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

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

  1. DCockey
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    DCockey Senior Member

    Comparing automobile aerodynamics and mast aerodynamics is fundamentally flawed for two reasons. First the Reynolds number for the flow around the car is much larger than for flow around a mast, everal orders of magnitude or so, with the flow around the automobile being fully turbulent for almost the entire length of the automobile.

    But more importantly the flow around an automobile is inherently 3D. Experiments have shown that the flow is inherently different than the flow around a 2D shape with the same profile. 3D shapes like automobiles generally do no have periodic vortex shedding.
  2. gggGuest
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    gggGuest ...

    I fear if you haven't read and learned from the references you've been given, but instead continue to believe the arrant nonsense you've just posted then there's really very little point in continuing the thread.

    Have you really not yet grasped that the reason why a mast with a sail behind it has so much less drag, even drive in some circumstances, is nothing at all to do with whether it approximates a tear drop shape, and everything to do with the total flow of air round the system, and in particular that vortex creation is utterly different?
  3. MoeJoe
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    MoeJoe Junior Member

    So how much performance loss is expected? If only, say 5% boat speed, perhaps a good tradeoff for a recreational sailboat if rig handling becomes much easier.
  4. MikeJohns
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    MikeJohns Senior Member

    These are the better shapes from Hoerner. I'd expect that Sailing Upwind an attached sail will very effectively change the angle of attack and the drag coeff will be close to that illustrated.

    Without the sail you need to consider your mast shape as a poor foil shape, and look at the drag component. I suspect you'll find that given the angle of attack that even an aerofoil section like your teardrop shape will be operating stalled much of the time and will have a significant drag.

    Attached Files:

    Last edited: Nov 18, 2013
  5. upchurchmr
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    upchurchmr Senior Member

    Calling "everyone's bluff" with no facts is no worth anything.
    You don't seem to be listening, but sometimes that is how things get clarified.

    How about you do a test.
    The famous aircraft designer Burt Rutan developed his early designs by putting a platform on top of his station wagon, attaching a model on the roof with a scale to measure drag and lift. He would drive it on an abandoned airfield runway.

    One person drive, one read the instruments. You can easily vary wind speed by car speed vs wind speed and angle of attack by driving across the prevailing winds, or if the wind speed is high enough, just park the car at the desired angle.

    Put up a bare mast, measure different speeds and angles.
    Put up a sail (sorry, edited from "mast") behind it and repeat the measurements.
    The mast and sail does not need to be really big, get something off a typical 16'er (and not a catamaran - those get tall).

    Show us your set up and measurements.

    BTW, whose claim are you objecting to anyway? Can you tell us the article or person who made the claim?
  6. pbmaise
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    pbmaise Senior Member

    Aloha to all

    I did indeed post who made the claim, where the thread is, and anyone can review the question in context. See my initial post and right click on the arrow next to groper.

    I have seen the claim enough to break this out into a separate thread.

    jonr - In reply to what is the point? The point is that the argument against aft-mast rigs is starting to get more heated now that manufacturers are starting to build them and offer them. They are no longer just experiments. I obviously have one, and am defending it from attack. Attack has come in the form of a claim putting a mast up without a sail behind it, has far more drag than a mast that has a sail behind it.

    Here is the critical part of the claim again.

    the drag of the mast itself is largely offset by the sail attached to it

    In other words, the drag caused by the mast, goes way down after a sail is added behind it. In most versions of new aft-mast rigs, like mine, there is no sail behind the mast. It is part of the reason I have been able to design hand-tight reactive compounded standing rigging with no lee or weather side.

    Thank you for your good comments. Yes, I am putting things to the test. In another thread I have started I am discussing the rigging I designed. To make sure what I was doing worked I built a scale model of both a Bermuda rig and the rig I designed. Scale model to me is a 20 foot high mast. Since my interest was mast compression I used a digital scale at the bottom.

    DCockey You wrote: Near the top of Bermuda sail the flow will not be close to two dimensional so my guess is there will not be vortex shedding with the sail up. Okay, that is my whole point, I don't endorse guessing when it comes to making a claim. Yes I am making a claim too, however, my claim I think is backed up by 80 years of wind tunnel testing on cars. Further, the last time I checked, the top of the mast of a typical sailing yacht is chock full of stuff that cause vortexes.

    gggGuest .. You are dancing pretty close to personal attack. However, I take it in good stead. When calling for the bluff, you must show your hand. It doesn't mean to attack the other person. Another way to saying call I call your bluff, is back up your statement with research. I did indeed follow links that people gave me to see if the research was flawed. That is exactly what I did with daiquiri and his references to flow around round masts. I showed that round masts is not in contention. A brick behind a round mast would decrease the mast drag.

