Published: 18-09-2009
From its headquarters in Milan, APM Keels works on major superyacht projects around the world. The company was founded in 1993 by Matteo Caglieris and Giampiero Galli and the partners have brought their company to a position of global prominence in the advanced field of movable and fixed keel designs and constructions.
The major yards with which APM works include Baltic Yachts, Green Marine, Royal Huisman, Nautor, Pendennis, Vitters, Wally and McConaghy. All these builders create yachts for high-profile owners with a wide range of demands, from greater speed to easier access to ports and anchorages. These custom projects typically take five to ten months from design development to finished construction and APM handles on average about seven or eight of a year.
Despite the high-end nature of his work, Matteo Caglieris is convinced that many people have an ‘out-of-sight, out-of-mind’ approach to keels, and do not realise how advanced they have become. In this fascinating article, he looks at how the requirements of racing and cruising boats are coming ever closer together. Matteo then examines the technologies which underpin keel designs before looking in more detail at recent developments in lifting and canting keels. Written in non-techno language, it offers you an excellent overview of the state of the art in the keel sector.
The keel is one of the least known and least well understood parts of a yacht. Located below the water surface and always hidden from view when a yacht is at sea, we only get to see a keel when boats are hauled out on dry land. Perhaps this is why the public at large is mostly unaware of the extraordinary evolution that the design and construction of a modern yacht’s appendages has gone through over the past 15 years. And this evolution continues today, aided by new technologies and new materials and by the growing desire of owners and sailors to take their yachts to higher performance levels.
As always nothing is really new because someone has surely tried it before - a good example are fin keels that can be found on some yachts built in the late 1800s. We may therefore find one day that someone had already used canting keels a century ago; nevertheless this new wave of keel design is fairly recent and can be linked to the growing size of modern cruisers and increasing sophistication of one-design, handicap and America’s Cup racers.
In fact these two worlds of racers and cruisers are coming closer every day, borrowing ideas from each other. The latest generation of performance cruisers is perhaps the best test bed for new keel technologies. In fact, in a wave first pioneered by a few one-offs, then launched by Wally Yachts and later followed by more traditional builders in search of alternatives (such as Baltic and Nautor’s Swan) lifting and canting keels have been fitted to an increasing number of recent launches.
In the process, the centreboards common a few years back have been replaced by a new generation of hydraulically lifting keels. These are made of a thin metal blade fitted at the bottom with a lead bulb that carries most of the weight. Taking the game a step further, canting keels provide for lateral movement of the whole blade and bulb. This allows a better righting moment when the boat is heeled without the need to resort to a longer blade and deeper positioned bulb.
A lifting keel solves the practical problem of how to access shallow draft locations. But for boats that have been designed with a deep keel for maximum performance, a canting keel actually increases the performance.
Technology matters
Without the contrasting action opposed by the wing-like plane of a keel, a boat would just slide laterally when the wind hits her sails and would tend to capsize without the aid of the ballast that keeps it upright. A sailboat’s keel performs therefore a double action: advancing the boat and righting it.
To meet these two requirements, modern keels are composed of a long fin and an olive-shaped ballast bulb attached to its lower end - almost like having an airplane wing with a heavy truck hanging at the end of it – a rather challenging configuration for us who have to design and build the keel.
To satisfy the first requirement – transforming lateral push of the wind into advancement - hydrodynamics and wing geometry call for low tolerance manufacturing, a small profile of the fin with a short chord and reduced thickness, and for increased depth. To satisfy the second requirement – keeping the boat upright – we need mechanical strength, stiffness and again increased depth.
Since depth is such an important factor there has been an obvious push towards longer keels as the demand for higher performance increased but this trend soon ran into the obvious obstacle of draft limitations in everyday use. Racing boats, which were willing to accept the trade-off between limited usability and increased performance, were soon blocked by class rulings imposing maximum draft limits.
Cruising yachts on the other hand which do not have the constraint of the rules still have to deal with practical issues such as the average depth of most marinas and the need to get closer inshore for better coastal cruising. Fifteen years ago a large cruising yacht measured about 65’ – 70’ with a draft around three meters, today the same type of boat measures on average 90’ to 100’ with drafts around five meters and pushing towards six to increase performance.
