The loop keel concept is described in UK patent GB2408487. The keel may be described as a closed loop under the hull of the vessel to which it is fitted such that the hull provides part of the closed loop. The keel may be configured to generate a continuous load per unit length around the loop thus forming a closed circulation system when the vessel is upright. If the vessel is heeled one side of the loop is lifted clear of the water surface and, if the force per unit length is such as to generate a higher pressure within the loop than within the free stream, a dynamic righting moment results.

This dynamic righting moment allows the yacht to generate more power as it creates a stiffer yacht. Where a yacht is fitted with the loop keel it will sail with the same heel angle as a fin keel, but in stronger winds and hence generating more power. Alternatively where you sail against a fin keel in the same wind strength you will either heel less or be able to carry more sail and again, therefore, more power.

For a yacht to sail close to the wind it needs to have an efficient keel design. The efficiency of a keel is very closely linked to its 'aspect ratio' and, as most sailors are aware, the deep slim keel of racing yachts give the best performance upwind.

As the loop keel has two limbs joined together in a bulb, it can spread the load between each limb. There are a number of aircraft wing designs that also spread the load between two limbs. The most obvious is the Bi-Plane, but there are other less well known designs such as the Ringwing and Boxwing. These designs all have one thing in common, they allow the aircraft to behave as though it has a greater wingspan than it actually does. Winglets also help to create this effect of a wider span and are becoming extremely common on commercial aircraft.

Boeing 737 with winglets

The Loop Keel allows you to design a yacht that has an effective aspect ratio that is 40% greater than it's actual geometry would suggest. This means that you can design a yacht with a 2m deep keel that performs as though it had a 2.8m deep keel.

When the vessel is upright the force system within the keel does not shed vorticity due to the closed circulation loop. When the keel develops sideforce,usually associated with heeling, then some vortex drag will result in addition to the dynamic righting moment.

A secondary effect of the circulation system is to vary the apparent mass of the vessel. A circulation which generates a force away from the enclosed flow path retards the flow within the loop and this has an additional displacement effect which manifests itself as an increase in the apparent mass of the vessel. If the forces are in the opposite sense the effect is to cancel some of the mass of the vessel and also to reduce wave drag.

These effects also change the sea-keeping properties. The use of a trailing edge flap on the limbs of the keel allow this apparent mass variation to be controlled at will, when running, for example, the apparent mass may be reduced in moderate conditions to reduce wave drag. In survival conditions it may be increased to provide a more comfortable motion for the crew and less sensitive handling.

A normal keel stalls when the flow separates from the keel and it stops producing lift. A keel will stall out for a number of different reasons, but normally from trying to sail to close to the wind and/or when sailing at low speeds. The loop keel is both angled back and in, which has the effect of making it highly stall resistant.