Aftmast rigs???

Discussion in 'Sailboats' started by jdardozzi, May 28, 2002.

  1. brian eiland
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    brian eiland Senior Member

    Wishbone-Shape Masted Vessel

    I think if you look back thru this subject thread you will find a number of references to this vessel outside your door. Look at posting number #28 in particular for some cross reference subject threads.

    I've even managed to contact both the new owners of the tall and short versions of this rig design and encouraged their communication with one another, as well as this forum
  2. brian eiland
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    brian eiland Senior Member

    I guess I'm not enough of a computer person to find 'the formulas' you used. Can you send these to me?

    I believe you have been a little less than fair with some of your comparisions on 'an equal basis'.
    1) What would your formulas tell you about the backstay tension in the aft-mast rig if you did not include a second luff tension equal to the forstay tension?
    2) Or what mast compression might be expected in the sloop rig if it were a cutter?
    3) How does you main halyard tension affect the backstay tension in the slooop rig?
  3. Andrew_Davis
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    Andrew_Davis Junior Member

    Thanks for the tip...I had no idea that there could be more than one of these! I understand now that I exist in virtual isolation here...dwelling in Plato's Cave. This thread is amazing. It really sustains the central thesis (if I ever had one) of my blog:
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    MAINSTAY Junior Member

    Mast Compression spreadsheet

    Hi Brian,
    It's good to hear from you.

    To see the formulas, download the SS and in the Excel main task bar, click on View, then within that dropdown menu click on Formula Bar. That sets up a line (fx) where the formula in a cell appears when you click on a cell.

    This SS was a special case study. It shows that mast compression is NOT double, as some thought obvious. To prove this I used identical sailplans. This was for two purposes: to eliminate any differences and complications that might have been said to prejudice the results in my favor; and to simplify the task to a 2D analysis so I could get it done quickly. I have left it open for a 3d analysis, but I need someone with real rigging expertise for that.

    As set up the SS does not analyze anything but masthead rigs, and then only considers the longitudinal forces. But considering this limitation, try entering 0 for the mainluff stay and 0 for the main halyard. J still has to be measured to a point directly below the masthead, the angle of the mast is controlled by L which will have to be measured to the point where the upper backstay line intersects the deck line, well aft of the hull. Anyway, use it first on the jib and upper backstay for the compression due to the stays in the upper mast. Then repeat using the innerstay and lower backstay for compression in the lower mast due to the inner stays. Then add them.

    DO NOT DESIGN to this compression. It does not include compression caused by the backstay spreader, or that caused by the shrouds and sails.
    Larry Modes
    Last edited: Apr 15, 2008
  5. brian eiland
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    brian eiland Senior Member

    Photo of Mast-Aft Trimaran

    Thanks, I'll get back on this subject when I have a bit more time.

    Meantime here is a note & photo I recieved recently from a gentleman at SAIL mag;

    Hello Brian
    Found you via, which I troll from time to time just for the
    hell of it. On your site I found the answer to a puzzle that's been bugging
    me since January, when I first saw this boat at Culebra, of Puerto Rico.
    Unfortunately we headed out early next morning before I could track the
    owner down to find out more about his boat.

    Peter Nielsen

    SAIL Magazine
    98 North Washington Street
    Boston, MA 02114, USA

    Attached Files:

  6. masalai
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    masalai masalai

    Brian, re your post #95, I see no great advantage there as 14 stays?, including one attached to a "bridle" at the stern... I cannot see any advantage there...
  7. brian eiland
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    brian eiland Senior Member

    I believe you are also seeing a number of hayard lines or whatever, not all stays. Haven't taken time to inventory them.
  8. brian eiland
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    brian eiland Senior Member

    Aft-mast Origination & Justification

    Over the past number of years I have attempted to make a positive case for my aft-mast {mast-aft} sailing rig configuration, both within, and aside of the technical discussions of sail aerodynamics. I now see some renewed technical discussions of the headsail/mainsail interaction, and lift/drag factors of cruising boat rigs under the Sail Aerodynamics subject thread; and some interesting new participants with respectable technical backgrounds.

    I’m going to refrain from re-entering these technical micro-flow discussions at this time, and watch where they go. Rather I will choose to do a review of why I pursued my alternative rig configuration for cruising vessels based upon the real-time, observable phenomena that we experience as sailors.

