Sailing boats' Stability, STIX and Old Ratios

[Vega]

Originally Posted by YachtManuals

I have seen this mistaken argument in several places. In fact any object floating on the inclined surface of a wave is accellerated sideways (in the direction on wave travel) so that apparent gravity is perpendicular to the surface of the water. The experiment would show that a pendelum suspended from anything floating on waves will always point straight down to the "deck" of the object. This means that deep keels and wide hulls both "heal" the same amount on the side of waves (assuming only the wave action). This can also be explained by the motion of a water particle on the surface of waves: they describe a circular motion. Otherwise the water on the wave surface would be rushing down to the bottom of the trough. In fact the water on the surface is perfectly happy to stay put, because it experiences the force of gravity as always pointing down perpendicular to the SURFACE of the wave, just like the vessel.

Yes, I had posted an image that shows otherwise. That image belongs to a presentation on boat stability by Professor Paul Miller (D. Eng.P.E.). He is the Professor of Naval Architecture at the United States Naval Academy.

Here we have that image again:

[Guillermo]

From Marchaj's "Seaworthiness, the forgotten factor"

[Man Overboard]

YachtManuals :
In fact any object floating on the inclined surface of a wave is accellerated sideways (in the direction on wave travel) so that apparent gravity is perpendicular to the surface of the water.

This is not necessarily true. An object floating on the surface does not respond the same as a particle of water immersed within the orbital flow of a wave pattern. We often explain wave motion by the circular motion of water particles; I believe it is more instructive to view waves as an orbital energy flow(s) through the surface of a body of water. This energy flow passes through the water in a rotation manner, and in doing so, acts upon water particles in suspension. Each particle of water moves in a direction that is a result of two components of acceleration: ‘g’ acceleration do to gravity, and ‘a’ centrifugal acceleration.

Centrifugal acceleration can be shown by the equation below_

The source of rotational energy flow is due to the fact that the energy flow uses water as a vehicle for propagation. Energy in effect passes from one particle of water to the next (very quickly) and in doing so imparts motion. That motion is resisted by gravity, and viscosity. Because there is less resistance do to viscosity at the very surface, than deeper within the water column directly below the orbit, the water moves more readily, and establishes a rotational flow. This rotational flow increases in magnitude as long as it is being energized by air flow across the surface. It should be mentioned that the gravity component both de-accelerates, and accelerates the water particles depending where they are in the rotational energy field. It is much easer, and more efficient for the energy to pass to adjacent water particles of microscopic size than to pass to large floating objects. In addition, floating objects have various other forces acting on them, especially a boat. A deeply immersed boat such as the example posted by Guillermo may be subject to subsurface energy propagation. (Sub-surface orbital flow) Although there is orbital forces imparted to the vessel, there may also be other forces that have an over riding influence: healing forces, components of lift from the keel, rudder, and sails, etc. Gravitational and buoyancy forces on the boat as a whole may have a much great magnitude of force than the energy absorbed by angular velocity. It should be noted, in the case of sailboats, the components of lift from the sails, rudder, and keel also impart rotational energy fields which disturb and alter wave patterns. A moving sailboat creates gyroscopic energy forces that resist and dampens the impact of waves. A fast boat can be more stable than a slow boat. (but not always)

[MikeJohns]

Originally Posted by YachtManuals

……………..In fact any object floating on the inclined surface of a wave is accellerated (sic) sideways (in the direction on wave travel) so that apparent gravity is perpendicular to the surface of the water............

Any attempt to simplify vessel-wave interaction to such trivial “Fact” is not a sensible approach. Also I would certainly question the source of this mis-information.

[Guillermo]

Another new light kid in the block, with an strange categorization.

Sun Odissey 36i is said to be a Category A boat (So STIX >= 32), but with the posted info I cannot get more than a 26,67 figure for STIX, even assuming downflooding angle coincides with AVS (+/- 123º). Minimum AVS for this boat to qualify for A category = 117,6º

Stated displacement of 5725 kg is for lightship condition so I'm estimating a MOC of 6193 kg and I'm assuming posted curve is for that load condition. Considered keel is the standard 1,94 m one.

