AVS of 116 degrees

[extravert]

That boat recently reviewed in YM (can't remember what it was called, lots of wood though) was RCD category A, but had an AVS of only 116 degrees. That doesn't sound very much to me, a knock down and a little bit more, and then it's on its way to upside down. Do other peeps think that was rather low for a supposedly ocean going boat? If it did go over, would it realistically ever come back, or would it happily bob around upside down? If it ended up inverted with the mast intact, with the now removed heeling force from having no sail/rig in the air, and a vastly increased water resistance to rolling from the immersed rig, sounds like it would be there to stay. What say you?

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[bedouin]

Yesl, an AVS of 116 degrees sounds a bit low. However that is really only on e point on the graph, and the whole thing needs to be examined to show the true story.

IMHO when looking at the curve the two important issues are the area under that part of the curve that is above the axis (good) and the area above that part of the curve that is below the axis (bad).

After all what really matters is how much force is required to cause either a knockdown or inversion - which is really more to do with the area under the curve than the actual AVS.

The more a boat relies on form stability rather than pure ballast, the lower the AVS will be but that does not necessarily make the design unsafe. A cat is a good example - they have an AVS of 90 (i guess) and once capsized cannot be righted, but that doesn't make them inherently unsafe.

I haven't seen the YM article, so I can't comment on whether I would regard the boat as safe or not, but I am just trying to make the point that there is more to stability than a single figure.

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[Magic_Sailor]

Yes, I do think it sounds low. But, I find this whole area rather confusing - what with the addition of STIX which is meant to take a number of different criteria into account.

Add to this the fact that any modification will affect these numbers and the problem of ascertaining good/bad gets even more compulcated. For instance, I've just added (or am adding) a liferaft, extra achors, chain etc for safety purposes - all of which must adversely affect AVS/STIX.



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[FlyingSpud]

When they do these tests do they work on a ‘clean’ boat?

If they do, then when people move in I assume the AVS falls as they hook on all sorts of heavy stuff above the water line and it will end up substantially lower than the official figure. I am sure I read somewhere about in mast furling knocking quite a few degrees off the figure on its own. I wonder if anyone has done any rersearch into the average effect of all this gear?

So would a boat with this sort of figure would pop back even if 'only' knocked flat?

The other vital point is that the higher the figure the more likely the boat will eventually self right, given the normal conditions one gets on a knock down, a high AVS almost guarantees the boat will come back even if totally inverted (unless the keel fell off or something like that)


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[Twister_Ken]

Agreed AVS at 116 does seem worryingly low, but I'm not sure whether that means it is more or less likely to stay inverted. I suspect that two key factors in that equation are beam (wider equals more stable inverted????) and how buoyant the (now) underwater parts are, so that a wheelhouse yacht with an area of buoyancy low below the inverted waterline might pop up more quickly than a flush decked boat. Another factor would be whether the rig stays in place. Obviously a mast with all that heavy stuff (roller reefing, radar, radar reflector, lights, aerials at the top(bottom) of it, 50 feet under water, represents a fairly substantial lever arm for the keel to overcome.

Whatever, 116 would worry me for offshore sailing, especially as, as others have said, you could probably subtract several of those degrees once the optional extras get bolted on.

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[jamesjermain]

I have great sympathy with the concerns of YM readers and Scuttlebutters about the low AVS which is acceptable under the RCD for CAtegory A (Offshore). The RYA regards an AVS in teh high 130s as more appropriate and I am inclined to agree.

The people responsible for the RCD painted themselves into a corner by insisting on only four categories. While applauding their desire t keep it simple, the result is that anything from a 32ft light displacement cruiser/racer can be in the same category as a 48ft heavy displacement world cruiser.

The problem they faced when drawing up the perameters for the four categories was, what do we do about all the existing boats in which their owners quite confidently and successfully cross the Channel, North Sea, Biscay and even the Atlantic? To suddenly condemn them as coastal cruisers would be to cause consternation and undue alarm all round. So we have this huge, catch-all category which really needs to be sub-divided into Offshore, Deep Sea and Ocean.

