Hi Gordon, thanks for the very comprehensive reply. Knowing that you had the same boat and often provided advice on the forum I hoped that you might be one of the members able to provide some useful advice although I wasn’t aware that you were in the process at the moment. I have been undertaking a fair amount of research, reading online and talking to several of the manufacturers and I think that I am making progress.
We have been looking into the option of replacing the asymmetric snuffer with a furler to simplify sail handling and in the hope that controlling the sail can be moved back to the cockpit from the foredeck. It follows therefore that, if we go down this route, the solution must be as close to bullet proof as possible, and that seems to be the issue. The furler drums and top swivels on the market all seem to be satisfactory and pretty similar, accepting the differences in drum size and ease with which a sail change can be made. The issue with top down furling (I am not worrying for now about a code zero) seems to fall into two areas. Wrapping the asymmetric around a furled foresail (largely down to care and technique) and getting a back-wrap in the furled, or worse partially furled, sail. This would be a problem short handed, requiring the sail to be stowed partly deployed, to be sorted out later on the quay. The problem of back-wrapping seems to be inherent in the design of top down furlers. The torsion cable transfers torque (from the drum) to the top swivel to start the furl. Since the cable can not have an infinite torsional stiffness there will always be some “windup”, twist between the drum and top swivel. The tighter the AT cable (increasing halyard tension), the more torsionally stiff it becomes (decreasing “windup”) BUT the increased load acting at the upper swivel makes the swivel harder to rotate and therefore has the opposite effect, increasing the twist. There is therefore a sweet spot, too tight or too loose and trouble is more likely. It seems to me therefore that to maximise the size of this sweet spot, to minimise risk, the system requires as torsionally stiff an AT cable as possible. Unfortunately this is where data becomes very hard to find. Selden claim that their cable is “three times as stiff as their nearest competitor” but without offering any evidence. Haken (technical) have advised me that they can not quote a value for the torsional stiffness of their “reflex” cable (“we do not make the cable ourselves and anyway very few customers would understand the information”). So I am left trying to guess by looking at the construction of the cables available. Certainly to me the Harken reflex cable with its integral stainless braid would seem a satisfactory solution. In any case I have concluded that a larger diameter cable will always be better than a small one. The same logic probably also applies to the top swivel, it seems to me important that the axial load (tension) applied to this should be as far from its “maximum working load” as possible. The second approach to this problem of back-wrap seems to be to surround the AT cable with some sort of sleeve to allow the cable to rotate within a furled or partially furled sail without dragging the sailcloth back on its self. Barden fitted a row of plastic balls along the cable but I am advised this is not very satisfactory and was instituted early in the development of these systems to overcome the problem of insufficient torsional rigidity. I would be interested if anyone has experience. I believe foam sleeves have also been tried.
So I am coming to the conclusion that a reflex cable combined with as large (or at least as “smooth”) a top swivel as possible and a well made lower drum will provide the smallest chance of a mishap. I hope, in the interests of my bank balance, that someone can provide a counter argument.
I am hoping to use our existing North asymmetric but I have not been able to find any information on the characteristics of an asymmetric which make it more, or for that matter, less suitable for use on a furler.