    So here are a few other observations I based my thought processes on:

    If properly set up with reasonably good sail shapes and a tight forestay, the jib-headed sailing rig will outpoint the uni-rig vessel. Overwhelmingly I believe the majority of sailors would agree that the jib and/or genoa headed vessel will go to windward better than under mainsail alone….just good old practical observations from multiple sources. Hopefully we’ve come to the understanding that this phenomena is a result of the interaction of the leading sail and its following sail….the two sail combination producing a more favorable wind flow to the headsail that allows it to point slightly higher….again supported by multiple sources of which I will only sight two at this time.

    I’ve referenced this experimental work by Hall Spars before, but I will again for a re-emphasis of their findings: Eric responds, “ first, of course, the boat would be improved upwind with a No.1 jib. Generally, we could not point as high as the others here (Block Island) and therefore had difficulty holding lanes.”
    …again, good solid real-life experimental results from a respectable source

    2) In racing situations we’ve often seen, or made use of, this ‘extra lift’ phenomena when we sought to make use of the ‘safe leeward position’ and force our windward opponent into a pinching situation. Even while we are just leeward of him in the race, the flow disturbance off his rig gives us a slightly better lift to weather.

    3) Numerous other examples are available that I just didn’t have time to reference.

    So, like any respectable sailor, I wanted at least the ‘capability’ of good weatherly performance to come to my rescue when I really might need it. I believe this aftmast design is capable of good windward performance, perhaps even exceptional in some cases.

    Here is where a real argument will arise….mastheaded vs. fractional, big genoa vs. jib, how much overlap, etc.

    From practical experiences I would be willing to bet that most all of us would prefer a fairly good size genoa sail in light-airs, for either pointing, reaching, or even running (particularly if we have no dedicated downwind sails). A Bermuda rigged cruising boat without a good size headsail or furling reacher is destined to be a motorboat in anything other than substantial breezes.

    So I wanted my new design cruising rig to have a good size headsail, a genoa. And since I was not intending to utilize a rotating mast, why not make it a mastheaded sail where I could get the max sail area in the lowest CE form. On a multihull vessel I’ve got a good wide sheeting base to make better use of this genoa on a reach or a run.

    Again, from practical experiences and theory both, I recognized the effectiveness of a good leading edge sail. I wanted one, even at the expense of a self-tacking headsail. This was to be a cruising design, not a racing vessel requiring multiple tacking capabilities. Fractional rigged designs are a requirement if you intend to have a rotating spar, or a self-tacking jib. I did not want to limit my effectively good headsail size in difference to the smaller self-tacking jib nor rotating mast. Besides, since I was looking to eliminate the mainsail, I saw no need for a rotating mast.

    Please note I utilized the word “headsail” to speak of the most forward of the sails on my rig, even thought some would say I have a second ‘headsail’…a staysail.

    Some sailors might term my second sail a ‘second headsail’, or an inner ‘staysail’ as one would find on a cutter rig. I’ve chosen to call it the ‘main-staysail’ because I have no traditional mainsail to perform the functions of the ‘following sail’ subject I addressed above (the interaction of the leading sail and its following sail....the two sail combination producing a more favorable wind flow to the headsail ). In recognition of this need for a good helper sail, I chose a cutter type configuration with a good parallel slot arrangement. Regardless of what you believe about the ‘slot theory’, etc, eventually you will have to come to the conclusion that with a multiple-sail vessel, the leading sail is helped more by the trailing sail than vice-versa.

    So I wanted my second trailing sail to 1) help my leading edge genoa, 2) be self-tacking, and 3) be of a size and disposition that it be easy to handle. I had had experiences with a standard staysail on my personal ketch/cutter-rigged vessel, and I knew I needed to improve upon the staysail’s self-worth. I knew it needed to be separated some greater distance from the headsail to be more usable in a greater number of conditions, so it needed to be moved back. Could it be made a bit bigger and substitute for the combination of staysail and mainsail of the traditional cutter rig?? Why not, particularly since I desired to get rid of the traditional mainsail. And lets make it a bit bigger, locate it over the center of the vessel such that it could be utilized alone in higher wind conditions, make it self-tacking, and make it roller furl.

    At this stage I realized I had a net overall lost of sail area compared to the traditional sloop or cutter. But wait a minute; I always had a deep appreciation for the ketch rig for a cruising vessel design…..lets add a mizzen.

    To those cruising sailors that have had the pleasure of utilizing a ketch rig, I do not think I need to sell them on the concept. In fact I think most of them would join me in the praise of this rig configuration. Besides the full sail configuration, they are happy to be sailed under genoa/mizzen, with the mainsail stowed or reefed, and under mainsail alone, or with a combo of genoa, mizzen staysail and mizzen sail. This is a balanced and very versatile cruising rig.

    Of course they do require two masts and correspondingly some extra amount of rigging.

