Sailboats, wide sterns and bow-down trim - what is too much

- We'll put a spoiler on the back, I saw it on TV, and it'll be a Formula 1 car.

- Well, no, things don't work that way.

And conversely

The humblest boat in the port surfed safely and with control, reaching a top speed of 20 knots in a one-minute surf. I repeat: controlled by a very clumsy tiller pilot, thanks to:

(A1) longitudinal balance: when heeling, the bow didn't sink
(A2) longitudinal balance: when the wave arrived from the stern, the bow didn't sink

(B) the sum of the lateral hydro forces is aft of the center of gravity

(C) when pushed by the Froude-Krylov Force (the Wave induced surge Force) and pulled by the Almighty Force of the Earth (sir Isaak Newton = mass in kg x 9.8 x sin Wave slope) the Hull, upon acceleration, adopted an optimal bow-up and stern-down attitude, thanks to

C1) dynamic Lift under the bow and
C2) dynamic Suction under the stern, a stern which wasn't too difficult to submerge in the big ocean wave.

 

(The stern suction, the enemy of this industry... is the friend of slow and heavy yachts)

For a slow yacht (and without a 4 million Euro budget) the best approach is

(A) a superb longitudinal balance like a good classic
(B) good balance on the yaw axis, and
(C) creating hydrodynamic Suction at the stern, a stern that doesn't offer much resistance to sinking.



A wide stern is for high-performance racing sailboats with large budgets, or for yachts owned by multimillionaires, or for small 1-3 ton sport sailboats.

 

Plywood Epoxy Windsurfboard
44 square meters per Ton
0.60 Froude 8.8 knots

1) Lift: 3000 Newtons
4) Suction: 1000 Newtons

On a powerful sports sailboat a very wide stern makes sense

i) It allows for hoisting that much sail area
ii) It eliminates lateral low pressures (suction) at the stern
iii) It reduces drag at high speed (> 0.6 Froude) and
iv) Because it doesn't dip much, it allows for better control of the Angle of Attack

The advantage of suction is that it doesn't depend on the angle of attack; it's much more robust. Emphasizing suction is the logical solution for a large or slow or heavy yacht.

 

A slow yacht with

a) a wide stern to facilitate a large area where the vertical component of the Froude-Krylov Force lifts the stern and sinks the bow

a stern that's easy to lift and difficult to sink in the wave, and

b) on top of that unbalanced longitudinally, and

c) to top it all off with a large keel to trip it up ... because the keel hydroDynamic center is forward the yacht center of gravity ...

is madness, but typical in this very strange industry, strange seen from the aeronautical industry.

 

A problem known for a century... and this industry has managed to make it worse.

What will be the next crazy thing we see: a motoryacht with a single 75-meter-high mast at the bow, unable to hold its bow into the wind?

 

Another typical example of such heel attitude : mathematically speaking the trim is bow down by a lot, but actually the bow remains at the water surface.

 

A yacht that is

- slow relative to the speed of the waves

and most typical yachts 5-10 tons that hoist around 10 square meters per ton are slow relative to the speed of the waves

+ with a very wide stern, and the Froude-Krylov Force depends on the encounter speed, and its vertical component depends on the Area and the position on the wave

- with its Center of Gravity (CG) forward of the center of Flotation (CF) where the Pitch axis of rotation passes and, moreover,

- when heeling, the bow dips further because its heavy quarters shift the center of Buoyancy aft

- and a big modern keel capable of creating a monstrous lateral force forward of the yacht's center of gravity

a hidro-Dynamic disaster

 

And vice versa

The Mini Transats were completely unbalanced longitudinally until yesterday afternoon

and it wasn't too bad

(1) because a Mini Transat hoists a huuuge amount of sail and sails at or near the speed of the waves: the collision speed is zero, very low, or low, and

(2) Because at the speed it sails, it creates hydrodynamic Lift and sails on top of the bow wave: it doesn't dig its bow into the bow wave.

 

Aleksey Krylov - Wikipedia https://en.wikipedia.org/wiki/Aleksey_Krylov

Froude and Krylov push the slow-moving yacht forward and stern up, and Sir Isaac Newton drags the yacht badly (yaw, roll, and pitch) with the Almighty Force of the Earth ...

