Sailing boats' Stability, STIX and Old Ratios

Here you are, Antonio:

NEPTUN 32
data from: http://www.caputatse.de/boot/neptun_klasse/werft_prospekt_neptun.pdf

INPUT
Lh = 9,60 m
Lwl = 7,50 m
Bmax = 3,00 m
Bwl = 2,70 m (?)
Draught T = 1,60 m
Body draught Tc = 0,50 m (?)
Moulded depth H = 1,75 m (?)
Disp = 3500 kg
Ballast = 1500 kg
Sail area = 40 m2
Mast height = 11 m (?)
Heeling Arm = 5,04 m (?)
Power = 15 KW


OUTPUT
Length/Beam Ratio (2Lwl + Lh)/3B = 2,73
Lwl/Bwl Ratio Lwl/Bwl = 2,78
Ballast/Disp Ratio W/Disp = 0,43
Displacement/Length Ratio D/L = 231,40
Sail Area/Disp. Ratio SA/D = 17,63
Power/ Disp. Ratio HP/D = 2,64 HP/tonne
Hull speed HSPD = 6,65 Kn
Velocity Ratio VR = 0,98
Capsize Safety Factor CSF = 1,99
Motion Comfort Ratio MCR = 21,12
Roll Period T = 2,18 Sec
Roll Acceleration Acc = 0,15 G's
Stability Index SI = 0,81
Angle of Vanishing Stability AVS = 125 º

Estimated STIX = +/- 37 (to be taken with great care)


Cheers.

 

Thanks a lot !!!!!

 

Antonio:
Please note SA/D ratio is 17,63

Cheers.

 

NEPTUN 32

INPUT

Lh = 9,60 m
Lwl = 7,50 m
Bmax = 3,00 m
Bwl = 2,70 m (?)
Draught T = 1,60 m
Body draught Tc = 0,50 m (?)
Moulded depth H = 1,75 m (?)
Disp = 3500 kg
Ballast = 1500 kg
Sail area = 40 m2
Mast height = 11 m (?)
Heeling Arm = 5,04 m (?)
Power = 15 KW

OUTPUT

Length/Beam Ratio (2Lwl + Lh)/3B = 2,73
Lwl/Bwl Ratio Lwl/Bwl = 2,78
Ballast/Disp Ratio W/Disp = 0,43
Displacement/Length Ratio D/L = 231,40
Sail Area/Disp. Ratio SA/D = 11,46
Power/ Disp. Ratio HP/D = 2,64 HP/ton
Hull speed HSPD = 6,65 Kn
Velocity Ratio VR = 0,98
Capsize Safety Factor CSF = 1,99
Motion Comfort Ratio MCR = 21,12
Roll Period T = 2,18 Sec
Roll Acceleration Acc = 0,15 G's
Stability Index SI = 0,81
Angle of Vanishing Stability AVS = 125 º

Estimated STIX = +/- 37


PUMA 34

INPUT

Lh = 10,35 m
Lwl = 8,20 m
Bmax = 3,35 m
Bwl = 3,02 m
Draught T = 1,85 m
Body draught Tc = 0,63 m
Moulded depth H = 1,75 m
Disp = 5400 kg
Ballast = 2400 kg
Sail area = 55 m2 (WILD GUESS)
Mast height = 15.5 m (WILD GUESS)
Heeling Arm = 6.94 m (WILD GUESS)
Power = 13,5 KW

OUTPUT

Length/Beam Ratio (2Lwl + Lh)/3B = 2,66
Lwl/Bwl Ratio Lwl/Bwl = 2,72
Length/Draught Ratio Lh/T = 5,59
Beam/Draught Ratio Bmax/T = 1,81
WL beam/Body draught Bwl/Tc = 4,79
Ballast/Disp Ratio W/Disp = 0,44
Displacement/Length Ratio D/L = 273,17
Sail Area/Disp. Ratio SA/D = 18,16
Sail Area/Wetted surface SA/WS = 2,43
Power/ Disp. Ratio HP/D = 1,54 HP/ton
Hull speed HSPD = 6,95 Kn
Potential Maximum Speed PMS = 7,82 Kn
Velocity Ratio VR = 1,13
Best motoring speed (1.1) CSPD = 5,71 Kn
Capsize Safety Factor CSF = 1,93
Motion Comfort Ratio MCR = 25,85
Roll Period T = 2,96 Sec
Roll Acceleration Acc = 0,10 G's
Stability Index SI = 0,88
Angle of Vanishing Stability AVS = 126 º
Dellenbaugh Angle DA = 24,06 º (14 kn wind)
Wind pressure coefficient WPC = 0,88

Estimated STIX 37,14


FIRST 345

INPUT

Loa = 11,00 m
Lh = 10,55 m
Lwl = 9,08 m
Bmax = 3,49 m
Bwl = 3,14 m
Draught T = 1,45 m
Body draught Tc = 0,57 m
Moulded depth H = 1,75 m
Disp = 5700 kg
Ballast = 2050 kg
Sail area = 50,73 m2
Mast height = 15,01 m
Heeling Arm = 6,58 m
Power = 21 KW