    Could I add a mizzen sail onto my double-headsail configuration and come up with a ketch style rig?? Wow, I believe so!! I could even term it a ‘single-masted ketch’ !. The idea was born.:idea:

    There is another big plus for a ketch rig on a multihull. The overall center of effort, CE, is lowered by a considerable amount compared to the sloop rig, and particularly the fractional sloop rig. Have a look at the illustrations below, and the illustration I will be providing for a big tri project I was consulting on. The rig heights can be a good 25% lower, and the overturning moments considerably reduced.

    The overall sail area could even be increased on this lower aspect ratio rig. Besides, too much emphasis has been placed on hi-aspect ratio sailplans that are really only good for windward work. Marchaj, et al, have shown the virtues of low-aspect ratio sails for off-wind sailing….often two times more efficient!! Lets design cruising rigs for cruising sailors that….

    rarely want to go out bashing to windward. Here is an interesting first hand ‘cruiser’s analysis’. A Liveaboard Cruiser for the Real World
    Having voyaged over 30,000 miles I have come to the conclusion that idyllic trade wind sailing with steady winds of 15 to 25 knots for 24 hours is a dream, or a myth made up by writers of cruising stories.
    . And here a Capt’s experience with multiple head sails (cutter rig)

    Chris White has very high praises for a ketch rig onboard a multihull. Have a look at this separate posting below. I excerpted a few observations by Chris White during one of his voyages onboard his client’s 63’ foot ketch rigged catamaran, Concept 63. :cool:

    My own experience; I have been the owner of a few sailboats, and one of them was a ketch-rigged vessel…more specifically a staysail-cutter ketch. For a cruising boat I really liked this ketch rig. It broke my total sail area down into more manageable size sails, it lowered my overall rig height, it allowed for helm balance by ‘tweaking’ the mizzen sail, it allowed for ‘mainless’ sailing under headsail/mizzen combo, and, had it been roller furling, it would have been even easier to sail single-hand. I went thru a particularly nasty offshore storm by initially running off downwind with just the small staysail, then upon full fatigue, lying slightly upwind under a backed staysail and reefed mizzen. Here’s a subject thread on going ‘mainless’

    What did I dislike? I always thought the cutter staysail jib was too small, and it was marginalized by its too-close proximity to the headsail. And I was disappointed in the strip-area of the mainsail behind the spar that appeared to do nothing for forward drive. Now remember this was back in 1973, 35 years ago. I searched out as much info as I could find on cutter-rigged vessels, and I had heard of some aft-mast experiments in AYRS. My webpage “Sail Propulsion” describes a few other items/processes that influenced my selection of this aft-mast configuration.

    So now I’d come up with a single-masted ketch that I thought even looked very presentable….not too radical. But it had a mast canting forward!! That would prove to be a VERY tough sell to the conservative sailor. Sailors are a very conservative bunch that cling to tradition.

    Just by happenstance a sailing friend/professional captain came across a magazine clipping of a vessel named Diomedea Exulans. That clipping is in the archive section of my website, and I’ve posted it below. Here was a vessel with almost the same mast inclination as mine. This vessel’s rig design reinforced my thoughts about the possibilities of a forward raked mast

    I had chosen a 10-degree forward cant to keep things in proportion, and allow for fairly good size twin headsails that would not overlap the spreaders. This cant could be reduced. In fact there was a problem with adapting this style rig to an existing in-build trimaran where I decided to reduce this cant to 6-degrees, and increase the size of the mizzen sail. The point is, the 10-degree cant is not set in stone, nor is the placement of the mizzen sail on the vertical backstay, nor do the spreaders need to be straight athwartships verses possibly raked back, etc, etc. There may be extra-light weight synthetic runners added to the tip of the aft jumper strut to counter the mizzen’s head from adding a torque force thru the aft strut to the mast at the hounds. And there could be other rigging additions as well.

    What is needed is a ‘mapping of the rigging forces’ in this rig design, which might lead to subtle modifications that would optimize this rig. Personally I do not have the computer skills to set this program up. I had hoped that an adventurous client would come along with the funds to do this stress mapping and FEA as required to optimize this rig concept.

    Is it worth the effort?? I would argue that it is a form of the ketch rig with lots of virtues for the cruising vessel, and particularly the multihull vessels. All three sails can be roller furled for great ease of handling. It should go the weather, and reach very well. It may well have more rigging members added to it than I have shown in the preliminary drawings, but it should still have less rigging than a conventional two-masted ketch.

    The single biggest concern voiced about this mast-aft rig configuration is the large amount of backstay(s) tension it will experience as a result of the geometry and the need to maintain tight forestays, and then the extra compression loading this will impart to the mast.

