sail aerodynamics

[Gary Baigent]

I'll keep it simple for you CT: If a sailor enters a racing class that is not one design, then he knows that unless he is ahead of the play in skill, innovation and development then he is doomed to fail.
He knows that.
Repeat: he fargo trucking knows that is the situation when he gets invovlved.
You sound like some nanny chook who has taken it upon yourself to protect the poor sensitive chickens who know no better.
I think the "chickens" can sort things out themselves, can stand quite okay on their own, don't need you to spread your fluffy wing over them.
In fact, to them, your attitude is degrading and ridiculous.

 

[CT249]

Perhaps we just didn't get the news over here in the USA about this prolific use of full batten mains at such an early stage,....I just don't recall it??

Perhaps you can point out this 'common use' in the early days before multihulls??

The only problem is selecting what pics to use as evidence.

The most popular small junior dinghy in Oz for years was the local version of the Sabot, normally with full battens;

The most popular junior/intermediate trainer in Australia until about 1970 was the 1930s era VJ. Look, full battens on the plan!
Click for Full Size Image

The Moth was the most popular singlehander and it had full battens well before modern cats arrived. This is boat No 3, from 1928

The most popular young adult's crewed class designed before the modern cat arrived (during WW2) was probably the Gwen 12 - full battens.

The most popular widespread large dinghy, the Heavyweight Sharpie - with full battens.

Perhaps the most popular dinghy in NZ before the modern cat arrived was the Idle Along - with full battens.

Later I'll put up some pics showing that full battens were very common in some of the most popular classes in France and Germany well before the modern cat arrived.

It is clear that full battens are NOT some new-fangled idea that mono sailors were too conservative to take on - generations of use have grown up with them, just as generations of people in some countries grew up with rotating masts. If it is implied that people do not use rotating masts or full battens because of conservatism then it is simply wrong.

 

[CT249]

I'll keep it simple for you CT: If a sailor enters a racing class that is not one design, then he knows that unless he is ahead of the play in skill, innovation and development then he is doomed to fail.
He knows that.
Repeat: he fargo trucking knows that is the situation when he gets invovlved.
You sound like some nanny chook who has taken it upon yourself to protect the poor sensitive chickens who know no better.
I think the "chickens" can sort things out themselves, can stand quite okay on their own, don't need you to spread your fluffy wing over them.
In fact, to them, your attitude is degrading and ridiculous.

Oh, those of us who sail development classes at championship level can stick up for ourselves, Gary. It's just that some of us don't like it when people like you
abuse us for sticking up for our ideals by changing rules when it is necessary to keep classes alive.

The strange thing is that when development class sailors sort things out themselves, by changing rules, some people who don't even sail the same sort of boat and weren't even there
then throw abuse from the sidelines. It seems completely hypocritical to say that development class sailors can sort things out and then abuse them when they do so.

Anyway, after yet another post from you with no information and content and nothing but abuse, I'll put you back on ignore. And you can go on and look at the tiny fleets that remain of
once-strong and wonderful development classes like the Rs and Javelins, and see what destruction is wrought by those who fail to think about long-term class survival.

 

[brian eiland]

Touche on the batten question

 

[tspeer]

For a heavy displacement boat, maximum thrust is what you want. Thrust is related to lift, drag & apparent wind angle by

Thrust = Lift * Sin(Angle) - Drag * Cos(Angle)

When the angle is 90 degrees, then maximum lift is what you want. When the angle is less than 90 degrees, it pays to reduce the drag, but when the angle is greater than 90 degrees, it pays to add drag.

The only time it pays to maximize L/D is in the limit when the hydrodynamic drag is zero, which never happens when sailing in water & is only approximately true for a very efficient iceboat.

That's not really true. It helps to improve the lift/drag ratio even when sailing off the wind. The fundamental sailing performance relationships are:

Vb = Vt * sin(gamma - beta)/sin(beta)
beta = arctan(D_aero/L_aero) + arctan(D_hydro/L_hydro)

Vb = boat speed
Vt = true wind speed
gamma = point of sail (gamma = 0 is head to wind)
beta = apparent wind angle (measured between the yacht's course and the apparent wind vector)

Speed is improved when the apparent wind angle is reduced, and reducing the aerodynamic drag (at constant lift) will reduce the apparent wind angle at all points of sail.

