Was AVS a journalists passing fancy? (Are you there JJ?)

[nicho]

For a number of years, new boat tests highlighted the AVS of the particular vessel concerned. Many AWB's were criticised for having an AVS of "only" 120/125 degrees, and it was oft insinuated that these boats were not capable of being subjected to rough weather. Indeed, some have even questioned their safety outside of coastal waters.

In the YM that has just landed on my doorstep is a test of the Catalina 42 MK11, carried out by JJ. This boat has "a practical durability, matured across the Oceans of the World". The insinuation is that this particular boat being reasonably heavy in displacement, is a sturdy and reliable vessel for Ocean passages.

I'm interested to know then, why (as far as I can see) there is no criticism of the boats amazingly low AVS of 105 degrees, INDEED not even a mention of this "shortcoming", which should, according to some past boat reports, render it unsuitable for offshore passages.........

Is AVS no longer an important issue to boat testers??

 

[Stemar]

I've often thought the same thing about OVNIs. I know it's not the only thing that counts, but I think I'd like to come back afrom a good few degrees more than that!

Anyone know the avs for my renowned bluewater cruiser, the Snapdragon 24? /forums/images/graemlins/wink.gif

 

[PeteMcK]

So when the spreaders touch the water, she's on the point of going over...?

 

[zefender]

I'd have thought that seaworthiness is based on the amount of effort required to get a boat to 'tip over', not the actual point at which it might - if you know what I mean.

 

[AlexL]

yeah could be right there Zefender. I'm no expert at this but i reakon that if one takes a wave with a given amount of energy, some boats will be tipped more than others by this wave. Lets say for arguments sake you have a lightweight, 'Tippy', boat but with an AVS of 140 degrees, and a heavy, stiff boat with an AVS of 105 degrees. Lets lash each boat with our new 'standard wave' and see what happens. The tippy boat may get tipped over to say 110 degrees - 30 degrees away from the point of no return, and the stiff boat may get tipped over to only 70 degrees by the same wave - 40 degrees away from its point of no return. So in this situation the boat with the lower AVS appears to be more 'seaworthy'. This is probably an over simplification but my engineering brain says that this sort of thing must be close to the truth, so this is a very longwinded way of saying that AVS is only part of the story.

 

[StugeronSteve]

Re: Sorry if I\'m stating the obvious but...

.....isn't this demonstrated by the stability curve , which usually accompany YM tests, and show the effort required to progressively heel a boat?

 

[Sailfree]

I agree and if the journals made more of a comment on the AVS and buyers voted with their wallets the manufacturers would respond by improving this aspect of boat design.

From my reading of I think Heavy Weather Sailing ALL boats capsize if hit broadside on by a breaking wave = to or > than the water line length (from memory based on research from the Wolfston? Intitute, Southampton University). The AVS dictates how prone they are to capsize before this point and can contribute to how quickly they right themselves after (less applicable to beamy flat deck boat that are the opposite to lifeboat topdeck design-unstable when inverted).

I think it was an Elan 31 tested a few years ago that had an AVS of 138 and I thought good boats are improving but unfortunately it is not a trend.

Probably though we are all wrong and the sucessful major manufacturers know that the people who buy the AWB boats will never choose to go out in bad weather, consider it unlikely they will be caught out on their weekend and summer trips and would not be prepared to pay any extra for improved AVS's.

I personally would like the option!

 

[zefender]

Maybe you're right Nicho. This site has a calculator to work out AVS of any boat. It also mentions that 100 is about the AVS required of an inland water boat. So praps journos might make comment, as I agree they used to. But I'm not entirely sure the method of calculation (it comes from Adlard Coles' Heavy Weather Sailing I believe, but there are other formulae) is entirely appropriate for a flatter bottomed, fat arsed 'modern cruiser'. The AVS is I believe based on boats at rest in flat calm. Maybe the physics of the modern design when underway, mess up the figures - a bit. So the AVS might look a bit crap, but actually, in anger, isn't.

 

[nicho]

Good point Alex, but the point I'm making, as the owner of an AWB with an AVS of around 120 degrees, many journo's (particularly PBO's David Harding) have seemed to insinuate the AVS to be almost the "be all and end all" of the seaworthiness of a boat, and really marked down those that are on the low side, and relying on form for stability. Now, it seems in recent reports, and in particular this one, the AVS in no longer an important issue. Just wondered why, that's all!