    This subject alone will consume more time and space than I care to give it with this individual posting here…so I will delay it for awhile till I post some other material I have collected on the aft mast subject.

    But let me leave you with the analogy that came to my mind when I considered this aft-staying subject. When I looked at the profile view of the upper portion of my mast aft rig I saw what appeared to be an analogous situation to the athwartship staying of a mast by conventional spreaders and shrouds. My aft ‘jumper strut’ was acting as a spreader, and my masthead backstay was the upper shroud wire….and like a conventional rig, my lower backstay was a ‘diagonal’. The genoa’s headstay pull was analogous to the aft/sideways force of a twisted-off mainsail headboard, and the cutter sail headstay loading could be considered analogous to the headboard pull of a conventional reefed mainsail. These forces can be accounted for in a conventional rig, so why not with my aftmast arrangement??

    In fact I can utilize a larger ‘cap shroud angle’ for my upper backstay than might be considered prudent with a conventional rig as I don’t experience the same degree of torsional instability that a conventional rig might experience. A larger ‘backstay angle’ could possible represent a smaller backstay load.

    Oh well….more later. (sure would be nice to do a three dimensional space frame computer modeling of the rig where lots of the variables could be changed around to arrive at the most ideal form of this rig).

    Attached Files:

    Last edited: May 1, 2008
  9. brian eiland
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    brian eiland Senior Member

    Ketch Rigged Catamaran

    …excerpts from Chris White’s sailing report aboard his client’s Concept 63, ketch rigged catamaran design HERON
    Sunday, October 25, 1998, I joined Bill Shuman owner/builder of the new Concept 63 catamaran HERON and crew Joan Welsh for a sail down the east coast of the USA.

    ……text break…..

    Immediately, I was struck by the way Heron slides along at 10 knots in relatively light conditions. Our speed varied from 9 to 11 knots sailing close hauled in 12 to 14 knots of true wind speed with a rolling swell. During the first night the wind shifted to NE and fell away to near calm so we motorsailed into the next morning. Our first days run was about 230 n.miles.
    As we neared Cape Hatteras the wind built stronger out of the northeast. For a while early on day two we had 25 knots of wind astern with waves of 6' to 8'. These waves were large enough to surf and we had a number of nice rides producing 16 to 18 knots of boat speed under full working sail. The Alpha autopilot did a fine job of steering but was not quite as good at catching waves as an attentive helmsman. In a ketch rig it pays to sail a very broad reach rather than a dead run since this prevents the mainsail from being blanketed from the mizzen. This we did and the sails were drawing well with only an occasional backwinding of the jib. The jib was snatch blocked to the rail giving a nice wide lead while main and mizzen each used a combination vang/preventer led to the leeward rail which provides the sail control of a 28' long traveler without the weight and expense.

    ……text break….

    However, we had to jibe back to fetch Diamond Shoals 30 miles to our south but did so a little too early. After dark, with dinner in the oven, we strayed back into the Gulf Stream. The water temperature immediately shot up to 83 degrees! Over a period of 15 to 20 minutes the waves grew to 10' or so and steepened such that tops were falling over and the wind built to near gale force. Wow, that was quick! It was fun to let Heron strut her stuff for a little while, and strut she did with prolonged surfing rides generating 20 knots or more of speed. But it was also getting a little raucous below decks and with a full mizzen Heron was developing a lot of weather helm. We decided to get some sail off (double reef in the mizzen and rolled up about 20% of the jib) and jibe back to the SW to get out of the worst of the current

    ……text break….

    Now that we were around the corner we were able to head more toward the west which brought the apparent wind up closer to the starboard beam. Heron loved this! With the wind direction NNE at 20 gusting to 25 we took off on a beam reach at a steady 14 knots occasionally reaching 17 in the puffs. The moon was bright, the wind now cold since it was coming off the land rather than the warm Gulf Stream and we were streaking along dry and comfortable with Cape Lookout 70 miles ahead but getting rapidly closer. But in the wee hours the wind once again fell away. Near Cape Lookout we finally gave up pure sailing for motorsailing at nearly 10 knots by running a single engine at 2300 rpm with light wind on the beam.

    ……text break….

