Aftmast rigs???

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

  1. Dutch Peter
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    Dutch Peter Senior Member

    No, they're not. That's the difference between a lee or luff helm. It's actually considered safer when a boat will luff up, otherwise the boat will run away when your down in the water.
  2. brian eiland
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    brian eiland Senior Member

    Bows Driven Down

    First I would like to note to 'gggGuest' that that quote he attributed to me should really be attributed to the fellows at WindSpeedYachts whose website I got this discussion from.

    Next I would comment that he is correct that there is a moment force created by the sail force centers (other than kites) above the hull resistance centers that does tend to drive our bows under. But the contributions by the headsails can be so much less than that of the Bermudian mainsail that it may appear as a bow lifting experience. Many sailors have expressed this sensation. Even Herreshoff comented on these bow burying forces aboard his catamaran, and the lifting of the bows by the headsail.: go to and click on the 1988 heading of Herrshoff's Catamaran Be sure to scroll back to page 4.

    You might also visit another tread on this forum at "Herreshoff's catamaran reasoning"
  3. brian eiland
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    brian eiland Senior Member

    Staysail (no main) Use in Extreme Conditions

    ....just an observation....
    14 Jan 2005

    Back at Cape Horn, MacArthur went round early yesterday morning in the
    sort of epic conditions for which that feared headland is infamous. With
    winds gusting to more than 60 knots, MacArthur's 75ft trimaran, B & Q, was
    sailing under staysail only, yet was still hitting more than 30 knots of
    boat speed as it surfed down huge waves squeezing their way between the
    tip of South America and Antarctica.

    These were some of the roughest and most dangerous conditions of
    MacArthur's voyage and by the time B & Q got past the Horn, about 35 miles
    south of the lighthouse, MacArthur had collapsed exhausted in her bunk.
  4. brian eiland
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    brian eiland Senior Member

    Arvel Gentry's slot explaination

    The gentleman who most deserves credit for finally getting the explainations of sail aerodynamics corrected, and upon which several excellent books by Tom Whidden and C.A. Marchaj are based has recently updated his website to include many of the technical papers and magazine articles he wrote on the subject originally. I had mentioned his name, Arvel Gentry, in these postings previously, but I could not make a direct reference to his many documents as they were not posted on his site at that time.

    From his site, "I got involved in the technical aspect of sailing because I started racing. Reading the sailing books and magazines, I began to realize that most of what was written about the aerodynamics of sails was wrong, or certainly very misleading."

    "The explanations for how lift is generated were based on popular myths. The description of the interactions between a jib and mainsail, the 'slot effect', did not make much aerodynamic sense."

    "I was soon launched on a quest to discover how our sails really worked. Over the years this resulted in a number of technical sailing papers and magazine articles. The technical sailing papers are archived in this section of my web site. My magazine articles can be reached from my Home page."

    "If you are interested in sailing aerodynamics and how your sails work, you have come to the right place. All of my sailing technical papers and magazine articles are archived on the Technical Papers and Magazine Articles pages."

    Arvel Gentry's updated website
  5. brian eiland
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    brian eiland Senior Member

    Something different...a gentleman wrote to me recently;

    I have been interested in the idea of a forward raked mast with an
    unusual sail layout for quite some time now, originally seeing the
    idea at a student design show. The boat looked like the sailboat
    equivalent to a future speculating auto show "Concept Car", with wild
    ideas that gave little concession to practicality.

    Have a look at this futuristic design
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    FAST FRED Senior Member

    The best reason for an aft stepped mast is it will be used to mount the Flopper Stoppers in that location anyway, for comfort under power.

    Most M/S dont strive for FANTASTICASTIC WINDWARD ability , they just try for a higher than adverage transit speed.

    Modern roller furling gear would be a good gamble with large forestaysail & staysail

    and a fully battened main , as they M/S so well.

  7. scot
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    scot New Member

    Revisiting aerodynamics...

    I ran across the pro/con discussion by Brian and Tom quite some time ago, and recently the mast aft rig has popped back into my mind. I have some ideas that I have not seen mentioned before, and I thought I'd bring them up and let Brian and Tom have at them.

    To summarize, mast aft advantages:
    Staysails have less turbulence than a mast, and so generate more lift.
    Staysails on parallel stays will provide good slotted wing performance.
    Staysails are easy to roller furl, allowing continuously variable reefing.

