Loop Keel - Development History

Our rigorous testing procedures combine tank test models with testing scaled down and full-scale versions in natural, outdoor conditions to obtain qualitative and quantitative results.

The starting point is the conviction that we have developed a theoretical concept that offers specific benefits and improved craft performance. The next step is to develop a set of rules that can be implemented when designing the full-scale version. The actual size is irrelevant at this stage, as the rules should be capable of being applied to any sized version. Having decided on the size of model we require, we construct it according to our defined set of rules

Our starting point is the conviction that we have developed a theoretical concept that offers specific benefits and improved craft performance. The next step is to develop a set of rules that can be implemented when designing the full-scale version. The actual size is irrelevant at this stage, as the rules should be capable of being applied to any sized version. Having decided on the size of model we require, we construct it according to our defined set of rules.

The advantage of testing a model in natural conditions is that it will quickly reveal any flaw in the original theory. On the flipside, the disadvantage f testing in this way is the subjective nature of the results and the difficulty of obtaining accurate numbers and comparisons. In these circumstances, any problems highlighted by the tests are normally resolved by careful observation, such as examining film footage of wave patterns.

The tank testing approach differs in that we build a model designed to produce the same performance results as we would expect from a much larger craft, without having to construct a full-scale version. The test tank model is not designed to be taken outside the tank environment, but to be a scale replica of a much larger craft operating under specific conditions.

The end result of our testing of the loop keel is that we have now developed a set of rules that we can apply to a much larger yacht and know what its performance will be in advance of any actual testing.

We built this early prototype using a laser hull as the test platform.

We used a basic steel bulb weighing about 50 kgs with the load taken back to the hull with 50mm by 25mm steel beams bent to the correct profile. The actually foil was made from wood and epoxy grp. This was filled and faired to the correct final profile. We used a Clark Y section for this model, which was not ideal.

We also acquired a second hull on to which we attached a fin keel with the same area as the loop keel. The keel weight was identical and the foil profile was uncambered Clark Y with a thickness distribution to match P1.

The next step was to carry out tank tests at Southampton University's Wolfson Unit. For this test series, we attached a loop keel to a medium displacement yacht hull and were able to compare our results with the Wolfson Unit's existing data on this hull fitted with a single fin keel.

This was the first time that we used a sharp leading edge on the keel (we say sharp, but the actual foil has a slightly rounded tip, maybe radius .2mm). As the leading edge was sharp we were also concerned about stalling. To reduce the chances of this the section was also swept forward as it enters the hull. This has two effects:

1. To reduce wave drag by effectively stretching the section out.

2. To reduce the likelihood of stall by reducing the chord-wise loading at the leading edge and therefore lowering any pressure differential. It also allowed the formation of a leading edge vortex, similar to the leading edge root extension found on fighter aircraft designed for high alpha (angle of attach) flight.

For this prototype, we took the original Laser and replaced the Clark Y sections with a similar section to the tank test versions P3A and P3B to produce an elliptical chordal load distribution..

The keel position was moved back 200mm.

At this stage of the loop keel's development, we fitted a bump on the underside of the hull. The advantage of this is that it avoids the complexity of fitting control and flaps into the keel arms. Instead, it is a control consisting of nothing more than two flaps controlled by a small piston, which can fit into any recess in the hull.

This model was created to test the effect of Toe In (negative Toe Out) and to see whether it cancelled some of the Laser's normal added mass. We used the original hull and fitted an elliptical foil section.

This was by far the most complicated model that we built. Using another Laser hull, the keel was swept back and the depth of the foil reduced. The flaps were recessed into the structure, but as the keel bulb remained in the same place, there was no change in the craft's centre of mass.

The flaps were recessed into the structure and all metalwork was made from 316 stainless steel, including the flap hinges. We used bicycle cables for the control runs and fed the controls back to the cockpit on the craft.

We tested the P2, P6 and P7 prototypes at the Gosport Test Tank. As these were actual sailing craft, they had a significant righting moment from the keel bulbs. This meant that we had to use counterweights (see below) to negate most of this heeling moment

As ours were actual sailing craft they had a significant righting moment from the keel bulbs and this meant that we had to use counterweights (see above) to negate most of this heeling moment.

Sailing testing has been an intrinsic part of our testing programme and absolutely critical to the most important aspect of sailing, the feel of the craft.

This aspect of testing has been conducted at Grafham Water Sailing Club, where we have always received fantastic support from the Club's coxswains and office staff. Their valuable contribution in terms of providing practical support and access to facilities has made all the difference as we have always tested at Grafham Water in the worst weather conditions possible.