    Late in the day the breeze came around toward the SW and gradually built in strength. This was a great opportunity to see Heron sail upwind. We strapped the sheets in tight put both daggerboards down, set the autopilot and watched in awe as she powered up past 10 knots to 11.5 hard on the wind with full sail in about 18 knots of breeze. We had a wonderful fresh yellowfin tuna dinner in the main saloon watching the sun set while Bill's beautiful new machine devoured the miles toward Georgia. Under autopilot we were barreling along upwind enjoying our meal and spectacular view at the same time Joan's nearly full wine glass rested peacefully on the smooth table top without a ripple inside. Before dark I had a good chance to look at the masts for movement. With about 25+ knots apparent and full sail the rig was very stable. The masts were very straight and the leeward rigging still reasonably snug. A small amount of movement was seen in the mizzen masthead but this is to be expected with a long cantilever masthead. Most cats suffer from too much headstay sag which makes windward sailing less productive or requires a running backstay to remove. Heron has no running backstays but her rig is so efficient and stable due to the wide chainplate spacing and resulting large shroud angles that headstay sag is very minimal

    …….text break

    The NOAA forecasters had changed the predicted wind direction 4 times in the last 12 hours so the next wind was anybody's guess. My guess was that because the only wind direction that they had forgotten to predict was southeast, the wind was surely to arrive from that quadrant. Well, by mid afternoon we were having a beautiful sail with 12 knots of SE wind, beam reaching along toward the Sea Islands of Georgia where Bill and Joan planned to cruise a few days and I planned to depart.
    As evening rolled into night we saw some of the nicest sailing that I have ever had the pleasure to enjoy. The moon was nearly full, the wind a gentle breeze from the port bow and the ocean absolutely flat. By now we were far enough south so that it was warm. I spent hours of my watch sitting in the trampoline near the windward bow watching the slender hulls slice cleanly through the water at a steady 8 to 9 knots
    The wind, while very steady in direction became progressively lighter. By 3 am it was 7 knots by my best estimate (no wind speed instruments on board). It is always hard to evaluate performance of a new design without having a boat of known ability sailing alongside but these were ideal conditions to see how Heron sails to windward in light wind. By recording GPS and knotmeter speeds and headings over several minutes and averaging the readings I was able to get consistent results with little data scatter. We also tried several daggerboard settings and found that in this light wind it seems that Heron's best windward performance was obtained by having only one daggerboard fully down. Our best upwind VMG seemed to be at 5.93 knots boatspeed at an angle of about 53 degrees to the true wind. Pretty respectable for a conservative ketch rigged cruising cat in 7 knots of wind

    ……text break….

    It was a great sail and very instructive for me. We saw a variety of conditions although the weather was generally light for the trip. Heron, with her long fine hulls, covers ground very well. I really like her rig, which although modest in size, is efficient and easy to handle. Going upwind in stronger conditions I had complete confidence in the spars which are extremely well supported by the long swept spreaders and efficient shroud angles. Light air performance was the big surprise. I knew that she'd be fast in a breeze but I did not fully appreciate how well she would sail in light air. This feature I ascribe to her more slender-than-normal hulls which are just so easy to move through the water.
    Heron demonstrated that the catamaran Achilles' heel, underwing clearance and related pounding, could be dealt with successfully
    . We saw (actually felt) a few kicks to the belly in the sloppy conditions rounding Cape Hatteras but they were less frequent and less severe than most cruising cats that I've sailed. Sailing upwind in waves there would be a rumble of water noise every now and then as a wave top was mashed between the hull and wing intersection but it was easy to ignore. For her weight and overall beam the C-63 design has fairly generous underwing clearance. But it seems that the larger advantage is in her slender hulls which create much smaller (almost non-existent) bow waves. It seems to me that that hull waves are often responsible for a lot of the underwing slamming as they cause existing seas to peak upward at exactly the wrong time as the lowest part of the wing passes over them. (BE noted: my observations as well)
    Another issue of importance in a cruising cat is no wind, or very light wind. Racing boats are disqualified if they use the engine. Consequently boats designed to race (and the cruising boats that emulate the racing designs) have sail plans optimized for light air, which are often too large and too fragile for offshore cruising. Cruising boats, on the other hand, use the engine when the wind quits. And the time spent motoring, or motorsailing, is often quite significant. The term motorsailer has had negative connotations for decades. Normally motorsailers neither SAIL nor MOTOR very well. So they've been viewed with some disdain as neither fish nor fowl. But I view the Concept 63 design as a motorsailer that works. Her power performance with twin 50 HP diesels is quite good with 10 knots average speed at an easy 2750 rpm. Fuel consumption is very moderate and she achieves about 5 miles per gallon at 10 knots (typical for catamaran power boats is 3 mpg or less). But the real benefits happen when there is some wind too. Running one engine often is all that is needed to bring the apparent wind forward to make the sails work harder and the combination provides much better results than either motoring or sailing alone. And of course when there is wind you can shut off the noise makers and enjoy superb sailing at faster speeds than any reasonable engine could provide. I know that there is now considerable interest in power catamarans, with all the builders coming out with updated models. But honestly, there is nothing in the world quite as nice as shutting off the damn engines and SAILING.
    In terms of weight or cost Heron is no more boat than the typical 50' cruising cat, nor does she require any more effort to sail. But by drawing out the hulls to 63' in length substantial benefits are gained in performance and comfort. This combined with her 3' draft, the ability to pass under 65' bridges and excellent performance under power make her an incredibly versatile and pleasing cruising boat.
    Chris White
    Chris White Designs

    Brian added: I agree wholeheartedly with all of the above !!!

