Naca Airfoils shape and rocker oft flat bottomed boats

Hello Boatmic,

Thank you for your comment, which provides some practical insights. However, I must clarify that my goal isn't to improve an existing boat, not even my own. I simply wanted to understand why some flat-bottom boats exhibit good performance, i.e., low resistance, assuming the performance of the sails is set equal for all boats. These flat bottomed boats perform better under certain conditions than theory would suggest, without any clear explanation.

The Goose in the video showed foam emerging from under the stern, leading me to suspect that a transition in flow conditions under the bottom occurs there. A video from the underside without disturbing the flow there would be a good thing. That's all. I neither could nor wanted to improve an Optimist or a Goose. I'm just curious to understand these phenomena. I've been curious for over 70 years May be to long

 

Carlos II, thanks for the videos and the comments. My hangliders were from the old style, triangular. Glide ratio 4 to 1, one had to run long and fast.
I remember the Sinah of Buttler, very equilibrated, the book told us/me. Selfsteering.

Best Dieter

 

Well, the obsession with drag is one of the things that has destroyed sailboat design. A tenth of a knot is certainly a gigantic amount in a race/Regattas, and a meaningless amount with respect to good or bad performance at sea.



Residual Resistance of Displacement Vessels https://www.mdpi.com/2077-1312/8/6/400

Anyway: Drag on a sailboat can be recreated as the sum of

(A) moving/displacing the water to the sides, what we could call "main part" of Michell 1898
+
(B) passing the water underneath in a modified Savitsky adjusted to sailboats 2025



I mean that there is a trade-off between passing water over the sides and passing water underneath, and in some cases the latter may be more beneficial.

Maybe that's what you're looking for about flat bottomed sailboats.

 

There is one crew member in the bow, that is, say 75-80 kilos in a sailboat of 800-880 kilos loaded crossing the Atlantic with two crew members; but it is the same conclusion:

It is completely absurd to sail with the ballast of a sailboat and the Center of Gravity of a sailboat forward of the Pitch axis of rotation, with waves lifting the stern and sails sinking the bow.

(A few years ago a boom killed a 75 year old sailor crossing the Atlantic in a new yacht. One more case of the typical nonsense that is a sailboat)*

We need to move the mast aft, the ballast aft, the center of gravity aft, and a friendlier bow.

And Low pressure (L) suction, at the stern, which fulfills the function of the propeller of an engine: Bow up



To create Lift at the Bow we need Angle of Attack, part of the angle of attack is provided by the inclined shape of the hull profile, and another part is provided by the small Hull Wave (which can be destroyed by the big Sea Wave).

The good thing about Low Pressure is that it does not depend on the Angle of Attack.

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*: 73: "On the 26th/27th [nov 2021] around midnight, a mayday call was sent requesting a medical evacuation. Unfortunately, the injured crew member was pronounced dead before outside assistance could be provided."

Force 7-8, Waves 4-5 m high

Pitch ---> Yaw : Boom ---> Head

 

The Wright brothers of the outlaw gang (Yaw, Roll, Pitch) managed to control Roll and Yaw, but the lack of Longitudinal Balance caused the pilot to kill himself.

The Bleriot XI is the first modern airplane where the pilot did not play head or tail to kill himself.

 

Longitudinal Balance

Airplane: control that the nose does not go Up.

Slow sailboat: control that the bow does not go Down.

 

In the video on the previous page under Force 7 conditions we hear a young Englishman (with a restrained expression) say:

-"Freaking Frustrating"

The image and the sound are cut (and then in silence we hear Captain Haddock) and then the young Englishman (of restrained emotion) says: -unstable boat, even with me at the helm I can't get to go straight down a Wave.

 

(How is it possible that we have been circling around two axes of rotation for a hundred years: Yaw and Pitch)

Sailboat (relatively slow hoisting 10-15 square meters of sail per ton) properly Balanced.

CE: Center of Force of Sails

CF: Center of Flotation

(through which the Pitch axis passes)

CB: Center of the volume of water displaced by the stationary hull.

