Rotating Wing Mast – theoretical discussion

rotating mast

Hi Charles,
If you put the mast axis near its front it will be difficult to rotate the mast when you tack. The compression from the battons pushes the boom forward, this pushes the mast trailing egde forward and "around". If the mast axis is fwd then the batton moment is large and may not rotate when you want it to. The mast will need some sort of "spanner" to control rotation. If you have the mast axis near the trailing edge this moment is reduced and the mast may auto rotate. I'd make provision for two or three mounts starting as near the trailing edge as possible for you to try and pick the best one. Or have the mast mounted on a plate (that is connected to the spanner) that has the mount so it can be adjustable. On really big rotating masts like the french tris they have the boom go the the deck at the mast base as the boom loads are so big the mast can't rotate. have you seen mast spanners? Some are as simple as a pipe on the mast connected the boom with some rope so the boom pulls the mast around, on bigger boats the spanner is controlled with block and tackle or hydraulics. I did a Google Image search for "mast spanners" and it picked up a few good images of these.

Hope this helps. Peter S

 

rotating mast

Hi Charles,
I looked at your diagram again and since you are having a round bottom section on the mast I'd make a bearing for the boom end that went around the Dia60mm section that would end at the tack of the sail and become round. At this point I'd make a bearing that goes around the mast and connects to the boom. This will allow the boom to freely rotate independant of the mast rotation. A spanner would then control the mast rotation. This will allow easy control of the boom as it will be independant of the batten forces. You have the sail in tension and yes the cloth is in tension but the dominant mast rotation loads are from the battens. This type of mast and boom arrangment has much more load on it and standard goosenecks fail. So a substantial bearing/gooseneck is needed for this type of mast/boom connection. A simple spanner like a NS14 could be used to syncronise the mast/boom if needed but my guess is that you will need a resonable spanner to control the mast.

Cheers Peter S

 

I've got the idea of the mast spanner now, from looking on google - thanks for the suggestion. I'm not so clear about what you're thinking in terms of the location and action of the boom pivot point and the influence that the batten (compression?) forces have on this. Could you possibly do a quick sketch and attach it...? I can see that if the boom pivots under the sail luff groove then the batten 'tension' can't have any effect on the angle between the boom and mast because there is no induced moment - this is perhaps the best configuration to keep things simple.

 

Hi Charles,
The Battens are under compression and push the mast forward. As the mast has to be over rotated (say 45-50degs) the mast is being held on the lee side of the boat by the batten load. Lets call this the batten torque. It is not a small force to deal with, on my NS14 you cannot straighten the mast by hand using the spanner when its under load (if the mast axis is fwd), so your boat will have much bigger loads. When you want to tack the batten torque must be overcome to get the mast to rotate to the other side. If the boom vertical axis and the mast vertical axis agree there is no batten torque. I'm not sure how to get images into the page so I'll read up on that now.

Peter

 

The pivot location doesn't depend on just the section at the foot. It also depends on where the pivot axis goes up the mast. For example, if you bring all three stays together ahead of the leading edge at the hounds, as is common practice with landyachts, then the mast is effectively raked back with regard to the pivot axis. The pivot at the base can be well aft of the quarter-chord, as you've indicated, because it gets its aerodynamic balance from the top of the mast, whose area lies behind the axis.

The most critical time for the stability of a rotating mast is when it is not being restrained by the sail, such as when tacking. So it's a good idea for the mast to be aerodynamically stable. Although very few masts are made this way, it's also a good idea for the center of gravity of the mast to be on or ahead of the pivot. This is almost impossible to achieve unless there is some ballast on a short boom extending from the leading edge - just above the hounds would be a good location. Mass balancing the mast prevents flutter when tacking. Flutter can cause a very large amount of drag - I've pitchpoled a landyacht backwards that way.

Besides the sail, the thrust at the gooseneck can have a significant influence on mast rotation, so the position of the main sheet and vang/kicking strap can also be important. I once had a scow whose mast rotation was tuned by moving the mainsheet blocks forward and aft on the boom (but it wasn't a very powerful means of controlling the mast).