    Attached Files:

  10. brian eiland
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    brian eiland Senior Member

    Diomedea Exulans

    Unusual but practical…much original thinking on the part of both her owner and her designers.

    Designer’s Comments:

    Diomedea Exulans is the result of a joint effort on the part of a yacht owner and a firm of yacht designers to venture beyond conventional concepts in order to achieve results not attainable by conventional means. Both believed that the features incorporated in the dsesign were dictated by sound reasoning, and several seasons’ experience with the boat appears to confirm this belief. This is a point that requires emphasis at a time when a great deal that is unconventional around us is intended largely to surprise or amaze…yachts as well as yippies.



    Owner’s Comments:

    THE GREAT WANDERING ALBATROSS, Diomedea Exulans according to Linnaeus, long puzzled scientists by sustaining flight without movement of the wings. In the light of modern analysis the bird is seen to accomplish a wonderously elegant maneuver based on his extraordinary aerodynamic efficiency—which only “appears” to be perpetual motion (But it surely does look like it until a vector analysis is made of the wind velocity gradient). Modem sailors are much more puzzled by the vessel which bears this name. She is, perhaps, best described as a “staysail ketch.”

    Her arrangement was devised to reduce—or eliminate work on the physical limitations, and to take advantage of the aerodynamic efficiency of sails with wire leading edges (having no spar at the leading edge)—this efficiency to compensate the penalties for high freeboard and great beam. At the same time it permitted all sail management, including hoisting, lowering and reefing to be accomplished from the cockpit leaves the cockpit except to “unreef” or to get a gasket on the mainstaysail after lowering.

    The whole rig lays down on the pulpit and slides aft to stow. One man can do this jo two are better. Two men are required to raise the rig; shipyard facilities are not necessary.

    In answer to the inevitable question as to the positions and attitudes of the masts: The main mast is stepped far aft to eliminate overlap and permit the sail to ‘be self The arrangement of the mizzen puts the sail on vertical “jackstay” and eliminates any sail over the cockpit.

    There are a number of collateral features which tend to reduce manpower requirements, hut equipment and finish are very simple: contrary to rumors, there are no power winches or auto-pilots. The result is a very roomy and easily managed cruiser only requiring help in picking up the mooring in a breeze. Otherwise the owner considers her a singlehander—for a slightly younger man.

    Vice-Admiral USN (retired)
    Seawanhaka Corinthian Yacht Club,
    Oyster Bay, N.Y.

    Attached Files:

  11. Alexluk
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    Alexluk New Member

    Hello Brian,

    I'm new in this forum, but I do search a lot about catamaran. After reading and comparing lot of different designs, I perfer to have a catamaran which can be easy handle.
    According to your idea and concept of ‘single-masted ketch’ , I try to build by first catamaran, and I should to express respect to you; in the below is the blog of my builder.

    I may become a genie pig, but I think besides the theory, a real experiment can provide more improvement in sail boat.

    I hope I can share it with you and give my input to the ‘single-masted ketch’ design.

  12. yipster
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    yipster designer

    a great essay and i do like that motorcat sailing design
    for some more rig ideas look in my gallery
  13. brian eiland
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    brian eiland Senior Member

    Hello Alex,
    I tried to send you a private message but you left no email address on your 'profile'.
    I'm going to wish you the best of luck with your project, but let you know I'm not pleased that ***** has chosen to act as though this idea of a aft-mast is his own with no reference to any other sites discussing this idea.

    I will also make you aware that I've done some research on Thai building, particularly since I recently married a Thai lady and intend to retire to Thailand. and I will make you aware of some concerns in a private email.
    Regards, Brian
  14. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Sails in Combination

    ...I thought it might be interesting to revisit this portion of Paul Bogataj's paper

    Each sail by itself is much simpler than the combination of a foresail and mainsail as in the sloop rig. The sails are operating so close to each other that they both have significant interaction with the other. The most interesting feature of this is that the two sails together produce more force to pull the boat than the sum of their forces if they were each alone.

