Big Waves

[Swift]

Good Morning all

There's a big swell running of Sydney today so I thought it might be a good time to start a new thread to follow up on Diana's comments on her bumpy ride to Broken Bay. BTW well done Diana!

As Phillip pointed out Seabreeze provides excellent information. One of the guys from the site explained to me that the forecast page is driven by information gathered from a number of sources and various features are switched on an off when they reach threshold points (like rain clouds etc.) It's all totally automatic and that's why forecasts will occasionally be different from BOM forecasts. For more detail and informed opinion on sea conditions the surfing web sites like Swellnet can be very useful. The Swellnet commentary for today is as follows: "Well, here it is. One of the biggest swells we've seen in Sydney-town all year. To be honest it's hard to put a size on it, as there's (obviously) no-one out,......, The swell is very strong, and there's a lot of water moving around - today is a day only for experienced surfers. Don't fool yourself otherwise". However, Seabreeze is showing a forecast of slightly over 2 meters which does not seem that big, so what's going on?

Wave heights forecast by both the BOM and Seabreeze are the significant wave height which is a calculation of the variable local wind wave combined with the more consistent underlying swell. It's the average of one third of the highest waves which means that some waves must be bigger to make up the average. Maybe 10% of the waves will be about 30% higher but the probable maximum wave height is 100% greater than the significant wave height. So there you are out in your boat thinking of the 2 metre forecast but suddenly out of nowhere you find yourself on a 4 metre face thinking where the hell did that come from? But actually it's consistent with the forecast.

A good site to see real wave heights is Manly Hydraulics Laboratory - http://www.mhl.nsw.gov.au/www/welcome.html. Check out the graph for the last 4 days on the page http://www.mhl.nsw.gov.au/htbin/wave_da ... ion=Sydney. The green line is the significant wave height and the red line is the maximum wave height. Have a look at last night, this shows waves up to 6 metres. Note the big spike on the 13th. It shows that a wave in excess of 8 metres came through. Because the significant wave height at the time was 4 meters an 8 meter wave is completely normal, or to be totally accurate as this wave was slightly bigger at 8.4 meters it would be, by definition, a freak wave, but it's only a freak by a very small margin. So a good rule of thumb when looking at wave forecasts is to be aware that you may at times get some waves or sets of waves that are double the forecasted height.

Cheers
Keith

[storm petrel]

G'Day All,

Great to see Jessica Watson back safe and sound!

As a to wave height, I don't think it is the height that is the problem when sailing but the steepness of the faces and whether the crests are breaking. As a life-long surfer, I have been out on some huge days (e.g. Fairy Bower on the evening when Manly pool was washed away in 1974) and I know that once you are out the back, beyond the breaking waves, things are pretty calm (unless you are unlucky enough to have a big set break on your head). So when I go sailing, the swell size doesn't really worry me too much. I do however take careful note of the predicted wind strength and its direction wrt the swell. When the swell is up I try to avoid shallower water (where the waves are steeper), places where the current is running against the swell direction which also steepens the waves and anywhere where the wind is going to be strong enough to cause the crests to seriously break. Seabreeze, swellnet and BOM forcasts are all great sites that make coastal hopping much safer.

Wish I was sailing,
Mark

[Dolphin]

Further to that,
Francis Chichester in the 60's was hit by a rogue wave that rolled him to 120degrees and also dislodged lifeboats from the liner Canberra. He enlisted a mathematician who calculated that every wave in 300,000 is 4 times the height of the average waves. If you consider that waves are about 8 seconds apart that means that every 28 days you are going to get a "rogue wave".
Also a wave will break in 1.5 to 2 times its depth. That means that a 2m wave will break in 3 to 4m of water. Also when the trough to length ratio is less than 1:14 it will also "break", or "crest". It's important to realise this if you are crossing a barrred entrance. You may go out on a high tide and not be able to return because the tide has dropped and the bar is breaking.
The bureau gives a disclaimer, with every broadcast following the 98 Sydney Hobart race, that says the wind can be 40% stronger and the wave heights can be twice the height.
The wind force on a sail is the area times the velocity squared (F=A x V^2). If the wind speed increases 40% ie from 10 to 14 knots the force is twice that of the 10 knot speed. (140% x 140% = 200%)
TO estimate the height of the waves raise your eye level until you can see the surrounding waves whilst the boat is in the trough. Most yachties overestimate the wave height because they are sitting down and the waves seem to tower over them.
Off the coast the current sets south at about 1 to 2 knots whereas in close, within 500m, it can set north. In a southerly this can mean the difference between breaking waves out wide and nice rolling swells in close against the bricks.
I hope this helps.
Greg.

[storm petrel]

Hi All,

Good points about the East coast current and how it affects the sea state.

