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

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

  1. michael pierzga
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    michael pierzga Senior Member

    This crazy reactive rigging dream has so much junk in the air that each morning you will have to sweep up all the dead seabirds who collide with it at night

    Ive notified Greenpeace.........
     
  2. pbmaise
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    pbmaise Senior Member

    Safety not speed. Did I mention that?

    Aloha All

    CT 249
    Thank you for your detailed review of my thread.
    I took particular note of the line you wrote that read:
    Jib-only rigs or mast-aft rigs have been tried many times over the decades by legends like Sherman Hoyt with the 6 Metre Atrocia, or Dick Carter with Vendredi Trieze. They have failed to work as well as a bermudan sloop. When reality and theory collide, reality wins.

    How do you define failed to work as well? For most on this site, it is the fastest boat that defines success. I looked up Vendredi Trieze. Dick Carter was a solo sailor attempting a record using three jibs on a huge sailboat. That doesn't sound like a cruising sailor to me.

    After all these years of development, the fact that cruising sailors crossing oceans still regularly break the mast on a Bermuda sloop rig tells me there is room for improvement.

    My multihull is so big it just barely fits in the ballroom of my old home. LOL I thought that should lighten things up a bit. Doesn't your home have a third floor ballroom? Mine did. I tend to buy big things. The loads on lines for a 65 trimaran with a 79 foot full baton main sail, are more than just a little bit higher than most cruising boats. I suspect the friends you mention do not sail a big multi-hull. If they are, I really want to know if they make windward progress when reefed on the third reef when fully loaded for cruising.

    There are several ways to define if one rig is better than another. Compare mine to the Lagoon 620. Hands down I think the 620 would be faster to the finish. There was one here on Cebu Island during the super-typhoon and the mast on that boat stood too. However, it must be pointed out that the winds it experienced were far lighter. Now, what about maintenance costs, initial expenses, and number of crew required to safely reef. You can't even reach the end of the boom on a 620 without leaning way out over the lower level.

    The scientific article showing an aft-mast rig with two jib sails performed better in a detailed wind tunnel test. Isn't attaching well to this post. You may have to Google search for WIND TUNNEL AND CFD INVESTIGATION OF UNCONVENTIONAL RIGS

    This research speaks directly to the question of this thread.

    Again it concluded that despite the naked mast of the aft-mast rig, that overall the two jib versions performed better. According to scientific research the supposed extra drag of a naked mast, even a non-rotating mast wing mast, wasn't a big problem.

    Regarding wind surfers, I thought being lifted 100% out of the water and going air borne was part of the fun. Certainly they need to maintain contact with the water to have drive in a particular direction other than downwind.

    I think must cruisers would be quite happy with lots of lift from their sails, and YES I will join the camp that says assymetric spinnakers provide lift. Provided the foot is allowed to travel high enough above the deck level.

    You write an excellent report on Bermuda rigs have advanced in speed. Again, and I say over and over again. I would rather make slow windward progress, play the piano, and sleep comfortable knowing any of my crew can single handed tack, or reef without leaving the cockpit.

    You harp against the crab claw sail as being slow failed in speed trials? So?
    Again you only demonstrate that speed is your game. I'm fine with that.

    I've been on deck enough in a squall and being attached with a harness isn't what I like to do. That is exactly what a big Bermuda rig would force me and a crew to do on a big multi-hull.

    Remember the only real speed trial is the one of life. I almost laughed in the face of that famous Frenchman who just got demasted yet again. He was trying to be nice and helpful and encouraging with me. He said it would be no problem for a sailboat my size to handle a 40 meter carbon fiber mast.

    michael pierzga
    How did you know my nickname? In the Marshalls they termed me Crazy Phil for being determined enough to sail all the way to Thailand using street light pole purchased at the salvage yard. Not once during the trip did I feel unsafe, and I made 11 knots on that light pole. Somedays were 180 nautical miles made good.

    I may be Crazy Phil, but this business of watching lee stays go loose will vex more people now that they found that someone solved it. They are going to be wondering, is it a good idea for all that load to be on my weather side?
     

