Mast building.

Discussion in 'Multihulls' started by redreuben, Aug 18, 2012.

  1. Eric Sponberg
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    Eric Sponberg Senior Member

    Boy, a lot of words have gone across the screen since yesterday. Let's see if I can catch up.

    First, more summary of Milgram's paper. He did 2D water flow tests with a bronze, rigid sail foil with and without a wood elliptical section mast attached to it. The model was of rectangular planform, 506 mm long with a chord of 229 mm. The point of the testing was to determine how much drag does the addition of a mast section make when added to the front of the foil in an orientation similar to that of a soft sail attached to a mast. Milgram fully admits that there are many 2D limitations in his testing. I do not own the copyright to this paper, and I don't know if it is yet in the public domain, so I hesitate to provide a copy of it here. But it may be available from SNAME, and so for those of you who want to obtain a copy: It was published in Marine Technology, vol. 15, no. 1, January, 1978, pgs 35-42. Title: "Effects of Masts on the Aerodynamcis of Sail Sections." Author Jerome H. Milgram.

    The important finding from the paper is:

    "In spite of the shortcomings associated with the limited data given here, several important new findings have been made. First of all, the drag due to the addition of a mast to a sail is the result of complicated flow intereaction and is not given by the sum of mast drag and sail drag as measured separately from each structure in the free stream."

    The point is, mast drag is significant. And all during the history of mast building in the world (and I'll qualify that with stayed rigs) The notion and the practice has always been to reduce the size of the mast to reduce drag. Witness the B&R rig with its cross-staying--adding more wires to make a smaller section more rigid.

    The same principle applies to my free-standing wingmast designs. I always try to make them as small as possible, consistent strength and stiffness. And the reason for that is because wingmasts produce lift even when there is no sail attached, and customers have a real concern about how much their boat is going to go sailing around the mooring when they are not on it. If not considered in the design, it's a real problem.

    Then you evolve into the racing rigs that get more and more wing for less and less soft sail. One has only to look back at the 1988 America's Cup campaign where Dennis Conner built two identical catamarans, Stars and Stripes 1 and 2, one with a solid wing rig, and the other with a soft sail-wing rig. The solid wing rig was chosen because it was the better performing rig. But, they could not leave the boat alone on the water. Every night, the boat was lifted onto the hard and tipped over on its side to protect the wing from damage and the boat sailing around by itself. In the current America's Cup, the AC45's all have rigid wings because they are more powerful. But they also have night watches on board to control the boats every night so that they don't go sailing away or get damaged when not racing. Someone is on board or in attendance with the boats 24/7. A cruising sailor is not going to go to that effort. He wants a powerful rig that he can control, reef, and put to sleep without any extra help.

    So, getting into the area of A, B, and C class catamarans, particularly in the Little America's Cup, they've all gone to rigid wings, and really there is not comparison to a soft sail rig. I don't know the intricacies of the rigid wings because I have never been hired or asked to participate in such campaigns. The engineering problems are similar in that you are dealing with very light, high strength mast structures, and one always has to be aware of buckling phenomena as well as ultimate strength.

    As for rig proportions in the catamaran classes, I cannot comment, I am here to learn. I do report what my experience with wingmasts has been to date, and in that regard, I always strive for the smallest mast section size consistent with strength and stiffness.

  2. cavalier mk2
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    cavalier mk2 Senior Member

    Slots and slats

    Hello Gary I've seen some airfoil tests that illustrate the benefit of slots or gaps in various sections. Has anyone tried a larger standoff than track where the soft sail join the wing?

    James in extreme cases consider having a garden of weeds, its much less work! When people go mad with sprays the result looks and probably is like agent orange.
  3. idkfa
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    idkfa Senior Member

    Consider two sails of similar plan form,

    One with a small dia round mast, say 6% of sail chord.

    The second with a wingmast say 20% of sail chord, but with a thicker section, 8-9%.