... the yacht lies flat like a boxer on the canvas, the autopilot gets burned or the helmsman's brain gets fried, the sailing is very, very unpleasant, the sailor in the cabin gets hit, and at the limit the yacht capsizes

"Freaking Frustrating" that is Fuxxx Frustrating

 

Conclusion

Answer to the question posed

It depends on the speed, and that depends on the type of sailboat and also on the crew: the ability to hoist sails for good speed and multiple helmsmen.

And this is The root of the problem: the wide sterns of high-performance racing yachts and yachts with large, experienced crews are imitated on slow yachts with small, inexperienced crews.

As stated above: put a stern spoiler on an Amish wagon and suddenly it's a Formula 1 car. A superstition or a crazy belief or, to put it more simply: a current trend that suits large aft cabins and a large bathing platform

 

Right, and without actual CE and CLR it's very difficult to say anything about the YAW moment.
Thanks, found another similar video. Looks like the twin rudder is a necessity.

 

Unfortunately CarlosK2 has hijacked your thread like he does throughout the forum. This is interesting topic. When the boat heels, the wide sterns make a large part of the hull come out of the water on the windward side creating a large righting moment. With two rudders the steering does not suffer. I see a parallel to a proa.

 

The answer

The answer to whether a little or a lot of longitudinal imbalance in a sailing yacht is acceptable depends on:

(A) Speed; a MiniTransat that hoist 60 square meters per ton can be very unbalanced, and it's not a cause for concern.

But at this point, it's important to consider that designing a fast sailboat larger than 1-3 tons is extremely difficult.

At 3-3.5 tons, we're already talking about 180,000-300,000 Euros, and at 8 tons, it's 4-7 million Euros.

(B) Number of expert helmsmen

(C) Price of the autopilot

At one extreme, we would have a powerful 1-ton sports sailboat that hoist 44 square meters per ton And ... And superb longitudinal balance, almost like a classic British design by Thomas Harrison Butler: it would glide over the sea with silky smoothness and be like a well-behaved, obedient child.

At the other extreme, we would have a slow vessel that carries the house on its back, hoists 10 square meters per ton, and handles so badly that not even a very expensive autopilot can control the yacht. And on top of that, there isn't a handful of professional or semi-professional helmsmen, but a single helmsman, a seasick wife, and some seasick invited friends.

What's left for us to see?

After seeing slow, clunky vessels with the wide sterns of a MiniTransat, what's left for us to see in this baffling industry?

Perhaps a large motor yacht with an enormous engine room and, on top of that, a solitary 75-meter-high mast at the bow, incapable of turning the bow into the wind.



Boat Design

Earl Boebert, Paper PDF

https://www.google.com/url?sa=t&sou...r4kDegQIGBAD&usg=AOvVaw2_n5ZVRFZ3FsdBaso_3uRA

Turner

Turner Theory https://www.boatdesign.net/threads/turner-theory.33052/

Center of Flotation

center of flotation calculation and implications? https://www.boatdesign.net/threads/center-of-flotation-calculation-and-implications.30857/

- I'm a very busy man and I don't have time for those convoluted, century-old schemes.

-Okay

Simple way for lazy people:

1) Calculate the longitudinal position of the Center of Flotation -Achtung!- without cheating: the yacht isn't always going to be moored in port with the crew drinking gin-tonics, keep in mind that a large ocean wave will soak the entire stern, and consider the effect of heeling

B) Place the Center of Buoyancy slightly aft of the Center of Flotation

C) Place the Yacht's Center of Gravity slightly aft of the Center of Buoyancy.



Example

This client wants a sports sailboat with a very very wide, gigantic stern like a MiniTransat

The center of Flotation is at 58% LWL

(curiously the same as an 1979 IOR yacht with a seemingly very narrow stern, but heavy quarters and a very sharp bow, because in this case the Bow is very wide)

Well

Center of Buoyancy: 60%
Center of Gravity: 62%

62% like a Billonaire's Dinghy





Look at the position of the keel and the mast.

And this is an extremely powerful yacht.

Designing a slow yacht with a wide stern and the keel forward is criminal.

Boat Design

Earl Boebert, Paper PDF

https://www.google.com/url?sa=t&sou...r4kDegQIGBAD&usg=AOvVaw2_n5ZVRFZ3FsdBaso_3uRA

What the author said

The author said: If I were to write it again, I would insist even more on placing the center of Buoyancy at the center of Flotation.