This increase in boat size and deeper draft prompted the development of lifting keels that have become sophisticated machines requiring extensive design and testing to make sure that everything works as it should. When they are fully extended they must be exactly equivalent to a fixed keel, totally devoid of any mechanical clearance, and also capable of sustaining a grounding impact without compromising the safety of the yacht. You should also be able to move them even when the boat’s power sources are out of commission.
Their installation inside the hull, their lifting mechanism and every other component that is located inside the hull must be sufficiently light so as not to raise the centre of gravity and compromise the boat’s stability. Last but not least, the keel must have a very thin profile like all modern keels.
Finding viable solutions to these and other problems are the design challenges we face at APM every day. We have moved from keels fitted only with an internal lifting dagger board to fully retracting keels via a continuous development of the systems that guide, control and block the movement.
Currently we use a prismatic and conical blocking and guiding mechanism and three in-line cylinders for lifting. For the future we are looking at an up-and-down standardized mechanism for each type of boat. This mechanism will be an integral part of the hull, leaving the naval architect free to design the keel shape he prefers. A further step will be the combination of the lifting and canting mechanisms in a single device; something that today only exists as a design concept and in some early experimental prototypes.
Lifting keel
The optimal performance for a keel is linked to its length. For a fast 100 ft cruiser this length is five to six metres. This deep draft creates problems for the yacht’s access to harbours and anchorages. Lifting keels are designed to solve this problem by reducing the yacht’s draft to three to four metres. They are composed of a wing-like fin,
NC-machined in special steel, and a lead bulb attached to its end. They are designed to be fully deployed when the yacht is sailing and partially retract when it comes back into port.
Lifting is accomplished by two to three hydraulic cylinders, located inside the fin. Guidance and containment inside the hull are through a carbon box with low friction internal guides and surfaces coupled with adjustable bronze sliders. Blockage when deployed is by a stainless steel conical head. Blockage when retracted is by means of hydraulic pins or large manual screws. Lifting keels go from fixed models with an internal lifting carbon daggerboard to fully retracting keels that move in and out of a special box fitted to the hull.
Canting keel
This wing-like fin with a lead bulb attached to its end can pivot laterally. These keels can be inclined (canted) to windward when the boat is heeled, dramatically increasing the righting moment. The effect on speed is remarkable and immediate. The maximum canting angle is about 35?, which, added to 25? of boat heel, give around 60? of total angle of the lead bulb from the vertical plane. At this high angle the keel cannot effectively oppose its surface plane against the yacht’s leeway; this work must be carried out by a daggerboard. Usually this is a lightweight retractable carbon wing and there is one fitted to each side of the hull.
A canting keel has a structural arm and a head housing the pivoting pin and the attachments for the hydraulic cylinders. Inside the hull is a box with the cylinders used for movement. The structural arm is covered by a fairing surface shaped like a wing. Canting keels have the sole purpose of increasing performance. Their design and construction varies depending on the type of yacht they are fitted on. A canting keel fitted on a BOC Open 60’ is therefore very different from that mounted on a fast 100 ft cruiser.
The wing shapes of the keel fins and their exacting geometrical requirements call for high precision machining and the use of special steel alloys. From the steel reinforced lead keels of 15 years ago we have moved progressively towards materials with increasingly high tensile strength, typically in the order of 1000 MPa (or 100 Kg/mm²).
We are not talking here of special applications because for cases like the America’s Cup yachts these parameters have been widely surpassed and we are testing steel alloys that have close to twice that strength. Manufacturing parts from these high-end materials requires special technologies that are both elaborate and costly whether you are casting or forging using special heat treatments or else putting together thick plates with massive welding.
Accurate wing shapes can only be obtained by precision milling of the entire wetted surface with large numerically controlled milling machines that work, in various stages, for weeks on end, carving out the final shape that is within a tenth of a millimetre of the computer designed geometry. Such precision level is higher than that required in aeronautical construction because water is denser than air and thus penalises any surface imperfection. The same milling techniques are applied to lead bulbs.
Over the last five years, the choice of steel alloys has grown wider and their resistance has increased by 30 to 40%. This has allowed us to build keels that are longer and thinner. It can be expected that in the near future steel alloys will become even more resistant. The obvious limitation is the high cost that today, for a lifting keel on a 100 ft yacht, is between 300 and 500,000 euros. High design and production costs make up a large part of this price and they should come down in the future as the number of keels produced increases.