Progressing from the Legend flames below

[Twister_Ken]

For me, one thing which has come out of these threads started by the Legend stability curve (or lack thereof) that I hadn't realised before is the probable cause of capsize. Apart from striking something or being struck, the most likely cause of capsize would seem to be a beam sea breaking on the side deck. Two pieces of logic kick-in at this point, each leading further.

1. Boats that rely heavily on form stability (relatively low ballast ratio, relatively broad beam) are automatically more vulnerable to breaking beamers. The weight of water on a beamy side deck will create a longer lever than the equivalent on a narrow beamed boat; and the relatively light weight and smaller surface areas of the keel of a 'form stable' boat is less able to resist that lever. Hence the likelihood of capsize in breaking beam seas is increased, and the speed of recovery from capsize will be slowed, allowing more time for flooding.

2. That in survival conditions, any boat needs to have a strategy which minimises the risk of being struck by a breaking beam wave. This would seem to put sensible tactics into four types
a) running before the wind
b) sailing into the wind
c) heaving-to to keep the wind well forward of the beam or
d) deploying some sort of drogue/sea anchor from the bow.

Lying ahull, which is often mentioned, would not seem a good idea in reality because it exposes the boat to beam seas.

Choices a) and b) require active helming and boat management, which supposes a fit and aware crew. From experience many 'form stable' boats are very uncomfortable to take to windward in high winds, and will tire the crew very quickly. Conversely, some of the 'traditional' hull forms roll heavily when sailing downwind, again making the crew suffer, not to mention the possibilty of a roll-induced broach across the wave train. So perhaps, if caught out and wishing to take 'active' survival tactics, a traditional boat should be helmed to windward, whereas a form stable boat should be helmed downwind, sea room permitting.

Similarly, with the more passive choices c) and d), some modern boats are difficult to heave to in a stable fashion, with wind well forward of the beam. Consequently, the passive survival tactic for such a boat may well be to lie to a sea anchor, whereas the traditional boat owner has a choice of either method.

Of course, rogue seas coming in from the beam would screw up any of these considerations!

Comments please - these threads seem to be a good tutorial for all of us.

PS - I'm off to re-read Heavy Weather Sailing

 

[burgundyben]

At last.

Ken

I'm absolutely delighted to see that seamanship has been brought into this discusion, bout time too.

It crossed my mind earleir today, its 7.20 pm here in China, how would Suhali have fared if we measured her against the different ratings that have been mentioned? Robin Knox Johnston saw some pretty impressive weather on his 'Around Alone' in '69.

In a nearly 20 year boating career, of which five years I did it as a living, I have been out in 70 knots twice and knocked right over once, in both those cases seamanship was equally as important as the boat. Again late last year I had to raise a helicopter to take a ill crew member to hospital form the middle of Biscay, the most stable boat in the world was no more use to me there than 505.

I had the luck to work for a sailing school through my teenage years where seamanship came above all else, it gave me a good grounding (pun intended).

I reccomend anyone looking for something to read, find a cope of KNR Around Alone, puts more recent single handed round the world's into perspective.




burgundyben, coolest of men, by all I'm called, cool burgundyben.

 

[webcraft]

I may be wrong, Ken, and am prepared to be told so, but I don't think it's a beam sea breaking on the side deck - it's simply a breaking wave of sufficient height hitting the boat beam-on that is likely to cause a capsize.

The odds of a wave actually beginning to break at exactly the right moment to hit the side deck must be vanishingly small. My understanding is that any wave with a large breaking crest is likely to capsize a yacht if striking it beam-on. The crest is a mass of water moving rapidly in the horizontal plane, unlike the predominantly up and down motion of a non-breaking wave.

There is a generally accepted figure as to the height of the aforementioned breaking wave, although obviously this is only a guide. Essentially, I believe that any vessel being hit beam-on by a breaking sea with a height more than two thirds of the vessel's waterline length will probably be knocked down and possibly capsized.

This means - as you point out - that lying ahull is a dubious tactic in storm conditions. In a boat with a low angle of vanishing stability that relies heavily on form stability it is tantamount to suicide.

However, without wishing to harp on - if for any reason the worst happens, then you will come up more certainly and quicker in a boat with a Contessa-like stability curve.

Keep the wet side down,

Nick

 

[Guest]

At last Ken, you have finally added some sense to this thread. IMHO seamanship is a large factor in this thread, probably the biggest. Yes by all means anyone can get an unexpected knock down, but if your fool enough not to realise WHY and what you should do about it to stop it happening again, then you really shouldn't be out there in the first place. If your going to place all your faith in a theorectical stability curve to stay up right when the boats motion should have told you long ago that you need to do something before you get into serious trouble and your feet wet then I would suggest that you might think of giving up sailing and take up ditch crawling in a narrow boat<s>

 

[stiknstring]

Judging not from experience but from written articles, the perceived wisdom seems to be as you state, except that a breaking wave height of one -third, not two -thirds the waterline length seems to be the criteria for a roll inducing wave.

Interesting debate

 

[bedouin]

There's lots of good stuff written on this.

Surprisingly the conclusion of studies on the chance of being rolled is that the design of the boat was not particularly important. If you're caught beam on to a breaking wave then you are likely to be rolled whatever your hull shape.

