brian eiland: Slots between Sails on Aftmast Rig

We know that the restriction presented by the slot tends to divert more air around the two sides of the slot, i.e. the windward side of the main and the leeward side of the genoa. This higher flow rate on the lee side of the headsail increases its effectiveness. Now if we also overlap the mainsail with the trailing edge of the headsail, we further increase the effectiveness of the headsail, as it is able to carry this increased flow rate much further aft along its span than if it was to have to dump its flow at free stream velocities up at the leading edge of the mainsail. This overlap is important. Now imagine looking down on the sailing rigs from directly overhead and evaluating the cross-sections at various vertical levels ( I probably should draw a diagram of this view and post it on my website). You would discover that my two parallel headstays present uniformity in both the slot between the two sails, as well as the overlap of these two sails. And this uniformity is consistent from the foot of the sails up to the hounds. Significantly this is no-where near the case with the traditional Bermudan rig, either fractional or masthead configured. In both of these cases the throat of the slot is decreasing in size as we move vertically up the mast, while the wind velocity is increasing with this heightsort of a reverse of what we might desire. And the overlap of the necessarily hollow-leached headsail is at best really only effective at the bottom 1/3 of the sailplan. Seems there are many more questions of the compatible interactivity of the main and the headsail of the Bermudan configuration than with my twin headsail arrangement. In this comparison I think my rig configuration will prove significantly superior. Note also that this nice uniform genoa overlap is attained while utilizing only what amounts to a 110-120% genoa rather than a radical 150-180% sail (Bermuda rig designations). And the foot areas are fully compatible, unlike the raised boom region of the Bermuda rig. Above the hounds the natural twist in apparent wind should help to make this upper portion of my genoa be a more productive sail area, certainly more productive than the thin strip of Bermudan mainsail hidden behind a mast structure on a fractional rig vessel. Just possibly the slot formed between my bare mast and the genoa sail in this upper region may create an upwash that could assist this productive task. One might equate the twin headsail portion of my rig to the old cutter arrangements. In fact the old cutter rigs many times demonstrated a superiority to the standard sloop when a reasonable open slot was designed between the two sails (too close a slot was an Achilles heel of the cutter arrangement). I also find it interesting that the latest state-of-the-art Volvo 60s appear to be evolving from their fractional rig plans to a masthead arrangement with their new flat cambered Code Zero sails (large genoas?) for close reaching work. I think this speaks to the superiority of the masthead verses fractional rig. Just when we thought evolution was favoring a smaller jib/larger main, things re-adjust. Evolution doesnt always follow mother natures preferred path, it can get skewed off- -course a bit following rules put in place by handicappers. While on this flow subject, I wish to bring up one other matter. Almost no one including the textbooks addresses the triangular nature of the slot (throat of the slot) in the traditional Bermudan rig, and how this might redirect (divert) some portions of the airflow up or down vertically (3 dimensions)? Everything is treated in a 2 dimensional manner, in a plane parallel to the waters surface. Wouldnt this more restrictive slot at the upper regions combined with an increased flow velocity, tend to deflect some airflow in a vertical manner? Ive certainly seen it with my telltails..and if my Bermudan mainsail is diverting wind upwards it must be pushing back downward on my mainsail (for every action there is an opposite and equal reaction). Or how about that lift I seem to experience with some headsails. Where is the theoretical data to suggest and/or collaborate these observations? Things in nature dont always follow our assumed calculated rules. http://boatdesign.net/forums/showthread.php?t=623

Posted by: brian eiland - website: http://www.RunningTideYachts.com