OUTPUTS

Length/Beam Ratio (2Lwl + Lh)/3B = 2,74
Lwl/Bwl Ratio Lwl/Bwl = 2,89
Length/Draught Ratio Lh/T = 7,28
Beam/Draught Ratio Bmax/T = 2,41
WL beam/Body draught Bwl/Tc = 4,69
Ballast/Disp Ratio W/Disp = 0,36
Displacement/Length Ratio D/L = 212,37
Sail Area/Disp. Ratio SA/D = 16,16
Sail Area/Wetted surface SA/WS = 2,05
Power/ Disp. Ratio HP/D = 2,27 HP/ton
Hull speed HSPD = 7,31 Kn
Potential Maximum Speed PMS = 7,90 Kn
Velocity Ratio VR = 1,08
Best motoring speed (1.1) CSPD = 6,00 Kn
Capsize Safety Factor CSF = 1,97
Motion Comfort Ratio MCR = 24,00
Roll Period T = 2,78 Sec
Roll Acceleration Acc = 0,12 G's
Stability Index SI = 0,80
Angle of Vanishing Stability AVS = 120 º
Dellenbaugh Angle DA = 16,13 º (14 kn wind)
Wind pressure coefficient WPC = 1,31

Estimated STIX 38,68

My friend wants a safety boat for the Atlantic Crossing, but rather the smallest the better. Wich would be your choice Guillermo?

 

Puma 34, if in good condition.

Cheers.

 

I thought the same, but it seems the Neptun has an incredible and safety superstructure, unless you have another information. Both have the same STIX....

 

I find Neptun's MCR and SI may indicate a harsh behaviour. But you know:
"Horses for courses...."

Cheers.

(Caution: Do not trust my estimated STIX numbers at all. Still in needing of thorough tests and development)

 

I have felt that but didn't know why! Thanks again!

 

Some comments from the peanut gallery.

STIX does have some interesting features. One number that classifies marketabily, can be used to compare boats of vastly different hull form with respect to safety at sea (Guillermo has convinced me that comparing very similar boats in fine detail, as in many of the examples in this thread, is probably not its strong point), and also gives the layman an indication whether a particular design has emphasized safety (does STIX exceed boat length in feet). That's a lot to ask from one number. I think it has been very cleverly constructed. Am I wrong in thinking that this is the first widely available piece of boat data whose presentation is not controlled by the owners of the intellectual property. A precedent of a sort? (then again, how widely distributed is it?)

Regarding the conveyence of this information to the public, there would appear to be some conficts of interest here. The guild nature of the profession of boat designers is a fact of life and pushing an agenda contrary to that profession's traditions and views of what constitutes its best interest is going to be hard work. Adding more independant variables to the equation would involve getting more data from designers. Adding more variables should also reduce the distribution of STIX numbers, and this in itself is an important consideration. Changing the formula for STIX in a way that improved all of its current uses would be quite an accomplishment. That said, I like the "u" suffix proposed to designate the presence of the unsinkability factor. Has anyone floated this balloon in other arenas?

White Knight- Yes, the boat buyer would love to have a simple system to compare similarly priced and sized boats, but should we expect boat designers to provide the formula?

I'd rather they keep at each other and keep building better boats. I think the formula you seek is a very personal thing, like a fingeprint.

Guillermo, would you place a note in you original post that the spreadsheet has evolved. I downloaded several versions in succession as I discovered them. Thanks in advance.

What is "heft ratio" (vertical distance from fridge to cupholder? ) I couldn't find this anywhere but in this thread.

Have there been any studies in the distribution of STIX numbers? Has the distribution changed since the formula was enacted?

 

I don't know, but certainly not me.

Unluckily I cannot do that, as those posts are old ones and editing function is only available for relatively recent ones

Its a ratio similar to the MCR (Motion Comfort Ratio), developed by a member of these forums, but unlucikly I don't remember now his nick.

He wrote:
"Heft ratio works like this: 20*disp. vol./(Length * Beam^2). 'Beam' is taken as the (Waterline Beam + the beam 1/8th the waterline Beam up)/2. I consider a HR of 1.0 a good compromise. I like higher numbers such as 1.5 to 2.0, but thats just my personal taste. For multis, I add the Beams of the two most leeward hulls together to figure their HR."

It has the vertue of being adimensional, so you get the same figure either in metric or imperial units.

What do you mean by the 'distribution of STIX numbers'?

Cheers.

 

Guillermo,

I was thinking of a simple scattergram. Boat length on the x axis, STIX number on the y axis, and a bunch of dots representing boats in production at any given time. Wondering how much STIX scores have shifted over time.

 

Phil,
I don't know if somebody has worked out such scattergram and then follow up its variations with time. Not easy, as STIX is relatively new and not all boat manufacturers and/or designers make it available to the public. As a matter of fact there is a clear tendency not to inform about it. Most manufacturers just inform about the design category.

Cheers.

 

All of this is extremely interesting, and I really want to learn about the calculations and other things involved in the boat building/design world.

Does anybody have books or litterature that I could read and study on the matter...

or colleges that specify boat engineering and design?
-Dan

 

Dave Gerr: Boat Strength and The Nature of Boats

Lars Larsson and Rolf Eliasson: Principles of Yacht Design

Skene's Elements of Yacht Design



Cheers.