Stix Calculation

[Battleship 100]

Does anyone know if we need are STIX calc, for a Cav36 to get Cat 2 for the RNI?

It looks to me like the book says we do but I think I have read somewhere that a proven design may not.

 

Plenty have been offshore and ours has been pitchpoled in its early life and righted itself, there was also the well documented rolling and subsequent self righting (rig intact) of Camelia in Boating New Zealand last year.

 

It's built like a brick shithouse and has about 3.5 tonne of ballast so i'm sure the numbers would stack up fine, but would like to put the dollars towards some more usfull equipment than a piece of paper.

[Jon 360]

From memory that is only one way of proving stability

Before 2005 Suva race we had to get an inclination test done on the 1104 even though they have done 1000s of offshore miles both cruising and racing.

 

But I would suggest talking to your inspector and ask what he thinks is the best way to go about it, He may be happy to go with the proven design concept but if not you have time to get it sorted some other way.

[Battleship 100]

How was the incline test done?

Did you simply winch the mast down?

Also what sort of evidence do you need that it has been done?

[rigger 47]

How was the incline test done?

Did you simply winch the mast down?

Also what sort of evidence do you need that it has been done?

 

an explanation of an incline test

 

www.gidb.itu.edu.tr/staff/akyildiz/Inclining%20test.doc

[Battleship 100]

Thanks Rigger, do you have a copy in English?

 

Seriously tho its sounds pretty complicated, I guess it would have to be done by a proffessional.

 

Any designers here that would care to comment?

[Changed 10]

B B-W did mine for me on the computer. Very reasonable and once done worked for the other boats as well. All they had to provide was freeboard measurements, keel depth and bulb weights (from memory).

[rigger 47]

there is always someone on this site that can offer more

[Guest]

We once did one ourselves on a trailer sailer. Had the mast tip at the water and held it down with one finger.

[Fish 0]

There is a Stability Index from "fit to fleet data" on an ORC handicap certificate. This has been accepted as a way of demonstrating stability previously but only to CAT 3 as far as I know. I wouldn't want to comment on how that would go for CAT 2 / RNI.

 

ORC did only cost about $70 but has gone up to soemthing like $250

[PaulR 3]

. . . ours has been pitchpoled in its early life and righted itself, there was also the well documented rolling and subsequent self righting (rig intact) of Camelia in Boating New Zealand last year.

 

I would say STIX could be a waste of time, because the "BEEN THERE, DONE THAT and SURVIVED" should be worth heaps more than any theoretical calculations, no matter who does them or how much it costs.

 

 

Mind you, the alternative could be equally valid. The STIX may be vital as your boat may be tooooooo tender, and may need more lead, shorter mast, better rum, better food, better crew, lighter halyards, lighter sails . . . . and heaps more stability factors.

[Jon 360]

How was the incline test done?

Did you simply winch the mast down?

Also what sort of evidence do you need that it has been done?

 

Kevin Dibley did it

All they did was setup a small clear hose as a water level across the stern then added 20ltr container (20kgs) to the end of the spin pole out at right angles and measured the angle of heel. this was done twice more times eg 40kgs then 60kgs each side and from this he calculated a self righting curve

We were told before hand that we wouldn't pass bit it pissed in and other 1104s have since used this data

so it may pay to check other owners as someone has possible already do this.

As far as pulling the boat down this doesn't need to happen and is probably going to damage something.The boat probably only heeled about 5deg.

[Battleship 100]

Does Kevin Dibley stil do that sort of thing?

Can you share what it cost you? (PM if you like).

 

I have contacted the owner of Camelia who runs the Cav36 website but he hasn't ever needed a calc done and goes offshore almost every year.

[Jon 360]

Does Kevin Dibley stil do that sort of thing?

Can you share what it cost you? (PM if you like).

 

I have contacted the owner of Camelia who runs the Cav36 website but he hasn't ever needed a calc done and goes offshore almost every year.

 

 

I would talk to Camelia safety inspector first and see if he needs one to do yours

As to cost I am racing with the owner tommorrow so will check as I can't remember even though I got it done.

[Guest]

What Danaide said but use a digital spirit level. You might need quite a few 20l containers though.

 

http://www.trademe.co.nz/Building-renovation/Tools/Hand-tools/Levelling/auction-292842036.htm

 

Quite handy for other things as well. BBW did me the calcs for Black Fun.

 

Surely someone has the calcs for a Cav 36?