CG: Center of Gravity

H_CK: HydroDynamic center of the keel-centerboard

H_CLR: HydroDynamic center of the calculated sum of

(A) Hull with 3-6 degrees of Yaw and 15 degrees of Heel
+
(B) Keel-centerboard, and
+
(C) Rudder without moving it, without touching it, without looking at it.

 

(Classic solution)

-I don't want to

-I don't want to move the Mast aft and center it at 50% LWL (1/2)

-Okay, then we offer you the Classic solution



(Modern solution)

 

Sailboat, Yacht.

Artifact designed for sailing in summer with 10-15 knots of wind.

With F 7 the artifact tends to be insufferable, and with F 10 desperate for its bad behavior you jump into the water so that a helicopter can pick you up and the sailboat turns and crosses the ocean alone.

Sailboats to enjoy (?) You are crazy

Our heroic tradition and our tribal superstitions demand sailboats for brave sailors without fear.

 

Bonus Track

The capsizing of a sailing ship prophesied by Scott Russell in the 19th century

John Scott Russell - Wikipedia https://en.m.wikipedia.org/wiki/John_Scott_Russell

The paradox of transverse hydroStatic stability

The more difficult it is for the Wind to capsize a sailboat that holds astronomical amounts of sail ("Stability under Canvas") the easier it is for the Waves to capsize it.

This problem of the modern sports sailboat, how to stand still safely in the waves, has a really cheap solution: take the rudder blades out of the water and anchor in the sea like Robert Manry.

 

The empirical reality is that there is no interest in sailing yachts beyond the big competitions with millionaire budgets and beyond summering in Mallorca.

In fact, and without going any further, the well-known Principles of Yacht Design after five editions, there are no Waves, no Hull Yaw Moment, nothing, nothing that reminds us of the Real world beyond 10-15 knots of wind.

So welcome not the wide bows, but the Scow, at least a bow difficult to sink, or better: let's consider the monohull as impossible and welcome the catamaran.



-Nah, before delivering the Monohull to the bonfire of St. John on the summer solstice to burn the old and bring forth the new.... I firmly believe that the Monohull is salvageable.

 

From my point of view

Design Category E, Polar Regions Exploration

Design Category Aa, 10 Tons to enjoy Force 10 in the South Indian Ocean

Design Category Ab: 1 Ton to Force 8

Design Category C: Coastal cruiser

Design Category D: Dinghy

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(We forgive the supreme pontiff Van Oossanen for having invented that spherical joke, whichever way you look at it, called "STIX")

Van Oossanen Naval Architects - Naval Architechts, Yacht Designers, Hydrodynamicists and Consultants https://oossanen.nl/

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| Yacht Stability – What is STIX?Performance Boating https://www.performanceboating.com.au/yacht-stability-what-is-stix/

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(Now that nobody listens to us: to begin with, the supreme pontiff confuses the Wind and the Waves. And the capsize of the huge IMOCA "Hugo Boss" is the empirical confirmation. And for another, he leaves out the whole bulk of the matter: good or bad behavior, as we see in the video of the desperate crew jumping into the water. And for more joke makes three different categories of objectively small sailboats of 1-3 tons measuring the size of its length that is like making different categories of short men of 1.60, 1.62 and 1.64 meters tall using as a criterion the height instead of the color of the hair or the smell of the feet)

(And the bulk of the safety issues do not depend on the size of the length)

 

Wow, CarlosK2, good sources and explanations.
You seem to have some experience and knowledge in this field that I miss - still.
But it is saturday morning, my wife rushes around me shouting: Today we have to .... The garden is the last point, I did not hear the word sailing.

Tomorrow it is all rain here, also on the lake. So I will have time to read your comments and sources.
Ah, wait: The toilet for a guest will not spend water without waiting 6 hrs for a refill, she shouts additionally.
I guess, at least one hour of next sunday I have to look for the failure. But then ...
My little wife is the best one on earth - for me
Best - Dieter