    Mast aft disadvantages:
    Bare mast has lots of drag, not practical to streamline for all points of sail.
    Forward inclined mast generates huge backstay tension.

    So first let me address the drag issue. Why not wrap the mast in a rigid wingsail with a short chord, maybe a NACA 0012. Allow the sail to rotate about the mast 360 degrees, and balance it so that it points into the wind. This will significantly reduce the drag of the mast on any point of sail, and the rigid wingsail won't flutter and thus shouldn't need to be reefed, as feathered it will produce less resistance than a typical mast with a reefed or stowed mainsail.

    Next, on stay tension, what if you were to use an aft rake rather than a forward rake? This would put the base of the mast near amidships rather than near the stern, but it would equalize the forestay and backstay tension. It moves the COE significantly forward, but with Brian's "mizzen staysail" you could correct for that. The resulting rig is a hybrid of the cutter (which puts the mast pretty far aft to begin with) and Brian's mast aft rig, with it's mizzen staysail. Unlike a traditional cutter, the mainsail is a wingsail, and thus doesn't have the "shadow" of the mast to contend with. Since rigid windsails need to be super-high aspect ratio to be light, the mizzen staysail takes up the slack behind the mast, providing sail area lost by the high aspect wingsail.

    The disadvantage of this setup is that the mast, rather than being out of the way at the aft of the cockpit, is back near the center, but by basing it on a structural arch that formed part of the cabin roof, then it could be worked around. The advantages seem to be pretty good:

    1. The three primary sails (jib, fore staysail, and mizzen staysail) are all fairly high aspect ratio sails that can be quickly roller furled
    2. The foresails can interact with each other, and when close hauled with the rigid wingsail as well
    3. The rig tensions shouldn't be significantly greater than a standard sloop or cutter rig
    4. The rig can be sailed with one sail (fore staysail, fethered wingsail), two (jib and mizzen staysail), three (all staysails), or all four sails, if the wingsail is sheeted in.

    Despite the apparently complexity of the 4-sail rig, it should be fairly easy to handle. The wingsail can always be left unsheeted during tacking and jibing, and the mizzen and fore staysails can be provided with booms to make them self taking; the jib can overlap, and be furled partially or fully during the tack to keep it out of the way. For jibing, all sails can be quickly furled, the wingsail can rotate forward of the mast, and the sails can be quickly redeployed after the jibe.

    Any comments? It seems like this hybrid addresses Tom's issues about aerodynamics and preserves most of the advantages of Brian's mast aft rig. If anyone sees any holes in the idea, please point them out, and I'll see if I can address them. I'll also work on a sketch of the concept; I think the wingsail might have to be split at the level of the spreaders/inner forestay, and it might be the case that the upper portion is independent of the lower, but so far that's the only part of the rig that's not fairly straightforward.

  8. tspeer
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    tspeer Senior Member

    You're almost describing a cutter rig with a wingmast.

    It's not clear to me that there's any advantage to having a rotating wingmast by itself instead of having a sail attached to its trailing edge. A wingmast/sail combination provides more leading edge thrust than a staysail because of the greater thickness of the leading edge.

    Nobody yet in this thread has offered up any quantitative estimate as to the performance advantages of their design over a conventional sloop or cutter rig designed to the same requirements, or shown how there's any weight savings in the structure. I can understand the desire to simplify sail handling with roller furling staysails. But any improvement in performance is by definition a quantitative issue, not a qualitative one.
  9. brian eiland
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    brian eiland Senior Member

    Hello again Tom. Once again I must beg off on a ‘quantitative’ reply to the advantages of the headsail arrangement on my mast aft rig concept. I believe just as with the rigging load questions, a straight-forward quantitative calculation as to the superiority of the efficiency of the headsail over the mainsail may well be an incalculable quantity from a purely mathematical standpoint. So I have to rely on a variety of observations gathered from history, from pass designer’s applications, from current theorist, from real time sailor’s experiences, etc.

    So I will present a few of those observations:
    1) Lets begin with a ‘pictorial’ example. Rick Loheed recently posted these images here, “I found a post that had a reference to JavaFoil, Martin Hepperle's 'relatively simple' inviscid foil analysis program that will do multi-element airfoils. It is great for illustration purposes here. Clearly it shows the affect of the whole system- actually, as a cascade of foils. Further Aft foils must have more incidence- but when incidence is added, they help increase circulation around the whole system, increasing the forward foils effectiveness by inducing more incidence and accelerating more mass about the whole mess".
    Which sail looks most effective at driving you forward.

