Construction options for a 150kt wing mast?

Discussion in 'Boat Design' started by 556Geoff, Mar 13, 2007.

  1. 556Geoff
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    556Geoff Junior Member

    I'm planning to build some prototype masts this summer for some ice boats next winter. The plan is to build some wing masts for the Nite Class iceboat and to tune up between sail and wing mast designs. The Nite Class has 67'sq of sail on an 18'mast with a 7'3" boom. These masts wouldn't see 150kt apparent winds but the funding behind the project is to build a much larger wing mast from what we learn from the smaller masts. A much larger (40' mast) for a 40 long x 30wide skeeter that has been chasing the 145mph record, would be the eventual platform for whatever mast or masts we end up building. Ice boats can go from four to six times the speed of the wind and the true winds needed to get in the ball park are around 35kt!
     
  2. 556Geoff
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    556Geoff Junior Member

    The current plan is to build a two element mast 17' x 4' which includes the second part or flap. The mast to flap ratio is projected to be 60% / 40%. Also, the two elements would be attached not vented like the C Class Cats do. As I understand, venting or slotting the two elements gives the C Class rig more power to sail lower down wind, there would be no need for that here. The one-off construction plan is to build the mast sections out of shaped styrofoam with a central boxed spar inside. Skin the sections with carbon fiber than waste away the foam with acetone to reduce weight and open up the interior.
     
  3. 556Geoff
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    556Geoff Junior Member

    The traditional ice boat wing mast has been the stripper mast or bent plywd mast. The newer masts have been glass or carbon, over foam. But ice boat wing masts are no more than 18"for&aft with 5'-7' of sail behind that.
     
  4. 556Geoff
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    556Geoff Junior Member

    We can talk all day about shapes, but what is going to hold up to 100+kts of apparent wind? To add to the insanity, you can't leave anything up or the boat might take off (like the 50' Duece did a few weeks ago and run over another boat) or the mast would shake to death in those winds too. You need to design a strut to mount at the base of the mast and at right angles, that is as long as 1/2 the width of the boat (20'), that you attach a line that leads to a winch and the other side of the strute has the halyard attached to it. Still, raising a wing mast in any kind of wind will certainly catch your attention! One last crazy idea, what about putting the hounds at the top of the mast (putting the whole mast in compression)? Some advantages to this non-traditional setup are: a very clean (aerodynamic) foil, no fighting forces between where you hang the hounds on the mast and your mast rotating system, and finally all the large ice boats have tip-over wires run to the mast head to help keep the mast intact if the boat turns over and slides along the ice at 100+mph! Ice boating sure is exciting and some times fun?
     
  5. tspeer
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    tspeer Senior Member

    The structural design of a wingmast is conceptually no different than for any other mast. You have to estimate the loads and size the structure accordingly. The wingmast rotates, and that's a different wrinkle, but at 150 kt, the mast is going to be pretty well flattened out because the problem at that speed is not providing lift, it's cutting the drag to the minimum.

    The steady side force on the sail at 150 kt won't be much higher than it is at 80 kt - both are limited by what it takes to fly a runner. Dynamic gust loads will be higher in the frequency range that is above the response time of the pilot, so you may want to ratio those loads with the velocity. Once you've figured the applied loads, you can get the compression on the mast.

    The chordwise distribution of the lift between the mast and sail can be estimated using a program like XFOIL. The spanwise distribution of lift can be estimated using a spreadsheet like this one.

    Simple beam theory will allow you to engineer the structure of the mast. There have been many papers presented at the Chesapeake Sailing Yacht Symposium on mast design.

    There's really no excuse for building a mast that will flutter like that. If you ballast the mast so its center of gravity is on or ahead of the pivot axis, it will be far less susceptible to flutter. It can be left up, and it won't flutter when coming through a tack, either. Perhaps the best way to mount the ballast is to have a short, sturdy strut projecting forward just above the hounds. Both the ballast strut mount and the hounds can take advantage of the same skin reinforcement.

    If you're leaving the mast up, you might want to consider dollies like the landyachts use. On ice, this would consist of a piece of plywood with a couple of cleats forming a slot on the top the receive the runner, and it would have one or two runners underneath that were oriented at right angles to the line from the rear runner to the front runner. You'd make one dolly for each side and slip them under the runners to park the boat. This allows the boat to rotate about the front runner like a boat on water that is moored by the bow. Since the rear runners are nearly sideways, they block any forward or aft movement. Landyachts not only leave their mast up, but often leave the sail up with such an arrangement (althought their dollies have wheels, not runners!). In the morning, you'll often times see neat arcs or circles showing the yacht has weathervaned to all points of the compass during the night.