 

[nicho]

Re: Sorry if I\'m stating the obvious but...

True Steve, and I'm no expert on thses matters, but compare the curve of this Catalina to almost any other boat tested, and in my view, it makes uncomfortable reading.

 

[nicho]

As a flatter bottomed, fat arsed skipper, I tend to agree. However, like you I would rather like to get a comment from JJ as to why, for the first time, such a low AVS was not even mentioned. Perhaps there are new views on the subject of boat stability we don't know about?

 

[StugeronSteve]

Re: Sorry if I\'m stating the obvious but...

Mags waiting for me at home I'll give it a look tonight.

 

[PeteMcK]

Re: Sorry if I\'m stating the obvious but...

Resistance to being capsized by a given wave is about: a) shape of the heeling moment v. heeling angle curve; b) energy (i.e., area) under the curve; c) roll moment of inertia (flywheel effect). The paradox is that what's good for (c) - a heavy mast, for example - can appear to be very bad for (a) and (b).

I'm sure a few of us have been knocked down by the wind to positions where our spreaders weren't that far off the water, and the thought that combining that situation with a fairly small wave would push us beyond the AVS of 105 degrees would be very disconcerting to me. Spinnaker, 25 knots true, broad reaching, surfing 2-3m waves : 10, 12, 14 knots - great fun. Broach!

 

[PeteMcK]

The calculator is a wild approximation - it's impossible to estimate the AVS easily as well as accurately. The Contessa 32, for instance, can be shown to have an AVS as high as 180, measured - heeled - light, but loaded with add-ons above the initial CofG, that number falls dramatically. Every boat is different and it's essential to do an initial heeling trial (a degree or two) to be able to establish the CofG for every case. You need to know the displacement accurately too.

 

[Guest]

If I remember correctly from reading about the Wolfson Report, it is hard to say that the initial form stability noticeably helps to stop a boat being tipped by a wave. Because it takes only a very small breaking wave to tip a boat especially if it is caught beam-on, pretty much any boat irrespective of its form is going to be tipped over in that situation.

I don't recall exactly the height of breaking wave required to tip a boat, but I think it was something around 3 or 4 ft, breaking at the 'wrong' moment, to tip a 30 footer, anyway, it was an amazingly small breaking wave.

The question is therefore, not so much whether form or other kinds of stability stops you being tipped, but rather the ability of a higher AVS to right the boat promptly and/or make it less likely that the boat goes completely turtle (or if she does anyway, that she doesn't stay there for long).

Dinghy sailors know that when a boat's mast and sails hits the water, there is a tendency of the boat to pause before (perhaps) going completely turtle. This is of course what's known as a "knock-down" rather than an inversion, the sort of thing that may easily happen in a broach. When the boat's mast has hit the water, the boat is not at an angle of 90 degrees, rather more like 110 (look at the angles of semi-capsized dinghies' masts). Personally I would want for a boat that in that situation is trying to right herself, rather than having neutral or negative stability, something with an AVS of well over 120 degrees.

Another aspect to consider is windage on the hull - in a gale, the fatter the hull, the more that the wind is (assuming you've been knocked down to leeward) preventing the boat from righting. Since AVS is calculated in calm conditions (and without roller reefing radar domes etc. in the rigging), it's probably worth subtracting quite a bit from the theoretical AVS figure as a safety margin.

 

[nicho]

I agree with all that Simon, and I'm not saying that the AVS figure is not important to the "capsizability"(?) of a boat. I'm more interested as to why an AVS as low as 105 degrees does not even get a mention, as if it's no longer considered important. Perhaps it was just a oversight by JJ, or maybe any comment got lost in the editing. Hopefully James might let us know.

 

[Guest]

I understand, and agree 100% with your question. I think that it is part of a trend with AVS's - until recently, because of the Wolfson Report all AVS's were shown with the Contessa's alongside as comparison. But this made modern mass manufactured boats look bad, and as the lessons of the Fastnet were forgotten, the Contessa's AVS as a comparison was quietly put to one side a couple of years ago.