---

I would like those who defend a slow yacht with a center of gravity forward of the pitch axis that passes through the center of Flotation ... and a huge keel forward of the center of Gravity to tell us their reasons, in addition to the well-known ones: (1) large cabins to rent the yacht in the Mediterranean during the summer to partying young people, (2) a brochure with a beautiful lady in a swimsuit on a spectacular bathing platform, and (3) imitating high-performance sailing yachts as has been done since 1850.



The Satanita - Wikipedia https://en.wikipedia.org/wiki/The_Satanita

"Satanita," what a name!

Max Michael Munk's Moment (MMMM)

+ 1) due to leeway/yaw
+ 2) due to heel/roll
+ 3) due to bow down =

huge hydrodynamic yaw moment plus the aerodynamic moment due to the force of the sails and their leverage due to the heel

The yacht is uncontrollable and sinks another yacht.

We can do worse ?

Yeeeah, of course we can.

We install a large, modern keel capable of generating enormous lateral force and position the hydrodynamic center of that keel forward of the yacht's center of gravity.

And a very wide stern so that when sailing downwind, the vertical component of the Froude-Krylov force will lift the stern and sink the bow.



"Satanita"

"heavy quarters" in relation to the bow

Roll/Heel = trim Bow-Down

and Mr. Max Michael Munk goes crazy





Aleksey Krylov - Wikipedia https://en.wikipedia.org/wiki/Aleksey_Krylov

A Slow Yacht

Wave force depends primarily on the interaction speed between the yacht's speed (v) and the wave's speed (c), as well as the area (A) and the position on the wave:

on a 6-meter sailboat, the vertical component will be at the center of Flotation or slightly aft; on a 12-14 meter yacht, the vertical component will be aft of the center of Flotation, a center that has also shifted aft with the arrival of the wave and/or due to the yacht's heel.

(A) The force of the Wind, the force of the Sea, and the force of the Earth push the bow down

The pitch rotation axis passes through the center of the flotation (CF)

and, on top of that

(B) when the yacht heels, the stern lift and the bow dips.

Furthermore Roll, Pitch and Yaw reinforce each other in feedback loops

Result: seasick wife, seasick guests, the very expensive autopilot unable to control the newly purchased yacht, and the owner (a naval architect, to add insult to injury) condemned to 16 hours at the helm.



Pogo 2

A MiniTransat is so fast that its significant hydrostatic longitudinal imbalance isn't a concern because Downwind the waves don't push it and its behavior depends on dynamic forces.

However, a good semi-classic longitudinal balance is worthwhile to avoid relying on a very expensive autopilot and, above all, to improve upwind comfort.

Upwind Comfort in a 1 Ton Sportboat

This can be enhanced by positioning the sailor near the center of Flotation, and providing a superb 60 cm backrest, and carrying 220 liters of water Ballast, which makes the boat more powerful and much more comfortable by shifting the center of gravity even further aft.

1 Ton Plywood Epoxy Windsurfboard
964 Newtons





Pogo 1

The widespread desire to emulate high-performance racing sailboats ...

I think the solution lies in going to the origin: a MiniTransat.

A more comfortable and safer MiniTransat, that's all.

Less is more.

Why carry around a whole house and a seasick wife when she can comfortably travel by bus, train, or plane

Without spending 4-5 million euros, this is the solution i see for a beautiful, modern sailboat with an immensely wide stern.

 

5-10 ton yacht house (man and wife) freed from the obsession of imitating high-performance racing sailboats

+ Spoon bow, now even used on high-performance racing sailboats to mimic the "scow" design; wide bow

+ Ballast and center of gravity of the yacht aft (!) of the center of Flotation; twin keels

+ Hull profile designed to create (H) high pressure (lift) forward and a lot of (L) low pressure (suction) under the stern to dip the stern into the waves and adopt a proper hull attitude: stern down, bow up, crossing an ocean with large waves

+ Jib, StaySail with boom (!), big Gaff for downwind, and mizzen for "jib [StaySail] and jigger" upwind in strong winds

 

The Globetrotters https://ericwsponberg.com/boat-designs/globetrotters/

This family

In other words: well-balanced yachts with comfortable rig