Even so, I get this:

Base Length Factor (LBS) = 10,207
Displacement Length Factor (FDL) = 0,971
Beam Displacement Factor (FBD) = 1,061
Knockdown Recovery Factor (FKR) = 1,056
Inversion Recovery Factor (FIR) = 1,015
Dynamic Stability Factor (FDS) = 0,574 (because of the poor GZ curve)
Vaw Not aplicable
Wind Moment Factor (FWM) = 1,000
Downflooding Factor (FDF) = 1,250

Delta = 0

STIX = 26,667

DESIGN CATEGORY B

How the hell they get a STIX of 32 or higher...? Do the posted GZ curve correspond to MOC?

Once again: It would be very nice if boatbuilders and magazines decide to provide more complete and clear info on stability.

And now here its 'old ratios'

Length/Beam Ratio L/B = 2,83
Lwl/Bwl Ratio Lwl/Bwl = 3,05
Ballast/Disp Ratio W/Disp = 0,25
Displacement/Length Ratio D/L = 181,30
Sail Area/Disp. Ratio SA/D = 17,39
Capsize Safety Factor CSF = 1,97
Motion Comfort Ratio MCR = 23,51
Heft Ratio HF = 0,86
Roll Acceleration Acc = 0,12 G's
Stability Index SI = 0,77

[Vega]

Originally Posted by Guillermo

Another new light kid in the block with an strange categorization.

Sun Odissey 36i is said to be a Category A boat (So STIX >= 32), but with the posted info I cannot get more than a 26,67 figure for STIX, even assuming downflooding angle coincides with AVS (+/- 123º). Minimum AVS for this boat to qualify for A category = 117,6º

Stated displacement of 5725 kg is for lightship condition so I'm estimating a MOC of 6193 kg and I'm assuming posted curve is for that load condition. Considered keel is the standard 1,94 m one.

.....

How the hell they get a STIX of 32 or higher...? Do the posted GZ curve correspond to MOC?

Once again: It would be very nice if boatbuilders and magazines decide to provide more complete and clear info on stability.

It looks to me a nice boat with a good stability curve.

If you ask Jeanneau, or even any Jeanneau dealer they would provide you with information on boat stability, in lightship and loaded condition.

It is obvious that the magazine as messed out with that curve (it is not the first time). I believe Jeanneau provided a correct one.
You should know that boat, with those characteristics (beamy, with a bulb and that ballast) could not have a Max GZ lower than 0.4m. I believe that it is a RM curve and that where is 0.4 should be 4.

Of course I can ask Jeanneau for the correct curve, but so can you .

[Guillermo]

The GZ curve could very well be that one, precisely due to the beam, low ballast/displacement ratio, high freeboard and upwards volumes. But I'll ask them and PBO as you suggest.

Cheers.

[Vega]

Originally Posted by Guillermo

The GZ curve could very well be that one, precisely due to the beam, ... But I'll ask them and PBO as you suggest.

Beam increases form stability and therefore increases MaxGZ and MAXRM. Why do you think Open 60's and Class 40 are boats with large transoms, or as you like to call them beamy boats?
Take a look at the MaxGZ of a cat...it's huge and all form stability (beam).

The problem with large transom boats is not MaxGZ, but AVS and inverted stability. The Jeanneau 36i comes out with fling colors, with a reasonably good inverted stability and with an AVS (125º) slightly above average for a modern boat.

Look at this RM curve. Again the magazine messed up in several ways . They call it a GZ curve and the units.... well, but I hope you can understand it. It is of a powerful and fast boat, but this time with a very low AVS (109º) and a big inverted stability.

I believe you should focus on boats like these. (it's a Glamorous and beautiful new model and I like the design, with the exception of the stability curve )

[Guillermo]

I'll answer you when I get the info back from Jeanneaou or PBO.

[Vega]

Originally Posted by Guillermo

The GZ curve could very well be that one, precisely due to the beam, low ballast/displacement ratio, high freeboard and upwards volumes. But I'll ask them and PBO as you suggest.
…..

I'll answer you when I get the info back from Jeanneaou or PBO.

Well, if you insist, but I am absolutely sure that 0.39 is not the MaxGZ value. The Max GZ of this kind of boats (jeanneau 36i) lies between 0.6 and 0.8. Most common value is a little bit over 0.7.