    Surely the windmasted one has less drag, due to a much smaller separation bubble, on leeward side.

    Is that not the point of having a wingmast?
  4. Paul B

    Paul B Previous Member

    Ben and Eric Hall from Hallpsars have done an experiment that tells us something about this topic. They took a J90 and installed a stayless rotating wing mast. They tried multiple configurations.

    What they found was they could not hold a lane upwind compared to sisterships with conventional fractional sloop rigs. They did have running backs to offset the headstay forces, but still could not point as well as the sisterships.

    This was written up in an article by Ben Hall in Yacht Racing Magazine.

    Another interesting comparison is in the Open 60 fleet. It seems the later boats were going away from the rotating wing masts. The speed difference, if any, is not worth the complication.

    On a multihull, with much greater speed potential and a naturally wider staying base, the wing mast rigs make more sense.
  5. Gary Baigent
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    Gary Baigent Senior Member

    hi Cav, I remember Eric Tabarly trying a many slotted spinnaker (with flaps over the slots to increase airflow) and this was carried on the first Paul Ricard foil trimaran, 1979 - but in the end they dropped the idea and went back to conventional spinnakers.
    That was a spinnaker but to my knowledge, there has been no attempts at slots set in the trailing soft sail on a wing mast rig (like some kite developments).
    However brilliant Aussie Lindsay Cunningham had multiple slots in one of his full wing C Class cats, Victoria (which easily beat the US Patient Lady) and then later on Yellow Pages (which also won for a number of years until Steve Clark's Gogito arrived)... but those slots (and zap flaps) were positioned in the 50 and 75% area (rough guess) behind the main leading no. 1 element.
    Imo, the fast air area between wing mast and soft sail is exactly the position NOT to have a slot; a little further aft, okay, but not near the leading element.
    Paul, do you have figures of rotating wing and non rotating masts from the recent Open 60's? Just guessing, I think the winning-est boats had rotating wing/soft sails.
  6. Paul B

    Paul B Previous Member

    I recall the French tried a slot (of sorts) behind the mast on one of their AC efforts. The mainsail stood off from the back of the mast at each slider. I think the gap was less than a foot. Didn't seem to work too well, wasn't repeated by them or anyone else.

    I don't follow those boats too much, but I do recall Seahorse articles about the latest round of boats. The "small" boats, like Safan, were trying to save weight and complexity, so went with pretty standard fixed rigs. The other end of the spectrum was the latest Juan K boat, which I think also went with a conventional rig.

    Even in the heyday of the otating rigs wasn't the fleet split about 50/50?

    Regardless, the Open 60s are probably the best place to use a rotating mast on a leadmine monohull. They don't really go upwind anyway. So if they are not sold on the concept it is probably not going to be so useful downstream from them.
  7. cavalier mk2
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    cavalier mk2 Senior Member

    STOL airplanes with the slot near the leading edge that they close after takeoff are what got me thinking about it. Probably the soft sail join is too far aft for a similar effect. Farther back would be working like more of a flap.
  8. Gary Baigent
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    Gary Baigent Senior Member

    A leading edge canard foil out front, that is a different thing; zero attack when beating and angled, cambered up for reaching and downwind. That was tried on the French C Otip and it worked. Like an aircraft taking off or landing to gain very high lift.
    Different animal than soft slots in mainsail though.
  9. Petros
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    Petros Senior Member

    leading edge slots on STOL aircraft allow for higher angle of attack without stalling, they create very high lifts, but drag goes way up (which is desirable on a short landing). I think the best performance is best lift to drag ratio, not just lift.

    trailing edge flaps, and slotted flaps, also increase lift, but also at the cost of a lot of drag. Again, good for landing on a short runway, but not sure it would be useful in creating thrust to drive a sailboat.
  10. redreuben
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    redreuben redreuben