When surfing on the East Coast of Aust. there are some days that seem quite small that have occasional sets of waves that are several times bigger than the average. these are usually the days when fishermen are wept from the rocks. Interestingly, when surfing in Hawaii I noticed that these 'big' big-set types of days are the rule rather than the exception. I guess we can thank the continental shelf for protecting us 'coastal sailors' from the sort of rough waves that Chichester encountered. Being turned 120 degrees wouldn't be much fun!

Cheers
Mark
Storm Petrel

[Phillip]

Hi All,

Mark is right, it's the steepness of the wave that causes the problem.

In the early 90's I was returning from Seal Rocks to Forster in a 25' diveboat (had two desiel powered sterndrives) during a black NE'er against the current and eventually off Cape Hawke I literally drove the boat straight through the face of a 3m high wave, the boat filled to our knees before the scuppers were able to handle the amount of water we had taken in over the cabin. We reckoned that for a moment the whole boat was just about underwater.

Now if I had been there in aTop Hat, it would have been a very rough sail/motor to say the least!
I just hope I am never silly enough to allow myself to be caught out in a NE storm like that one.

Phillip
SEAKA

[SeaLady]

Ok, so from all of this and my own thinking.

Go outside the heads, or anywhere only in 1 metre swells max, 10 knots max wind, 25-30 deg C air temperature with guarranteed no southerlies to creep up on you. Never heel over so far that you spill your bubbly.

Yeah sure!!!

I have been caught in a southerly that came in much earlier than predicted going across the heads in Sydney.
40 knot winds and waves breaking completely over the Top Hat. And I mean completely. Got very wet. And scared. Normally I can see a southerly coming but this one just hit. Hours before it was due. I had planned to be safely back at my mooring.

Once I got around Bradleys all was calm. Would never have picked it.

I do agree that it can be hard to estimate the height of the waves, but when all you can see is a wall of water it looks very very big. And the wind dissapears as the wall of water is in the way.

Also regarding big surfing waves. I am not a surfer but a couple of years ago went to Hawaii to the beach where they film all those big wave movies.
Those people are mad.
I realised why the scenes of film are all so short. The waves break almost on the land.
So they only get to ride the waves for a few seconds before they hit the brown and green stuff and nasty coral.

My waves coming up were nice and rolling and I was going with them. So it was quite pleasant in a way and no pounding.

Regarding being close to the rocks to avoid swells??? am I a wimp? but I would prefer to have more than less than 500 metres between me and the coast.

Diana

[karl010203]

At the risk of revealing myself as a complete novice

I had the very interesting experience about 18 months ago where I had the day off work, mid week, and decided to go for a solo sail overnight... out of Mooloolaba.

I checked the bom website, said nothing unusual (was stronger winds) and had from memory 2 - 3 m swells offshore and the usual disclaimer in regards to the significant wave height.

As I headed out towards the bar the was a lot of white water on the bar itself which was a little unusual for Mooloolaba but at this point I didn't give it much thought and picked a path through it.

Got out through the bar after a bit of wave dodging - which caught me by surprise - always fun when your top speed is 6 knots... and mooloolaba is a great bar so not a problem normally had...

After motoring about a Nm, it was evident the weather was actually pretty extreme - the waves were huge and standing up due to shallow water. Solitude was falling off the back of some waves and punching the bow deep into the next and I was dodging breaking waves in deeper water - Some waves seemed to be spreader height!!! (I call them waves rather than rolling ocean swells - as they were standing up and threatening to break, or breaking.)

At this stage I decided probably not the most ideal or comfortable day and headed back in - much to delight to onlookers at bar, which took a bit of work to control speed, and avoid whitewater.

Getting back to the safety of the computer looked at the mooloolaba wave bouy and it showed significant wave height 8 - 9 metres!!!

http://www.derm.qld.gov.au/environmenta ... /index.php

So what did this novice learn:

1. I was far too casual and just happy I didn't have kids on board. (or wife!)

2. Tophats punch well above there weight division - and are extremely capable.

3. In my opinion, and it may just be my lack of understanding and knowledge, the info on BOM is actually too generic for my liking - and certainly did not show this!! (am I the only one who gets frustrated with reports that show the likes of winds SE/NE 10 - 15kts... ?)

4. I now prepare differently - I check wave height - from Wave Bouy - and get sea and wind conditions through Predictwind, which shows the wind direction on a local level nicely during the day and which seems really accurate. I know there are a stack of other similiar sites also.

5. Take time to make own assesment of conditions, sit at the bar, watch waves etc. Dont get too excited about just getting the hell away from the laptop and work phone!

6. Was a stupid thing to do and I am sure if I had an insurance claim as a result they would not have paid given the conditions.

7. I can see why a drogue could be an important safety item controlling speed.


It was a great learning experience and gave me faith that the boat can handle these conditions... and I am sure it is still much safer than working and in my case the occupational potential dangers of papercuts, coffee burns and death by powerpoint- ha ha.

[Tales]

The RYA Weather Handbook - Southern Hemisphere Edition by Chris Tibbs is an excellent book which explains, among other things, why local conditions are different from the BoM forecast.
Cheers,
Tom