[SloopJohnB 322]

an explanation of an incline test

 

http://www.gidb.itu.edu.tr/staff/akyild ... 20test.doc

 

Sounds like the good old inclination tests we use to do for the IOR rating.

 

We would strap the inclinometer across the pushpit, swing the spinnaker of the side at Bmax and hang 20 litre containers (full of seawater) until the set angle of heel was reached.

 

The grey matter is working hard here but I think it was 2 and 5 degress.

 

The digital spirit level would have saved a lot of mucking around.

 

I spent a whole day doing freeboards and inclining tests on 50ft, moving 500kg of lead around to get the optimum L/D/RM ratios

[Brett Bakewell-White 0]

The cheapest way to prove stability up to and including Cat 2 is to get an ORC Club rating. Forms are on the YNZ website.

 

You will only need to prove stability by inclination and then GZ or STIX if the boat fails using the ORC estimate. STIX is by far the best and most thorough assessment as it takes into account cabin and cockpit volume and the sail plan.

 

The ORC Club method is not acceptable for Cat 1.

 

History and ocean miles are only a measure of good luck, not stability. If I successfully crossed the Southern Motorway on a skateboard half a dozen times, it would not make it safe.

 

Unless you have rolled your boat upside down and had it self right you have not proved it's stabuility in practice. It is a common misconception that a boat that feels stiff at the small angles of heel that we experience as sailors (up to 30deg) is a stable boat - often the opposite is the case. Safty requirements are all about large angles of heel, essentially upside down. It is very rare for a yacht to ever experience this.

 

Designs from the IOR period of the Cav36 etc often struggle to pass the stability requirements as the IOR punished stability with rating so designers deliberatly designed boats that were less stable in order to get a better rating.

 

Inclining is a very simple procedure and is used on everything from trailer yachts to oil tankers. We have a written procedure available to clients so they can do the inclining themselves and then we check the results and do the calcs. This saves the cost of getting me out of bed at 4am to do the inclining for them!

[Kiteroa 8]

If you're looking to determine if your boat is safe or not, skip the ORC stability measurement. It's a bureaucratic farce and will give you a number that is a guess. And not even an educated guess. The tested stability curve for my boat is nothing remotely similar the ORC derived number. And it doesn't matter that it's only $110 or whatever. The ORC number isn't accurate.

 

Either do a pull down test with a load cell or have a proper calculation done with some known design information.

[Battleship 100]

Thanks for the reply Brett, I would rather get a proper calc done so if/when in future we want to get Cat1 it won't be an issue.

Sounds like your simple test would be the best way to go then it's done and I can move on.

[Battleship 100]

thanks,WT

Not really trying to see if it's safe or not, as stated earlier the boat has been rolled and righted in the past, and they are built pretty strong so have a good reputation as an offshore boat.

With a just shy of 50% ballast ratio I will be pretty dissapointed if it doesn't pass the stability requirements.

[Brett Bakewell-White 0]

Ballast ratios are another misconception regarding stability - again IOR and some later IMS boats had some quite healthy ballast ratios, but also featured wooden, or hollow keels with all the ballast in the bilge (some boats even had lead liners in the cabintop!)

 

WT, you are correct in saying that the ORC Club stability index is a calculated assessment, but to say it has no basis is not correct. Because it is an estimate it is not accepted for Cat 1, however before it was considered at all a fair amount of time was spent comparing actual inclination data with the ORC calculated figures over a wide range of boats and the results were remarkably accurate, far more so than was expected. The ORC library contains thousands of designs and so usually is able to make a very good data match.

 

YNZ worked hard to try and make as many methods available as possible that are appropriate to the boat, budget, and category - as an owner I think from memory there are 5 different options of proving stability. Without doubt the best method is to do a full inclination and then use the results in combination with a computer model and complete a GZ curve, or a STIX calculation, but this takes a fair amount of time and can be expensive as a result. This would be over the top and therefore inappropriate to enforce for Cat 3 for instance, and so other methods such as the ORC calculation are more cost effective and appropriate.

 

Without the introduction of the STIX method there are a number of 'safe' designs that could not achieve Cat 1 and therefore race offshore - these are largely from the 70's/80's where boats lighter by reducting ballast and prior to the widespread use of bulbs, examples being Farr 1104, Farr 38, Farr 1020. These boats do not meet Cat 1 using the GZ curve, but do pass using STIX where sail area and cabintop volume etc are taken into account. Stability is affected by much more than just the amount of lead, things like freeboard, beam etc.