It can be seen from the diagrams below that a loop keel generates an additional element of righting moment as it heels. This is in contrast to a fin keel, which tends to generate a destabilising moment as the yacht heels

Dynamic Righting

This additional righting moment will be seen as a raised area of water within the loop that is a direct result of this force. This mass of water is continually being 'lifted' by the keel to replace the water that has passed through it.

The result of this is that a yacht with the loop keel will generate more power as it heels. In the examples above you can see that the loop keel and the fin keel have the same degree of heel, but with the loop keel operating in a higher wind strength. With the same wind strength, the yacht with the loop keel will heel less.

Draft is one of the key characteristics of a yacht. Too much draft and you cannot sail into shallow rivers and estuaries, too little and your sailing performance upwind suffers. This is one of the fundamental trade-offs made when designing a yacht.

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 draft and, as most sailors are aware, the deep slim keel of racing yachts give the best performance upwind. One of the main advantages of the loop keel is that it enables a yacht to be designed with an effective draft that is 40% greater than its actual geometry would suggest, so a two metre deep keel will perform as though the yacht had a 2.8 metre deep fin keel.

This 'extra' depth is a result of the two keel limbs (joined together at the tip), spreading the load equally between both surfaces and resulting in an improvement in the overall lifting efficiency.

When a yacht heels, it moves the centre of effort of the sails outboard, to leeward. At the same time, the rudder is also heeled and reduced in effectiveness. This can lead to that yacht 'broaching', where the sails overpower the rudder and the yacht turns up into the wind. As it turns into the wind, the force on the sails will increase temporarily and this, in turn, can lead to the yacht tipping over to a near-horizontal position.

Broaching can happen very quickly and unexpectedly, leading to potentially dangerous situations for crew on board. It also increases a yacht's vulnerability to capsizing as it will normally take the yacht beyond its point of maximum righting moment.

The swept back configuration of the loop keel removes any tendency to broach as the forward part of the keel comes out of the water when heeled. The part of the keel raised above the waterline is effectively taken out of action and if the keel assembly is provided with sweep, then the effect of the removal of the top, weather part of the keel is to move the lateral centre of pressure aft. This effect can be used to reduce the effects of heel on weather (or lee) helm, cancelling the effect of the sails moving outboard and significantly enhancing resistance to broaching.

The Loop Keel makes a yacht more stable by interacting with the water flowing past the yacht. This interaction 'attaches' the water to the keel in a 'bound vortex' and means that any movement of the keel also has to move this attached water. This allows the mass of that water to be treated as if it had been added to the yacht.

You will the most flexibility over the amount of mass you can add to the yacht by using a small flap on the trailing edge of the keel. For simplicity If you then the hull shape alone then you can design a yacht so that mass is added as the yacht heels ands comes off as the yacht sails more upright.

This creates a medium displacement yacht where, if the weather conditions are unfavourable, one changed setting will transform it into a heavy displacement craft. Consequently, there are no constraints at the design stage, with the loop keel giving you the ability to vary the craft at will, whilst actually sailing.

Stalling occurs when the flow separates from the keel and it stops producing lift. A traditional keel will stall for a number of reasons, most commonly from trying to sail too close to the wind and/or sailing at low speeds.. The loop keel is both angled back and in, which has the effect of making it highly stall resistant.

The loop keel avoids the problems associated with carrying the loads from the keel to the mast. Moreover, as the loop keel allows the chain plates from the mast to be directly attached to the keel, the yacht's floor can be greatly reduced in depth and weight.

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