Naca Airfoils shape and rocker oft flat bottomed boats

[Dieter51]

Hi CT429 – having a rough day?

I'm not a particular fan of Bolger, but saying he wasn't a good sailor, had no significant training, and that some experienced sailors would judge his boats as poor seems a bit exaggerated, don't you think?

By the way, I'm not referring to Bolger but primarily to Howard I. Chappelle's “American Small Sailing Craft.” And I'm not interested in hearing that other rocker curves of flat-bottom boats have been successfully sailed. I want to know why the typical rocker curve of flat-bottom boats, where the bottom slopes flatly at the front and then steeply rises towards the stern, sometimes over the magical 15 degrees, has been so successful that it’s found in almost all flat-bottom boats. (Exceptions prove the rule.)

That's all I wanted to know, but it seems there is no definitive answer.

And of course, potentially dangerous boats in professional hands are relatively safe, otherwise this profession wouldn’t exist anymore. If 3,000 fishermen work several hours a day with such boats for 300 days a year and “only” 50 lose their lives, that’s a good, though still very unfortunate, ratio for such a high-risk activity, which primarily depends on changing weather conditions.

Translated by Chat GPT4.0

 

[CarlosK2]

"the bottom slopes flatly at the front"

To create High pressure ... thanks to an Angle of Attack

"and then steeply rises towards the stern"

To create Low pressure

It's Fluid Mechanics 1.0

Ludwig Prandtl – Wikipedia https://de.m.wikipedia.org/wiki/Ludwig_Prandtl

Why is such a simple scientific fact so difficult to understand or accept

(your gods prevent you from accepting simple facts of Fluid Mechanics. I don't understand it. What's next, doubting gravity and Newton?)

 

[CarlosK2]

Boat Design Dot Net Academy

Final Exam for small sailboat designer

Ladies and gentlemen you have half an hour, whoever finishes first can get up and hand in your exam for the title of small sailboat designer.

First question

Calculate the Force in Newtons produced by the sum of High (H) pressure and Low (L) pressure on a 4 square meter Sail, Aspect Ratio 3, with an Angle of Attack (AoA) of 20 degrees, a Wind of 20 knots and a 20 percent "arrow / chord" ratio.

Second question

Calculate in Newton meters the turning/Yaw moment of a hull of a sailboat with a Displacement of 1 ton and Leeway/Yaw Angle of 6 degrees and sailing at a Speed of 7 knots.

 

[CarlosK2]

Ladies and gentlemen ...

the smartest girl in the class warns me that the first question is rather strange, well, have mercy on this old fogey.

Calculate with a bow "string/chord arrow" ratio of 10 percent.

 

[CarlosK2]

Special Question

On a 5.7 m LWL 2.4 m Beam sailboat going down a Wave at 8 knots speed ...

Calculate the Force (F) in Newtons produced by a 6 degree Yaw/leeway on a sharp bow with a 30 cm stem (forefoot) created by a 3 degree Pitch Angle (P)

 

[CarlosK2]

1) around 320 Newtons
2) around 734 Newtons meters in a very crude estimate as the shape of the hull is not indicated.

3) A huge amount, and the sailboat sails really bad with the waves created by Force 7:

35:30-

Philosophical Question

Why sailboat design is a sum of ignorance, fantasies and superstitions

Philosophical Question, 2

Is sailboat design just another piece of this beautiful asteroid and horrible world of troglodytes created by an enormous blind force called "God"

 

[CarlosK2]

Pitch Axis passes through center of Flotation: CF

A) The Wind Force on the Sails (CE) with its Horizontal component multiplied by its leverage arm + its Vertical component multiplied by its leverage arm ... sinks the bow

B) The Force of the Earth (CG, Center of Gravity) ... sinks the bow

C) The Force of the Sea ... sinks the bow

And a sharp bow produces a lot of sideways/Lateral Force as soon as the sailboat yaws, and is insufferable for the helmsman.

 

[CarlosK2]

1) Longitudinal Balance

2) we seek and try to achieve the maximum Longitudinal Metacentric height we can

3) a hull profile that creates High pressure (H) forward and Low pressure (L) aft

4) a spoon-shaped bow that skims the water

 

[CarlosK2]

This humble sailboat was transformed in the Atlantic into a prodigious Surfboard ...

And Surfed with force 7 high and steep waves reaching in a controlled manner and without any problem a peak of 20 knots (!) while an old retired sailor smiled.

