Force 7 Ocean Wave

20 knots Force 7 Ocean Wave
20 knots WindSurf Board

round figures

Drag: 3000 Newtons

Force of the Sea: 0-200 Newtons*
Force of the Earth: 2000 Newtons
Force of the Wind: 1000 Newtons

*: After a long and very entertaining and funny discussion for several days this summer, DeepSeek and I have come to the conclusion that the "surf-riding" model of large vessels based on the Froude-Krylov force is not suitable for small boats.

Why does top/peak speed last so little time?

Because the good surfing zone is really small.

If the boat speed is greater than the wave speed, then the boat enters a low-slope zone and the counter-current zone.

If the boat speed is less than the wave speed, then the fast wave overtakes the boat.

A young Force 7 wave starts sailing at 12-14 knots and then, as it grows, it sails at 18-20 knots.

 

It's a tangle of acceleration, deceleration, and forces whose result is not instantaneous, that is, at the end: time and inertia.

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But the vertical component of the Froude-Krylov force near the crest of the wave is important.

The new 45-foot yacht has its center of gravity forward of the center of Flotation, stationary (!) in port.

Now let's consider the longitudinal position of its center of Flotation, heeled, and/or with the entire wide stern wetted by the big ocean wave.

The vertical component of the Froude-Krylov force (the Force of the Sea) has an enormous lever arm.

Furthermore, Roll/Heel also causes a bow-down Pitch.

The hydrodynamic CLR (with Roll/Heel and Pitch bow down) moves very forward of the yacht center of gravity.

This is enough

But

if we add current or countercurrent ...

And

if we add current and countercurrent...

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In short

a slow yacht of 10 square meters per ton with the keel and center of flotation as in 1979 ... and that imitates a high-performance sailboat of 40 square meters per ton because so the aft cabins are larger.

 

The 45-foot yacht is sailing so slowly relative to its LWL (low Froude number) that when the wave's push/thrust hits, the yacht crashes and plunges its bow into its bow wave.

It is unable to climb over its bow-wave and overcome it.

The funny thing is that the ocean wave can be our enemy or our great friend.

Our great friend frees the boat from sailing sunk in the hull wave.

The ocean wave frees the boat and sends it into Surfing.

 

5.7 m Plywood Epoxy

Here we see it

Being small, the law of the square and the cube works in its favor

Square–cube law - Wikipedia https://en.m.wikipedia.org/wiki/Square%E2%80%93cube_law

The ocean-wave frees the boat from the hull-wave and the boat glides in harmony

Now we need NASA's committee of scientists to transform this into something reasonable, truly functional, safe and comfortable.

 

If this boat capsizes and turns over, it becomes a stable catamaran (!)

We improved passive safety with the following measures:

+ 10 cm of freeboard

+ a deep cockpit, completely open in the S8-S10 area where the seawater ballast tanks for Upwind will be located

+ a deeper keel, 1.75 m instead of 1.3 m

+ a fully enclosed mast, which in a small, light boat has a huge impact on rollover resistance and righting.

 

With the deep cockpit, we gain a lot of comfort and a sense of security, protection, and real safety.

And the sailor sitting close to the center of gravity and the Pitch axis, unlike in the MiniTransat Pogo 2 in the video above this thread.

 

A cockpit that's a real cockpit, like a 1917 Curtiss.

Curtiss JN Jenny - Wikipedia https://en.m.wikipedia.org/wiki/Curtiss_JN_Jenny

We placed the mast 40 cm from the cockpit.

And the bowsprit rotates aft.

Of course, the sailor has to lift his butt off the comfortable seat.

A cruelty on the part of the designer.

 

Typical French 1964 style

dad and mom sleep at the stern, and the children at the bow

Not only is this what I've known since I was 6, but it's also the same as the sailboat I own now: an old French MiniTransat.

This makes no sense to either 1 grumpy old man or 1 young person.

So, we urgently reconvene the NASA scientific committee meeting.

 

(Obviously when the boat is sailing the Keel is down)

impact against an UFO

10 Closed compartment with very low density material

(18 Closed compartment empty + 4 cargo boxes + 2 Water Ballast)

Up to S4 is the bed for sleeping at anchor and in port, and where, for example, a young man can maneuver with a young woman. It's also where you can sit across with a good cushion to read, listen to music, or write.