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  3. CT 249
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    CT 249 Senior Member

    Who needs to "solve" the issue of lee stays going loose? Lots of boats sail around with them loose and do perfectly well. In fact in some highly competitive classes I've sailed, we have found that the boat goes fastest when the rigging tension is adjusted so that the lee shroud is slightly loose at all times.

    While this is racing boat experience and it does vary from boat to boat, it demonstrates that there is no big problem with lee shrouds that are loose. And if loose lee shrouds are a problem then there were already known solutions.

    Just to repeat - the reason that I refer to racing experience is to demonstrate that racing sailors ARE aware of mast-aft rigs and have been open-minded enough to try them, and secondly to demonstrate that the claimed aerodynamic benefits do not appear to occur in real life.
     
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  4. groper
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    groper Senior Member

    Phillip,

    Noone is debating whether or not a mast aft rig might be easier to sail, better for cruising, or any of the points you raised earlier... this thread was started, by you, purely about whether the naked mast vs a mast with sail attached had a better lift to drag ratio. The explanations have been made, the information given, and there is not 1 peice of evidence that suggests that there is the possibility of less total drag with a naked mast type of rig, whether it be mast aft or any other type of naked mast rig.

    You called the bluff... well there is no bluff, ALL evidence backs up the notion that less drag is possible when using a rotating mast with sail attached. The racing results and world records are the proving grounds for which style of rig is most efficient, with highest lift to drag ratio.

    To put this into perspective for your style of cruising, think about this;

    If you had an extremely efficient rig with a very high lift to drag ratio, you would be able to cruise along gently just as you are now, but with less sail area and a shorter mast. Why run a 72ft masted rig when your not interested in speed? Why not run say, a 50ft mast, with a high power to drag ratio design, for the same net propulsive force?

    So when you look at a racing boat - yes you see a very overpowered sailboat compared to a cruising boat. This doesnt nessesarily mean its the bermudan rig causing it to be overpowered, but rather the total size of it.

    Therefore It stands to reason that, if you would like to minimize rig size for easier handling, then really shouldnt you be opting for the most *efficient* rig so you can can get *enough* power for your needs, from the smallest possible package?
     
  5. MilesP
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    MilesP New Member

    My understanding is that the spiral wrap you mention just makes sthe vortext shed consistently on one side. The spiral wrap seen on industrial flues (chimneys) causes the vortices to shed on different sides at different heights no matter what the wind direction in order to prevent the vortext shedding on one side and then the other. This needs to be prevented because if it takes place at the natural frequency of the flue, the vibration can cause failures. The Tacoma Narrows bridge was an example of the vortex shedding at the natural frequency of the structure and leading to failure.
     
  6. brian eiland
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    brian eiland Senior Member

    Drag of the Bare Mast

    I was just now reading thru this subject thread and found that surprisingly I had never entered the discussion,....particularly considering the aero-drag implications that have been directed at the bare-mast of my mast-aft rig proposal. I must have been asleep, or off doing something else :eek:

    I have long recognized that the bare-mast represents more drag (and potentially much more drag) than the situation with a sail or some other drag reducing feature attached to it. Since my mast-aft rig uses no sail attached to the mast I've had to search for other solutions. In trying to do so, and in an effort to keep it simply, I came up with a couple of ideas that I believe I posted on the forum,....but I've got to go look for them...:confused::confused:
     
  7. Kojii
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    Kojii All is remodelling

    Concur.
     
  8. daiquiri
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    daiquiri Engineering and Design

    Yes, and the helical strakes do even more. By breaking the vortex street made of long and large vortices into a one made of smaller ones, they also decrease the total energy imparted to the vortex street.

    Everything else being equal, the energy of a vortex is proportional to X^n, where X is the characteristic length of the vortex and n depends of the geometry and type of the flow. Hence, by creating smaller vortices one also creates a flow with smaller vortex drag, though other drag components might increase by introducing the spiral.