    Earlier, upwash was identified as the increase in flow angle immediately upstream of a wing. There is also a corresponding change in angle, called downwash, just behind a wing, where the flow leaving the wing has been turned to an angle lower than the original flow. This is the cause of the well known “bad-air” that a boat just to windward and behind another boat experiences.

    The mainsail of a sloop rig operates in the downwash of the forward sail, causing the flow angle approaching the mainsail to be significantly reduced from what it would be otherwise. This decreases the amount of force that the mainsail produces. The observed affect commonly referred to as “backwinding” is partially a result of downwash from the foresail, but is also due to the higher pressure on the windward side of the genoa being very close to the forward, leeward side of the mainsail, causing the flexible material of the mainsail to move away from that higher pressure.

    The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail. Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity (lower pressure) and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses.

    There is a converse affect to a windward boat receiving “bad air” (downwash) from a boat ahead and to leeward. A leeward boat gains additional upwash (“good-air”?) from a boat just to windward and slightly behind that acts like a lifting windshift until it moves ahead of the windward boat. This is the same phenomenon from which a foresail of a sloop rig benefits.

    Another consequence of the difference in flow angles that the two sails experience in each others’ presence is that the mainsail must be trimmed to a much closer angle with the boat’s centerline than the foresail, which is able to be trimmed to a lead position well outboard. This angle represents the difference in upwash on the foresail and downwash on the mainsail due to each other.

    On a masthead rig, where the forestay is attached to the top of the mast and both sails taper to basically zero chord length at their heads in a similar fashion, the interference effects of the sails on each other are similar along the entire height of the mast. The mainsail ends up being rather tightly trimmed all the way up becauseof the genoa’s downwash, and the genoa gains from favorable upwash all the way up.

    A fractional rig has the more complicated characteristic that the top of foresail is not as high as the top of the mainsail. This means that the top of the foresail is very close to the front of the mainsail at a height where there is still an ample amount of chord length in the mainsail. As the foresail luff approaches the mainsail luff, the upwash on the foresail due to the mainsail increases, because the low pressure behind the mainsail has more affect the closer the flow gets to it. This causes the top of the foresail to experience even more upwash and contributes to a fractional rig’s foresail being trimmed more twisted than a masthead rig’s foresail.

    The top of the main on a fractional rig extends well above the foresail, leaving the upper portion of the mainsail free to experience the apparent wind without the downwash interference of the foresail. Apparent wind toward the top of the mast comes from a much higher angle, so the mainsail above the foresail experiences much higher wind angles than the lower portion of the mainsail where the genoa is causing substantial downwash. This change in flow angle with height on a mainsail is quite dramatic with a fractional rig and leads to trimming a fractional rig’s mainsail with more twist than a masthead rig’s mainsail

    ...and a couple of flow sketches (please excuse their older rough nature)

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  15. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Brion Toss, on 'Mizzens'

    ….wandering thru a library the other day I happened on a few books from which I extracted some interesting passages. This one is from Brion Toss’s book, “The Complete Rigger’s Apprentice"

    A Portfolio of Rigs

    If there’s one thing that characterizes rig design, it is endless variation. Riggers and designers take the apparently simple task of holding a mast up and render it in more, and weirder ways than you'd ever think possible. With such a profusion of structures it can get confusing out there when you're trying to make decisions for your boat.

    The good news is that sensible variations are responses to sensible considerations; hull type, climate, sailor's temperament, and other factors inform how a finished rig looks. So if you understand those factors you'll be well along in understanding design. The following portfolio is intended to illuminate design decisions, and to show some (mostly) appropriate results. Soak it up, and then turn new eyes on your rig.

    In this sloop-happy world, mizzenmasts don’t get a lot of respect. Ketches and yawls generally don’t go to weather as well as their single-masted cousins, and so are viewed by many sailors as inefficient—that is, by those whose sole definition of “efficient” is “able to tack through 70 degrees.”

    “The elaborations of elegance are at least as fascinating, and more various, more democratic, more healthy, more practical….though less glamorous….than the elaborations of power.” Wendell Berry

    But a mizzen can be more than just an extra mast. It can be evidence that designer and owner have decided that versatility and comforting redundancy offset a loss of absolute weatherliness. That the expense and complexity of an added mast is off-set by reduced size, expense, and labor-intensiveness of the mainmast. That any inconvenience and clutter,…the mizzen of a ketch does sit right in the boat's busiest work area…can be more than offset by a center of effort lower than that of a comparable sloop, by less sharply focused hull stresses, by a more versatile sail plan, and by increased power on a reach. This last reason is why so many of the vessels in the most recent Whitbread Round-the-World Race were ketch-rigged.

