Sailing boats' Stability, STIX and Old Ratios

[Guillermo]

Originally Posted by Guillermo

...Now I learn Nauticat 38 is a Category B boat! (http://www.nauticat.com/Default.aspx...Specifications).

I wrote Nauticat to ask about this, and here the response:

"Hello Mr. Gefaell
The stability of the boat is more than enough for category A, but it is the side doors, which makes it extremely difficult to get it in category A, as those are considered as flooding openings.
I hope this explains the situation. The same thing with our Nauticat-331 and Nauticat-44.
best regards
NAUTICAT YACHTS OY
Veli Kärjä"

Here we have a decent builder and another STIX 'strange' situation. Thanks a lot Veli.

[Mikey]

There we are... also the Nauticat 44 is a category B boat but Multichine 28 is a category A boat

Something is just not quite right here.

Mikey

[fcfc]

Funny, this http://www.formenti-forsea.it/en/53.htm is in the same category as the nauticat 44.

This http://www.floatingneutrinos.com/son...wn%20Hall.html is also an interesting "thing" to cross atlantic, too.

[Guillermo]

Originally Posted by fcfc

Funny, this http://www.formenti-forsea.it/en/53.htm is in the same category as the nauticat 44.

There are several models of Ribs around there categorized as B. There is not an STIX calculation and number for them, but an specific norm: ISO 6185-4. But I'm interested to know how a 5.35 m length one is able to be B categorized, as there is not a norm usable for that under 8 m length, if I'm not wrong.

Originally Posted by fcfc

Most hard factor in powerboats is the required righting moment at 30° of 25 000 N.m for category A and 7 000 N.m for category B.

If you take a mini 6.5, I think most with STIX in category B due to unsinkability, remove the mast and put an engine It would not be in category B in powerboats, because it would not have enough rigthing moment at 30 °. An empty 1 T boat would need 70 cm GZ at 30°.

But a 3,5 tonnes one will only need 0,2 m. Weight, always weight...!

Cheers.

[Guillermo]

Too busy these days to actively participate in the Forums.
Just a quick post to inform I have posted an improved release of the STIX spreadsheet calculator, available at the "Spreadsheet Library". I'll keep working on that (as time allows) to make it more complete and useful.
Cheers.

[Guillermo]

Brian Trenhaille of Hawaii Marine has developed a very complete spreadsheet to estimate a lot of your sailing boat's characteristics. It doesn't calculate STIX, but a lot of other useful info.

http://hawaii-marine.com/templates/P...escription.htm

From there:
"This spreadsheet takes you input values for your sail boat and automatically calculates many of your sail boat's numerical characteristics (ratios, coefficients and parameters). Then this spreadsheet presents various target values allowing you to quickly check and compare your sailboat's characteristics with other similar successful sail boats.

There are several advantages and benefits to this calculative approach. Some of these advantages are listed below.

First, this method saves time. Many of computations are quickly generated behind the scenes on your input data. Also to modify, simply change a value in the spreadsheet and it will automatically and immediately recalculates all affected values.
bullet

Second, this approach provides clear and neat documentation.

Third this approach is cost effective because the calculative approach is already developed for you, research time is minimized to the familiarization of concepts when necessary and not to time consuming development activities.

Fourth, this template is kept simple, it contains no Excel macros and there is no Visual Basic code utilized in it's creation. Also advanced Excel features such "Goal Seek," "Solver" and "Scenarios" are not used.

Fifth, because this is a spreadsheet and not a program, the users can easily modify it to suit their particular needs."

[Guillermo]

I like this 40 footer
http://www.nordship.dk/index.php?id=...S40&menuid=344

Specification here: http://www.nordship.dk/nordship/file...07-02-2006.pdf

Some estimated numbers for her:

Basic data from Yachting World and Nordship's site.

Overall Hull Length = 12,2 m
Length Waterline = 10,8 m
Flooded Buoyancy = (Y/N) N
Beam Waterline = 3,51 m
Beam = 3,9 m
Displacement MOC = 11100 kg (best guess)
Displacement Max = 12948 kg (best guess)
Height of CE above DWL = 7,52 m
Height of CLR below DWL = 0,8 m
Angle of vanishing stability = 140 deg
Downflooding angle = 120 deg (best guess)
GZ at downflooding angle = 0,25 m
GZ at 90 degrees = 0,67 m
Sail Area = 78 sqm (site) & 95,9 sqm (YW) -- I,J,P,E missing
Area to flooding (Agz) = 68 m.deg
Area to AVS = 70 m.deg


STIX factors and figure for MOC and 78 sqm SA:

Base Length Factor (LBS) = 11,267
Displacement Length Factor (FDL) = 1,040
Beam Displacement Factor (FBD) = 1,061
Knockdown Recovery Factor (FKR) = 1,435
Inversion Recovery Factor (FIR) = 1,186
Dynamic Stability Factor (FDS) = 1,231
Vaw (Not aplicable)
Wind Moment Factor (FWM) = 1,000
Downflooding Factor (FDF) = 1,250
Delta = 0

STIX = 54,984 (Wow!)
Note: Maybe this figure is some enters lower due to the 120º guess on downflooding angle. If it were 110º, STIX would be around 53 and around 49 if 100º.