    Two sets of stays sounds like a lot of extra drag, to me. The drag of a stay is tremendous. And the key to high boat-speed/wind-speed ratios is to reduce parasite drag to the absolute minimum.

    A masthead rig might make sense for a rigid wingmast. If the mast is flexible, a fractional rig may be required to get the mast to bend the right way.
     
  6. 556Geoff
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    556Geoff Junior Member

    Thanks Tom for your advise. I'm following up on the symposium site.

    Weather vaining the boat using dollies might very well work but I think we would have to ice drill anchor the bow down, to do it.

    Your thought about center of mass being ahead of the pivot point to reduce the flutter tendency is interesting. Wouldn't just keeping the centers of area and lift, aft of the pivot axis work just as well?
     

  7. tspeer
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    tspeer Senior Member

    No, you don't, any more than a landyacht needs to be tied down and needs only the friction of the wheels.

    The front runner resists sideways motion, but the rear runners don't because they are nearly at right angles to the boat. So if the wind pushes sideways on it, the rear starts to move, but the front doesn't, which makes it start to turn. When the boat is pointing straight into the wind, there is no more turning tendency, but by then the rear runners are resisting the force of the wind, like a skater doing a mohawk turn.

    Having the aerodynamic center aft of the pivot is necessary for static stability - the tendency to weathervane. But it's not sufficient for dynamic stability. Flutter is a dynamic instability, as anyone who's seen it can attest!

    Flutter is a combination of rotation about the pivot axis and sideways motion - rotation about a longitudinal axis at the pivot. The stays don't hold the mast rigidly in position. The flexibilty of the boat - especially the plank - allows the mast to tilt a bit to leeward when there's a side load on the mast. When these two degrees of freedom couple together that's when flutter can occur.

    Imagine the mast is upright, stationary and pointing into the wind. Now hit it with a gust from the side. The mast now has a nonzero angle of attack and develops lift, accelerating it to the side. The center of lift, also being behind the pivot will start the mast rotating into the gust. But if the center of mass is behind the pivot, the acceleration of the pivot line will make the center of mass lag behind and increase the angle of attack, slowing the weathervaning of the mast. If the aerodynamic center is ahead of the center of mass (not too difficult to do, since the aerodynamic center is near the quarter chord), then the mast won't weathervane at all, initially. The angle of attack will actually increase at first.

    The sideways velocity of the mast creates an apparent wind coming from the opposite side of the original gust. The mast wants to weathervane into this apparent wind instead of the true wind. So the aerodynamic moment has dropped off compared to what you might expect at the same point in the mast's motion. The lift has dropped off, too. But the inertia of the mast carries it on.

    As the mast moves to the side, the stays tighten up, slowing the mast and accelerating it back the other way. If the center of mass is behind the pivot axis, then the mast will whip around and acquire an angle of attack driving it back the other way, in addition to the pull from the stay. The mast goes shooting past center, weathervaning, but too late. When it hits the stops on the other side, it whips around and comes back even faster. The sideways motion and the rotational motion are phased so that one reinforces the other. This is what causes flutter.

    Now consider the case when the center of mass is ahead of the pivot axis. When the mast is initially accelerated, the aerodynamic moment wants to weathervane, as before. But now, as the center of mass lags behind the motion of the pivot, it is adding to the weathervaning instead of opposing it. This adds lead to the weathervaning, compared to the previous case. The mast isn't driven so far, so when the stay tightens up, the mast comes to a stop sooner.

    But now when the tension on the stay accelerates the mast the other way, the center of mass ahead of the pivot tends to stop the mast from overshooting. The phasing of the rotation and the sideways motion is completely different. Before, the rotation lagged behind the sideways motion. Now the rotation leads the sideways motion. This helps to cancel and stabilize the dynamic motion of the mast.

    If the center of mass is behind the pivot, there will be some speed at which the mast will flutter. The lift and moment go up with the square of the wind speed, but the stiffness of the structure does not. At higher wind speeds, the structure becomes comparatively softer, changing the relative frequencies of the lateral motion compared to the rotational motion. When those frequencies converge, that determines the flutter speed. By the same token, slop in the rig will also make the rig more prone to flutter, since the slop allows more sideways motion.

    The importance of mass-balancing aerodynamic surfaces has long been known to aeronautical engineers. If you see a picture of airliners on the assembly line, the whole aicraft only has primer applied to it. Except for the rudder, which is fully painted with that part of the airline's logo. The reason the rudder is painted is because it was mass balanced before it was hung, and the balancing had to be done after it was painted so as to take into account the weight of the paint. Nearly every control surface has some portion that projects ahead of the pivot - the ballast is located there; sometimes made of tungsten because it's denser than lead.
     
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