I call your bluff. The naked mast drag. A myth?

Discussion in 'Sailboats' started by pbmaise, Nov 17, 2013.

  1. brian eiland
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    brian eiland Senior Member

    One of the primary reasons I set about developing my aft-mast configuration that had no mainsail at all...
     
  2. CT249
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    CT249 Senior Member

    As I recall it, the French boat didn't actually perform well with the luff set back. And that's not surprising, considering that many other people have effectively experimented for decades with having the luff aft of the mast. Outside-mast mainsail furlers were tried and proved slow. Extremely popular classes like Mirrors and Herons used to lash the mainsail luff to the mast, and that proved slow - they've now moved to luff grooves. After all, the old systems of lashings and mast loops used to allow the mainsail luff to sit behind the mast - if it had been faster why would almost every class have dumped the system?

    If the mast is such a disaster why does the CFD show that it's not, why do aerodynamicists indicate that it's not, and why is there not such a huge benefit from wing masts in most boats?

    If restraining the rotation of a Hobie mast is so slow, why do the Hobie class tuning guides - written from years of experience in tight OD racing - tell sailors that they SHOULD restrain the rotation in many situations? Why do the tuning guides for other cat classes and Tasar dinghies with wing masts say that rotation should be restrained?
    Why have wingmasts failed to perform in development classes like the Canoe, 18 Foot SKiff, 12 Foot Skiff, Merlin Rocket etc?
     
  3. LP
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    LP Flying Boatman

    Good discussion everyone. On a more basic level, I had my own small epifany as a result of participating. Once stated, it instantly becomes completely self evident. A luffing is a visually display of the vortex sheets coming off a mast.:idea: With that being said, maybe it does matter what the first couple of feet behind the mast is doing. (At uncivilized) Even a little bit of luffing is a sign of vortex development (drag) that can be reduced if the sail is properly trimmed. While this area of the sail is not producing effective lift, perhaps the sail is still cleaning up the airflow around the mast so that drag is reduced.
     
  4. UNCIVILIZED
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    UNCIVILIZED DIY Junkyard MadScientist

    IIRC, the ideal mainsail trim has things set up such that there is a small section of the sail just behind the mast, which is shaped in a small (inverted from the rest of the sail) curve to windward.
    Perhaps such a curve is the embodiment of ideal trim in the main having that part of the sail shaped thusly so that there is a modicum of flow attachment from the mast tube's air flow patterns, onto & working jointly with the main in order to optimize lift.

    And if such is the case (likely), then I think that trim wise, the fluttering of that part of the main is due to the fact that aerodynamically, naught is in a steady enough state on a boat to allow "the perfect curve" to remain in the mainsail for more than a few seconds.

    Thinking on it, odds are that if one were able to achieve & maintain such perfect trim shape, with a constant small section of the main behind the spar, smoothly inversely curved to windward, it's purpose & effect, would to a degree, be mimicking the efficiency of solid foil, wing mains. Or at least reducing drag & turbulence by a good bit.

    I'm sure that it's an easy enough to look up, aerodynamic info bit. Ditto on doing a real world test (in or out of a wind tunnel) to physically see & measure as much.
    Anyone want to dig into the AYRS archives? ;-) Assuming, that is, that someone hasn't already provided such empirical data & science, earlier on in this thread.
     
  5. brian eiland
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    brian eiland Senior Member

    Agreed,.... I also believe that the naked mast will create more drag than one with a sail attached. But is that extra drag comparable to the lost in drive by that ineffective vertical strip of mainsail area behind the mast?

    I've been on many boats where I physically tried to 'feel' that pressure difference between the leeward and windward sides of the mainsail in that area to try and determine what amount of drive we might be getting from that strip of sail area. On older fixed-mast boats I just don't detect a significant pressure difference, so I deduced that that strip of sail area is NOT very productive in providing forward drive. On most rotating mast boats I detect a very substantial pressure difference. On smaller boats with rotating mast I feel a very certain gain in performance with a rotating mast. Just go sailing on a small beach cat with the rotating mast restrained from rotating, .....then let it rotate.....tell me you don't feel the difference !!

    Agreed.

    Exactly my sentiments, why not keep the mast heights at a lower figure if at all possible. I went back to this posting of mine to borrow the image of the two rigs super imposed over one another,....the aft-mast 'ketch' & a fractional rigged rotating mast sloop rig. If I remember correctly there is a 12-15 foot difference in the height of those 2 mast
     

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  6. CT249
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    CT249 Senior Member

    "Just go sailing on a small beach cat with the rotating mast restrained from rotating, .....then let it rotate.....tell me you don't feel the difference !!"

    OK, I'll tell you - I don't feel the difference. I know there IS a difference in some conditions, but it's quite small IME - which is why some top sailors take the rotation OUT in some conditions. They are not doing it to slow the boat down!

    Can I ask again why, if rotating the mast makes so much difference, so many wing masts classes DO restrict rotation, to the stage where some of them try to take ALL of the rotation out in some conditions?

    Secondly, if the luffing of the mainsail luff is caused by the mast, why does an innefficient round mast on a Laser cause LESS luffing than the wing mast on a fast cat or a Tasar, all else being equal?

    If the luffing of the mainsail is caused by the mast, why can one normally eliminate it by dumping the jib or pulling the traveller to windward?

    If conventional masts are so bad, why did the creator of the most popular wingmasted sloop dinghy want to replace the wing mast with a round tube?

