Aftmast rigs???

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

  1. brian eiland
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    brian eiland Senior Member

    Wind tunnel and CFD investigation of unconventional aftmast rigs

    INTERESTING RESULTS

    I'm going to summarize some of this paper here in case it should be taken down, or disappear from the web, as many things tend to do nowadays.

    ABSTRACT
    This paper presents research activities carried out by the authors to investigate aerodynamic behavior of several unconventional sailplans,... in comparison to the sloop traditional solution. In particular an 'A-shaped' mast placed in the stern area of the yacht has been considered in single-jib and double-jib configurations. Wind Tunnel tests and performance prediction analyses have been performed in order to compare different configurations.

    ...a most interesting solution, including major potential development, appears to be the one configured on an “A” shaped mast, placed in the stern area of the yacht. In this way, what was the mainsail is now transformed into another jib (in the double jibs configuration) or even completely removed (single jib configuration).

    ....Many attempts to reduce drawbacks of aerodynamic interference of the mast on sails, both in sloops and in multiple mast sailplans, have been made in the past. On the experimental yacht Amoco Procyon by O. Harken
    .....
    (BE note: They were apparently quite knowledgeable about the Procyon rig, and several others, but made no mention of my numerous postings on the subject of aftmast?)

    So far the purpose of this research, conducted by a cooperation between Mechanics Department of Politecnico di Milano and IDEA Department of Facoltà di Architettura di Pescara, was to compare through wind-tunnel tests, a traditional sloop sail plan with a (similarly sized) “A” shaped stern mast sail plan, both in the single-jib and the double-jib configuration.



    Two complete scaled models for both single rigged yacht and traditional sloop yacht have been built and tested in the Politecnico di Milano Twisted Flow Wind Tunnel.
    The yacht model, consisting of yacht hull body (above the waterline) with deck, mast, rigging and sails, is mounted on a six component balance, which is fitted on the turntable of the wind tunnel. The turntable is automatically operated from the control room enabling a 360° range of headings. In order to correlate force measurement readings and the sail shape and in order to provide input data for CFD calculations, an in-house photogrammetric measuring system has been developed to recover flying shapes during tests.

    The traditional sloop yacht rig used as a reference is a Comet 51’a Vallicelli Yacht Design & Co 51 feet IMS cruiser-racer sailing yacht, winner of 2007 IMS Italian Championship.
    Wind tunnel tests were performed using a 1:10 scaled model of this yacht where a mainsail with the maximum IMS rule allowed roach and 100% non overlapping jib have been used.
    A-frame Rig Model Setup, ps less contrast.jpg

    single jib configuration, ps.jpg

    Single Jib model, ps.jpg

    double jib, no overlap, ps.jpg

    Double Jib model without overlap, ps.jpg

    double jib with overlap, ps.jpg

    Double Jib model with overlap, ps.jpg

    Apparent wind angles were chosen to be 22°, 27°, 32° and 42° which cover the upwind range. Tests were conducted in upright condition. For each apparent wind angle tested the first task was to determine the maximum driving force potentially achievable At the same time the influence of the sails trimming changes was observed using the data acquisition program that visualizes the forces acting on yacht model in real time.

    At the end, some runs were performed on the bare hull and rigging (without sails) for both yacht models at different apparent wind angles and in different heeling conditions in order to measure windage. These values are subtracted from each of the measured data points in order to produce the sail force coefficients.


    From a pure aerodynamic point of view the relative performance of different rigs can be compared by comparing the driving force at similar apparent wind
    angles and heeling moment. From these figures unconventional rigs seem to perform better than the standard sloop configuration.

    As can be seen at closer AWA unconventional solutions are better than the standard sloop and in particular the two jib configuration with overlap seems to be able to produce higher driving force (at dynamic pressure =1) at the same heeling moment.

    As can be seen unconventional configurations have aerodynamic centre of effort which are lower than the standard sloop and in particular the two jibs without overlap sailplan has the lowest values.

    In order to gain further understanding of the sailplans aerodynamic behavior numerical simulations have been carried out using RANS method.

    For each design scenario performance prediction have been carried out in 4-20 Knots true wind speed. Figure 24 shows a comparison in terms of optimal VMG in close hauled condition between standard sloop and
    unconventional rig with reference to both single and double jib configurations. In particular figure 24 refers to full scale case with the same sails tested in the scaled model.

    The results obtained confirm that the double jib configuration performance is better than the standard sloop up to 10 knots TWS, while the single jib
    performance is pretty similar to sloop configuration. In windier conditions all the unconventional rig solution are faster and in particular the double jib with overlap gives the best performance.

