AVS, GZ curve & RCD category

jonathankent

jonathankent

Well-Known Member
Joined
23 Dec 2005
Messages
1,733
Location
Cornwall
Visit site
Bit of a long shot, but you never know with the knowledge on here....

Are there any general rules of thumb that can be followed to have an approximate GZ curve and AVS? I have no idea what my boat can do, but out of curiosity I would like to know, if possible. I would be a bit happier knowing how far I can push her /forums/images/graemlins/smile.gif

Does anyone know of a site/database where I could try and find out what RCD category my boat is? I am guessing probably category C, but thats just my guess, and I would like to have an idea what she can reasonably take.

Any knowledge gratefully received.

Boat spec as follows:
LOA 19' LWL 16' Beam 6' 3" Draft 2'
Displacement 1568lbs Ballast 545lbs
Sail area 150 sq. ft
Classed as a cruiser-racer with ratio of 17.7
 
My background is in engineering / physics so I like playing with numbers like that. Nevertheless I would be very nervous of using RCD categories or AVS as any sort of guide out there on the water. Maybe they are useful for comparing boats but for saying what any of them can do, in your hands, in real conditions - forget it!

Biggest determinant of safety - all other things being equal - is size. Properply handled, bigger is always safer. Effectively, resistance to capsize goes up by the cube of the length.
 
Some of what you want is on here:
AVS etc

I would take the answers with a lot of caution. You have to know how the centre of bouyancy moves as the boat heels and the details of the weight distribution (and whether it moves when the boat heels), to do it properly.
 
[ QUOTE ]
Biggest determinant of safety - all other things being equal - is size. Properply handled, bigger is always safer. Effectively, resistance to capsize goes up by the cube of the length.

[/ QUOTE ]
Can you quote any references?

The work I am aware of seems to suggest that it is more linear than cubic
 
Re: AVS, GZ curve & RCD category

Thought it was related to beam.

You can be capsized by a wave which is equal or bigger than your beam. That's what the RYA 'Stability'course states.
 
bedouin - was asked the same question last time I made the comment about and tried to find the reference then. I failed, but I can clearly remember reading it in a paper about stability particularly relating to multi hulls when I was buying my last cat.

I'll have another look. My suspicion is that the cube of the length is simply a proxy for the weight and volume of the hull. This would make sense since the method of capsize is not simply pressure on the sails of a mono but the transfer of energy from a breaking wave. And that energy has to overcome the rotational inertia of the boat plus any hydraulicl resistance etc. And these would be weight as well as dimension dependant

All this is why the AVS for a given category decreases as the length of the boat increases. Or why ships, with much lower AVS than small yachts rarely if ever capsize.

P.S. Found one reference which is from an investigation done by the US coastguard into capsize and the use of drag devices. I quote

" Results of these early model tests indicated that size played an important role in the ability of a sailboat to resist capsize. This is not surprising if you consider that the kinetic energy necessary to capsize any boat design will vary as the fourth power of the boat length. Thus a 60-foot sailboat requires sixteen times as much kinetic energy from a moving wave crest as a 30-footer in order to capsize."

Its an interesting paper - you'll probably find it via Google. I dont have the web sdetails since I downloaded it a few years ago.
 
To do it properly you need to either have some linesplan drawings of the boat or take lines from the boat and develope the linesplans. Lots of work but can be done with some of the free software available on the internet. The GZ curve depends on the shape of the boat and the position of the centre of gravity. On such a small, boat this is tricky since the walking ballast makes up a significant proportion of the ballast. Of course the position of the bums can be assumed and a simple inclining experient used to determine the CoG of the hull and fittings.

All this (and a whole lot more) goes into the RCD mixing pot. But what is the RCD? Does it mean that Frank Dye should not have sailed a Wayfarer to Iceland? Your boat is only as safe as you want to make it and what conditions you want to go out in. Having a letter after its name won't make your boat any more capable. Over time you will work out what you and the boat are comfortable with. You will fall out of the cockpit before you reach the AVS on most sailing boats, so how useful is it? And if you leave the companion way open, the Angle of downflooding would be more important. Boats full of water have an RCD rating of Z!!
 
[ QUOTE ]
You will fall out of the cockpit before you reach the AVS on most sailing boats, so how useful is it?

[/ QUOTE ]
If one has any sense then one would be clipped on in that sort of weather, so that falling out of the cockpit hopefully wouldn't be fatal.

AVS is obviously important in an extreme Fastnet 1979-type scenario where the boat has turned turtle, to make sure that she doesnt stay upside down too long. But if you're saying that that won't happen to most of us, you may be right, but it doesn't mean that boats shouldn't be designed to cope with it.

