Fossati, Politecnico di Milano Wind Tunnel

Let's look at some conclusions now that we have accurate pressure measurements.

 

The first conclusion is the confirmation of the magnificent and let's say incredible painstaking scientific work done in 1936 by KSM Davidson of the Stevens Institute in New Jersey, published in the Society of Naval Architects and Marine Engineers

 

A sailboat builds 1 Wing with 2 elements (StaySail + MainSail) and has a Lift Coefficient (CL) between 1.5 and 0.8

And the Drag Coefficient (Cd) is 0.064 + 0.131 x CL squared

 

(KSM Davidson)

 

Sailing against the wind you can produce 1000 Newtons of Lift by paying 200 Newtons of Drag, a ratio of 5 times

But when opening the course Lift and Drag tend to be similar: Sails then are Sails

And downwind the bulk of the force is the Drag component: the sails are parachutes, as clearly seen in a Spinnaker

 

Small conclusions

The leading edge of the Jib/StaySail is not as good as some dreamed, and the leading edge formed by the Mast is not as bad as some feared.

 

Small or not so small conclusion

The so-called "Davidson CE" is so good that it even allows you to explain a schooner

 

As Ludwig Prandtl taught us...

the blade of a rudder, a daggerboard, a wing...

can be replaced by a column of fluid interacting in the flow of a fluid

so it does four things four

1) diverts the fluid, the so-called Upwash (which in other cases would have to be called Sidewash)

2) accelerates the fluid (= Low pressure)

3) diverts fluid, Downwash (sidewash)

4) slows down the fluid (= High pressure)

(such a schooner is a rig in the upwash of the aftersail. And the aftmost sail is in the downwash of the forward rig)

 

Why

why build 1 Wing with 2 elements (StaySail + Mainsail)

Well, in order to be able to work with a very large (from an aeronautical point of view) (25-35 degrees) Angle of Attack

 

(Modern Yacht)

Mainsail center = 0.4 P, 1/2
Jib center = geometric center

CE is located in the vertical of the Center of Buoyancy (CB) an idea that appears in the old Howard L. Chapelle books.

CB which is between CF (Center of Flotation) and CG (Yacht Center of Gravity)

And the Hydrodynamic center of the Keel (HC_K) more or less below the vertical of the Center of Gravity of the Yacht

 

But

But if the HydroDynamic turning/Yaw moment of the Hull is very large...

Then we have to move the Sails forward

No, Nein, No

If the HydroDynamic Yaw Moment of the Hull (Max Michael Munk Moment) is very large...

Max Munk - Wikipedia https://en.m.wikipedia.org/wiki/Max_Munk

draw another Bow ....
and draw another Stern

Thomas Harrison Butler - Wikipedia https://en.m.wikipedia.org/wiki/Thomas_Harrison_Butler

 

The lateral pressure of the water in traditional hulls was around 38% LWL and in the vertical of 38% LWL is the center of the force of the sails located at

0.38 Mast
0.43 Chord (Jib Luff - MainSail Leech)

 

The traditional sailboats were balanced for sailing... and were unbalanced from a HydroDynamic point of view

That's why every sailor knew that in a gale you shouldn't run with the waves with a traditional sailboat

Well-made classic sailboats like those of Thomas H. Butler ...

1) there is no mismatch between Center of Flotation (CF) and Center of Buoyancy (CB)

2) the HydroDynamic Yaw Moment of the Hull (the Munk Moment) is low

3) A delta wing-shaped profile moves the water pressure aft when the sailboat yaws, and

4) If in a big wave accelerate flying toward the center of the earth, the hull attitude is stern down; bow up.

 

As all the widows of F-104 pilots know, it is more dangerous to fly based on high pressure than on low pressure.

The problem of the modern flyer is that without an angle of attack we do not have high pressure in the bow to take off.

 

So let's rely on a good sum of

a gigantic longitudinal metacentric height (= 3 LWL)

the launching ramp or wedge created by water piling up on the bow

a well thought out bow to create high vertical pressure

good Longitudinal balance (CF, CB, CG)

And a little help provided by the low pressure, suction, at the stern