Ratios related to 10500 kg lighship condition, 3700 kg ballast and 2 m keel. Figures in () are for full load:

Length/Beam Ratio L/B = 2,88 as: (0,7*LWL +0,3* LOA)/Bmax
Lwl/Bwl Ratio Lwl/Bwl = 3,08
Ballast/Disp Ratio W/Disp = 0,35 (0,29)
Displacement/Length Ratio D/L = 232,49 (around 280?)
Sail Area/Disp. Ratio SA/D = 16,53 (14,37)
Sail Area/Wetted surface SA/WS = 2,33 -- best guess for WS: 33,44 sqm
Power/ Disp. Ratio HP/D = 2,41 HP/ton (1,95)
'Hull speed' HSPD = 7,98 Kn
Potential Maximum Speed PMS = 9,15 Kn (8,1)
Velocity Ratio VR = 1,15 (1,02)
Efficient motoring speed (1.1 S/L) EMSPD = 6,55 Kn
Capsize Safety Factor CSF = 1,8 (1,68)
Motion Comfort Ratio MCR = 32,35 (39,90)
Heft Ratio HF = 1,1 (1,36)
Roll Period T = 3,67 Sec (4,41)
Roll Acceleration Acc = 0,08 G's (0,05)
Stability Index SI = 0,94 (1,13)

Something like a 10% more of sail area would have been nice to my taste, just to bring SA/D ratio to around 18 for lightship and 16 for full load, to make her more agile in light winds. STIX would still between 48 to 53 (depending on Dfl angle) which are yet very nice figures for this size of boat nowadays (Anyhow I would have not been discontent with present SA figure).

[Vega]

Yes, it is a good boat. I have said that two years ago when the boat was released.

http://www.boatdesign.net/forums/sho...+40#post121613

http://www.boatdesign.net/forums/sho...+40#post114563

http://www.boatdesign.net/forums/sho...p+40#post82925

[Guillermo]

OK. I'll buy this one and you buy the Pogo 40

[Guillermo]

Some interesting (estimated) numbers for CIGALE 14 (http://www.alubat.com/voilier%20alu%20cigale14.php):

Lightship (7000 kg)

Length/Beam Ratio L/B = 3,03
Lwl/Bwl Ratio = 3,33
Ballast/Disp Ratio W/Disp = 0,47
Displacement/Length Ratio D/L = 79,36
Sail Area/Disp. Ratio SA/D = 25,56
Sail Area/Wetted surface SA/WS = 2,23
Power/ Disp. Ratio HP/D = 3,26 HP/ton
Hull speed HSPD = 8,92 Kn
Potential Maximum Speed PMS = 11,16 Kn
Velocity Ratio VR = 1,25
Best motoring speed (1.1) CSPD = 7,32 Kn
Capsize Safety Factor CSF = 2,37
Motion Comfort Ratio MCR = 14,65
Heft Ratio HF = 0,48
Roll Period T = 1,86 Sec
Roll Acceleration Acc = 0,36 G's
Stability Index SI = 0,41
Angle of Vanishing Stability AVS = 116 º


Full load (9300 kg?) 6 pax. No water ballast considered.

Ballast/Disp Ratio W/Disp = 0,35
Displacement/Length Ratio D/L = 105,43
Sail Area/Disp. Ratio SA/D = 21,15
Sail Area/Wetted surface SA/WS = 2,13
Power/ Disp. Ratio HP/D = 2,45 HP/ton
Hull speed HSPD = 8,92 Kn
Potential Maximum Speed PMS = 10,4 Kn
Velocity Ratio VR = 1,17
Best motoring speed (1.1) CSPD = 7,32 Kn
Capsize Safety Factor CSF = 2,16
Motion Comfort Ratio MCR = 19,46
Heft Ratio HF = 0,64
Roll Period T = 2,38 Sec
Roll Acceleration Acc = 0,22 G's
Stability Index SI = 0,53
Angle of Vanishing Stability AVS = 114 º

More info at: http://www.northseamaritime.com/Page/ALUc14.htm

A pity not having a GZ curve for this boat.

