Sailing boats' Stability, STIX and Old Ratios

[Guillermo]

Hi, nikmik
Those data are not enough to know it. Have a look at the attached STIX spreadsheet to find out what the necessary data are.
Regards.

[neelie]

Maybe this is thread drift - if so, I apologize but, I guess the subject being Angle of Vanishing Stability it is semi relevant.

The crux of my question, is just how relevant is this number?

Retirement beckons and I am now looking at a couple of cruising boats, HR46 and an Alubat Ovni 455. Two extremes and yet both equally competent cruisers.

One has an AVS of say 140-ish plus and the other may be lucky to have one of 110.

It is fair to say that capsize past 90 degrees will in all likelihood be wave induced.

The center board fraternity say that with their boards up, there is markedly less propensity for the wave to "trip" the boat up. Assuming this is true (and I don't know for sure that it is), then an AVS of 110 would seem quite proper would it not?

So in the real world of compromises, should this difference in AVS influence ones choice of boat?

Years ago, I would have said yes, but now I have become cynical of ratios and "magic" numbers, All the ratios can be manipulated by manufacturers putting out data that is not exactly correct.

I am now of the opinion that real world experience is the only measure. Am I wrong?

[nikmik]

Neelie
My idea is that I make a boat this feature before retirement. I am now in the design phase. then all the screams of finance unfortunately

[MikeJohns]

Originally Posted by neelie

............
So in the real world of compromises, should this difference in AVS influence ones choice of boat?..............

I am now of the opinion that real world experience is the only measure. Am I wrong?

There are many factors that affect the safety of a crafts crew. AVS is one of them.
It is however well proven that the risk of the boat inverting in waves is closely related to the area of the RM curve from 90 degrees to AVS. Also the deck edge is the real 'tripping device' not the keel.

Also another very important issue is what happens after the craft inverts, or the time to re-right. Studies verified with comparison to real world events show a good general relationship between time to recover and AVS.
At 110 degrees the boat will generally be close to killing crew trapped on deck before a wave large enough to re-right it comes along. Once the AVS gets over around 140 degrees it takes seconds and not minutes and you'll even be conscious if you were trapped on deck under the boat.

There are plenty of other design features too that affect safety.

Also there are factors like the accuracy of forecasting of wind and waves, the experience of the crew and the condition of the gear. If you can avoid survival situations and keep a craft within it's sensible limits of safety then you can sail anything safely.

STIX is not the greatest measure of craft safety, it's too easy to fudge the measure IMO.

[Guillermo]

Originally Posted by MikeJohns

If you can avoid survival situations and keep a craft within it's sensible limits of safety then you can sail anything safely.

STIX is not the greatest measure of craft safety, it's too easy to fudge the measure IMO.

Agree.

[Tad]

Originally Posted by MikeJohns

STIX is not the greatest measure of craft safety, it's too easy to fudge the measure IMO.

IE is the 110 degree AVS of the Ovni with the keel up or down?

[neelie]

The center board of the Ovnis are not ballasted and would have only a small effect on the righting moment.

As far as the AVS of 110 for the Ovni, I stress that I do NOT know this to be fact. One of the best kept secrets in the yachting world is the AVS of the Ovni series.

I agree AVS is but one of many variables, I am just trying to get a subjective feel for how much weight it merits in the grand scheme of things.

I saw a table of Length vs desired AVS for Cat 1 boats. For 15 meters, the recommended minimum AVS was 121.8. Can anyone calculate the AVS to such precision?

[souljour2000]

I just got a hold of a Morgan-designed CB '66 Columbia 40...and I'm looking for input I suppose because this model had only 55 made and doesn't come up much as a bluewater boat and she deserves some consideration since her 15 minutes of fame back in the 60's...as a derivative of "paper tiger" and "sabre"...
Not withstanding the issues with her deck which needs replacement/re-cored in most of the foredeck and the fact that the valiant 40 was probably built a bit more robustly in terms of bulkhead tabbings and hull/deck flange,etc.....I other wise think the Columbia 40's numbers stand with the Valiant 40 or many other boats...add to her 8400 lbs lead the diesel hung low below the salon sole and she is stiff and has a low cabin profile...any thoughts..?.I feel I got a good blue-water platform(at the right price)given that she needs alot of work...Don't wanna be a thread -killer though my rep precedes itself I suppose...

