Force 7 Ocean Wave

I'm going to try to organize my thoughts on an offshore sailboat design here.

My reflection begins with a young Force 7 summer ocean wave in the Atlantic sailing from Madeira to the Canary Islands.

 

Franz Josef von Gerstner

Franz Josef Gerstner - Wikipedia https://en.m.wikipedia.org/wiki/Franz_Josef_Gerstner

 

Pierre Gutelle, The Design of Sailing Yacht



"the most important phenomena to understand when considering the behaviour [the bad behaviour] of a boat in seas"

 

At the top, there are 4 knots of orbital current, and at the bottom, there are 4 knots of counter-current.

If the sailboat is sailing at, say, 7 knots, the rudder blade sees 3 knots and the bow feels 11 knots.

And the hydrodynamic forces depend on the square (!) of the speed.

The difference is simply huuuge: the sailboat is stuck in a nutcracker.

 

Roger Taylor, MingMing

"Despite a lifetime's advocacy of minimal ocean cruising, I could still be caught off-guard by the reality of a tiny boat sailing effortlessly in massive seas"

Because

a tiny boat (< 20 ft, < 6 m) ... sail on the same zone of the same wave

 

...And now you know why bar crossings are so dangerous. This is why you trail drogues in high seas (not necessairly high winds)...to keep steerage. FWIW, small boats have the same problem in choppy bays and sounds, hense the loss of small sailboats in San Francisco Bay.



Edit to Add: https://www.sciencedirect.com/science/article/pii/S0029801821009380?via=ihub

 

The second most important phenomenon is the wave's steepness.

A Force 7 ocean wave, like the wave drawn here following Franz Joseph von Gerstner, can have a steepness of 12 degrees.

And the sine of 12 degrees is 0.20

In other words, the Almighty Force of the Earth propels the sailboat with a force in Newtons equal to 20% of its mass in kilograms multiplied by 9.8 in the metric system.

Thus, a sailboat that was accustomed to (and even designed) to move with, a wind force of 3-4% of its displacement is propelled by a force equivalent to 20% of its displacement.

Moreover, it is a sum of forces

The Force of the Earth
The Force of the Wind
The Force of the Sea

 

The importance of the Froude-Krylov Force is controversial when we talk about a small boat; but it is very real on the crest of the wave, with its horizontal and vertical components.

Thus, the important phenomena are:

A) orbital current and countercurrent

B) wave slope, which unleashes the Almighty Force of the Earth

C) wind force on the hull and sails with a large lever arm, and

D) Froude-Krylov impact or push, with its horizontal and vertical components.

 

A small sailboat of 18.7-20 feet (5.7-6.1 meters) is a great size for good dynamic behavior in open seas with big waves.

Small size has great potential.

Now, I'm going to argue against a sailboat under 5.7-6 meters (18.7-19.7 feet)

 

A wind force on what? How do you compare wind force (speed in common language use) to displacement which is weight? They are different units so it makes no sense.

 

Weight directly translates to force; it is an expression of force.

Wind speed can only be translated to force when area and density are included. Wing/foil lift dynamics makes it more complicated, but everything can be reduced to Newton's of force.

-Will

 

A small boat is a great idea because it offers a versatile and powerful rig of more than 20 square meters per ton, and it also tends to perform well in typical Force 7 and Force 6 summer ocean waves in the open sea.

That is, the exact opposite of the typical insufferable dinosaur: rig that is not versatile, 10 square meters per ton, and which on top of that easily gets caught in young Force 7 and Force 6 ocean waves.

Furthermore, a small boat allows for retractable appendages and a mast 40 cm from the comfortable and safe cockpit, so that the sailor never leaves it, not even to drop or retrieve the anchor, hoist a gennaker, or hoist a jib.

In other words, the boat acts as an exoskeleton at the other end of the Victorian-era yacht.

6 meters is a maximum that is quickly lost with a smaller size.

With a smaller size, nothing is gained and two very important objectives are not achieved:

D/L < 150

A large, very large

Longitudinal Metacentric Height

 

<150 D/L

It's an essential goal to achieve everything that comes with gliding over the sea:

beauty, pleasure, enjoyment, serenity, the peace of mind that comes from feeling something is working well, and the feeling of accomplishment and triumph.

 

Longitudinal Metacentric Height

In a 6-meter light boat, with careful design, a Longitudinal Metacentric Height of astronomical size can be achieved: 18-20 meters

3 (!) x LWL

18-20 meters of longitudinal metacentric height in a 6-meter boat means ...

Pitch -1° Bow down ...

= Center of Buoyancy moves 30 cm forward: 5% LWL

This gives 1 Ton Displacement a lever arm of 30 cm, an essential element for Stability of the balance on the Pitch axis.

 

Pitch Control

We achieve this by combining three pieces of the puzzle:

1) A huge Longitudinal Metacentric Height that provides us with a high baseline safety reserve

2) The longitudinal position of the sailboat center of Gravity aft of the center of Buoyancy, which is aft of the center of Flotation

L_CG > L_CB > L_CF

L_CG: 62% LWL
L_CB: 60% LWL
L_CF: 58% LWL

With this, we simultaneously achieve two things:

2a) that Roll dont provoke Pitch bow down

2b) eliminates the tendency to Pitch bow down due to Froude-Krylov push when the wave hits the stern

3) Hydrodynamic Trim capable of withstanding the Force of the Sails and their enormous lever arm.