    So there is a double advantage with helical strakes (and other similar vortex-disrupting devices):
    1) smaller vortices have less drag, which means smaller vortex-induced loads on the structure
    2) smaller vortices have higher shedding frequency, hence moving the frequency of the excitation force farther away from the natural frequency of the structure.
    Cheers
     
  9. DCockey
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    DCockey Senior Member

    Do you know of any experimental or even good CFD results for helical wrapped vs bare cylinders? I have not heard of drag reduction by helical wrapping a cylinder though your rationale is plausible. As you mentioned drag due to other changes in the flow may increase enough to offset any drag reduction due to less energy shed into larger vortices.

    The helical strakes/wrapping can do more than just change the frequency of vortex shedding. It can fundamentally change the flow field so that large alternating vortices parallel to the cylinder (Karmen vortex street) are no longer shed. When the frequency of the vortex street coincides with a low order bending frequency of the structure, than not only will the structure be excited but the bending of the structure will also increase the magnitude of the shed vortices, which in turn increases the bending of the structure, and so forth.
     
  10. DCockey
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    DCockey Senior Member

  11. daiquiri
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    daiquiri Engineering and Design

    I admit to have written that part in a not very clear way.
    What I wanted to say is that the drag component due to vortex shedding is reduced, thanks to the reduction of the characteristic size of the vortices. The overall drag (vortex drag plus friction, form etc.) of the cylinder is very probably increased by adding strakes, but this increase comes in the stationary (non-oscillatory) part of the drag breakdown chart - which is an advantage in this case (smaller alternating force on the structure, which is the one that annoys the structural designer).

    Regarding the published data - it was an argument which was treated during my university days, back 15-16 yrs ago. So I don't have any printed data at hand, but I am pretty sure a Google search would yield something useful.

    Edit: just did a quick search. The very first result I have found was this one: http://www.2hoffshore.com/documents...fectiveness-at-Very-High-Incidence-Angles.pdf . At the page 2 it says:
    "The drag coefficient of a cylinder fitted with strakes is independent of the Reynolds number, that is it keeps practically unchanged when going from sub-critical to critical and post-critical flow conditions (Cowdrey and Lawes, 1959)."
    If it gets confirmed by other papers, it could be an indication that the main drag component with strakes is no more vortex-dependent, since it is decoupled form the Reynolds number. That would mean exactly what the vortex-size vs. energy rationale suggests - with helical strakes the vortex drag gets reduced and displaced by other, stationary drag forms.
    I agree, but we are fundamentally saying the same thing. It depends if you want to consider it from a purely aerodynamic point of view (assuming the structure roughly rigid), or you want to go one step further and include the aeroelasticity too. :)

    Cheers
     
  12. LP
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    LP Flying Boatman

    I think that Daquiri's post in #4 clearly shows that the is most drag associated with a bare mast. You have unfortunately thrown all masts ever built or designed into a single category. For a tear-dropped or streamlined mast section, you may actually be right, but this is best left to empirical data to concur or refute. A round section has 20 times the drag of a properly streamlined section so anything that improves the flow around a round (or unstreamlined) section is going to reduce overall drag. Or, to express this properly, a round section has the same drag as a properly streamlined section, 20 times it's thickness.

    Your desire to apply automotive aerodynamics to sail and rig design is a misplaced comparison IMHO. There is certainly a different set SORs when it come to the design of a car versus the design of a mast. To diverge into another realm of aerodynamics, how many aircraft wings end in a blunt edges that are a high percentage of their overall thickness? Conversely, how many suitcases or bags of groceries would fit into the boot or trunk of a car that was designed strictly to aerodynamic and minimum drag requirements. Try to envision the shapes of the vehicles that have set land speed records at Bonneville, to pull in an automotive reference.

    I think that stating that the kammback is an efficient aerodynamic shape is like saying the a submerged, square transom is an efficient hydrodynamic shape for a non-planing sailboat.

    Mast design has certainly progressed and there are many designs that diverge from the basic "round mast and sail" shape. Perhaps, with an aerodynamically efficient mast shape, your statement could be true, but to apply it to all masts in inappropriate.