    Ad Hoc - In reply to your comment. One needs a mast to sail...AFTER...the storm has past. Therefore, mast drag greatly interests me. Air flows around my mast exceed 100 mph just a few days ago. I frequently see 50 mph and plus.

    Oh, I must admit I really don't want to dig back into what a Reynolds number is. However, isn't it the same for the same air speed, same temperature, and same density? The drag caused by my mast is several thousand pounds. The worst condition my mast and rigging has to survive is when it is naked.

    philSweet Thank you as always. I want to add the third situation for bare pole mast. It is an aft-mast rig with only a big foresail. I will admit to something here. When calculating the load caused by my mast in a blow, I used the cross sectional area exposed as if it was a wall blocking the wind. To this I added g forces associated with the mast being pounded by seas. There is one sub - issue to what I just wrote. What g forces do you use? They obviously must relate to the height of the mast, and the type of sailboat. I've been slammed back and forth so hard on a 100 foot plus mast that I was bruised. This was on a monohull in fairly calm seas. On a 40 foot trimaran with a far shorter mast, the g forces seen by the mast would be far lower. I've been having trouble finding research on this area in part because there is a maker of sailboats called g force.

  7. upchurchmr
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    upchurchmr Senior Member


    Great that you made a scale model.
    Now by putting a scale on the forestay and one shroud you could publish the data for drag to support your claim.
    A little cost and time, but I would think you would want the answer, since it interests you enough to start this thread.

    I guess that means I'm calling your bluff back.
  8. daiquiri
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    daiquiri Engineering and Design


    Regarding the Reynolds number - yes, it is a number which relates the speed, the characteristic length and the viscosity. A very important number in aerodynamics, serves (among other things) to:
    1) assess in which turbulence / vorticity regime does a body operate at a certain air speed
    2) assess how similar and comparable are two airflows around geometrically similar bodies
    3) help correctly extrapolate data from scale models to full-size objects.​
    An airflow at Re=100.000 is radically different respect to an airflow at Re=3.000.000, even if the body is the same in two cases. The former one is representative of a sailboat mast at 15 kts, the latter one of a car moving at 100 km/h. Even if two bodies were identical (which they are absolutely not), the two airflows at such different Reynolds numbers cannot be directly and acritically compared.
    This is what DCockey has been trying to tell you.

    So, you have a scale model - great! Now you can perform few experiments and see where the truth (approximately) is. Just remember to scale the speed accordingly. You have to ensure that the Reynolds number of the two airflows (real stuff vs. scale model) is the same, in order to get similar airflows and comparable flow data. Let's ignore the laminar/turbulent issue for a moment.
    The speed-length relationship which has to be satisfied is:
    Vmodel * Lmodel = Vfullsize * Lfullsize​
    So if you have a 4:1 scale model, for example, you have to test it at 4 times the wind speed seen by the full-size boat. I hope the scale of your model is not too small, or you'll have big problems with laminar to turbulent transition, which might make your test model data unreliable.

    I have seen the pdf presentations of your rig in this thread: http://www.boatdesign.net/forums/sailboats/aftmast-rigs-623-38.html#post663229.
    Regardless of the outcome of this (pretty much theoretical) discussion, I want to congratulate with you for the job done. :)

  9. groper
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    groper Senior Member

    Just noticed this thread... Seems my comments were quoted, but perhaps mis communicated...

    So let me clarify pbmaise...

    My short comments need to be read in context.
    The idea is that a mast with a sail attached, ie a mainsail, presents less drag than the same sail with the same mast when the 2 are separated significantly, so as not to act as a foil and slot model.

    So imagine 2 identical bermudan classic sail plans... on the second, take the mast way out the back or out to the sides (a-frame) and support the mainsail on a vertical wire to deck level. The second configuration will always have higher drag than the conventional rig even if the masts are tear drop shapely...