A significant criticism of some designs has been not so much to AVS, but rather their stability the wrong way up.

To my mind the best storm survival tactics are to double-up the warps and head to the bar!

 

[Boatman]

Webcraft I agree with most of what you have said however you comment

"I believe that any vessel being hit beam-on by a breaking sea with a height more than two thirds of the vessel's waterline length will probably be knocked down and possibly capsized."

I think if I remember correctly it's not 2/3 but a very small 1/3 so a 3meter wave beam on will cause problems on a 10m yacht.

 

[Boatman]

Ken,

Excellent book, if a bit slow to start, you will be reading it a number of times

 

[pandroid]

You are absolutely right. Ted Penny, who did much of the work on behalf of Southampton University when they were coming up with GZ curves, after the 79 Fastnet, has a video that they made showing the response of various hull forms to breaking waves. You'll no doubt be delighted to know (if you didnt already) that the ideal hull form was the Twister, which 'skips' sideways over the waves rather than being rolled by them, mainly due to its narrow beam. He has one horrendous video, shot from a helicopter, of a real yacht (with someone on it) being rolled by one of these waves.

Wind alone is not usually sufficient to roll a yacht. Once the boat reaches 80 or 90 degress, all drive has gone out of the sail, and the rudder has lost steerage. The problem stems from the fact that wind generates waves! In the case of the Fastnet, one factor was a large wind shift, (caused by a passing front) which set up a 'cross' (and thus confused, and higher) wave train.

 

[AndrewB]

Heavy weather tactics.

I agree entirely with your analysis, but given sufficient sea-room and a medium-long keel there are only two tactics as far as I'm concerned. I've had more practice than I would wish, but even so, a gale is not a time you feel like experimenting.

(1) Heave-to, under storm stay-sail alone. For this purpose the storm stay-sail needs to be a lot SMALLER than a conventional spitfire labelled as a storm jib which is designed for clawing off a lee shore. This tactic is probably only suitable with a longer-keeled boat.

(2) Running downwind with same storm stay-sail, towing a series drogue (or warps) to keep speed to a minimum consistent with maintaining steerage (under 4 knots). Pick up speed, and broaching is inevitable. Fin keel yachts seem to prefer wind slightly on the quarter and may be less vulnerable to a broach.

I can't imagine setting a drogue or parachute anchor over the bow in heavy weather. Also, it pins you down more than I would want.

Incidentally, if you are caught by a high breaking wave on the beam, the force and noise is almost unbelievable. I've seen water pour through a shut portlight like it wasn't there - the portlight was perfectly watertight afterwards.

IMHO its not being inverted that's the worry, it's the thought of not quickly re-righting. All ocean going yachts must be thought through with the possibility of capsize. Yes, it can happen even on the Canaries-West Indies milk run!

 

[Guest]

Re: Capsize

Two knockdowns and a right over.

There is a lot of sense in this thread. But there is never one solution. It is the steepness of the wave, in my opinion that causes more problems than anything. If the slop on the leading side is 45 degrees theen one has almost no inherent static stability and one hopes that the wave, which is proably doing 20 knots will pass before your yacht goes over because if it does the other side will bring her up.

The problem in big steep waves is that part on the top in which there can be found no force of buoyancy because it consists mostly of bubbles.

I think, (but do not KNOW, because nobody does) that bad to moderate seas can be coped with best by running, towing something which should have a bit of depth. It is when the seas get both BIG and SHORT (an example is a north-easterly over the strong flowing gulf stream) that your family needs a good insurance policy. When I designed Fare Well, we built her with 6mm steel and made her so that in extremis, she could play at submarines. In those conditions (waves >30ft, wind against current), then one can only play at submarines. We survived. Three old folk in their sixties aboard. And we were not all that uncomfortable.

William Cooper

 

[zefender]

Nice post Ken (and subsequent ones too). My only (humble) criticism of the Heavy Weather Sailing book is that it seems to be very biased towards heavy displacement tactics. There is a small piece suggesting that with a fit crew, to keep a lighter boat sailing seems to be the best option. But I was still left feeling a bit puzzled about whether it is just foolish to venture out in a modern, lighter displacement design or a gap in the text.

 

[Bergman]

Not sure I understand this.

Ken made the point that wide beam made vessel more prone to being rolled by breaking wave on beam.

This formula relates vulnerability to waterline length - not to beam.

Does this mean that a vertical stemmed, sugar scoop sterned lightly ballasted fin keeled 30ft boat with LWL of 29 ft will be less likely to being rolled than 30ft boat with long keel heavy ballast and a LWL of 25 ft.

Doesn't sound quite right to me or am I missing something.

 

[bedouin]

As far as breaking waves are concerned - the outcome of the research is that the hull form didn't make that much difference.

They found that any hull could be capsized by a breaking wave of height 2/3rds of Length (I presume LOA is what matters rather than LWL).

 

[Stardust]

Also read Maurice Griffiths about some of the advantages of shoal draft in extreme conditions. He reckons that one of their main advantages is that in extreme seas they make so much lee way that they are likely to leave a slick to windward that will trip a threatening sea. Equally, if they are struck they are more likely to be shoved aside rather than rolled by being tripped by a deep keel locked into static water whilst the hull is pushed over by a big sea. Like everything its not the ships but the men in 'em that counts.