It is true that maximizing the aerodynamic L/D by itself does not always maximize performance because you need to take into account the corresponding effect on the hydrodynamic lift/drag ratio. This is because the hydrodynamic lift is entirely dependent on the applied load from the aerodynamics. For example, when sailing dead downwind, the hydrodynamic lift/drag ratio is zero because there's no aerodynamic side load, which means the hydrodynamic lift is zero.

Especially for craft in which the added hydrodynamic drag due to additional side force is low, it can pay to operate at levels of lift above the point of maximum aerodynamic lift/drag ratio because the added side force improves the hydrodynamic lift/drag ratio, and the performance is better despite the increased aerodynamic and hydrodynamic drag.

Interactions like this are why it really takes a velocity prediction program (VPP) to say what the sensitivity in performance is to any change in the boat.

 

[Doug Halsey]

Tom : I beg to differ ...

Since it's getting late here, I'll confine my comments to one simple counterexample & save my longer explanation for later.

Consider a boat sailing dead downwind. The driving force from the sails consists of their aerodynamic drag only. There is no aerodynamic lift or hydrodynamic sideforce. The larger the aerodynamic drag, the faster the boat goes. That's why symmetric spinnakers were invented & why low-aspect ratio sails can be very fast downwind.

I'm sure you'll agree that increasing the drag helps in this case, but you might be tempted to think that it's a very special case & it wouldn't happen at any other angle. But if more drag helps at gamma = 180 degrees, is it so implausible to think that it might also help at 179 degrees, or maybe even somewhat smaller angles?

Let's not confuse the issue by suggesting that the boat could have a better VMG by sailing at a higher angle. We're discussing heavy displacement boats, where it isn't uncommon to have totally convex speed polars.

 

[brian eiland]

Isn't this the situation where the aircraft aerodynamicist experiences some problems interpreting the aerodynamics of sailing?

Or perhaps I have worded this incorrectly.

What I am trying to get at is the fact that in sailing we are operating at such low velocities that many aircraft foils & designers
really never need to contend with....on the very low fringes of flying aerodynamics. Then you add in the concurrent hydrodynamics
of our sailing situations, that the aircraft guys do not have to contend with, then I begin to question some of their axioms,...such as recently proposed...

....that its the trailing edges of the foils(sails) that control almost all the lift of the foil, with little or no contribution by the leading portions or interactions of the sails??

Additional edit:
While the trailing portions of the airfoil very well might be controlling the substantial portion of the lift generated by the airfoil, BUT is it really controlling that
relatively small portion of the lift that pushes our sailboats FORWARD?....that portion of aerodynamic lift that the airplane guys have no concern for?
Is our airfoil on the sailing boat oriented in the correct direction (sail trim and incidence angle), verses what the aircraft designer might be seeking out with their wing orientations?

 

[Doug Halsey]

Before I write anything else, let me repeat what I wrote on this subject earlier (Post #585, Page 39) :

I also question how much 'lift' we need from our sails considering that the very large percentage of the lift force on our sails is directed athwartships rather than in a forward driving direction

That's an interesting subject in its own right. It's important to know what we're trying to achieve.

If you were writing a VPP code (which balances the aerodynamic & hydrodynamic forces), it would be important to know that the maximum speed on a given point of sail would be when you trimmed the sails in such a way as to minimize the sum of the aerodynamic & the hydrodynamic drag angles. Each drag angle is defined as the arc cotangent of the lift/drag ratio. While that nails it down for the code, it doesn't give a practical sailor much to sink his teeth into.

Looking at some special cases can help though.

The simplest special case is the idealized iceboat, which assumes that the runners can provide whatever sideforce is necessary, but with zero drag. In that case, the maximum speed in any wind direction is obtained by trimming the sail for maximum lift/drag ratio. (where it should be emphasized that the lift & drag from all sources need to be included, & limitations on heeling moment would of course need to be considered). This might be a practical result for iceboats, but usually not for softwater sailors.

Another special case which is much more commonly applicable is the case where the hydrodynamic drag due to sideforce is a small part of the total hydrodynamic drag. This is the situation much of the time, especially for heavy keelboats where the drag is dominated by wavemaking drag, but also in many circumstances for light-displacement boats. In this case, the sails should be trimmed in order to maximize the thrust in the forward direction (again, subject to heeling moment limitations).

To get the thrust & heeling force, a straightforward rotation of coordinates is applied :

T = L * sin(beta) - D * cos(beta)

S = L * cos(beta) + D * sin(beta)

where L & D are the aerodynamic lift & drag components (including the sail + everything else in the air), T & S are the components in the forward & side directions, and beta is the apparent wind angle.

