AVS - Angle of Vanishing Stability

[stravaigin]

Hi - the MCA code for category D sailing vessels requires that the range of positive stability extend to 123 degrees.
We would be interested to learn from Blue Water cruising folk what is considered an actual / acceptable AVS given that liveaboards tend to have a heavier displacement than the original designer's criteria [which can reduce the range]
Would much appreciate "actual" rather than theoretical findings! Thanks! M&M

 

[KellysEye]

I've got no idea what ours is (with or without extra weight) and don't really care to be honest. In my experience most blue water cruisers look for a solid boat with seaworthy lines, the categories are generally viewed as nonsense hijacked by the French. Out of interest why are you asking?

 

[silver-fox]

Be warned down that road lies madness!!!!

I started to include stability considerations in my boat choice, only to find that its not that easy, for example what does stability actually mean? Is the AVS a key factor? (After all a log will have a higher AVS than any boat but its hardly stable is it? A raft has at most a 90 degee AVS but is very stable!

My pursuit of knowledge quickly led me to the conclusion I needed a much greater grounding in applied physics than I have.

Another stumbling block was lack of data unless the boat in question was a relatively new production boat.


You might find these two web pages useful
Sailboat design

Ted Brewer speak some sense too

Ted Brewer Sailboat design

The conclusion I came to in the end was that a moderate proven design would cut the mustard......sigh.....which is where I started.

I am still taking the paracetomol.

 

[douglas_family]

I've read a bunch of articles on stability displacement ratio's and other such nubers of little interest when it comes to buying a boat. the cruical questions are :

Whats my budget?
Where do i want to go?
Who is going to be my crew?
whats my cruising kity?

The answers to these bring others like one hull or 2 or even 3?
What type of construction?
how deep should the draft be?
what is my preferred rig?
Should i buy new or second hand?
how much will fitting out cost / how long will it take?
how important is resale value?

Then of course you get down to the real questions:

Whats available?
Which compromise is acceptable?
Which was does your partner like?
Which one did you fall for!

Good luck

 

[trouville]

for once i dont know what to say?But i try not to strain the boat eccessivly

On the French canals there was a lot of stress.

 

[roly_voya]

179.5 or enough to be absolutly sure she will be self righting - but only if you are going somewhere you might suffer a knockdown. It is basically a measure of how long you might sit upside down waiting for the next wave to knock you upright again but if you think about an AVS of 120 then if you get nocked flat, sails on the water down a wave front of only 30deg she will keep going... Not aplesant thought. 'Classic' designs such as the contessa tend to be closer to 160. If you are looking at one of those graphs supplied for the RCD then look at the area below the line not just the AVS as that is a guide to how much energy is needed to right the boat. The RCD is not a guide to seaworthyness or really much else.

 

[boatmike]

Perfectly true. While not a Naval Architect I have been involved in the design of vessels and the MCA recommendations must be taken in context. Most commercial vessels (especially catamarans) don't have an AVS anywhere near this but the force needed to get them past about 30 degrees is enormous! This ruling would in fact, taken in isolation, eliminate every multihull on the planet along with trawlers, cruise ships, container vessels and ferries. What I think they are getting at is there are an increasing number of light monohulls being built now that are so beamy in relation to their draught that they lose self righting capabilities at a lot less than 120 degrees. They are saying 120 is therefore considered to be a minimum for a sailing monohull and at least indicates self righting. I agree with you that classic designs have higher than this and were always considered good "sea boats" because of this. Increasing beam has reduced this number now though.... Beware of boats with exessive beam that are very "stiff" in moderate winds but would never survive a knock down. They can lull you into a false sense of security until that big wave comes along and a gust of wind can blow you down. So look at the AVS... it is important, but only part of the story and rather depends on how and where you sail.