In any event...I think I got my girl finally... thanks be to the sea-gods...and though a bit slower than some she's easy on the eyes...and could be even more with some work...
this is from Sailcalcpro .....I think the sail area might be off though for one thing..it' seems high..and I've seen lower numbers elsewhere...

Performance Comparison
LOA Columbia 40
39.16
Valiant 40
39.34
LWL Columbia 40
27.75
Valiant 40
33.88
Beam Columbia 40
10.66
Valiant 40
12.32
Displacement Columbia 40
18200
Valiant 40
22306
Sail Area Columbia 40
747
Valiant 40
768
Capsize Ratio Columbia 40
1.62
Valiant 40
1.75
Hull Speed Columbia 40
7.06
Valiant 40
7.8
Sail Area to Displacement Columbia 40
17.27
Valiant 40
15.51
Displacement to LWL Columbia 40
380
Valiant 40
256
LWL to Beam Columbia 40
2.6
Valiant 40
2.75
Motion Comfort Columbia 40
38.29
Valiant 40
33.96
Pounds/Inch Columbia 40
1057
Valiant 40
1491

[neelie]

Sounds ok to me, if you're happy with your boat, then you're golden. And in the end that's all that matters. Life is full of compromises and the best anyone can hope for is a bit of happiness.

Otherwise get a 20ft diameter steel ball welded up in 1/2inch plate, attach a 20 ft keel with 10 tones of lead at the end. You'll have an AVS of 180 and be totally bulletproof. But in the end, will you be happy?

You have a nice boat, be happy with it.

[Guillermo]

The Columbia 40 is a very nice boat, and her numbers are pretty fine for a blue water cruiser. Enjoy it.

[souljour2000]

Thanks guys...I think she's gonna take me somewhere someday...I hope to get her rails wet this summer but I got a ton of projects...right now am waiting on PO to send me the fuel injectors in his possession...and then we'll pop those in and see what happens...so at least I know whether I can maneuver her under her own power...

[Michail]

I have been following this discussion for a while, and I found it very interesting. However, the boats which were covered are relatively large. I just wonder whether it is possible to design a small boat (19 - 20 feet LOA) with a STIX of say 35, which would put it into A Cat.

I have played with the formulae, and it seems that it is possible, provided that one accepts a few limitation of the boat (like being underpowered having SAD something like 15, heavy, relatively narrow, designed as submarine to comply with the downflood angle, having a superstructure for reduced inverted stability, etc.).

The standard microtoners of that size seem to fit the C Cat, and at best can approach B.

I wounder whether such a design would be feasable (in practical terms).

[TeddyDiver]

Originally Posted by Michail

I wounder whether such a design would be feasable (in practical terms).

Your have to design one so we can see the practicality thou can't see the point becouse such a small boat has other more limiting restrictions than the STIX number to be real ocean going vessel. Look at some smaller SAR boats, quite seaworthy designs but not meant for oceans either..

[Michail]

The idea is not to have a boat for long ocean passages, but for sailing in South Pacific near Chile, i.e. relatively short passages, but for any weather even beyond the minimum A class conditions.

The design requirements were the following:
- carry legally required safety and standard equipment (estimated at 130 kg., a long list of about 50 items, including life raft, tools, water, radios, anchor, etc)
- carry stores for two persons for a maximum of 30 days.
- carry crew of two.
I estimate that the net useful load capacity of the boat should be around 700kg. to satisfy the above. Assuming a sturdy fiberglass hull with keel, the empty boat could weight 1000 kg., and fully loaded, a maximum total of 1700 kg. Minimum sailing weight around 1350 kg. Using Elisson as a rough guide, an estimate of 5.8 meters LOA and 2.0 BOA would seem to be the lower limit to accomodate that weight without making the boat a simple square barge.

I like the idea of SAR boats, but I am talking about sailboat (mainly because I do not trust engines, and prefer sails).

[Guillermo]

Flicka 20 (http://www.flicka20.com/) is a proved oceancrossing little boat, but its mass under 3.000 kg doesn't allow it to enter Cat A.

There are several old threads in these forums about pocket and micro cruisers.

Cheers.