    Doesn't basic aerodynamic theory indicate that the luffing is generally caused by the air flowing over the jib, rather than the airflow over the mast? This is backed up by practical experience- on a well-sailed J/24 the mainsail luffs a lot more than on a well-sailed Etchells, yet both have the same mast section extrusion and both have similar boom lengths and roughly similar mainsail dimensions. If the luffing was down to the mast then why does one have much more luffing than the other? Surely the differences is due to other factors, such as the fact that one has a genoa and one does not.

    And while I'm at it - if conventional masts with round entries are so bad, why did windsurfers abandon the wing masts they used in the '80s and return to round conventional masts? Why did International Canoes, Merlin Rockets, R class dinghies, Z Class Renjollen, Gwen 12s, Cherubs, 12 Foot Skiffs and 18 Foot Skiffs all try and then abandon wing masts?
     
  7. Richard Woods
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    Richard Woods Woods Designs

    Rotating the mast also tightens the leech, and thus powers up the boat, so decreasing rotation is a way to "reef"

    the Firefly and Graduate both started with rotating masts but now have them fixed

    Richard Woods
     
  8. brian eiland
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    brian eiland Senior Member

    Before I reply to your entire posting I have just a couple of questions.
    In that quote above, are you saying that the sailors themselves 'take ALL of the rotation out in some conditions' ?,....or the class rules DO restrict rotation??

    Secondly, would you agree that the air flow over the leeward side of any sail is more important to its efficient development of lift, than the flow over the windward side?
     
  9. CT 249
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    CT 249 Senior Member

    The sailors restrict the rotation, sometimes playing with it gust-by-gust.

    Yes, conventional theory AIUI says that air flow over the leeward side is more important. However, reality teaches us that "cleaning up" the airflow over the leeward side of the luff is not really vitally important, as demonstrated (for example) in the fact that some cat champions take out just about all rotation in strong winds, and the fact that in other classes it's faster to have the mainsail luffing a bit than it is to change trim to remove the luffing section.
     
  10. Richard Woods
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    Richard Woods Woods Designs

    Air flow off the leech is important, no point having the whole sail stalled. As I said earlier light crews use less rotation in stronger winds to depower the rig

    RW
     
  11. CT 249
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    CT 249 Senior Member

    /\

    Yep; I'm just highlighting that if increased rotation brought the immense benefits that some claim, then no one would compromise those benefits by under-rotating in order to depower.

    Sure, as we know, rotating masts DO work, but experience seems to indicate that the advantage (even in optimum conditions) over a conventional mast is much smaller than often claimed, which leads to the conclusion that conventional masts are not as bad as often claimed. CFD and aero theory seem to back that up.
     
  12. brian eiland
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    brian eiland Senior Member

    I believe there are 2 subjects being intermingled here, a rotating mast as a leading edge component, and a rotating mast as a mainsail shaping element.

    Yes a rotating mast can be used as a sail shaping device by controlling its bend and amount of rotation to effectively flatten or deepen the shape of it's attached mainsail. So the rotating mast can be a 'complicating' factor, sometimes good, sometimes poor depending upon the user's experience and the sailmakers cut of the sail for that vessel.

    That is a bit more advanced than what I was addressing in this subject thread about a FIXED mast (no rotation) as a leading edge component to its mainsail.
    For instance here is an excerpt out of Principles of Yacht Design

    ...pages 156 & 157....Mast Interference
     
  13. CT 249
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    CT 249 Senior Member

    /\

    Yes, mast rotation does have an effect on draft as well as on leading edge flow. That has not been denied.

    The point is, as already noted, that top sailors will often sacrifice the improvement on leading edge flow, when they rotate the mast to reduce draft.

    Let's say the performance improvement created by the effect of the rotating mast on leading edge flow is "X" %. Let's also say the performance improvement created by de-rotating the mast to flatten the sail is "Y" %.

    Top sailors and sailmakers now accept that it is better to sacrifice a lot of X in order to improve Y. Therefore X cannot be a massive factor, in practise.

    Sure, wings ARE efficient in many craft, but the fact that they have also failed in so many other craft with different characteristics indicates that the advantage of cleaner leading-edge flow is so small that it is easily outweighed by other considerations, ergo the advantage cannot be as big as sometimes claimed.

    That section from POYD appears to look at the mainsail only. On this forum, Tom Speer, Mikko Brummer and (IIRC) Mark Drela have all said that the jib reduces the drag of the mast, therefore a diagram only showing a cat rig is not relevant to sloop rigs or mast-aft headsail-only rigs.





    __________________
     
  14. Zulu40
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    Zulu40 Junior Member

    that argument goes to drag
    it is far more important how the flow leaves the sail arrangement than how it enters. This is why virtually all sub-sonic aircraft are rather finer at the rear than at the front. In most cases the suction force is greater than the penetrating drag.

    There is also an interference relationship, a connection between the flow around several elements. Again in aircraft the relationship between wing surfaces of bi-plane designs or tail and mainplane; the attachment struts and wires. Each contributes to an alteration of an 'ideal' flow, making it less efficient as illustrated by the total drag product vector.
     
    Last edited: May 2, 2015

  15. brian eiland
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    brian eiland Senior Member

    How did they do that? I'm having trouble picturing it.


    (Oops, I just discovered page 5 of this discussion which I apparently have already entered the conversation. :eek::confused: Sorry about that I was reading the thread thru in an orderly fashion and did not realize that. )
     
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