    Seems like the original topic has taken a lurch sideways !
    All good however I have learned what I wanted to know thanks to the many posters especially Eric Sponberg.
    Carry on !
  11. groper
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    groper Senior Member

    a GOOD read on the entire sublect can be found here __>

    Ill take this quote from it;
    If the pressure increase on the mast’s windward surface is too great, which happens at low angles of attack, the flow separates and doesn’t reattach. This sets the minimum angle of attack for that shape. Small wingmasts have a much shorter distance between the peak near the leading edge and the joint at the sail. So the pressure increase is much steeper for small wingmasts. This means that a small wingmast has a narrower range of useable angle of attack between separation on the windward surface a low angles, and stall at high angles. Of course, the mast can be rotated to help alleviate this. However, the fact remains that a small wingmast will have a narrower “groove” than a large wingmast. This will make it more difficult to trim well, and it will be more affected by changes in the local flow angles along the mast, such as from gusts or wind shear. But it may also be lighter in weight and have less drag when it’s in the groove. And large wingmasts are dangerous for offshore craft because they can’t be reefed.

    and also;


    The aerodynamics of wingmasts are strongly influenced by boundary layer separation and reattachment, forming separation bubbles on both the windward and leeward sides. Transition from laminar flow to turbulent boundary layers typically takes place via laminar separation and turbulent reattachment at the Reynolds numbers investigated (200,000 to 2,000,000). For large wingmasts, the laminar separation point moves forward on the lee side with an increase in angle of attack.

    The sharp concave corners formed at the mast/sail junction cause steep adverse pressure gradients on the mast, leading to laminar separation or turbulent separation if the flow has already completed transition. The corner is especially severe on the windward side, leading to a large separation bubble that can span most of the windward side at low angles of attack.

    Under-rotating the mast can reduce the size of the windward separation bubble, but introduces a new separation bubble on the leeward side. The leeward separation bubble can be susceptible to bursting at high angles of attack, causing an abrupt leading edge stall.

    Over-rotating the mast causes a low pressure peak at the mast/sail junction. Large over-rotation angles do not increase the maximum lift but do cause a large increase in drag.

    Reasonable wingmast shapes can be designed by reflecting the leading edge of an existing airfoil about the mast chord. At high angles of attack, small wingmasts are as effective as large wingmasts, and the windward separation bubble can mask the effect of mast size. However, large wingmasts have less drag at low angles of attack and are therefore effective for high speed craft such as landyachts and iceboats.
  12. cavalier mk2
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    cavalier mk2 Senior Member

    Yes but deploying a slot to make a mark instead of tacking could win a race.
    I'm also trying to evaluate a beach cruiser rig with a round mast and the sail set back due to the mast attachment. Like a traditional gaff rig with hoops. In this case the flow is much better than expected but the sail has freedom to move to the leeside, smoothing airflow there as it isn't locked into a track. Perhaps the gap allows the pressure from a bubble to equalize...
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    Why not to have (totally independent of the mast) tack on a horse?
  14. cavalier mk2
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    cavalier mk2 Senior Member

    Actually old fashioned boom jaws do the same thing with less hardware....there is more to some things that evolved than meets the eye at first glance.

  15. arekisir
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    arekisir Junior Member

    Relevent question,

    Most of the commercial wingmasts available in extruded Al or CF tend to be something close to a standard 4 digit NACA section.

    Mr Speer suggests that for a wingmast / softsail the actual mast section could be derived by taking a known non symetical aerofoil & mirror the low pressure side about a line that passes through the leading edge of the aerofoil. I have not explained this very well but Mr Speer does in his paper referred to in this thread.

    My questions are:
    1.What sort of wingmast section do people use when the mast is about 10% of the length of the foot of the sail?

    2. For a typical modern mainsail plan with squaretop & minimal leech curve is it worth keeping the ratio of the mast to the chord length the same, ie taper the mast along it's whole length so that it always is say a fixed % of chord. Ignore all structural issues and ability to manufacture, would this help flow across sail?

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