From these relationships, you can see that maximum lift is the optimum at beta=90Deg (~beam reach) & maximum drag is optimum at beta=180Deg
(~dead run). At values of beta smaller than 90Deg., the optimum lift is somewhat less than maximum, but also larger than the point with max L/D.

This is illustrated by the attached figure from Marchaj's 1st book (Sailing Theory & Practice, which I recommend highly to anyone interested in this subject.

 

[Doug Halsey]

Rather than attempt yet another long explanation, I decided to dust off an old VPP code & get some numerical results. Unlike the special cases referred to earlier, these results are not limited to heavy displacement boats or cases with no drag due to side-force.

The calculation is for a lite-weight dinghy approximating an International 10-Square-Meter Canoe. The blue curve shows results with aerodynamic characteristics typical of a an isolated sail, while the red curve adds an arbitrary constant to the drag coefficient (& calls it parasite drag).

The curves cross at about Gamma=137 degrees. At angles smaller than that, the extra drag is detrimental, but at angles larger than that, it is definitely beneficial. It's not shown in the graph, but that's also the crossover point where the apparent wind angle becomes greater than 90 degrees.

I hope this will convince everyone that increasing the aerodynamic drag can sometimes be beneficial.

 

[brian eiland]

Wing Mast on J 90

There were a number of postings back on this page about wing mast, and specifically one experiment by Hall Spars,...their J-90 Blackwing experiment
http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-43.html

I have a response to a note I wrote to Eric Hall:
1. First to say is in my comments below, the “una rig” was a fully rotating airfoil shaped mast. Everything should be understood in that context.

2. It was inferior going to windward.

a) A lower frontal area airfoil section would have improved it (but would have had sideways stiffness problems).
b) Any rotation past in-line with the apparent wind slowed down the boat where added drag trumped added power (as flaps do on a plane)
c) There is no substitute for a jib to improve upwind performance
d) The jib seems to add lift while improving the flow around the main​

3. Beginning with a close reach the Blackwing rig was superior.

a) With the mast able to rotate, the wind “sees” a much better mast shape with less turbulence that a conventional non-rotating mast.
b) As soon as a jib is eased, its positive effect on the mainsail reduces quickly
c) Rotating past apparent wind adds substantial power this time trumping added drag
d) Adding a standard J90 A-sail made this system a real weapon!​

 

[powerabout]

seems like that was findings from 1960, was it?

 

[powerabout]

Mikka, I was just trying to answer the question "what is a typical monohull". Thanks but I'm no expert, just someone who has studied the history.



Powerabout - there's a long history of wing masts in small performance monos that show that there IS a downside (in fact more than one) to wingmasts in such classes. That is why they have been tried and discarded in 18 Foot Skiffs, International Canoes, R Class, Merlin Rockets, 12 Foot Skiffs, Gwen 12s, Z Class renjollen (if I recall correctly), Cherubs, Suicides, Bembridge Redwings (I think) and other classes and types such as windsurfers.

The normal problem is the lack of gust response with a wing mast, because the thick chord means that it's hard to get the mast to bend automatically while staying strong enough to survive. Weight is also a problem. So wings have generally failed to provide enough all-round performance to be competitive. At times they have shown potential but in the end the problems are too much for them to be adopted, even when top class crews have spent several years and several different iterations. Even one of the most dedicated creators of dinghy wing masts, Frank Bethwaite, later concluded that simple non-rotating sections were better for modern assy boats.

Some have claimed that wingmasts give a 25% or more increase in lift, and a 60% decrease in drag. If they really did give such an advantage, they'd win just about every time. It's been proved time and time again that the on-water advantage, if it exists, is very marginal and that most classes lose more due to handling issues than they gain through the superior aerodynamics.

None of the above is meant to say that wingmasts don't work in certain classes, like the NS14s and MG14s with their small rigs, una-rigged Finns, or multis where many factors are different. I assume that in the Finns, the wing mast may work because the improved airflow over the mast is more significant in a una-rigged boat, and perhaps the unstayed rig and heavy, rounded hull mean that gust response is less of a problem. Mikka is clearly the expert there!

