Jib question

That's a rather drastic statement I think.

Lift and induced drag are uniquely related to the sail's circulation distribution in the potential flow (as rwatson pointed out), while profile drag is uniquely related to the total viscous dissipation in the sail's boundary layers and viscous wakes. There is no such clean separation of flowfield physics when working with drive and heel force components.

The clean division of flow physics between lift, induced drag, and profile drag in the relative-wind axes allows bringing in some powerful concepts like Trefftz-Plane theory and Boundary Layer theory into the sail design problem. I can't imagine designing a wing while working with force components in some arbitrary axes which have no special significance to the air flowfield. Yes, you want to know drive and heel when optimizing the boat/sail combination for any point of sail. But that doesn't mean the relative-wind axes should be neglected from the start.

 

I don’t know what you mean. The data I have referred to from Fig 4 is a comparison of the tack on CL vs the tack “to leeward”. The data from Fig 5 is the same, with the only change being the tacks have both been moved aft 6”. So comparing the L/D curves in Fig 4 is the same as comparing the L/D curves in Fig 5.

I don’t know what other settings you think are different. I don’t know if any other settings are different. If there are they were not documented.

I am glad to see you agree the report seems to indicate a flawed experiment.

What we have is a flawed report that has conflicting information. I wonder if you have looked at the Driving Force shown in Figs 11 and 12? What do you think about that data, especially in the 25 to 30 degree "upwind" angles?

What do you think the AWA will be for your boat when sailing upwind? Have you looked at those points of the graphs?

I think one of your main arguments for the tack to leeward is the more vertical headstay. I think you have stated that would increase the force.

So let’s look at an example. Say your boat has the tack on the CL. It sails along upwind in some amount of wind so that the heeling force is in equilibrium with the righting moment. Let’s say this is at a 15 degree heel angle.

Now let’s drop the tack to leeward such that the headstay is now 3 degrees more vertical. According to your theory there is more force on the sailplan. A good amount of that force is heeling force. So now the heeling equilibrium of the boat is destroyed.

To restore the equilibrium the boat would heel more. The extra heel angle would have a detrimental effect on the leeway angle. This increase in leeway will add hydro drag. Does the increase in drive from the more upright headstay overcome the increase in hydro drag?

Does the increase in force overcome the loss of VMG due to increase in leeway?

Now draw up the sailplans and look at the direction of the total resultant force for both conditions. Do you think that difference is going to help the VMG?

This starts to sound like faith and not science.


With people like Tom Speer and Mark Drela posting here I would really like to hear their opinions about the tack-to-leeward effect.

 

Good point well made. However, there is a dilemma about what to do with lift and drag data for sailing boats. With the apparent wind forward of square, you always want to minimise drag, because it resolves into heel force and negative drive - bad on both accounts. But when the wind is aft of the beam, drag produces heel and forward drive, so is a mixed blessing. Likewise, upwind, lift resolves into a heeling force and a positive driving force, but much more heel than drive.
So when thinking in terms of lift and drag, it isn't simply the case of wanting to maximise one and minimise the other, or even maximise the ratio between the two. However, when thinking about drive, heel force and the corresponding moments, it becomes easier to understand what you are trying to achieve, even if the directions are somewhat arbitrary in relation to the first principal fluid flow phenomena.

 

It may sound drastic, yes, but if you read this thread you may come to a conclusion that some drastic measures are needed to increase understanding about how the sail boat works ;-)

Yes, lift & drag certainly are important to people dealing with sail design - but yacht designers are dealing with sailboat performance... there maybe drive & heeling moment are more descriptive and direct than the aero lift & drag. It seems that many tend to confuse sail drag with the underwater boat drag, and draw conclusions from there.

I'm sure you are right - the relative-wind axes should not be neglected from the start. In that respect my statement is too drastic. In a way lift & drag axis are arbitrary axis to the sail boat performance, while drive & heel axes are directly relevant.

 

The relative wind axes have a fundamental connection to sailing performance, since the performance depends the apparent wind angle and the aerodynamic lift/drag ratio. However, neither the apparent wind or the true wind are uniform in direction along the span. How does one even define Vmg in the presence of a non-uniform wind?

 

That's the beauty of life. One never knows what comes along.

IMO, the fact that the true wind has a varying direction is irrelevant for all-round boats. Whatever it's direction, you want to design for maximum Vmg, in order to reach the upwind destination as quickly as possible. Once you have made the best boat for that (among others) task, the real-time optimization of the course, based on the momentary true wind, is in hands of the crew.
Different thing is if the boat is to be designed for, say, a speed record over a waterway with a well-defined and constant direction of the true wind, and where the shortest course joining the start and finish line lays at an arbitrary angle to a true wind. In that case Vmg criterion would be substituted by some other, more suitable for that particular case.

Cheers

 

When reading the thread "Canting Bowsprit?" on SA I couldn't help thinking about the relation to this thread.
Especially to the first post of Eric Sponberg in this thread.

http://forums.sailinganarchy.com/index.php?showtopic=140334

 

Thanks for the heads up..

 

I'm not sure you understand what I meant by nonuniform wind. Say there's a 10 deg difference in true wind direction between the deck and masthead. What direction do you choose to define what is Vmg?

When wind shear is present, the boat's speed polar will be asymmetrical - it will have different speeds and pointing angles on each tack. What is the meaning of best Vmg then?