If the mast rotation is induced by thrust from the boom, one way to control it is to have a spanner from the mast linked by a line to the boom itself. This allows the spanner to be pulled closer to the boom for less mast rotation or eased off for more mast rotation.

Most sailors find that they need some means of positive rotation - being able to forcibly establish the mast rotation where they want it - instead of depending on the loads from the boom. When the sail is sheeted out, it is hard to get enough rotation naturally, and when the sail is sheeted in, one often wants less. A spanner led either forward or aft from the mast can be used to control the mast with a tackle to the deck. Another approach is an arm on each side of the mast with lines led fore or aft from both sides, but this tends to get in the way of the crew.

As to how to determine the best rotation angle, many landsailors mount a small dinghy wind vane so the tail just brushes past the leading edge of the mast. The vane will typically be on one side or the other, depending on where the stagnation point is on the leading edge. Once the vane is on one side, it doesn't respond to further changes in the angle of attack because the flow is parallel to the mast near the surface. The vane really indicates the sign of the stagnation point location. When the vane is flicking back and forth more or less evenly, the stagnation point is right at the leading edge. Landsailors tend to trim so the vane is flicking a little more to the leeward side than the windward side, but that still positions the stagnation point close to the leading edge. This trim results in minimizing any separation bubbles between the mast and sail. The mast will be rotated with the leading edge past the apparent wind direction because of the upwash induced by the lift.

Cat rigs need more mast rotation than do sloop rigs because the jib turns the flow at the mast to be closer to the centerline of the hull. The mast angle for a sloop will be more aligned with the apparent wind direction.

 

From practical experience, using the 3 stays joined at the mast makes it really hard to rotate the mast in light winds, as the mechanical tension almost forces the mast back into fore and aft position.

Since light winds are a prime time to use some mast adjustment, it always seemed a poor way to do it.

 

As wing masts become a greater percentage of total chord, how is twist handled? It would seem that you need to twist the mast or have a very forgiving LE section/radius?

R

 

This is a simple and efficient way to set up a reasonably large 500mm chord wing mast. I have no trouble with light or heavy air rotation, beating or broad reaching.

 

For Mast3 on USA17, one of the requirements was that the mast section should have a stall angle of attack of 10 degrees, without the sail. This was intended to reduce the drag of the exposed mast above a reefed main, but it also had the effect of creating a certain amount of robustness to mast rotation when combined with a sail, too.

The big driver for twist is the jib. The jib makes the apparent wind angles at the foot much smaller than at the head, so it's really hard to get an untwisted mast to cover the whole range. The flow was attached on both sides of Mast3 for about most of the span, but there was a windward side separation bubble on roughly the bottom third of the mast. You just have to trim for the best compromise.

 

Yes, compromises. I built a catenary shaped trailing edge wind mast for my 32 cat Supplejack; and the differing chord shapes from mast base to peak were of course different .. and so there was plenty of compromising going on in getting the wools to fly at there compromised best. A straight luff makes things easier ... but still twist causes a compromise setting.
A 10 degree stall angle seems a very shallow angle to me. How did they do that with a thick section mast? I mean getting a good shape leading from wing mast to sail, a sweet flow line without a step, both sides?

 

I need to think on this a bit more. With a small chord mast, you can get near linear twist, as the mast chord becomes a larger percentage of the total twist in the mast becomes more important if the wing mast-sail is to have the designed profile? A jib screws this up requiring linear twist below the hounds and a different twist above the hounds?

For a practical wing mast on a smallish multi, where would you put the Leading Edge Flicker? Just above the hounds would let you trim the top 25% of the span, otherwise the stagnation point indicator would be in the slot... I can see that you might have the indicator flicking one way below the hounds and the other above the hounds?

Would changing the laminate schedule above the hounds force the mast to twist in the right direction under load? It would tend to over rotate/twist as a gust response, not sure that is what you want ...

Where is my tin foil hat and runes ...