S4-S5 are seats, but not for sitting, but for moving, and they are bare because they are part of the wet area. It is a simple board without a cushion or mattress.

On the sides behind a tarp are clothes and plastic boxes held with belts.

S5-S6, four cargo boxes for food with the typical round lids

They form the chart table, the kitchen counter, and the bunk for the ocean sailor who doesn't want to break his ribs.

On one side hangs dry clothes and on the other wet clothes. They are two open closets, a wonderful luxury on a small sailboat where it's common to get soaked and wet where you sleep.

 

In the cockpit, below the seats, there are two cargo compartments.

There are >60 liters of fresh water, a 100-amp LiFePO4 battery, storage for the outboard motor, the sea anchor, another anchor, and the last question: the toilet/WC.

In port, the last question is resolved by walking a bit, and offshore with a bucket or even without a bucket. But anchored close to land, it's a well-civilized invention under a tent-awning that contributes to comfort.

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The mast is tilted sharply aft: the navigator's berth is 60 cm wide and 50 cm where the mast rests on the spectacular chart table and galley.

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And the fenders go on top of the ballast tanks.

I mean, plenty of room for a lot of things is essential.

The worst thing about a small boat is living inside a storage room, so you have to protect the order and pleasant atmosphere of the cabin at all costs.

 

2 m (!) MainSail Traveller

3 ropes in 1 rope

Preventer - Sheet - Vang

This

This, along with dropping the anchor and picking it up without leaving the cockpit, is what excites me the most, along with being able to raise the large keel.

Since the price of these things is ridiculous, the system is built with a simple steel or aluminum "L" put as "V" and a cart with four wheels.

 

The seawater ballast tanks (WB) each have two typical round lids and drain into the cockpit floor with simple plugs.

The effect of water ballast in a small, light boat is spectacular.

The MiniTransat Pogo 2 doesn't have ballast tanks; it's absurd* that this isn't common.

1) Increases Displacement

2) Moves the center of gravity to windward

The sum of 1 + 2 turns a boat with a forward force of 400 Newtons into a boat with a force of 800 Newtons + 100% (!)

and moves the center of gravity aft, so

3) Withstands the force of the Sails

4) Smooths the passage of the wave

*: The tanks will be 230 liters, exactly like American Express, which since winning the Mini Transat in 1979 ...



Comparison with American Express Freeboard (equalizing length for better visualization): aft, the sea Water Ballast (WB) tanks determine the true Freeboard

 

Here is the correct longitudinal position of the center of gravity: 62% LWL

The Froude-Krylov thrust (surge: forward push) in this area of the wave is perhaps only about 200 Newtons.

But on the crest, the Froude-Krylov Force can be very strong, with both a horizontal and a vertical component.

The good surfing area is very small, and the waves are very diverse, so it's very difficult to predict how long the surfer can sustain the ride, carried in the arms of a large ocean wave.

 

There are no lateral curves at the stern: the curve you see isn't a lateral curve at all because the water comes from the bottom up.

The very wide stern has several functions.

The first is to increase the sail-carrying capacity.

The second is to prevent lateral dynamic Low pressures

That is: to sedate and send Mister Max Michael Munk to sleep.

Once the boat takes off (Froude number > 0.60) Munk Moment tends to zero because the bulk of the high pressures at the WindSurf Board bow are vertical.

In any case, to size the rudder, I made a cautious estimate of the Munk Moment with the boat heeled a Lot and only counting 1 rudder blade. A worse-case estimate could be made: with the bow down, the Munk Moment is doubled; but this design is so cautious that such an estimate would be pure exaggeration.

During acceleration and in the critical takeoff zone (0.50-0.60 Fn) the bow leaves the water and the Center of Flotation moves slightly aft (?)



The low chine on/at the bow makes the Magic Carpet wings larger and the hydroDynamic Lift coefficient higher.

 

"Ming Ming (...) she should by rights have been overwhelmed by the hungry combers ravaging her from astern. Not a bit of it. She shrugged them off with an easy balletic grace, light and airborne, an innocent beauty unfazed by the advancing packs of leering beasts"

(Roger Taylor, MingMing and the Art of Minimal Ocean Sailing)

The tiny boat has great potential, it is a Diamond in the rough