Jump to content

ORC Stability Number/s

Guest

By Guest,
in MarineTalk

Recommended Posts

Guest

Guest

Posted
Posted

Anyone know theirs or anyone elses?

Anywhere they are listed on-line?

 

I've been given a 930's one and was wondering how it stacks up next to some other boats. Is the 'tip truck' tag actually justified? Yeah it probably is but do they come back up faster or slower than others.

 

While here does anyone know if a ORC number can be related to a 'STIX' or 'Minimum level of positive stability' or a 'GZ Curve' or a 'IMS Stability Index'.

Link to post
Share on other sites
Guest

Guest

Posted
Posted

Long promised?? It's been all of 10 days there big fella.

 

Last Sunday was a go until the weather decided to stay a tad to blustery. We want to try with the sails up as well so 25-30 gust would have made that a tad more fun than we wanted.

 

It'll have to happen inside 2 weeks as after that she'll be sitting on stuff harder than water.

 

The 930's number is tad larger than Willows, which surprises me a tad, but knot quite as high as Fineleines, which doesn't surprise me at all.

 

118.7 on a close as dammit Class boat. It just has a slightly different keel and I'm checking to see where the lead in it is, I think it maybe a tad lower than a std Class keel. And just looking at her I think she has a bigger mast.

Link to post
Share on other sites
Battleship

Battleship 100

Posted
Posted

The 930's number is tad larger than Willows, which surprises me a tad, but knot quite as high as Fineleines, which doesn't surprise me at all.

 

118.7 on a close as dammit Class boat. It just has a slightly different keel and I'm checking to see where the lead in it is, I think it maybe a tad lower than a std Class keel. And just looking at her I think she has a bigger mast.

 

Why does that surprise you? Surely you didn't think the tip trucks would have greater stability :)

 

Anyway I wouldn't give much credence to the ORC theoretical numbers if they were close to true a Y11 would be a mission to set up for Cat1 or even Cat2.

Link to post
Share on other sites
Guest

Guest

Posted
Posted
KM

Just sent a email with some useless information, I also have ISO 8666, 11812 & 12217 if you want some more read. :wink:

Yeap got that lot Thanks, had a quick look but then my head exploded. Very Monty Python it was :)

Sent them to home to have a deeper suss later.

 

Why does that surprise you? Surely you didn't think the tip trucks would have greater stability :)

WTF??? You playing with my head also?? Isn't 118.7 higher i.e. stiffer than 116.5 or are the numbers some weird arse PCified work backwards thing?

 

I would have expected you to be stiffer than me hence have a larger number, isn't that the right way of thinking? I hope it is or wouldn't that make Fineline soft and floppy, you stiffest and the 930 in the middle somewhere?

 

And some wonder why I'd rather pull it over rather than have some random number :?

Link to post
Share on other sites
Guest

Guest

Posted
Posted

And I repeat - and people wonder why calculations make me nervous :) :)

 

Yes, doesn't seem to be a rime nor reason thing happening there. I'll read SJB's stuff, it may shed some light on which is stiffer, low or high.

Link to post
Share on other sites
B00B00

B00B00 310

Posted
Posted

Sorry just got onto this topic. Aren't you guys just talking about the angle vanishing stability? Or are you talking something completley different?

Has sweet F all to do with actuall stability, (well a bit) but more to do with hull and cabin shape.

Wide boats often suffer but narow boats with high wide cabins such as 930s will allways be ok even if they are actually tip trucks.

Cat one requires 115 degrees I think and cat zero such as the hobart needs 118 or something.

Think about boats like the lady liz police boat in wellington, self righting because of the floatation in the cabintop when upside down.

Link to post
Share on other sites
Jon

Jon 360

Posted
Posted

Willow it's crap really it just gives the theoretical angle of heal that it won't come back from.

Tippy boats that realiy on deep keels tend to have higher numbers than boats with high hull form stability (windara)

But then you get into the real world were deep narrow fin keels have the tipping effect were the bulb can be on the upward side of the wave when the hull is sliding down the other.

The idea is really to cut out the really unsuitable designs from offshore.

Link to post
Share on other sites
Bogan

Bogan 8

Posted
Posted

Find ORC rules here. Go to pg 6 to see how stability index is calculated.

 

Basic factor is the limit of posiitive stability (the angle of heel at which the boat is balanced - neither inclined to return upright, nor capsize). This has two increments added: one is called "capsize increment" the other is called "size increment"

 

Capsize increment is defined as CI = 18.75 *( 2-((4*max beam)/(cube root of displacement))) (not sure of units, so hard to actually use this to try calculating one). However, this will favor a narrower heavier yacht over a wider lighter yacht. This probably accounts for the narrower yacht being less stable in the inverted condition. If displacement is kg and max beam is m, then a farr 1020 has a CI of about 19. But for the purpose of the stability index the maximum adjustment is +5.

 

Size increment is defined as SI = ((((3*(cube root of displacement))+second moment length)/3)-30)/3. Again, don't know the units or how they calculate second moment of length (although you can probably just approximate sailing waterline length). However you would expect this will favor heavy over light and long over short. If displacement is kg and length is in m, then a farr 1020, weighing 4300 and length 10m has an SI of about -4.

 

So for a Farr 1020 the stability index is the angle of vanishing stability in degrees +1.

Link to post
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing
×
×
  • Create New...