 

Hi RH,
The more telltales and flickers the better to get an understanding of the airflow. Once you have an understanding of the airflow and what you need you can remove the telltales and flickers that are not important. So you should then only have the airflow indicators of interest eventually. The concept of having the structure respond to air pressure is called aeroelastic twist coupling (AETC). Being a mast/rig engineer I spend most of my time solving the structural issues. I think that by the time you make the structure soft enough to acheive AETC it will be too soft to function as a mast. We have to decouple the structural requirements from the aero requirements. This is what is happening with Wing Sails. We use a structural spar for the structural (elastic) requirements (max stiffness/min weight) and put a very light element that is the right shape in place to fulfil the aero requirements (max lift/min drag). The light aero structure is relatively easy to twist using spanners or internal strings etc. Many of the discussions had by sailors over the last 50 years could be simplified if we uncoupled the structural and aero issues... they will never be able to be resolved together as they are in too much opposition. Much of the resistance to change is created by the racing rules which seek to limit innovation and the rules are based on arbitary conditions not based in the physics. Box rules have corrected much of this as we can design using good science vs trying to meet geometric constraints. Our sails probably would look like hang glider wings if the rules permitted two sided soft sails and in the classes that permit wing sails they will be more like USA17 and C class cats. Given time they will become lighter and friendlier as all the great minds like on this forum work through the issues and solve them. Currently we are at the end of a poor development path and hemmed in due to the racing rules and tradition vs good design that uses good science. In the AC33 they were looking for 0.5% differences to get a technical advantage (and spending small fortunes to find and realise them) I think we are going down this path with current rigs. Hopefully the AC34 is the begginning of a new era where sailors and sailing can open up and make great gains like the car and aero industry have done in the past. I was reading a paper the other day and it stated that the theoetical max lift coefficient is about twelve (12!) So we are along way from this at present! High lift aircraft wings are about Cl=4.0 Cheers Peter S

 

Thanks. Excellent post. I've done wing design for RC Gliders so controlling twist to what was designed in was the goal. I had never looked at a symmetrical profile that needed washout to change when the airframe was inverted.

The C-Class wings that twist the first element as well as the flap make perfect sense to me. For a wing mast with jib rotating the mast above the jib independently of the mast below the hounds would be interesting except for the vortex the change on profile would create and the practical requirement for a continuous luff to raise the sail.

Getting rid of the damn jib makes the whole thing easier to design as long as you can get the area you need whilst controlling the heeling arm. This leads me to think that for comparable weight and simplicity, a wing mast uni-rig might be a good rig. If you were to have a constant chord mast, the leech taper (reduced tip chord) would have the same effect as aerodynamic washout (I think).

With no rule to limit area, you don't need high CL and the high induced drag penalty that goes with it.

If you could get a CL of 1.6 or so with a wing mast and sail without a jib for upwind, you just add area with a screecher or A-sail off the wind and you shouldn't need the high CL's that require multi-element sail plans?

That leaves the mast area that cannot be reefed and could be a problem in a slip or at a mooring if the mast was not free to rotate through 360 deg.

For the small tri I'm planning to build, a 27 ft span x 1 ft chord wing mast would leave area equal to 50% of the jib up all the time. Having to drop the rig when not sailing does not appeal to me and a mooring is not a viable option.

If I'd be dropping the rig anyway, I might as well just design a wing and call it good.

R

 

Hi RH - There are many many 27'x12" chord wing masts out there on tris that never come down so I don't think this will be an operational problem. Bethwaithe published data on wingmast/sails and regularly got Cl=1.8 so 1.6 is easy. I think triangular jibs exist because the forestay is there, the sooner we get rid of standing rigging the better! I think the cat rig with schreecher is the go and minimise or remove standing rigging. Peter

 

The W-17 is 17' on deck with a 13'-10" beam. Standing rigging is simple, two shrouds and a forestay. Going with a free standing rig would require re-engineering the basic structure so not an option. It takes a bit of fiddling to keep the CE about right with a wing or a uni-rig. Moving the step forward is not a huge problem. Having rig choices is part of the design to begin with and a feature that I wish to keep. A cut down Hobie 18 rig makes it a relatively tame boat, like the WETA. A 27 x 1 wing mast and downwind sails should make it a hot rod.

From an aerodynamic point of view, sailing biplanes up wind makes little sense. Cat rigs make more sense.

If it turns out that the boat is underpowered as a Uni-rig, I can always move the mast back and add the jib.

I think I just talked myself out of building a wing ...

R