    2) Another JavaFoil analysis by Rick here, “Here is a comparison using Javafoil of a simple 15% camber (pretty high lift) 40% max camber location Jib/main combination arbitrarily loaded to near Max CL, and a 20% Clark 'Y' wingmast shape based on methods from Tom Speer's wingmast paper. For an input Aspect ratio of 10 for each case, this simulation shows a Max Cl of 2.11 for the main jib combo readily achieved for the combination, whereas the wingmast gets to a fairly typical Cl of 1.2 max

    2) Excerpted from aerodynamicist and North Sails consultant Paul Bogataj’s paper, “How Sails Work”
    ‘Sails in Combination’, “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.
    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

    3) Its been some 40 years ago that I did extensive study of some of the classic sailing boat design books, but I can distinctly remember how there was applied a ‘performance factor’ to the headsail of 1,3 to 1.5 more effective than its actual sail area when computing the CE of the sail plan. So even though the theory was not thoroughly understood at the time, real time observations came into the equation.

    4) Redcooprs was talking of the subject under the ‘Fraction Rig’ discussion (www), “However, the design of sailforms is very much a practical nature. In terms of sailing, the feel is that our jib supplies the driving power - and wind tunnel tests also show that it has a very large Cl compared to the main. The main on the other hand, is very responsible for the righting moment and general tuning of the boat.”

    5) etc
  10. yokebutt
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    yokebutt Boatbuilder

    I'm realizing from reading this thread and the masthead vs. fractional one that this type of rig would have the same type of "backwards" dynamic response in puffs, where the forestays sag and powers up the sails, just when you don't want it.

  11. brian eiland
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    brian eiland Senior Member

    MastAft Rig Outsells Conventional Rig

    I ran into an old friend at this year's Annapolis Sailboat Show, Rob Underwood who for years was managing director of Prout Catamarans.

    We had a few discussions, including one about the 'Prout' name brand being sold to an Asian boatyard I visited recently.

    Rob is now heavily involved at BroadBlue catamarans. As I was looking over their literature I noted that one of their models (the 385) was being offered with a standard sloop rig, and also with a mast-aft rig similar to the older Prout vessels.

    That prompted a question from me, what is the acceptance quotient of the aftmast rig compared with the std rig? Rob’s response almost surprised even me, "10 to 1 the folks choose the aft rig over the conventional one"

    I went on to ask why this might be? Taming the handling of the conventional mainsail seemed to be a primary motivation.

    There might be a marketing message here?

    Attached Files:

  12. Richard Hillsid
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    Richard Hillsid Senior Member

    I found this beuty in Brian Eiland's gallery

    Actually im quite interested on this subject and happy it popped up.

    I bought a old 100’ 100 ton small cargo ship with a old but nice ship engine. and have been toying with the idea of a sail for it, something that would give 4-5 knots and able to point 40-44 degrees to the wind and help keep out of harms way if something goes wrong with the engine.

    Also it should be able to save some fuel with favourable wind.

    No keel so plenty of leeway without the help of motor, but might give a nice ride.
  13. brian eiland
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    brian eiland Senior Member

  14. Richard Hillsid
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    Richard Hillsid Senior Member

    Brian no i wasn’t familiar with these posts, they lead me to more info thanks.

    Bear in mind im not trying to make a sailboat out of the cargo boat, its just that I have sailed my life and always trust a sail over motor. Now with this new toy im trying to be self reliant as I know I will be doing some adventures on this one too.

    The alternatives I have are a Latin rig or an aft mast, cant have anything to high or overpovered.
    The Latin rig gives me a mast forward for nav lights but requires a long boom, no problem but gives no manner of hoisting stuff.
    The aft wishbone gives nav light, radar and vhf stand, a hoisting platform and long luff I can hang some sails to give me the in this case 20 + hp. I need to keep out of harms way, with a wl of 92’ its not asking for 2 much.

  15. brian eiland
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    brian eiland Senior Member

    Aux Sail for Cargo Vessel

    Maybe you should consider a 'junk rig' or a modern square rig like Maltese Falcon. Both of these rigs could give you multiple vertical mast for hoisting stuff, and a low overturning moment.
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