My post was not quibbling with your observation, just commenting on AlexL's suggestion that the initial form stability was some kind of alternative to a healthy AVS further along the curve.

 

[webcraft]

This isn't the first Catalina reviewed in YM with CatA Ocean and a low AVS . . . there was a fair bit of comment on the previous one, which I think might also have been reviewed by JJ - and in the ensuing discussion here the implication was that in fact the vessel had actually been damned with faint praise in the review. The low AVS was, I seem to recall, mentioned in passing in that review. (Thread is over a year old, so beyond recall).

I haven't read this new Catalina test yet as we always get YM late here. However, I do know that the new stability measure, the STIX number, involves considerably more than just the AVS.

AVS still means something, though - and personally I would rather go bluewater in my old, small Albin Vega than in a shiny new Catalina with an AVS of 105 deg - but maybe there is something I don't know. It would certainly be interesting to hear the professionals' views on the issuing of Cat A to boats with this low an AVS.

The previous (still available) thread on AVS mentioned Ovnis, which also have a v.low AVS figure but have a reputation as great bluewater boats. Someone reckoned the underwater shape / lack of deep fin means they don't get rolled by a breaking wave the same as other boats, but I have to confess this sounded unlikely to me at the time.

Of course, B&Q has an AVS of less than 90 deg . . .

- Nick

 

[peterb]

Energy

[ QUOTE ]

I'm interested to know then, why (as far as I can see) there is no criticism of the boats amazingly low AVS of 105 degrees, INDEED not even a mention of this "shortcoming", which should, according to some past boat reports, render it unsuitable for offshore passages.........

[/ QUOTE ]

Not just the AVS. The area under the curve is a measure of how much energy it takes to get the boat to her AVS. Looking at the Catalina's curve, the area under the positive part of the curve is not much greater than that under the negative part, which means that she wouldn't recover easily from a capsize. In fact, if YM's curves are correct, the moment required to right from a capsize is greater than that required to capsize in the first place!

 

[webcraft]

AVS - a few bits of interesting info

I've been having another poke about on the web to see if there was anything new to say on AVS, but nothing seems to have changed. Any boat can capsize given the right (wrong) conditions, only some will come back up, and the MCA certainly used to recognise this when coding yachts for commercial use. Quotes and sources below:

QUOTE 1

If a boat is positioned into a breaking wave, most boats (wide and narrow beamed) can survive a 55% LOA (overall boat length) breaking wave. However, a 35% LOA breaking wave hitting a wide-beamed boat beam-on can easily capsize the boat. All yachts tested rolled to 130 degrees. No yacht, no matter how stable, could consistently resist capsizing when hit, beam-on, with a 55% LOA breaking wave . . .

2. Ultimate (Latent) Stability. This is the resistance to capsize and heel. One of the best predictors of ultimate stability is the "angle of vanishing stability" or the angle to which the boat can heel and still right itself. A dingy will have a stability range of about 80 degrees, an inland water boat should have a stability range of 100 degrees, and an offshore boat of at least 120** degrees. Boats which have a stability angle of less than 140 degrees may be left floating upside down once capsized. Boats with a higher angle will usually right themselves

- Source <font color="red"> SAILING USA </font>

<font color="blue"> ** many other sources quote 140 degrees here . . . </font>

QUOTE 2

Monohulls usually achieve maximum righting moment when heeled between 40 degrees and 70 degrees. As they heel, the wind is progressively spilt from the sails until at 90 degrees of heel there is very little heeling effect. Unless downflooding occurs, many monohulls will return to the upright after being knocked flat by a gust of wind in relatively smooth water. If the righting moment remains positive up to about 120 degrees (which usually means that the boat must have permanent ballast in the keel), it will normally recover even if totally inverted by breaking waves.

Source - <font color="red"> EUROPEAN BOATBUILDER </font>


INTERESTING FACT . . .

Another interesting snippet is that the original stability requirements of the MCA Code of Practice appear to have required an AVS of 128 deg for a Cat 0 certified 42ft yacht . . . although the stability calculations are considerably more complex nowadays. Source <font color="red"> HERE </font>

IMHO anyone who chooses to take a yacht with an AVS of less than 110 degrees out of the range of reliable weather forecasts is either ill-informed or a Vendee Globe competitor . . .

- Nick