    I think a lot of good information has come from this thread and I'm glad you started the thread. It's been quite informative with a lot of exceptional contributions. As for myself, I'm just agreeing to disagree. :cool:
     
  13. MikeJohns
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    MikeJohns Senior Member

    Remember that foil drag is a polar plot not a single fixed value and in this case it is a polar that extends from an inflow angle of say 40 to 180 degrees. Therefore an aerfoil that has a low Cd in a narrow range of conditions is hardly applicable in this case. Also structurally the stiffness requirements are going to present an equal, or even more area at any practical sailing inflow angle all of which changes any real world comparison.

    Only to windward an attached Sail (acting as a splitter plate) will certainly reduce drag by binding vortices and changing the inflow angle. But that may be compensated for by increased lift from a free flying sail particularly if the sail is relatively narrow and a lot of it's lift is reduced by the effect of the mast.

    From a very basic theoretical viewpoint, some polar plots are required to properly compare section shapes.
    Also consider that the drag vector becomes drive past 90 deg, and if you want to be thorough, an angled cylinder has an elliptical section in the plane of inclination.
     
  14. LP
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    LP Flying Boatman

    Mike,

    Thanks for the response. I don't disagree with anything that you've said, but I'm not sure how it applies. i.e. I'm not sure of the point(s) you are trying to make and perhaps my own point was vague. So just to be clear, I am in disagreement with the OP's original blanket type statements regarding bare masts verses masts with sails, but offered some legitimacy within certain boundaries. Also, I had rephrased the statement you quoted to this,

    I wasn't trying to promote the virtues of a streamlined section as much as expressing the extremely draggy attributes of a circular section.

    I think that we could include polars down to 10-15 degrees. Even with a boat that tacks to within 45 degrees of the wind, the relative wind will be much more aligned with boat centerline due to forward velocity and flow modification from additional sails. On a beam reach, depending on the boat, the relative wind may still be less then 45 degrees off of the bow. The further down wind we get, the less of a factor mast drag becomes. It's working to windward on a close reach where we see the highest relative wind velocities and the highest potential for wind resistance. This is where a foil section will do the most good, though I'm not really trying to make that argument.


    Not sure what you are getting at here. Sorry. Foil thickness?


    Agreed (Well maybe I disagree a little.), but...even with the wind at 90 degrees, the sail is still acting as a splitter plate despite it being asymmetric to the flow. It will still brake/reduce the vortex streets and by this time, we are close to running and any drag components may be helping to drive the boat.

    Absolutely.

    I was offering to the OP that there are times when a sail on a streamlined/foiled mast COULD present more drag that a bare streamlined/foiled mast. In his case, he has a rotating mast that could be aligned for least drag or even some lift production.
     
    Last edited: Mar 28, 2015

  15. UNCIVILIZED
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    UNCIVILIZED DIY Junkyard MadScientist

    Just saw this ancient thread, on my way to check something else, & Had to chime in.

    When you're talking about a conventional mast creating drag, in terms of anything resembling upwind sailing, there is ZERO denying that it definitely creates a large amount of same.
    Witness these examples:

    When trimmed properly, most of the leading edge & first 10%-20% of the "E" dimension of the sail, contribute little to any drive to the overall sail plan. And I say as much, for when a mainsail's properly trimmed, it's luff is fluttering (mildly luffing) about 50% of the time.
    Not enough so that it creates a huge amount of drag, but then again, said bit of canvas isn't contributing much drive either.

    In the '92 America's Cup, the French team actually "cut out", say a 1' or so deep section of their main, for about half of it's "I" measurement, & "affixed" said measured sail area, onto the sail's leach/roach. At that point they were not too far from getting knocked out of the standings, due to their earlier, repeatedly poor performances in the Round Robin tourney... So, more or less, they had little to lose, & at a minimum it gained them some good PR time for their sponsors via the controversy about it.

    But when you're racing at that level, you don't do things which you know will slow you down by so much as 1/100th of a knot. And they just said, "well, that part of a main's kind of a no drive contributor anyway, so let's put it somewhere where it's of use. The reason for the canvas in said locale (directly behind the mast) being a poor upwind contributor, is due to the drag shadow of the spar.
    Neat "trick", & IIRC, the rules committee quickly put the nix on modifying mains thusly.
     
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