Actually, quite frankly, it doesn't. The whole thing needs to be ditched and replaced with sensible building regulations and quality control.

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[extravert]

I agree that a bouyant volume (kept watertight) high up normally and low down inverted would help recovery, but surely it would also increase the AVS as well. This boat with an AVS of only 116 must mean that it doesn't have this feature, or else it would have a higher AVS.

From what I understand, the means that lifeboat designers use to get a high AVS and self-righting ability is just that described, a high up watertight volume. They certainly look like they are designed that way.

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[extravert]

Isn't RCD category A ocean and category B offshore?

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[Jacket]

J R Benford has designed a series of houseboats with two and a half storeys on a LOA of 35 feet (so almost as high as they are long!). Provided all the windows and doors (its a houseboat, so they are doors rather than companionways) are shut, it has an AVS of 178 degrees, almost totally due to the boyancy due to the high freeboard.

Bolger is another designer who uses high freeboard to get a resonable AVS on very shallow draft boats.

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[AndrewB]

Scrap the RCD? Hurrah!

Shout it loud in the mag, JJ!

Come to think of it though, every change in regulation, however well intentioned, always seems to get hijacked by the business interests and ends up making things worse. So perhaps its better to stay quiet after all.

 

[extravert]

Arctic traversing McGregors too?

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[Twister_Ken]

>I agree that a bouyant volume (kept watertight) high up normally and low down inverted would help recovery, but surely it would also increase the AVS as well.<

I don't think this is right. High up volume will be associated with high up weight, which will act to decrease the AVS, even though it helps boat pop back up after it has gone beyond the AVS. Or maybe not?

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[NigeCh]

CO26 - Rated as Cat \'C\'

Talk to Tanae Aebie and Brian Caldwell ... Less may be more but sometimes it is less. I'd rather do a transat in in CO26 than a Beneteau 476 ... or to be even blunter, than a Northwind 55 (RCD Cat 'A') - Simply unsailable and dangerous.

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[NigeCh]

PS ... I\'ll add a Farr 50 into this as well

OK downwind but a wet pig to windward ... almost verging, IMO, as another Cat 'A' boat that's unsuitalbe for blue water cruising.

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[NigeCh]

PPS

I think that John Neale is wrong with his list of best blue water boats - there are several on his list that couldn't make an Irish sea crossing in an F7.

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[Twister_Ken]

And what\'s more

If a Co 26's rudder ever falls off, it doesn't leave a hole in the boat, unlike the Hunter that was abandoned on the ARC.

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[extravert]

High up weight in the form of a sealed cabin or coachroof will decrease stability until it is immersed. Then it will increase the righting moment, given that the mass of the displaced water is more than the mass of the coachroof materials itself (likely). As most monohull AVS is more than 90 degrees, ie the cabin or coachroof will be immersed at the AVS, and this enclosed volume will increase the AVS. This is because the mass of the coachroof materials will be very much less than the mass of the displaced water.

Far a similar reason some dinghies have foam filled or sealed masts. Although this extra weight aloft decreases stability slightly in normal situations, it makes a big difference once it is immersed. Righting an inverted dinghy with a flooded mast is not for the faint hearted. Usually involves a MOB or two.

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[NigeCh]

Never did understand that

ie why they abandoned their boat ... there has to something more to explain why their boat was sinking.

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[webcraft]

Re: John Neale

Interesting that he doesn't include the Albin Vega on the list, even though it was his Pacific voyage in one that made his reputation. Many Vegas have circumnavigated successfully, so I am a bit surprised at this ommission.

Which of his recommendations in particular would you not cross the Irish Sea in a F7 in, Nige?

<font color=blue>Nick</font color=blue>
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</A>

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[NigeCh]

Don\'t like Island Packets

Here's the link to John Neale's choices:

http://www.mahina.com/cruise.html



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