[Guillermo]

And now an excellent design from Heyman Yacht Design, in Sweden, very much of my content!

One of the boats I'd buy or built, if only I could!
(SA sems somewhat low to me, but her ketch rig allows for an extra mizzen staysail)

Some preliminary numbers for her, asuming the stated displacement of 36 tonnes is for the MOC, and downflooding occurs at 115º.
Cp is asumed as 0,56 and Cwp as 0,7

ATOA 64
Loa = 22,30 m
Lh = 19,50 m
Lwl = 17,32 m
Bmax = 5,40 m
Bwl = 4,86 m
Draught T = 1,60 m
Body draught Tc = 0,95 m
Disp = 36000 kg (Asumed to be MOC)
Ballast = 10000 kg
Sail area = 146,4 m2 (estimated measuring I, J, P...etc, from drawings)
Power = 140 KW
Heeling Arm = 8,97 m
WS Appendages = 13,18 m2
WS Hull = 65,89 m2
Wetted Surface = 79,07 m2

Length/Beam Ratio L/B = 3,33
Lwl/Bwl Ratio Lw/Bw = 3,56
Ballast/Disp Ratio W/Disp = 0,28
Displacement/Length Ratio D/L = 193,26
Sail Area/Disp. Ratio SA/D = 13,64
Sail Area/Wetted surface SA/WS = 1,85
SA (metric)/ Power (Imp.) SA/HP = 0,77
Power/ Disp. Ratio HP/D = 2,40 HP/ton
Hull speed HSPD = 10,10 Kn
Potential Maximum Speed PMS = 9,8 Kn
Velocity Ratio VR = 0,97
Best motoring speed (1.1) CSPD = 8,29 Kn
Capsize Safety Factor CSF = 1,65
Motion Comfort Ratio MCR = 44,87
Heft Ratio HF = 1,24
Righting Moment/Beam RMB = 0,8
Roll Period T = 5,21 Sec
Roll Acceleration Acc = 0,06 G's
Stability Index SI = 0,97

Displacement MOC = 36000 kg
Displacement Max = 42295 kg
Height of CE above DWL = 8,33 m
Height of CLR below DWL = 0,64 m
Angle of vanishing stability = 132 deg
Downflooding angle = 115 deg (best guess)
GZ at downflooding angle = 0,24 m
GZ at 90 degrees = 0,61 m
Sail Area = 146,4 sq.m
Area to flooding (Agz) = 55,48 m.deg
Area to AVS = 58,67 m.deg

Base Length Factor (LBS) = 18,047
Displacement Length Factor (FDL) = 1,067
Beam Displacement Factor (FBD) = 1,061
Knockdown Recovery Factor (FKR) = 1,500
Inversion Recovery Factor (FIR) = 1,288
Dynamic Stability Factor (FDS) = 0,795
Vaw = Not aplicable
Wind Moment Factor (FWM) = 1,000
Downflooding Factor (FDF) = 1,250

Delta = 5

STIX = 75,140

I would like to clarify the displacement and downflooding angle doubts. Somebody?
More info at:
http://www.atoa64.com/

Cheers.

[Vega]

Originally Posted by Guillermo

Some interesting (estimated) numbers for CIGALE 14 (http://www.alubat.com/voilier%20alu%20cigale14.php):

Lightship (7000 kg)


Angle of Vanishing Stability AVS = 116 º

The Msc AVS = 121º

[Guillermo]

Excellent! Do you have the corresponding GZ curve?

[Guillermo]

Coming back to the very initial purpose of this thread:
From a Dave Gerr's article at the Westlawn Institute Magazine's September 2007 issue:
"Because complying with the STIX under ISO is now the law in Europe, there is a tendency to interpret STIX as a good indicator of seaworthy stability in the U.S. I personally don't believe that STIX is all that a reliable indicator. Nevertheless the STIX number is a reasonable marker, but it is not as reliable -in my opinion- as simply applying the basic concepts above (he's referring to several of the 'old' ratios and parameters) along with one or two others involving dynamic stability...for really accurate results by designers"
http://www.westlawn.edu/news/Westlaw...d03_Sept07.pdf

(Italics are mine. Thanks a lot, Perry!)

[YachtManuals]

Originally Posted by Vega

I will post two images that explain the differences in dynamic stability and in the RM curves between beamy boats relying more on form stability and narrow boats, reeling more on Ballast.

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.