class of boat was ultimately a compromise based on the reality of what the majority of sailing yachts were actually used for.
They offered many advantages but no longer offered the degree of seaworthiness and speed of boats created by designers such as Charles Ernest Nicholson, Laurent Giles or Francis Herreshoff (designers of the J Class Yacht and other yacht designs) based on full length keels.

The Modern Cruiser-Racer is therefore acknowledged to be less seaworthy than its long keeled predecessor but offers more comfort because of their wider beams and improved performance due to a reduction of ballast/weight and of course the introduction of the fin keel. The fin keel in all its guises is the keel that is ubiquitous and now dominates the yacht market

What does a keel do ?

Before we look at all the different alternatives in terms of keel shape, design and performance it is worthwhile briefly understanding the role of a keel and its two basic functions.
It provides ballast and counteracts the lateral forces generated by wind in the sails

Firstly what is ballast and why is it necessary?

It comes in lots of different forms. Pig Iron, steel, concrete, lead, or a mixture of metals and polymers All boats whether sail or motorised boats need some degree of ballast which is additional dense weight added to the vessel.

Without ballast most boats would be too buoyant, sit too high in the water rather like a cork. Consequently the vessel would be inherently unstable and capsize easily because of the upward forces provided by water under the hull. (buoyancy). Without ballast these upward forces would be far greater than the downward gravitational forces of gravity. Buoyancy is why boats float rather than sink when afloat. So ballast is designed to counteract counter balance the natural physics of a boat’s buoyancy provided by water and bring gravity and buoyancy into somewhere approaching equilibrium.

In boats without keels, ballast is placed at the lowest possible point of gravity in the bilges of a boat; as in the case of Falmouth Working Boats. Some of these boats still dredge for shellfish in the Carrick Roads Estuary in Cornwall and compete in regattas during Falmouth Week but only under sail. Without ballast these boats which carry a lot of canvas would be very unstable plus maintaining a desired course would be difficult.

In the case of sailing boats or sailing yachts with a keel the ballast is often built into in the keel which as it is situated under the hull is the lowest possible point of gravity. Additional ballast may also be placed in the bilges if required.

The weighted keel on a sailing yacht provides a righting movement as the boat heels due to the force of the wind in the sails. These forces are trying to push the boat over and as the vessel heels over, the shape and overall surface of the hull presented to the water changes so decreasing buoyancy and stability.

The weight of the keel produces an increased gravitational downward force or torque that counteracts these forces and helps the boat to return to a more stable upright position. Normally the heavier and deeper the keel the stronger the righting moment.
Secondly a keel prevents the boat from sliding sideways by counteracting the force of the wind. On a beam-reach tack for example the wind is directly trying to push the boat sideways. This is mitigated and countered by the keel which produces drag that resists that motion and keeps the boat pointing /moving on the desired course.

This article was last update on 24-10-2019 at 08:15