understand that it is very efficient to invest carbon in the diamonds rather than in the mast wall – which remains 0.15m from the centre of inertia. If this can be done with limited drag, stiffer diamond stays increase safety factors and reduce the deflections of a tube that would otherwise be considered rather slender (and light).
Left: the rig on Allegra flirts with the once sacrosanct minimum angle of shroud-tube intersection of 10 degrees, something that is made possible by improvements in carbon rigging as well as – in this case – clever use of wide-span diamond stays
In terms of mast engineering we were in crossover territory, since a mast engineered for hull flying will not be significantly heavier than a spar designed to a maxi- mum AWS. That said, for such a boat there always needs to be a substantial margin for a crew not releasing sheets or reefing at the prescribed point for both safety as well as warranty reasons. With a hull flyer the dynamics are by definition limited. For the Allegra 78 the tube weight is slightly north of 800kg (well below the norm for a mast like this), employing high-modulus carbon and a rather moderate tube size of 640x308mm. This has been achieved with conservative safety factors with the main contributor being ‘oversized’ diamond stays allied to a dock-tune with significant pre-bend. Pre-stressing the mast with such stiff diamonds results in smaller sideways deflections than the norm as well as less variation in sailing bend, making the job easier for the sailmaker in terms of creating a mainsail that will look good in a variety of load cases. Remarkably for a large mast there is also no requirement for an inner forestay, not even deep-reefed or hull fly- ing with the Solent with a racing tight mainsheet, or flying the reacher with a three-reefed main.
For cruising multihulls one of the key debates has been whether a rotating mast should be of the French spreader-less variety or should carry diamond stays. The advent of carbon rigging changes the game a bit, with the drag penalty of the additional length of diamond stays (compared to lower shrouds) greatly reduced due to the smaller rigging sizes that are now achievable. The reduced size of the stays also affects where it is most useful to invest the carbon: in the tube or in the diamonds. With the spreaders on Allegra extending 2.75m from the centre of inertia it is easy to intuitively
To further reduce the drag penalty of the rigging we gave our diamond stays airfoil sections. With a rotating mast this is particularly efficient since the airfoil diamond stay is turned together with the mast into the apparent wind. Air-foiled rigging is not new, but what is new on Allegra is that the airfoil is achieved using soft rather than solid carbon stays. Future Fibre’s latest carbon rigging also played a key role in reducing leeward rig fall-off on a boat that is relatively narrow – and where the cap shrouds teeter on the 10° angle of intersection with the spar that is often considered borderline for such a boat. With today’s stiffer and smaller- diameter carbon rigging I would say that this traditional 10° benchmark can no longer be regarded as a firm cut-off, and that in due course we will therefore see significantly taller rigs on cruising cats with narrow beam, vessels that will still have a relatively high righting moment due to the displacement that inevitably accom- panies a luxury interior. In the case of Allegra, crowning the above mentioned characteristics, we have employed an integral mast jack to deliver the correct tension to the diamonds and cap-shrouds and without the need for turnbuckles. This system allows a dock- tune to half the maximum spar compres- sion. Diamonds and shrouds are normally regarded as supporting components of the mast tube, but in this case one could perhaps take a philosophical wander and instead regard the tube as a supporting component of the rigging…
Summing up, we have a single-spreader rotating wing mast that is slightly heavier than a fixed rig, but not to a large extent. The fixed rig would only be lighter through more complicated rigging which means that the rotating wing mast also comes out similar in terms of price. In theoretical terms the rotating wing is 2-6% more efficient, depending on which study you read and what point of sail you are referring to. If you equate that to the weight you need to save to get the same performance gain, or the mast height you can get rid of for similar performance, this is a substantial number.
Finally, you will often hear people arguing that this is only a theoretical aero gain since the complication of rotating the rig will inevitably see it locked more or less permanently in the middle. Not so on Allegra… where mast rotation is among the control functions at the helm station where mainsheet, traveller, jib sheet and mast rotation are all managed hydrauli- cally – bringing dinghy-like adjustability to Allegra’s fortunate crew.
SEAHORSE 33
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