 

[CarlosK2]

Gliding is ...

falling ... (propelled by the powerful force of the earth towards the center of the earth: Tons x 9.8 Gravity x Sine of the Wave Angle) ... with style

Surf gliding is just Surf riding in bigger Waves

(falling with style)

(falling with style)

 

[CarlosK2]

Look

1:25-

A nice and cheerful video ... depressing from a scientific point of view: the bow sinks ... And on top of that it is a completely sharp bow (stem, forefoot)

A perfect HydroDynamic disaster with big waves

Which on the other hand proves that you can cross the Atlantic in anything that floats, for example in a well-inflated truck tire.

(The tragedy of sailboats is that they float, and that masks everything; the great luck in aircraft design is that airplanes fall and you have to be careful. It's not that we aeronautical engineers are smarter.)

(And look at this marvel of a bow in this hydrodynamic marvel by Thomas Harrison Butler.)

(Bow ... Up)

(Surf riding)

 

[CarlosK2]

(1 Ton WindSurf Board)

Drawing the forces to scale:

the hydroStatic Force number 4 is about 10,000 Newtons, 10 times larger than the hydroDynamic Force number 1 which is about 1000 Newtons in this 'photo' (Froude 0.60) just before take off and fly away.

 

[Dieter51]

Sorry, Carlos II. There was just another answer from XChat GPT 4.0.

I asked Chat GPT 4.0 the question I have about flat-bottomed boats. Here is the question and the answer:

Question:
For airplanes and boats, the wake is crucial in determining how much resistance the craft experiences. The wake doesn't create the resistance, but it indicates it. In sailboats that reach their maximum speed as displacement vessels, the waves in the wake become so strong that they break and show foam crests when there is more wind. A lot of energy is lost from the boat in this process. However, there are some flat-bottomed boats, sailboats that sail faster than their theoretical hull speed allows. This hull speed primarily depends on the length of the boat. Those that sail much faster than their hull speed allows, but do not plane, show a flat, wake-less trail, instead of which water exits under the transom with strong foam formation, as if a foaming agent were in the water. How is this phenomenon explained? (Goose from Michael Storer ist a good example)

Answer:
The phenomenon you described can be explained by the hydrodynamic and aerodynamic properties of these specific flat-bottomed boats, which sail faster than their theoretical hull speed. Here are some key points that contribute to the explanation:

1. Hull Speed and Its Limitations

• Hull Speed: The hull speed of a displacement boat is determined by the length of the boat and the resulting wavelength. The rule of thumb states that a displacement boat reaches its hull speed when the wavelength of the generated bow wave equals the waterline length of the boat.
• Speed Limitation: Upon reaching hull speed, the boat forms a large wave that it effectively "pulls along," leading to a significant increase in water resistance.

2. Dynamic Lift and Semi-Planing

• Dynamic Lift: Unlike displacement boats, flat-bottomed boats can generate significant dynamic lift. This reduces the wetted part of the hull and thus the water resistance. They move between the displacement mode and full planing, known as "semi-planing."
• Flatter Wave Formation: Since the hull displaces less water, the generated waves are flatter. This results in a less pronounced wake.

3. Turbulent Flow and Foam Formation

• Underwater Turbulence: At high speeds and with a flat bottom, these boats create turbulent flow under the hull. This turbulence causes the foam formation seen in the wake.
• Air Mixing in Water: The turbulence draws air into the water, leading to foam. This can be especially pronounced when the boat reaches high speed, and the point of water resistance shifts behind the transom.

4. Design and Weight

• Hull Design: Flat-bottomed boats often have a specific design that minimizes resistance and maximizes dynamic lift. The flatness of the bottom helps to lift the boat over the water rather than pushing through it.
• Weight Distribution: Balanced weight distribution helps to slightly lift the bow of the boat, which supports dynamic lift and further reduces resistance.

Summary
The phenomenon of a flat, wake-less trail and foam formation in flat-bottomed sailboats that sail faster than their theoretical hull speed is due to dynamic lift, the special shape of the hull, reduced water displacement, and the resulting turbulent flow under the boat. These factors allow the boat to exceed the limitations of traditional hull speed without transitioning into a fully planing state and explain the observed water and foam formation phenomena.

If this is the truth or if Chat GPT is guessing ...?

 

[Dieter51]

Here is my hypothesis:


Please take a look at the Goose by Michael Storer from minute 4:00 in the following video (

) before considering the possible explanation as to why the hulls of classic displacement boats are curved upward towards the stern, contrary to expectations. You can see the flat wake without the boat planing. The wake is characterized by strong foam instead of waves! It seems as if the boat is running through champagne with joy.

The possible explanation: The Goose also shows the typical curvature of the boat's hull, rising towards the stern over a wide area. At a certain point of this curvature, this shape leads to a peak in differential pressure over a broad front – a suction point where the flow detaches from the hull with foam formation.