A rather more likely scenario especially for offshore sailors is that the boat suffers a knockdown on her side so the mast hits the water. In that scenario, the boat may be heeling, say, 110 degrees, and there may be a lot of wind pressure on the hull, not to mention impact of waves. It is therefore reassuring to know that in such circumstances the boat would have enough reserve stability to make it unlikely that she is going to be knocked over further. For that reason, personally I'd feel uncomfortable with an AVS much less than 130, which after adding all deck equipment, rolling headsails etc. probably means a design AVS of 140 or so.

Most of us aren't even going to experience a knockdown, but it effects the way that we sail to know the boat's capabilities - I feel much more secure setting off in marginal weather with my current boat with a high AVS than in a previous boat that I sailed with a low AVS - it's a question of confidence in the boat. So even if we aren't knocked down, AVS is still important.

Another reason why stability is important, not only for offshore sailors, was demonstrated by that Polish-built boat that was knocked down in UK coastal waters a couple of months ago - where it failed to right itself as it should have done and a man died as a result. It was subsequently found to have been given the wrong RCD category because its stability charactgeristics weren't up to scratch.
 
[ QUOTE ]
but it effects the way that we sail to know the boat's capabilities

[/ QUOTE ]
This is exactly my thinking....

I have read the PBO report of my boat, right back from 1971. This would lead me to believe that it is absolutely fine in a blow and a bit of white water, even quoted that they couldn't get the toe rail down to the water.... believe me, you can and without trying. Its when I read this, look at what other small yachts do and then compare to my experiences that I am uncertain. I have been out in a F5 (first week after buying) and found out that she didn't really like that... mainly due to the sea more than the wind, but nevertheless decided not to deliberately go out in a F5. I have to say, after a couple of bad trips it has certainly knocked my confidence a little and to have something to reassure me, if nothing else, would certainly help.... and one of the links above said that it is at risk to capsizing doesn't help /forums/images/graemlins/frown.gif. I just want a few good days sailing to get back on track.
 
I guess we all have our confidence knocking from time to time. It's a good thing, reminds us that we're not the boss and makes us more careful in future. Bumping to windward in a F5 may or may not be a problem in a 19ft-er, more depends on the shape of the waves than the wind speed. But the more you get to know your boat in blowy conditions, the better you get to know how to tease her to windward eg. so much depends on sail combinations and sail shape (nice and flat), not pinching too much perhaps - try sailing slightly more off wind to increase drive to punch throught the waves, or reefing early, or not relying on a baggy genoa etc. etc. Most older (and therefore narrower) boats tend to heel quite readily (they don't have the 'form stability' to stop it) but once they're heeled they have drive to push themselves to windward (low freeboard in older designs helps).

But the thing is to gradually build up confidence by building up experience in ever greater windspeeds. Also working out where you can get experience with a ready run back into shelter again if it all gets too much.
 
Re: AVS, GZ curve & RCD category

It just comes back to me gaining confidence in this boat.... it does heel very easily, and its down to me to know that it is ok... right back when I started to sail it took me a while to gain the confidence and know that the boat is ok and will stay upright.. that took time, and then I stopped sailing for a few months, bought my boat, haven't used it too much and am almost starting at square one again. I have got next week on holiday to spend the week on the boat, but then they are forecasting F5-6 virtually all week and I don't feel I'm in the position yet to venture out in this.. I'll get there, just slowly.

Thanks for your input.
 
Re: AVS, GZ curve & RCD category

GZ curves, AVS's etc have been around a long time, but are only a relatively recent thing in pleasure boating. There are generations of sailors who had no idea what their AVS, GZ, KN, BMs were and yet they managed to sail safely. If fact how many people here know what their GZ curve looks like? And how many people who have a GZ curve know how it will change once they have fitted it out and how it might change with crew numbers and where those crew are (welly dangling or down below off watch)? My point is that the skipper has to get a feel for a boat and use their judgement as to what conditions are suitable. Those conditions will be variable with the skipper's experience, ability, guts and crew, it will depend on the level of fit, the condition of the equipment, the loading on the boat, the sea and wind conditions etc. It should not depend on the letters after a boat's name.

Of course extreme conditions mean that you are unlikely to have experience of how the boat is going to react. For real confidence, one should have a more detailed GZ curve generated for that particular load condition. But again, if you get water down below during a knockdown/capsize, the GZ curve is as useful as papier mache.

JMKENT,

I hope you manage to get out this week and have some great sailing and get some confidence back.
 
As someone said earlier, it depends on the shape and size of the waves, not windspeed as such. An an extreme example, behind the barrage in Cardiff Bay F5-F6 wouldn't bother you...the wind won't capsise you and waves are 1-2 ft at most. F5 offshore in deep open water might frighten you because of the shear size of the swell, but without the crests breaking it's safer than you think. Now, the short steep seas we can get with F5+ wind over tide in the shallow waters of the Bristol Channel takes a little more care...so would areas where overfalls occur.
 
You must log in or register to reply here.

Share this page

Top