    Because a small (under 33 feet or 11 meters) sloops and cutters already have relatively easily handled sails, mizzens are most appropriate on larger vessels. Crew laziness or non-agility, or a particularly large sail plan might justify a mizzen on smaller vessels.

    Regardless of vessel size, a mizzen always presents a challenge in rig design. How do you stay it adequately without interfering with the main? With few exceptions (see Sundeer below), there isn’t room between the mizzen-mast and the main boom for a forestay. There often isn’t even room for much of an angle on the forward-leading mizzen shrouds. And because the mizzen is so far aft, there’s also rarely room for a backstay. Designers have risen to these and other mizzen challenges with varying degrees of success. What follows is a spectrum of configurations analyzed for interrelationship.

    [...text on other designs from original book omitted here ]

    Yacht designer and world cruiser Steve Dashew brings mizzens into the New Age. There’s a high-aspect, double-spreader, intentionally bendy rig on his evolutionary ketch Sundeer. And there’s even a forestay and backstay, details more commonly associated with mainmasts.

    Modern details aside, this mizzen has a lot in common with the ones mentioned previously. Like them it’s a place to hang a staysail for reaching power, makes for a lower center of effort than a sloop of comparable sail area, and is part of a versatile easily-handled sail plan.

    But there are two other important mizzen virtues that Sundeer in particular exemplifies. One, mentioned briefly at the beginning of this essay is the mizzens helpmate-relationship with the main. Sloop proponents talk about a split rig’s ‘inefficiency’, then usually go on to how having a mizzen means you have to buy a whole extra mast, sails, and rigging. They admit only grudgingly that a ketch or a yawl might be easier to handle or more versatile. And they never mention that the main on a ketch can be much smaller and cheaper than it would he if it had to absorb the mizzen’s sail area. Nor do they take into account that the mizzen prolongs the main’s life by reducing the intensity of the cyclic loading that contributes to metal fatigue. On Sundeer the mizzen is over half the size of the mainsail. This is a big mizzen (20 to 40 percent of main is more typical) for a ketch. But any appreciable mizzen is a lot more than an extra mast stuck in the back of the boat.

    The other mizzen virtue has to do with the relationship of the mizzen to the hull. By distributing stress over a wider area, a split rig is kinder to its hull than a monomast. With many boats, this distribution advantage is qualified, since mizzens, at least on ketches, are often reefed or lowered first when the wind comes up, letting the main to deal with heavy weather. This is sometimes done because main and staysails provide more drive than mizzen and staysails, but most often it’s because, on most vessels, weather helm increases sharply with increased heel. Mizzens, being so far aft, only exacerbate weather helm, so down they come. But this is a design flaw in hull, not sail. A balanced hull like Sundeer’s does not suffer hull-induced weather helm as it heels.

    And on Sundeer, Dashew has gone a step further, intentionally matching hull and sail plan so that there is always a great deal of weather helm, all of it mizzen-induced. On most vessels this would result in a hard-to-steer boat, but Sundeer has a large balanced spade rudder, so the helm always feels neutral. Why do this? Because a big, properly shaped balanced rudder can provide lift, just like a keel. If it can provide enough lift, you can make the keel smaller and still go to weather well. So Sundeer’s rudder is helping the keel, just as the mizzen is helping the main. The net result is that this 67-foot LOD ketch draws only 6 feet loaded, yet will outpoint many sloops, especially in a breeze, when speed gives the rudder more lift. Balanced spade rudders are generally frowned on by cruisers as fragile, vulnerable things, but Sundeer’s is built around an 8-inch diameter rudder shaft (!), and has a sacrificial “crushable” bottom; it’s extremely unlikely that even a violent grounding would cause significant damage. (Brian’s note: Hot idea, loading up the rudders like we did with asymmetric catamaran hulls to reduce leeway !).

    It is unusual to have rig and hull so creatively interlinked, but it’s possible to optimize the performance of any split rig relative to the hull it sits in. On some boats this might involve flatter- or fuller-cut sails, adding a bowsprit, changing mast rake, etc. A qualified rigger or yacht designer can help you with particulars. Meanwhile, I hope this section has given you enough information to extrapolate from, whether it’s for a configuration that will allow you to disconnect a springstay, or to let you see force relationships more clearly, or just as an introduction to the next section.

    Mainmast are more than just great big mizzens. The loads they bear are a whole other order of intensity and complexity…. continued in book

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    Last edited: May 31, 2008
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