Dangerous swells - wave breaking formula

[Cruisingdad]

After reviewing the current wave height and period for Hanna, it made me start thinking about what point waves really become dangerous and/or start breaking.

Of course, I understand that all this is based upon the boat and its LOA, but worthy of discussion. FOr example, when you get (in my opinion) a tall sea (10+ feet) with a short period (a period well less than the height), it becomes very dangerous. You do not have time to do much correcting and the seas become fairly square. THose, in my opinion, are the worst as the square seas begin breaking off the top and spilling over.

So I am curious what others think? Where is you comfort level? Is there an easy formula for WvHt and APD (Average wave period) that when WvHt is divided by APD and exceeds a certain number, it becomes a breaker/dangerous?

For example, a wave hight of 30 feet may be beyond the imagination of many sailors. But is it dangerous? Maybe... but not if the period is 30 seconds or more (yes I am making this up for an example). It would smply be a long roller. However, if you had a 10 foot swell, which many of us have been in, that may not sound too bad. What if the period is 3-4 seconds? Better have the liferaft ready.

THoughts?

- CD

PS Hanna now has (off of Canaveral) an average WvHt (that means some higher and some smaller) of 18-20 feet, with APD at about 8 seconds.

20/8 = 2.5

[erps]

I've heard it brought up before about waves getting harry when the period in seconds is a smaller number than the height in feet.

[sailingdog]

I'd agree, once the period in seconds is down below the height in feet, they're getting awfully close together and steep. Of course, size of the boat has a lot to do with it... even 20' waves, 10 seconds apart aren't going to bother a boat that is 250' long much.

[Cruisingdad]

I have wondered about that too. The commercial guys like Sway or Boasun would know more, but I have seen pics of the bows ripped open on Cargo/tankers that they said was the result of breaking seas.

I cannot validate though.

[chucklesR]

I'm here to tell you that a even a 12 foot wave can be felt on a 90k ton, 960 foot long aircraft carrier.

20 footers make you secure your coffee cup, and 30 footers make you wish you'd joined the Air Farce.

Riding those kind of waves in a flat bottom amphibious ship is just plain bouncy (fun watching jarheads puke tho, till you have to clean it up).

I got caught out in 12 footers on USNS Apache once, the video I made from my stateroom, camera hanging on a bungee cord, stabilized with another cord to the side, is just plain hilarious

I don't know the period on any of those, just average heights are passed to crew.

[sailingdog]

chuckles-

post the video if you've still got it.

[astraeus]

Originally Posted by chucklesR

I'm here to tell you that a even a 12 foot wave can be felt on a 90k ton, 960 foot long aircraft carrier.

20 footers make you secure your coffee cup, and 30 footers make you wish you'd joined the Air Farce.

Riding those kind of waves in a flat bottom amphibious ship is just plain bouncy (fun watching jarheads puke tho, till you have to clean it up).

I got caught out in 12 footers on USNS Apache once, the video I made from my stateroom, camera hanging on a bungee cord, stabilized with another cord to the side, is just plain hilarious

I don't know the period on any of those, just average heights are passed to crew.

You can't tease us with a hilarious video and not post it

[lbdavis]

Chuckles...

[sailaway21]

Confused seas or the combination of wind versus current create the most unpleasant of conditions regardless of vessel. Likewise, shallow or shoaling water is to be avoided as the wave height increases exponentially.

As mentioned, a 10-12 swell is hardly worth mentioning the condition is so commonplace. But coupled with an opposing wind or tangential wind, things can get downright unpleasant in a hurry. You'll see far more breaking seas in such circumstances. The ride on a small vessel becomes decidedly uncomfortable under such circumstances and helming becomes arduous as well. Ships are less effected but start to lose their natural rolling rhythm.

Most anything over Force 5 ( 17-21 kts) and the wave height of wind waves starts to exceed the period. The breaking wave does not really begin to occur, absent shoaling/shallow water or a confused sea, until the wind reaches Force 9 (41-47 kts). You must factor in fetch and wind duration into this equation as well. 20' seas in Force 9 conditions are common but only half the potential height of wave possible given the wind force. You need about a day of the same wind direction over a long fetch to produce truly nasty waves from a given direction. On the other hand, and more common, the combination of a backing or veering wind with substantial swell will create much more spectacular and dangerous conditions at a lower overall wind velocity. These are far more likely to give the small boat a problem long before the breaking seas of pure wind waves from an extended Force 9 or greater.

Ships with their bows stove in have generally speaking been driven or otherwise mishandled. While the bow of a ship may well be it's strongest and most tightly framed structure it is not what one wishes to present to the sea in truly rough conditions. The bow is subject to pounding and panting stresses. Pounding can be viewed as the bow "slapping" down on the waves. It does to the bow what a belly-smacker does to a novice diver. Panting is the flexing inward and outwards of a flared bow as it descends and ascends through heavy seas. Neither condition is good. Taking green water over the bow, not just spray, is a portend of bad things to come. The problem is generally not the damage the water itself does, although it can on occasion be substantial, it is the sheer weight of the water on the foredeck that is the problem. When you bury the bow you essentially load 500-1000 tons of water foreward of number one hold. That does very interesting, not to mention dangerous, things to your longitudinal stress figures bearing in mind that you have several hundred feet of ship aft of that position of stress acting as a lever arm attempting to raise the bow. Tankers are not part of this discussion as they are essentially floating submarines, although they do ride better than bow on in many conditions. To see a tanker's entire deck covered by seas is hardly unusually due to design and cargo which yield high buoyancy and low freeboard.

Most modern ship hull forms actually ride their best when hove-to with the seas on the quarter. There they roll deeply but safely. That is, if she's loaded correctly. This is a condition that baffles the average sailor as it goes against his natural inclinations and preferences. A stiff ship in these conditions is quite dangerous. She will conduct snap rolls of a violent nature and may even break loose not only cargo but cargo gear and even such unthinkable things as the boiler or engine mounts. The tender ship with her long rolling period will ride the same conditions comfortably if still rolling deeply. It's rarely the depth of the roll, it's usually the speed or period of the roll that is determinate.

That's why dismasted sailboats quickly become uninhabitable. Absent even a bare poles condition with the mast alone producing windage the boat is free to roll rapidly and violently. The storm sails or the mast(s) alone provide a necessary damping force, as well as a portion of the mass of the vessel above 'G', via the wind that tempers the rolling of the boat in it's natural period.

Generally speaking, regardless whether boat or ship, the easiest motion that can be achieved is also usually the safest.

[Stillraining]

Another great post Sway..Tried to rep ya but need to spread some around I guess.