I was just trying to get my head around the success and failure is not the aerodynamics but other factors.
In beach cats if you have the rotation wrong the fleet goes past you.
Seeing how they work with the cunningham etc its a whole package of optimisation including rigging and sail cut that took years to get to where they all are today

 

[CT249]

I was just trying to get my head around the success and failure is not the aerodynamics but other factors.
In beach cats if you have the rotation wrong the fleet goes past you.
Seeing how they work with the cunningham etc its a whole package of optimisation including rigging and sail cut that took years to get to where they all are today

Sources like Bob Fisher's early book on cats indicate that it didn't take very long at all for wing masts to show their performance. There was very little regular racing in the C Class when wingmasts were adopted there and a tremendous amount of other development to do, yet wingmasts showed lots of potential very early on and quickly realised that potential.

That is quite a different situation to the one in mono dinghies, where wingmasts have been tried repeatedly but almost never won a series and have shown so many issues that the many champions who have tried to develop them have normally binned them. We also know that it's NOT a question of being closed-minded, as in Australia for example wingmasted dinghies are more popular than skiffs and very influential in skiff design, so it's not as in wingmasted monos are seen as freaks here. Here they are quite mainstream (the two most popular two-adult dinghies in the country use wingmasts, so they are as mainstream as Snipe and Vanguard 15 rigs are in the USA) and very well understood in many ways - and yet they still don't work in most monos, because of issues like gust response.

It doesn't seem odd that wingmasts work better on some types of craft than others - lots of rigs work better on some craft than on others. And in beach cats if you have the rotation "wrong" you still only go a fraction slower than the others, which underlines how over-blown some (not all!) estimates of the extra efficiency of wingmasts is.

The fact that there are such different answers to the under-rotate/over-rotate issue in heavy air sailing, and that they get such similar responses, also seems to indicate that some theories about the "superior" efficiency of wingmasts are overblown.

Just to repeat, this is NOT claiming that wingmasts don't work well in some boats, but the fact is that they have repeatedly been shown not to work in other types and surely we should be accepting that reality and adjusting the theories to fit, rather than saying that reality is wrong as some people do.

Cheers

 

[CT249]

Touche on the batten question

Cheers.

In order to save thread space, I won't put up more pics. However, in France the 9m2 Sharpie (one of the most popular of all boats from about 1930 up till about 1955) had full battens and a big roach. In Germany, full battens were standard on the popular J-Jolle 22 sq m dinghy and in all of the high-performance Rennjolle classes which were very popular from about 1920 to WW2.

Uffa Fox wrote about the superb Z Class Rennjolle (perhaps the fastest of all dinghies for decades) and the hassles the full battens caused.

In England, the most popular of all designers (Holt) had full battens in the popular Hornet class in the '50s, but they were abandoned because they made it hard to adjust sail shape for different conditions. However, for some decades many or most of the popular classes had one full batten, and very popular boats like many Moths and Holt's Solo were all full batten.

Of course the Canoes, the first popular small-boat racing class, had full battens from about 1860 for decades.

So the evidence is clear - in Europe, Australia, NZ and (in the very early days) America, full battens were completely accepted. However, they didn't always work well for various logical reasons, and they were therefore not universal and still aren't.

Since full battens (and rotating masts, for that matter) have been completely accepted in most countries for eons, "conservatism" can't be the reason they are not more widely used. Many of us grew up with them (for example, my father, my two brothers, my wife and I all had full battens on our first boats) and therefore if we took the conservative route, we'd stick blindly with them. If we don't use them, it's normally because they are not suitable for that particular application.

 

[tspeer]

Tom : I beg to differ ...

Since it's getting late here, I'll confine my comments to one simple counterexample & save my longer explanation for later.

Consider a boat sailing dead downwind. The driving force from the sails consists of their aerodynamic drag only. There is no aerodynamic lift or hydrodynamic sideforce. The larger the aerodynamic drag, the faster the boat goes. That's why symmetric spinnakers were invented & why low-aspect ratio sails can be very fast downwind....

Yes, you're right that if drag is all that's driving the boat, then the more drag the better. In that case the apparent wind angle is 180 deg, and the lift/drag ratios of both the hydrodynamics and aerodynamics are zero.

As the boat heads up from DDW, lift does start to come into it, and there will be a point of sail at which an incremental addition of drag has no effect on the performance. Above that point, any additional drag will be detrimental. Where that crossover point of sail is has to be determined by a VPP. It can't be determined by aerodynamics alone, because the lift/drag ratio of the hull is involved.

I suspect the crossover point of sail is pretty deep, even for a heavy keel boat. It would be interesting to see if it's as wide as the course for best downwind velocity made good (Vmg). Only for the case where the crossover point exceeded the best Vmg angle would there be a regime where additional aerodynamic drag would be beneficial.