ORC Stability Number/s

[Kiteroa 8]

Yup, and a Young 8.4 with 50% of it's displacement in the bulb, 2.1m down is all over rover at 106 deg. Which means Cat 2 is out of the question.

[Battleship 100]

Funny how things work out, it does make sense that a bulb keel has more righting moment than a boat with he same amount of lead in a conventional keel, I thought that our 3.7 tonne of lead might account for a bit but hard to go past the effectiveness of all that weight on the end of the keel instead.

I did notice that Dandide was a lot stiffer than us in the big waves off Cape Reinga able to carry a lot more rag comfortably form stability doesn't do as much when the seas are that big on the beam.

[Guest]

Ok so the bottom line is according to the maths a Ross930 is a more suitable offshore vessel than a Cav36, holy crap! glad we got that sorted.

Holy Crap is indeed the call Mr Willow. Doesn't that go against conventional thinking.

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

That's the problem, we don't know what we are/were talking about. We had some numbers but didn't know what or how the numbers relate to when applied to the book of Regs.

 

No matter what a calculator says I think I'd still rather be in Willows 'Tip Truck' boat than mine if 500nm from land and the weather goes super bad.

 

Weather looking good for Sunday. Cow Tipping looks fun but 930 Tipping should be a lot more interesting. Get that done and then only 2 little things I need to do before I can pull her out.

 

So in theory am I right in thinking that once we get the mast to 118 degrees +/- (based off the other 930) it should become neutral and if I push further she should want to roll over?

 

I wonder if she'll go to 120.1, I can get a Cat 0 then and head off to play with a V70's down in the 40's, Yeah Right!! Don't know we could carry enough Rum for all that ice

[Absolution 7]

KM you also mentioned that you were planning to do it with sails up. That will make your test invalid. You should be testing with no wind and no sails so that you are testing the static righting moment only. If you leave the sails up you might measure a whole lot more or a whole lot less righting moment depending on which way the wind is blowing.

 

For what its worth I still think its a bad idea to pull a 930 all the way down from the rig. Pulling down on the hounds is going to be a massive compression load on the rig until you get it down far enough that you can keep the load at 90degs from the mast.

 

A regular stability test only needs to heel the boat 20degrees or so which is far less load on the rig. The VO70 self righting testing that were mentioned are performed without a rig in the boat.

[markm 30]

Willow, nothing in itself wrong with the calculations (well some there are but I suspect more an issue of crap data in crap data out), the problem isn't the calculations, it's how people interpret it.

 

AVS isn't a measure of offshore worthyness, it's only a measure of how far you can roll and still come back. That a 930 can go over further than a Cav36 and come back doesn't make it a better offshore boat, only a boat that can roll over further and still come back.

 

Don't think of it as a measure of stability while sailing, it isn't. Much of the criticisms of AVS are around how a boat sails vs. their AVS number which is a bit like criticising an apple for not being a very tasty orange.

[Guest]

Sails down is primary and 'sails up if possible' after that. Assuming she isn't sitting on the bottom of the harbour by that stage. The sails up is so I can see what difference it makes as if push does come to shove the sails will actually be up at that time. But if anything looks to make that a silly idea it won't happen.

 

Yes whether the mast will make it seems to be an open question. But then if the mast fails a Stability test is pointless as we'll know the rig will go before she does fall over. The mast is helped a lot by the sort of semi-weird mainsail we have i.e. very heavy cloth fully battened. The plan we have developed will, we are reasonable sure, will have most of the load at 90 degrees most of the time so we'll see I suppose. But if it does come down we will have found a weak point I'd rather find in the harbour than further out.

 

Sure I can calculate but all that will do is make people who won't actually be on the boat happy they can say to a Accident investigation team they followed a rule set by some other bureaucrat. Tipping it over will make the people who will be aboard her is shite weather happier and more knowledgeable about the tool they have. There is quite a significant difference in the desired end result by the 2 groups. By tipping her over it will make both of those 2 groups happy, by calculating it will only make one group happy.

 

What I want to do with her is generally considered as a fruit loop idea in a 930. But the more I suss around and look at the detail I'm struggling to see why, the evidence just isn't there to back up the common thought. But as that mind-set is very much alive and well I am making sure as best I can that when the powers that be get asked to put pen to paper, any of this 'they can't do that' can be countered by actual fact.... or at least that's the target plan.

 

which is a bit like criticising an apple for not being a very tasty orange.

An other excellent Markisum

[Bogan 8]

Keep in mind, at 120 degrees the tip of your mast will be 6m underwater.

 

Does anyone have a stix number for a farr 1020?

[Guest]

Bugger, that much. I was wondering just how far it would be. That may prove a tad trickier than hoped for.

[Guest]

930 chatting last evening and one of the recently Turboed boats was reported to have a ORC number of 138

[SloopJohnB 322]

Bit more reading here KM.

 

http://www.boatdesign.net/forums/stability/

 

plenty.

[PaulR 3]

Sooooooo

 

Where in the world is KM's mast

 

Is there a reptile attached

 

Did the trip-truck tip

[Guest]

Had a small play but due to a baby arriving, rallies doing silly shite and a few things we got a bit short on hands so bevies and Bathurst was the winner on the day

 

I think the rig will be fine.... that's what I was playing with just to see.

[Guest]

Don't suppose any of you rocket scientists can translate this into something useful?

 

RM 1 - 44.12 kgm

RM 25 - 886.59 kgm

Displacement - 2200 kg

GZ (25) - 0.403 m

 

Calculate away to your hearts content

[chic014 0]

I could be wrong but that means:

 

at 1degree of heel there is 44.12kgm of righting moment.

 

at 25 degree of heel there is 886.59kgm of righting moment.

 

1kgm is 1 kg acting at a lever arm of 1m. So those numbers would be calculated by multiplying the pull down load in kg times by the horizontal lever arm. i.e pulldownload x mastheight x sin(heel angle).

 

Gz is the lever arm that the hull displacement has about the centre of gravity. That is simply calculated by lever arm = moment/load = 886.59/2200 = 0.402m

[Guest]

Ah so they aren't talking about kilos of milk fat then

 

So will the boat fall over and stay there?

 

Had a play on Sunday and we reckon at around 40-45 degrees, with reefed mainsail 50-55kts of wind isn't enough to push her over any further, she just sits there surprisingly tamely. She'll sit at that angle until the wind tapers back and then she pops up quite happily. We have been in that situation a few times and worked the angle to be just over 40 degrees of heel. A ruff measurement but it'll be close we think.