However, foam does not provide a surface on which differential pressure can continue to build, so the flow now follows freely without differential pressure and exits smoothly with foam at the transom. An initial wave formation in the middle of te wake, water coming up, which would otherwise increase with speed, does not occur initially. In this state, the boat does not behave like a typical displacement vessel but assumes a special state known from aerodynamics, albeit with no advantageous effect:

Some wing profiles are known as leading-edge stallers, where the flow detaches from the leading edge at a certain angle of attack over the entire wing and does not reattach even if the angle is reduced. The NACA 23012 profile, known for its good lift, drag, and low moment coefficients, is such a profile. Its history is perhaps the bloodiest of all profiles because such a stalling behavior without reattachment usually means severe nose-down or spinning tendencies.

For a boat, it is different. Here, foam formation in the boundary layer can create a state where the boat experiences drastically reduced resistance. On one hand, part of the pressure drag associated with wave formation is eliminated, and on the other hand, part of the frictional resistance is reduced because the boat loses a significant part of the frictional resistance over the foam bubbles in the rear area. In the past, the military tried to achieve this advantageous state by blowing compressed air through thousands of tiny holes.

With the Goose and other boats where the flat width of the hull is large enough, the upward curvature of the hull in the rear area is sufficient to achieve the desired effect without a compressor. But beware: If a radius were added to the transom of the Goose, a classic pressure equalization would occur from there forward, and the boat would revert to the normal state of a displacement vessel, with significantly higher pressure and frictional resistance.

So, my explanation is that the strong curvature in the stern of a displacement vessel does not have to be a disadvantage but can transition the boat into a low-resistance state. Not all boats and not always, but when the conditions are right. It is a special state that has received no attention so far, although its occurrence and effect were logically predictable and still are.

 

[BOATMIK]

I've spent most of my life racing very efficient boats - and now find myself sailing and coaching the Oz Goose where we have almost 120 boats sailing.

I would summarise the Goose's hull hydrodynamics like this.

The only bow entry angle is the angle of the bottom relative to the water surface. The first approx 600mm of the bottom rocker is completely straight to simplify fitting a 150mm wide piece of wood as the mast step (support).

Then the curve increased geometrically towards the back of the boat with the last part of the curve a tangent to the transom angle for clean water release.

This forces the maximum curve of the rocker to be placed well back. I think it is slightly too much but it allows a couple of practical results.

1. crew has to sit where the displacement is - this is surprisingly far back at about 4ft from the transom.
2. As others have noted the increase in curvature at the back results in quite strong downforce relative to the quite high lifting forces forward. So the bow lifts with speed - and lifts strongly. This is useful even though it may reduce ultimate speed slightly because even at speeds of 4.5 to 5 knots the effect can be felt and it keeps the wide bow out of the waves whether going upwind or sailing downwind.
3. The boat is happy with crews of 1, 2 or 3 adults. Making the racing equal for both one person and two person teams and allowing the boat to still have excellent response with a coach and two students (all adults).

As a result there are two ways of keeping the wide bow out of the waves.

• sit further back - which causes a great deal of drag observable as turbulence behind the boat
• sail faster - which means adopting the modern upwind method of NEVER pushing the tiller (pinching) in a gust, but easing the sail instead. If pinching in a gust the speed drops half a knot, the centreboard is less effective and the boat goes sideways and the bow hits the next three waves. But if sailed straight, ease sail to maintain or slightly increase speed and the bow stay well clear of even quite high waves.

After sailing so many seasons in the goose now and competing with other good sailors for the national titles I would say the hullform could be optimised for more speed.

A bit less rocker for solo sailors and then over the evolution of several boats reduce the amount of curvature to find the optimal hullshape.

But then one would be lonely. And the flexibility of the boat which comes from its non optimisation has so many benefits.

I think this is a general truth. I remember a conversation between Julian Bethwaite (the designer of 18ft skiffs and the 49er among others) and two i14 designers. Julian was saying that more speed was possible by optimising the 49er further - but that in strong winds when the advantages of the changes would be at the maximum the boat would be much less likely to get to the bottom mark without capsizing or some other drama.

Optimising is good, but sometimes it can be valuable to dial back a bit to get the sailing qualities you want in a boat.

And then achieving optimal hullforms hasn't even been possible in the development classes like the i14, NS14, Cherub, N12, Merlin rocket, Lowrider and foiling moths and more of the restricted designs. There is always one more increment every few years that brings the boats up to a new level.

National 12 history 1936 to the 2020s (youtube.com)