How to Design a modern Yacht

with care, with a lot of love, and calculating, with numbers, not with words

If one abandons the path of the Classical solution... one must be aware that one enters the swampy terrain of the modern yacht and all its dangers.

The first time i heard traditionalists and old fogeys criticize the modern yacht, it seemed to me that they were exaggerating, because they pointed out that when heeling the bow Pitch down. And i thought to myself: how is it possible that this detail creates so much imbalance. Later in this forum i read about this old controversy and Eric Sponberg's contributions. And later the puzzle came together by studying Hörner's two books on fluid mechanics: Fluid Dynamic Lift, Fluid Dynamic Drag.

Furthermore, before they limited access, i managed to download a lot of scientific articles from the Delft University of Technology. And to top it all off JW Slooff published his book.

Encouraged by these studies, i even ventured to create my own calculations, because i like to calculate by two or even three different ways to have bifocal vision. My impression is that when two paths of calculation give the same or similar result and fit with the empirical information, then one is touching reality and not trapped in one's own head, and that feeling of touching reality is nice, like knocking on wood.

 

In summary

(A) it must be avoided at all costs that when the sailboat heels the bow sinks, and

(B) the propensity to pitch must be avoided at all costs: the pitch, instead of fast, violent and deep, should be soft and small in depth

 

The first thing is not to cheat when playing solitaire

It is useless to cut out on cardboard the Area of the wet surface with the stern of the sailboat stopped in port ...

... because when sailing on the high seas the large Wave that comes from the stern will wet the entire stern.

 

We cut out the wet Area on cardboard to balance it on the sharp knife of an old sailor ...

to find the Center of the yacht

And this primordial Center is called ...

Center of Flotation

Google

site:boatdesign.net Center of Flotation

 

The problem that we have been dealing with since the end of the 19th century ...

The Satanita - Wikipedia https://en.m.wikipedia.org/wiki/The_Satanita

The problem

Pitch and Yaw control ...

recently (1990-) has been swept under the rug in the world of high competition because they hoist an astronomical amount of sail and their dynamics are different

The problem appears in yachts that are relatively slow in relation to the speed of the waves coming from the stern, yachts that hoist about 10 square meters per ton.

 

For 180-300 thousand Euros they don't give you even a fuxx place to sit comfortably in the cockpit

Why ?

Because the older man with money is sold a yacht that imitates the sports machine of a young athlete

imitating highly competitive sailboats has been the iron law since 1850

another is inertia: for example the bows of the small sailboats where we little children slept while mom and dad slept in the two stern bunk beds.

 

Munk Moment

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

Munk's Moment is greatly underestimated

Reviewing this story, it is evident that it was investigated because it affected the mega yachts of billionaires, otherwise it is likely that no one would have given it a minute of attention.

In a modern yacht we have the sum of three versions or variants of the Munk Moment

1) due to leeway/Yaw
2) due to heel/Roll
3) due to Pitch

and to complete the total hydrodynamic Yaw moment we have to take into account

4) the lateral force of a modern daggerboard that is capable of producing a lot of force

 

(10-6 meter high wave in the wave tank, instead of using radio control like Andrew Claughton in the Southampton wave tank, here in this research the models carried a small crew member)

Hull trim

In a small boat, a good hull attitude can be achieved simply by moving the crew towards the stern, thus achieving an Angle of Attack that, on a flat surface with sharp edges and large enough create hydrodynamic lift.

As soon as the sailboat is a little larger, the trim must be based on a good hydrostatic balance because the hydrostatic forces are usually about 10 times greater than the dynamic ones when starting or taking off at low speed

once the sailboat accelerates (> 0.5 Froude) the trim must be hydrodynamic and there are three ways to achieve it

1) based on High pressure like these images above

2) based on Low pressure, suction, at the stern

3) a sum of high pressures and low pressures

 

The key question here is ...

the Low pressure, the suction, which settles the highly curved stern of a heavy yacht well into the wave does not depend on the angle of attack

But

the High pressure in a light sailboat with flat surfaces depends completely and absolutely on the Angle of Attack

 

In a yacht that is going to surf based on low pressure, suction, at the stern, we can settle for a longitudinal metacentric height = 1 x LWL

In a light (150 D/L) or ultralight (100 D/L) yacht with flat surfaces and vertical stem we must aim for a longitudinal metacentric height = 3 x LWL

 

The latter is not precise nor is it intended to be a kind of universal law.

I just want to emphasize the great difference between the high pressures that depend on the angle of attack and the tolerant low pressures that adapt to almost anything

 

These melancholic notes looking back is perhaps like talking about dinosaurs in a near future world dominated by the catamaran ...

and the Scow

 

(Plywood Epoxy 1 Ton WindSurf board for an old fogey, Thomas Harrison Butler in Memoriam: Waterplane Area and Chine)

A clever chine (which also fits wonderfully with a Plywood boat) is a great hydroStatic and hydroDynamic aid/tool

In the future my light surf board will be considered a Scow or a proto Scow (?)

Don't know; but as soon as it begins to accelerate it produces more than 2200 Newtons of hydrodynamic Lift at the bow

 

1) hydrodynamic Lift, High pressure

2) (hydrostatic pressure of the bow wave, currently deleted from the model to make the calculation cleaner)

3) powerful force of the earth, which is the bulk of the force when surfing in mode "Surf Gliding" (that is: "falling with style")* keep in mind that with a 12 degree slope the earth propels the surfboard with a force equal to 20% (!) of its Displacement

4) hydroStatic

5) Low pressure (the drawing of that vector has been too vehement. The suction of the stern in this photo is about 1000 Newtons)

---
*:

 

The funny thing is that the dinosaur at the beginning of this thread has the Center of Flotation at 57% of the waterline without counting the part of the stern that would wet the Wave, and my surfboard with its wide stern (for at the same time minimize lateral Low pressures and minimize Roll) at 58% LWL