    CONCLUSIONS
    In the present paper an unconventional rig has been investigated in comparison with a standard sloop rig by means of wind tunnel tests. The traditional sloop yacht rig used as a reference is a Comet 51’a Vallicelli Yacht Design & Co 51 feet IMS cruiser-racer. Several unconventional configurations have been tested, all characterised by an “A” shaped stern mast without mainsail in single-jib and double-jib configurations. Aerodynamic data available from experiments have been used to perform some performance prediction at full scale by means of a VPP code.

    Both experimental tests and VPP calculation show that the double jib
    configuration with overlap gives the best performance and also the same configuration without overlap gives better results in comparison with the standard sloop solution.

    Numerical investigation have been carried out using RANS simulation in order to better understand the aerodynamic differences resulting from the experimental tests. Simulation results put in evidence a slat effect in the overlapping jibs configuration leading to more attached flow on the aft jib allowing for an higher pressure drop on the sailplan.
     
  2. brian eiland
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    brian eiland Senior Member

    J24 Testing Sessons

    I was just looking back thru some discussions on the 'Sail Aerodynamics' subject thread, and ran across this....;)


     
  3. markstrimaran
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    markstrimaran Senior Member

    Interesting read. Thanks for posting it. I been too busy for sailing. Cleaning up the shipyard.
     
  4. brian eiland
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    brian eiland Senior Member

    A real life such boat has been contributed to this subject thread in the past
    Orca
    http://www.boatdesign.net/forums/sailboats/aftmast-rigs-623-24.html#post415017
     

    Attached Files:

  5. brian eiland
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    brian eiland Senior Member

    I received this reply from one of the participants in this 'unconventional rig' test.

     
  6. Fanie
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    Fanie Fanie

    Hello Brian, we know this already.

    What is needed is to make up a rig and sail the daylights out of a couple of boats so it would catch on.

    I have plans to make up an aft mast rig, with a difference, but the economy is non existent.

    At this pace I'll get there when I'm 600 years old.

    LOL. The problem is, the girls will get on the boat, and not on me !
     
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  7. CT249
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    CT249 Senior Member

    If you know that, Fanie, then you must be right and every successful racing boat designer in the world must be wrong. That means that reckon you know more than Farr, VPLP, Irens, S&S, Peterson, Holland, AMAC, Reichel and Pugh, Botin, Carkeek, Bethwaite, Morrison, Bieker and dozens more, including aerodynamic experts from MIT, Boeing and other places.

    How did you get to know so much more than they did? If you know so much more about fast sailing than the world's top designers do, you must already have many championship wins under your belt. Which ones were they?

    Or is it possible that all the world's designers are actually quite smart, and that when they have rejected headsail-only rigs they did it because they don't actually go faster? And do you believe that Dick Carter, Gary Hoyt and others who tried (and then abandoned) the headsail-only rig didn't sail the daylights out of it? It's interesting to see that the latest boats by Vallicelli, the guys who commissioned the research, don't have double headsails. I suppose they can be added onto the long list of guys who don't know as much as you do about boat design. :)
     
  8. Fanie
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    Fanie Fanie

    Hello CT,

    I have never said anywhere I know more than any one else, did I ?

    But I did build a little aft mast boat and tested it. On the water, and did some experiments and did discover a few things about it. Another forum member actually sailed with me once, and had some good comments about the sail.

    Now this probably doesn't count any with any of the famous successful racing boat designers in the world - like I have to be one of them to achieve something, right ?

    So if one is not famous then one cannot achieve anything ? Is that the point you want to make ?

    Have you tried the aft mast sail ? I would like to see what you did, and perhaps learn something - if I may of course.
     
  9. CT249
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    CT249 Senior Member

    No, in sailboat design you don't have to be famous - in fact most of the greats were little known when they launched their first champion. That's the beauty of development classes - results and not reputation is what counts. What doesn't count is making huge claims that are not tested.

    Isn't saying "the mast-aft rig is superior" effectively saying that you do know more than the great designers? After all,you're saying that you know the mast-aft rig works better, but that they don't know it. That sounds like saying you know more than they do.

    With respect to your boat and experiments, you said yourself you didn't measure pointing, and you said yourself you hadn't done enough experiments. Also;

    * you were on an unusual boat, with a tiny rig which means that your tests may not translate to other boats;
    * you were on an inland lake, with very flat water, so you didn't test rough-water performance;
    * you were in fairly light winds, perhaps moderate at times, so you didn't test heavy-air performance;
    * you didn't appear to be travelling at anywhere like the speed of most development classes;
    * you never came across another comparable boat;
    * from the look of the angles and the rig (and the lack of a centreboard earlier) you didn't go upwind much if at all.
    * there was no testing about the way it accelerated, how it handled when pinched, how it gybed in 25 knots, how it could be powered up for 3 knots and depowered for 30 knots, what it was like in massive head seas, etc.