I suspect that the answer to the second question will resolve the first. You can pick a height (commonly 10m) and use the wind at that height as the reference wind direction. Differences in true wind speed from there are due to the wind gradient and differences in true wind direction from there are due to wind shear. If you set the boat on a given compass heading with a given sail trim, you can calculate the aerodynamic forces on the rig, and you can resolve them into components in any axis system. If you've done your sums right, it shouldn't matter whether you pick 10m or masthead as your reference wind - you should end up with the same total force magnitude and direction. What will be considered lift and drag will depend on the reference wind direction, but the total will be the same.

Similarly, the boatspeed polar will be the same, regardless of the choice of reference wind direction. It may look like you're pointing higher or lower if you pick 10 m, deck level or masthead as your reference wind, but the speed and direction through the water should be the same.

You want to minimize the time it takes to go upwind as far as the mark, but it is a constrained optimization because it's not enough to arrive at the windward location of the mark, you also have to arrive at the mark itself. If the mark is perfectly upwind from the start, then you need to sail equal crosswind distances in order to arrive at the mark. This puts a constraint on how much time you spend on each tack. Otherwise, with an asymmetrical polar, you'd maximize your upwind speed by staying on the favored tack all the time, and you'd end up way wide of the mark.

With a symmetrical polar, the crosswind velocity, Vmc, is proportional to the upwind velocity, Vmg, and the same on both tacks. As a result, it doesn't make any difference upwind how much time you spend on each tack, and all points along a line perpendicular to the wind direction are equal sailing time from the current position. That's not the case with an asymmetrical polar.

I haven't yet worked out a general principle for optimizing an asymmetrical polar. One approach is to pick the angles that result in the same crosstrack velocity on each tack and maximize the sum of the two Vmgs. That's equivalent to averaging the Vmgs of the two tacks. However, that falls apart when you consider a mark that is not perfectly upwind.

However one optimizes an asymmetrical polar, the same technique has to work with sailing to an offset mark with a symmetrical polar. We know the solution to the latter problem is to sail the same angles as to a perfectly upwind mark, but different amounts of time on each tack so the cross track distance works out.

I suspect the answer has to do with the non-convex nature of the velocity polar. If you use a straight line to bridge the lobes of the velocity polar, then you end up with a convex shape that shows the best speed in any direction. If your destination lies within the angle subtended by the straight line, then you have a jump discontinuity and have to spend various amounts of time at either point of tangency. This gives the right answer to the offset mark/symmetrical polar problem, and I suspect it is also the right answer to the windward mark/asymmetrical polar problem. But I haven't proven it to myself, yet.

 

I should add that if my speculation above is correct, then one definition of true wind angle is perpendicular to the line connecting the lobes of the speed polar. Vmg is then defined as the component of the speed perpendicular to that line.

Which also means that the direction of the true wind cannot be determined outside of the context of the boat that will be sailing in it! In a sheared wind, different boats would have different true wind angles, depending on how they were affected by the wind shear.

 

I don't think so. The situations are not congruent. You cannot transform all of the asymmetrical polar situations into the symmetrical polar with offset mark situation.

In the case of the symmetrical polar with offset mark, do the following construction-

1. On your polar, draw a vector to your offset mark to port.
2. Take a copy of the polar curve from the stbd side, rotate it 180 and slide it up the vector you drew. Locate it such that the curve is tangent to the port polar. This gives you a tangency point that shows what you need to do if you have to sail equal time on each tack. Of course you don't have to- so you can scale the transformed polar to represent differing amounts of time spent on each tack. The best result of a symmetrical polar will always be with a scale that produces a horizontal tangency. Ie the length along the vector, L, divided by (1 + scaling factor) will be a maximum.

In the case of asymmetrical polars, you are not guaranteed that the point of tangency will be horizontal at best VMG condition. This is true for dead upwind and offset marks. But even if the VMG differed by 15% port to stbd, If the max VMG occurs at about the same VMC, there is precious little in it. In the asymmetrical case, the radius af curvature of the the port and stbd polars come in to play.

This should probably go into a new thread.

 

Okay, I had a few minutes to doodle on Tspeer's asymmetrical polar problem.

Tspeer is right. Connecting the polars with a tangent line forms a set of the best possible outcomes toward any target whose angle intersects the tangent segment.

The proportion of time spent on the port tack is equal to the length of the tangent segment to stbd of the target vector divided by the total length of the tangent segment, and vis versa.

The strategy is to sail to the tangent points on each tack.

This is basically what "Wallying" is. If you are in a local wind field that is different from the expected average wind field in the future, you can form an asymmetrical polar based on your expectation for each tack. Instead of sailing the normal VMG for the current conditions, you should adjust it according to the above criteria. The difference between the angle indicated for current conditions and the one based on optimising given future expectations is called the "Wally Angle".

 

so i havn't read the entire article yet, er forum but correct me if im wrong, just a sailor and composite train irys student who just applied to west lawn, this idea is interesting, on say a volvo boat that has to heel over i assume to go up wind on and artificial hull of the chine or a e skow they do that right? but with the foward boards of a volvo with the out of the water keel of speed dream and then lower the tack, and you've created a new hull a new boat, with the exception that the mast would be still windward unless it to was on a traveler.
-ChrisCox