    The idea surrounds mast aft rigs, a-frame rigs, and/or sails only set on luff wires - such as head sails on typical Bermudan rigs - and the mast(s) dont do anything besides provide a structural support for the entire rig. they are therefore nothing more than parasitic drag. The comments I made suggest this is typically a higher drag configuration than what can be achieved via a rotating wingmast bermudan rig, especially as the masts in these unconventional configurations rarely, if ever?, are setup to rotate and be trimmed.

    Daiquiris explanation was great and backs up the idea well.

    I have yet to see one of these unconventional rigs, which employ naked masts, that could possibly achieve a lower drag configuration than is possible with a conventional, rotating wing mast, Bermudan rig using soft sails.

    Hope that clarifies things...
  10. pbmaise
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    pbmaise Senior Member


    gropher. My apologizes to you. I should have alerted you that I started a new thread largely based on your post and not given you a chance to clarify. This was bad form of me. Again I apologize.

    Thank you for telling me what I have is rare.
    You wrote: especially as the masts in these unconventional configurations rarely, if ever?, are setup to rotate and be trimmed.

    Well meet my rig HOT BuOYS. It is a 62 ft rotating wing mast set up to be trimmed. It is easier for me to trim the mast in the aft-mast position since I don't have to leave the cockpit. The mast is just inches behind the cockpit wall. It is true that the previous Bermuda configuration and 79ft mast could be trimmed within the cockpit too. However, it wasn't possible to reef it from within the cockpit. So doing the worst weather you had to go forward and stand on the pilot house roof to reef. Reefing now is simply pulling a line from the cockpit. This raises the lower spar within the lifting sail.

    It has largely been my conclusion after trying two different aft-masts. That a rotating aft-mast is the better way to go. If the mast isn't a rotating mast, it must be held extremely tight to the deck so no rocking takes place at the base. A slight rocking of my first mast was causing edge damage at the base. This in part is what led me to keep trying to compress the mast tighter and tighter to the deck. That first attempt using an 82 foot mast was a grand disaster. Well more like a whole lot of grand disaster.

    The rotating mast allows the mast to do some rocking without damaging the mast itself. It simply moves across the ball.

    I appreciate your clarification. The term you use "parasitic drag" reminds me of trucks. Egad, here he goes again. I'm sorry I have a good memory. I recall time and again being told about something called drafting. This is where a car comes up close behind a truck and travels in its slip stream. It helps reduce the fuel consumption of the car behind the truck. The term parasitic drag reminds me of this practice as I recall truckers were complaining that cars that were doing this were causing costing the trucks performance.

    In scaled wind-tunnel tests, driving 100 feet behind a semi at 55 mph will reduce drag on your car by 40%. The drag reduction increases as you approach the bumper of the truck until you get a 93% drag reduction at a distance of 2 feet.

    It turns out to have been a myth that the car doing this was causing the truck to have worse fuel consumption. The fact is it is the exact opposite.

    According to Discover online, driving in this "free ride zone" not only save fuel for the tailgating driver, but also for the trucker, who is getting a little high pressure push.

    Lets get a little more specific and now talk about upwinding, an aft-mast rig, and a rotating wing mast behind a large lifting sail supported by the mast. The way the way I designed my rig there isn't a forestay. The lifting sail is held aloft by simply attaching the halyard to the head and lifting the sail up. There is no hank on to a forestay, no wire, etc. It is a lifting sail that doesn't require high tension to shape it. In fact it has to be slack to perform.

    The fact that on my rig the mast behind the sail is a rotating wing mast means provided I am not lazy, I should get a little drive from it.

    Now the question is it parasitic. I will agree with gropher that an A frame mast is parasitic to the boat. Anything that isn't actively helping increase the drive of the boat is parasitic. My wind generator is parasitic too.

    HOWEVER, we are largely interested in the overall performance of the boat as a whole. Time and again I have heard, and I see pictures showing, that any mast, even a rotating mast, disturbs the first 1 to 2 feet of the mainsail.

    Putting the mast behind the sail, or off to the side as in the a-frame, will increase the average effectiveness of the sail. Therefore, it is entirely possible that even a poorly shaped naked mast set off to the side or rear will help the boat as a whole perform better, since the SAIL is more effective and has a better edge into the wind.

    Now lets get back to trucks and drafting.

    The big traditional lifting sail is essentially like a big truck that creates a draft behind it. The aft-mast is like the car that sneaks up into this draft. The car is running an engine, and the aft-mast is a wing mast that provides some drive too.

