The three causes of bow-down

The three causes of bow-down in a sailboat:

1) Sail Force = sum of the horizontal component multiplied by its lever arm (F_H x h) + vertical component (due to heeling) multiplied by its lever arm (F_V x d)

2) A wide stern and/or "heavy quarters": when heeling, it pushes more volume into the water at the stern than at the bow: the center of Buoyancy shifts aft and the bow shifts downward



3) The yacht's center of Gravity is forward of the center of Flotation (CF, which is where the pitch axis for hull trim passes), and the force of the wave (the Froude-Krylov force) is located aft

If, to complete the task, we place the hydrodynamic center of a huuuge modern Keel forward of the Yacht center of gravity ...

... we ensure that the yacht capsizes with a beautiful pirouette:

this is how 5 yachts capsized or disappeared into the sea in the 1997 demolition derby, where Aqua Quorum performed wonderfully.

 

And to capsize, it only takes one second.

To stumble on the hull and keel, it only takes one second. The hull and keel trip up the yacht.



Aleksey Krylov - Wikipedia https://en.wikipedia.org/wiki/Aleksey_Krylov

Mr. Froude and the bearded Krylov raise the great, wide stern ... and they give the yacht a push forward and leave it in Sir Isaac's hands to complete the task

and Sir Isaac Newton drags the yacht with the Almighty Force of the Earth = Newtons = mass x g x sin Wave slope

And the yacht, propelled by the force of the wind, the force of the sea, and the Almighty Force of the earth, performs a beautiful pirouette.

 

The problem

The problem isn't that the bow drops (Pitch for example -2°)

The problem is that Pitch is the trigger for a tremendous explosive cocktail formed by Pitch, Roll/Heel and Yaw/Leeway

Sighard F. Hoerner - Wikipedia https://en.wikipedia.org/wiki/Sighard_F._Hoerner

Hörner, Fluid Dynamic Lift
Hörner, Fluid Dynamic Drag

"cross flow" + Mister Max Michael Munk = huuuge hydroDynamic Yaw Moment of the Hull

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

+ huuuge Keel forward of Yacht gravity center

= the yacht capsizes

 

Solution: Do things right, like Aqua Quorum: the centered keel

Centered on the fundamental primordial Center of a sailing yacht: the Center of Flotation

google: site:boatdesign.net center of flotation

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And if we want more, we add a bow capable of creating a nice hydroDynamic Lift thanks to its size, Angle of Attack, and Speed

 

What's the point of building off-center, unbalanced yachts just to drive the sailor crazy?

Why not make sailboats with a little love and care and respect for science and good balance

For example:

- Longitudinal position of the Center of Gravity (CG)
- Longitudinal position of the Center of Flotation (CF)
- Longitudinal position of the Center of Buoyancy (CB)
- Longitudinal position of the hidroDynamic center of the Keel



or

For example:



Bow to the left

 

"What's the point of building off-center, unbalanced yachts just to drive the sailor crazy?"

Well, it's the same old story:

perhaps by ruining the design we can reduce the drag 46.873 Newtons at a speed of 0.3567 Froude, according to the Crazy Design 3.0 model.

The price: a nice somersault while sailing on a real sea with real big waves.

 

There are three types of surfing:

A) Surf riding: Froude-Krylov

B) Surf gliding: Isaac Newton

B1) The yacht starts with good speed because it is very powerful and lightweight, and it also generates hydrodynamic lift under the bow

B2) Surfing begins with the Froude-Krylov push and then continues under the force of the Earth.

In this second case, which is the case for large or heavy yachts, the Low pressure (hydroDynamic Suction) under the stern play a very important role in the good hull trim Bow Up Stern Down for surfing with control and safety.

 

B2

In this case, if the yacht cannot sail at high speed like a small sports yacht or a high-performance racing yacht...

then, since the start of surfing depends on the forward (and upward) push of the Froude-Krylov Force ...

a moderate or narrow stern relative to the bow is appropriate, that is, traditional shapes.

 

Surf riding: Froude-Krylov, or 0.8 Froude-Krylov + 0.2 Newton

Surf gliding: Isaac Newton, or 0.1 Froude-Krylov + 0.9 Newton

 

If the wave is gigantic with an enormous wavelength, then perhaps a heavy yacht could initiate surfing propelled by gravity.

But a Force 7 wave of just over 70 meters traveling at 10 meters per second takes about 3.5 seconds from trough to crest, and the steepest section is only part of the wave. There is little time to overcome inertia.

What I mean is that a slow or heavy yacht will most likely need the Froude-Krylov impulse (Wave induced surge) to initiate surfing.

And that's why it needs excellent longitudinal balance and must not facilitate the vertical push of the wave at the stern.

 

But it's the same thing, just in different forms.

The heavy surfboard (a 5-10 ton yacht, 10 square meters per ton) with its moderate stern would have its center of Flotation (CF) at 50% LWL and its center of Gravity at 54%, and when accelerating it creates high (H) pressure under the bow (hydrodynamic lift) and a lot of low (L) pressure (hydrodynamic suction) under the stern.

The fast, ultralight surfboard (1-3 ton sport yacht, 40 square meters per ton) with a gigantic wide stern would have its center of Flotation (CF) at 58% LWL and its center of Gravity at 62% LWL, and when accelerating it creates a lot of high (H) pressure under the bow and a little bit of low (L) pressure under the stern.

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only five things:

A1) Longitudinal balance: take into account the force of the sails and their large lever arm

A2) Longitudinal balance: ensure the bow doesn't dip when heeling, crucial for slow yachts and less so for high-performance racing yachts

A3) Longitudinal balance: center of gravity At or Aft of the center of Flotation

B) stability of lateral forces: the sum of the lateral forces (hull, keel, rudder) must be Aft of the yacht's center of gravity, calculated without moving the rudder, without touching the rudder, without looking at the rudder. The rudder is used to compensate for the imbalance created by the sails, not to fix a hydrodynamic disaster

[Aircraft longitudinal (Pitch) stability is very similar to a Yacht Yaw stability: hydro_CLR (Hull + Keel + Rudder) Aft center of Gravity]

C) When accelerating, the yacht should trim the hull stern down, Bow Up, to ride the big ocean waves safely and gracefully.

 

That would be an unstable state. It will return to a stable state where the CG and CF are vertically aligned. This is basic physics.

 

The rudder is used to steer the boat. The sails do not create imbalance per se. Further, the rudder is used for the dynamic imbalance that is intrinsic to a vessel operating at the interface of two turbulent fluids of very different density and viscosity. You are missing the most basic concepts.

 

And when considering the rudder, you must take into account:

(1) the effect of the keel on the rudder: what in aeronautics is called downwash, and here it would be the sidewash

(2) the effect of the hull/fuselage

(3) whether, when heeling, part of the rudder comes out of the water; and

(4) the orbital current of the wave

Total: calculate 1 rudder and install 2.