    So BY YOUR OWN ADMISSION your tests were not thorough, and from the vids it's obvious that there were other issues and that the testing you did didn't include the conditions that many boats encounter each day. Your testing cannot be compared to the testing that other people have done over many decades or even centuries, which range from the test of racing jib-only rigs across the Atlantic under sail (as Carter did), or racing mast-aft rigs in a hot class without success (as Sherman Hoyt did), or building and testing the mast-aft delta rig (as Garry Hoyt did). It also can't be compared to the hundreds of tests of sail proportion that go on each month in the development classes. It can't even be compared to the informal testing that traditional sailors (who also ignored the mast-aft headsail-only rig did over the centuries.

    So on the one hand we have your tests (which even you say were not enough) to say that the mast-aft headsail-only rig works, and on the other hand we have many much more thorough tests to prove that it doesn't work better than conventional tests. Why should we follow the unsatisfactory test and ignore the other ones?

    If the mast aft rig is so fast, why not prove it by racing the little tri in one of the many events on your dam? We who follow development classes are very, very, very easy to convince - all we need is proof on the racecourse. It's a bit as if I used some old water pipe to build an upright bicycle that steers from the back wheel, rode it around my suburb, didn't ride against anyone else or in a race, and then claimed that it proved that the Tour de France bicycle designers were all wrong.
     
  10. Yobarnacle
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    Yobarnacle Senior Member

    I am installing a low, tripod aft mast on my modified Albin 25. Hopefully launch date beginning of November.
    I'm planning balance with only a staysail to fill the large fore triangle, and additionally a hoisted to truck lateen yard, as a topsail, to increase the masts height/sail area in light airs.

    Not planning to race at all.

    I'll be happy if it sails, points, and is safe and maneuverable.
    I only hope to impress ME! :D

    Preference for the aft mast is entirely spatial, in my case. I have an aft cabin, center cockpit and only 25 feet long. An aftmast rig seems to me, a better use of available space and easier singlehanded sail handling. No need for foredeck work. Everything can be reached from the cockpit.
     
  11. Fanie
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    Fanie Fanie

    Hi Yobarnacle ,

    Never mind the negative comments. Won't you post a few pictures please, I would really like to see what you have there ?
     
  12. CT249
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    CT249 Senior Member

    Yep, negativity is bad - so why are so many people here so negative about the conventional rigs?

    There does seem to be a double standard. Conventional designs are regularity criticised here, often on false grounds - so why is it "negative" when claims about unconventional rigs are subject of scrutiny?
     
  13. brian eiland
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    brian eiland Senior Member

    I repeat again, it is not always a favorable situation for designers to 'go-off-the-beaten-path', either on a financial basis, or a reputation basis. If the current RULES don't allow unusual variations in the rigs (etc), then the boat's owner may suffer being disallowed to race, and possible even suffering a lost of resale value in his vessel. Subsequently the designer may in turn suffer in follow-on commissions to design, etc.

    So its not always a question as to whether a new idea has some viability, but rather will it fit in with the established 'norm' allowed under the rules.

    I posted a related question about some ideas that Herreshoff tried introducing to the yachting scene, and was rejected by rating rules:
    http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-46.html#post751483

    I really have forgotten whether they did or did not get a lot of time on there vessels to prove or disprove? I do recall that Gary Hoyt was really trying to sell his idea, and it was not happening fast enough to be financially viable at the time. So he shelved it and went on to other things.


    That is an interesting question, and I have written Andrea Vallicelli an email asking of his interpretation of this aero question I posted over here about the double headsail rigs:
    Slots Between Sails with Increasing Mast Height

    If he should answer, I will have to follow up with that question you pose.
     
  14. markstrimaran
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    markstrimaran Senior Member

    aft mast trailer sailor

    The reason I love an aft mast rig. Is mostly because the whole rig can can be raised well half asleep in less than 60 seconds, after arriving at a boat ramp.

    A comparable center mast would take two men 30 minutes.

    I can easily go under low bridges.

    Everybody knows that it's me from 2 miles away.

    I can drop the sails with two fingers, well reeling in a 10 pound fish.

    I can sail in crazy winds conditions and reef well after I should have.

    In light winds my sail area is huge. I can make 2 knots when others are stalled out.

    The crane function of the mast is great for loading cases of beer into the boat.
     
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  15. brian eiland
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    brian eiland Senior Member

    It's those important things......:D
     
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