    If drafting works for a truck and a car and both have better fuel milage, can something similar happen with a traditional lifting sail and wing mast behind it?

    I think that answer is a yes.

    Note: I really appreciate everyone's understanding that what I am doing is akin to reinventing the wheel. I question everything about sailing and have come up what I believe is a neat working cruising rig. I've been committed enough and spent enough of my money and time to wonder if I should be committed.

    Yes, I am almost to the point that I want to do a field test one on one against the Lagoon 620. I own the largest sailboat out here and that one is the next largest size.


    daiquiri You mentioned 15 knot winds. What are those? My rig and design calculations are all based upon super typhoon Paka. It was 145 knots (166 mph). This proved to be a good design basis since super typhoon Yolanda just hit within 30 miles of me and the eye packed higher winds. All we know is weather maps show we were hit with 100 mph plus. Trees in front and behind my boat were uprooted. I have the largest mast in the marina.

    The scale model was setup at the previous yacht club to measure mast compression. Drag across the mast wasn't examined. Remember, largely it isn't a big concern of mine since racing isn't my main goal. A sail boat that where

    Going to wind is second
    Affordability is third

    means racing for me is way down the list.

    Are all higher priorities. It is looking like it may be best for me to try and generate interest with a local university with a wind tunnel to answer the question with live testing.
  11. mij
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    mij Junior Member

  12. daiquiri
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    daiquiri Engineering and Design


    at this point you have to decide what you want to talk about. Structural design for strength or comparison of drag made by a mast vs. mast+sail?

    This is how this discussion has started, and what it was supposed to be about:
    Now you are writing about typhoons and winds of 166 mph. I am asking you at this point - now what is that?

    Frankly, I have a life to live and work to finish, and cannot afford to write a technical novel about every different and mutually unrelated issue you raise here with each new post. I hope you will understand.

    I'm out of this, good luck with your boat.

  13. DCockey
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    DCockey Senior Member

    The forward vehicle having less drag during "drafting" has been well known for many decades, and as you mentioned was the subject of wind tunnel testing several decades ago.

    The flow field of a truck with a car closely following is very, very different than the flow field of an aft mast and the sail ahead of it. In the case of the truck followed by the car the truck will have a large separated wake with recirculation inside the wake. When the car is close enough to the truck to be inside the separation bubble the presence of the car modifies the flow in the separation bubble and the result is lower drag on the truck and the car.

    In the case of a sail and aft mast the wake from the sail has an entirely different flow structure than the wake from a truck. The wake from the sail will be much thinner, will not have recirculation if the flow is attached to the sail to the luff. Even if there is a separation bubble on the surface of the sail near the luff the recirculation will not extend downstream of the sail.

    If the mast is close to the luff and in the wake of the sail the mast will modify the flow around the sail but the net effect would not decrease drag on the sail, and it could increase drag on the sail by triggering separation or increasing the extent of the separation bubble. If the mast is not in the wake of the sail then any interaction between the mast and sail will be completely different than a car closely following a truck.
  14. groper
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    groper Senior Member

    I agree with the others here pbmaise, a lot of what your considering is irrelevant or beside the point.

    The ease of which a rig can be handled is also completely besides the point of this thread, which is, the total lift to drag ratio of a sail rig. That's not to say its a very important consideration for a sailing vessel.

    Consider this, some of the worlds most brilliant engineers with access to very large budgets sponsored by the worlds richest companies, using the most advanced tools available, are employed to design the most efficient rigs for racing multihulls, to break world records, win races etc... The fastest yachts in the world all use a rotating wing mast Bermudan rig. If there was a more efficient rig, don't you think they would have found it and employed it in these yachts? You don't consider racing in your reasoning, but racing yachts and cars and trucks and and and.... Racing anything, is all about maximum power for minimum drag, which proves which rig can produce the most power for the least drag... if there is a better option, no one has discovered it just yet...

  15. upchurchmr
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    upchurchmr Senior Member


    What you say about high dollar programs has a lot of truth, but new advances comes from somewhere.
    Generally from someone willing to push and invest in their pet idea.
    Most of those ideas don't finally pan out, but the work and investigation MAY lead to another idea which becomes an advance.

    Lets not push too hard on the idea that if it hasn't been done - then it can't be done.
    Lets just ask the useful questions to test the idea. Or refine it.

    There are always new developments - where they come from is not always obvious.
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