A Max GZ of 0.39 for the Jeanneau 36i (as it is on that curb you have posted) is inevitably a big nonsense.

Assuming that curb (the one you posted for the Jeanneau) is a RM curve I will post a comparison between the RM curves of the Jeanneau 36i, the Bavaria 36 and the Halberg-Rassy 342.

The Bavaria is the lightest boat, with about 5T for a ballast of 1340kg, 3.59m beam and a draft of 1.90.
The Halberg-Rassy weights 5.3T for a ballast of 1950kg, a 3.42 beam and a draft of 1.82.
The Jeanneau 36i weight 5.7T for a ballast of 1571kg, a 3.59 beam a draft of 1.95 and is the only one with a bulb.

By the way, the Bavaria MaxGZ is around 0.73 and the Halberg-Rassy MaxGZ is around 0.7.

For the Bavaria curb I will consider the one that doesn’t consider the influence of the cabin. If that influence is considered the AVS jumps to 131º and the inverted stability is a lot smaller.

[Vega]

Well, I have not found out that curve, but I have found out the MaxRM value for the Jeanneau 36i: 3,9Txm, the same max value on that curve you have posted (0.39). With the MaxRM and the boat displacement you can calculate the MaxGZ

[Vega]

I believe that the Puzzle is almost complete.

I have found another published stability curve for the Jeanneau 36I. This one was published in the June edition of Yachting Monthly.

I believe that this one, published as a GZ is also a RM curve.

At least on this one they didn’t change the decimals on the numbers. Can someone explain to me what is the meaning and the units of these figures (from the curve)??? :

1000.000, 2000.000, 3000.000 and so on.

I believe that these numbers refer to the RM curve, the unit is kg and they have 3 decimals.

If this is correct then we have here for the MaxRM 3200kgxm (3.2Txm) a smaller value than the MaxRM from the curve posted by Guillermo (3.9Txm). The AVS is also different, in this curve the AVS is 130º and on the other one 125º.

These differences are consistent, for this type of boat, with the differences between a RM curve in Max load sailing condition (curve posted by Guillermo) and a RM curve for the same boat in Min. load sailing condition (this one). Bigger RM and smaller AVS on Max load condition and Smaller RM and bigger AVS on Min Sailing Condition.

This would make this boat even a safer boat. I had thought that the curve posted by Guillermo was the Min sailing condition curve and that the 125º AVS was for that condition.
After all, the boat’s AVS for Min.Sailing condition is 130º and that is not good, is very good for this type of boats. There are few modern boats of this type that have a 125ºAVS in Max load condition.

What do you say, Guillermo?

[Guillermo]

I say we have to wait for an answer from PBO/Jeanneau/Marc Lombard (I've written to all of them).
Perhaps you're right and curve is a RM one, but as PBO's tested version is the shoal draught one and posted info is not clear enough, we'd better wait.

[Vega]

Originally Posted by Guillermo

I say we have to wait for an answer from PBO/Jeanneau/Marc Lombard (I've written to all of them).
Perhaps you're right and curve is a RM one, but as PBO's tested version is the shoal draught one and posted info is not clear enough, we'd better wait.

I have received the stability curves from Jeanneau. They seem to agree with me that RM curves are more appropriated to judge the boat stability , because they only have sent me RM curves.

It looks that I was right.

The curve posted by you and published by PBO magazine is a RM curve from the GTE version (draft 1.94) with the boat in MaxSail condition. The curve published by Yachting Monthly and posted by me it is a RM curve with the boat in MinSailing Condition (the MinSailingcondition curve for the two different draft versions is identical).

[Guillermo]

It seems you have a nice relationship with Jeanneau, as I got nothing from them yet (nor from PBO or Marc Lombard). You got that info even in the middle of the 1st of May 'bridge'....! Unbelivable

You now may realize one the hazards of releasing RM curves instead of GZ ones: As the ISO norms use the GZ curves, even specialized and reputed magazines as YM and PBO go for the wrong.

(By the way: Your last posted curve seems to be also wrongly displaying units at the ordinates axis. Those cannot be mt)

Surprising to know the MLC gives a lower STIX than the MOC. Uses to be the contrary. I'll check the STIX numbers for your posted curves, when I get trustable data on considered displacements and downflooding angles. Can you provide that info?