Simplified Froude-Krylov Force

https://www.researchgate.net/public...yongsuru6_ziyoudunoFroude-Krylov_linojianyisu

This article/paper is in Japanese; what I want to point out is that very good simplifications of the Froude-Krylov Force can be made, such as the one I shared in another thread.

The issue is that the well-known manual, "Principles of Yacht Design", is already in its fifth edition and there are no real ocean waves. How is this possible?

The least a student or anyone interested in designing yachts/sailboats for ocean use should do is:

1) learn to draw waves: Franz Joseph von Gerstner

Franz Josef von Gerstner - Wikipedia, la enciclopedia libre https://es.wikipedia.org/wiki/Franz_Josef_von_Gerstner

2) calculate the orbital currents

3) sir Isaac Newton: Force of the Earth: Newtons = Yacht mass x g x sin Wave slope

4) Froude-Krylov: Force of the Wave: Surge and Pitch

 

The interesting thing is that if a boat is small relative to the wavelength of the ocean wave, and if the center of Flotation is forward of the center of Gravity, then it can even happen that when a large wave arrives from the stern, the vertical component of the Froude-Krylov Force lifts the bow instead of dipping it or having a neutral effect: the marvel of traditional and classical shapes with magnificent longitudinal balance.

But, of course, there's the problem: it doesn't mimic the sterns of high-performance racing yachts, there's no room for two large cabins to charter the yacht in the summer, and worst of all: it ruins the advertising brochure with a beautiful lady in a swimsuit on a spectacular bathing platform.

 

1966

maybe Staffan Mörling photo

Staffan Mörling – Wikipedia https://sv.wikipedia.org/wiki/Staffan_M%C3%B6rling

 

The wide stern makes sense on a fast sports yacht that hoists 40 square meters per ton: the Froude-Krylov Force tends towards zero.

Cap Martinique 2026 https://cap-martinique.geovoile.com/2026/viewer/

In "Cap Martinique", we see that the lead boat usually sails at 0.60 Froude (on the other hand, it should be noted that a modern 3.5-ton sports yacht already costs around 400,000 and its interior is a rather uninviting tent).

And nothing but inertia prohibits designing a fast sports sailboat with traditional longitudinal balance to the delight of the autopilot, safety, comfort, and functional beauty.

 

1 Ton WindSurf Board Plywood Epoxy

A still image taken at 0.60 Froude at the moment of takeoff, allowing the rider to glide on the wave at its speed.

At 0.60 Froude, we don't need the Froude-Krylov force to initiate surf-gliding.

 

Surf-gliding

The bulk of the propulsive force is provided by Sir Isaac Newton ... with the help of the Almighty Force of the Earth.

The ocean wave, unlike this wave created by a rock near the shore, would travel at great speed, and the boat would surf in the arms of the wave at the speed of the great wave.

---

Now, instead of a fast, one-ton surfboard with a huge, wide stern and classic longitudinal balance (like a fisherman that want to get back to port alive) ...

let's think of a slow cart carrying the house on its back like a snail, at the mercy of the Froude-Krylov force, and dragged along pitching, yawing, and listing by the Almighty Force of the Earth.

 

At rest they are always aligned vertically The CF will never be forward of the CG; it is basic physics.

 

Large yachts of 30 tons can be safer than small yachts of 1-8 tons.

Obviously, you'd have to be crazy to go surfing in the southern Indian Ocean with a 1-ton boat when the logical thing to do there is to use an 8-ton surfboard.

But for the Atlantic, an 8-ton ultralight sailboat is no safer than a 1-ton one if the sailboat remains stationary because the Froude-Krylov force has a larger surface area to act upon.

 

Let's review the safety of small yacht (1-8 tons) at sea:

1) Structural safety: hull, keel, mast, rudder

2) Passive safety

2A) Impact, significant water ingress, resistance to sinking
2B) Impact of a very rogue wave, capsizing and righting

3) Active safety

3A) Beating against the wind
3B) Hove-to: holding position
3C) Surfing

4) Maneuvering safety

 

(3.5-ton sports yacht in Cap Martinique 2026)

Let's compare 1 ton versus 8 tons.

4) The forces are on a human scale in the 1-ton boat, and well-designed is safer (in the open sea, from my point of view, it makes no sense to climb onto the deck of a 1-ton ultralight boat)

 

3C) In the Atlantic, they tie

3B) The smaller one ties or wins because it can hold its position like an ocean rowing boat, and of course, with its rudder blades out of the water like the modern sports yacht it is



(Cap Martinique 2026: JPK on a JPK 1050)

 

3A) The smallest one ties or wins with its seawater ballast if the 8-ton one is a snail cruiser
carrying its house on its back

2B) ties or wins, the mast does not survive in the 8-ton one

2A) The little one wins by a landslide

1) Smaller structures are safer and easier to oversize

 

Always remember to wax the board.

 

(Pogo 2)

In summary

Pegaso Z-102 - Wikipedia https://en.wikipedia.org/wiki/Pegaso_Z-102

Pegaso SLK-599, An improved Pogo 2, A 1-Ton WindSurfBoard Plywood Epoxy, more comfortable and safer, would be a magnificent sport sailboat, a magnificent SLK

Sporty, Lightweight, "Kurz", Compact

Mercedes-Benz SLK-Class (R170) - Wikipedia https://en.wikipedia.org/wiki/Mercedes-Benz_SLK-Class_(R170)

You don't need a committee of NASA scientists to put a comfortable seat in a MiniTransat, greatly improve its passive safety, and reconcile its Fundamental Centers (center of Flotation, center of Buoyancy, center of Gravity, hidroDynamic center of the Keel) as if it were a Classic British Yacht designed by Thomas Harrison Butler.

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

 

At which point, according to you, become unsafe because they are oversized.