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Old 15 November 2002, 09:18   #1
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Perceived Sea State and Freak Waves.

Just thought I would throw this in after watching the Horizon programme on BBC2 last night. Excellent programme overall, though admittedly it skimmed over a very complicated subject at times, but very interesting statistics.

For those with little sea experience, maybe some advise can be shared from the old crusty sea dogs to those with less experience going out over this winter!!!

When you are out in your RIB (or your sib even) have you noticed that the waves look really big when you are moving at any speed, but especially when they disappear under you at 10+ knots when you want to fly from one to the other.

Have you noticed that when you stop, the waves halve in their height, or so it appears.
I also should say (for your safety) stopping in big seas is not necessarily a good idea of course, keep the bow into the wind, do not turn beam (side) on!

Manos said in another thread something along the lines of enjoy the sea, but respect it.
You really would be a fool not to treat Mother Nature with respect!

How many of you have been out in weather that causes 15-m waves? Not me, fortunately, but I have seen lots of footage over the years through work.
15m is the regular size apparently that ship are designed to take hitting them, the weight of the water landing on the vessel is thought to be 15 tons / sq. meter and the vessels should take 30 tons / sq. meter with little damage.

The program actually went onto the freak waves that have been witnessed, recorded by radar satellites & sensors on oil rigs and are shown to be common, not freaks!!! These were 30m high, but worse still they have a deep trough either side, so you go down a wave and meet a vertical wall of water, a breaking wave, that sweeps into the vessel and itís a bit 50 / 50 if you pop up again.
The difference was that they estimated that the load of the wave was 100 tons / sq. meter.
Hopefully your engines are still running to keep into the wind if you make it through.

Now I bring this up for another reason because though its not the same, wave patterns can be odd for small boats too.

Some 5 years ago I let another RYA Power Instructor with limited rough weather experience drive an Avon 5.4m in a following sea to allow them gain experience of playing the throttle downwind. We were with two 4.8m wide tube Tornadoís (excellent sea boats (actually fantastic for size)) on a passage that would take 1.5 hours in a force 6-7.

After 15 minutes of practice we found a speed (15 knots max) that allowed the boat to float over the top of the waves that were about 1.5m in height. This was enough for me to be reading the waves very carefully from behind, in a boat that has a fairly flat bow (that does not lift much at all, when you add power!).

20 minutes later, there was suddenly a longer & deeper trough than before. We could not avoid it, so we fell down and met a steep wave, only about 2.5m in height, but by this time we were diving through the wave with 30cm of water coming over the top of the bow tubes and decelerated very quickly. The Instructor driving froze, I reached the throttle and put us into neutral and we popped up again, with water up to our knees.

Bad events go either very slow or very fast, this was fast.

We started to turn broadside, which was a concern at the time, but we had ballast, the boat was full of water, but would a RIB sink? The engine (Mariner 60hp 2s) was still running (fortunately) so we kept the boat pointing into the wind to prevent us being swamped while bailing out our swimming pool. After a few words of reassurance and an explanation to the other 2 boats that came back to us we carried on home.

I still believe that we were unlucky, maybe we did screw up, but we managed to quickly control the situation.

Any other stories that you care to share?

Tiger
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Old 15 November 2002, 16:04   #2
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filling up

if a rib fills up I have heard that even with elephant trunks sometimes these cant work, because the boat is so heavy full of water you cant get up enough speed for them to start draining ?

Is this the case ? Would you have to bail loads before you could start emptying the boat by planing off

When I did dinghy sailing years ago, after a capsize you had much the same problem with the transom flaps, you had to bail a bit first to lighten the boat before trying to plane off downwind for the transom flaps to work

Alesson possibly is to have something on board to bail with ?

What have others done when taking a fair a mount of water on board, personally I'm such a coward I stick to flat conditions
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Old 15 November 2002, 16:56   #3
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Re: filling up

Quote:
Originally posted by matiboy
What have others done when taking a fair a mount of water on board
- shouted at the skipper to turn the 'lecky bilge bump on

(sorry, although sorta true, I jest... It wasn't that bad, just a couple of inches )
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Old 16 November 2002, 03:08   #4
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Re: Perceived Sea State and Freak Waves.

Quote:
Originally posted by Tiger

When you are out in your RIB (or your sib even) have you noticed that the waves look really big when you are moving at any speed... Have you noticed that when you stop, the waves halve in their height, or so it appears.
And have you ever tried to photograph those monsters? Always VERY disappointing. My pics of the rough stuff always look pathetic. But that's probably because it was not half as bad as I thought it was.

Having said that, I guess I have sailed in some what to me have been very big seas. There have those when from top of crest to bottom of trough have been 30 to 50 feet.

That in itself does not make them dangerous. There are other factors including the distance from crest to crest in relation to the size of your craft that makes some waves far more dangerous than others.

There was the wreck of one super tanker that I came across on the South African coast - I'm sure Charles knows it well. Its' back had been broken when caught between, in relative terms, two very steep waves with a short space between them. The ship broke in half because it was supported at the sharp and blunt ends but not in the middle.

This situation came about because the south flowing Agulhas current was suddenly opposed by a sustained blast of wind, a Southerly Buster, and it soon heaped the seas up into violent, steep waves, with a very short interval.

I have been caught by a Southerly Buster in the Agulhas current and I can promise it is frighteningly dangerous.

But let me hand over to Charles on that one. I read on some site or other that his mate Malan won some hard-core RIB race in those conditions....
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Old 16 November 2002, 08:14   #5
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Well yes, Mike....the Agulhas current is well documented. Extremely notorious stretch of water to say the least. I have surfed and cruised off the Transkei coast which is more or less in the middle of this precarious current and can attest to its fickle nature. Picture the Alderney Race on a VERY bad day, and multiply this by 100 and you have some idea what you are facing. There is an annual rib/sib/zapcat/mako race which takes in the tail end of the Agulhas current and funnily enough is called 'The Trans Agulhas Race'. It is the most extreme test of endurance for man and boat and many do not even make it through the first day.
Take a look at this link and this one . The designer and builder of the Hysucat and good friend, Malan Conradie has used this race to test many of his cat and multi-hull creations, some with fantastic results and others not. In South Africa, this annual event generates an enormous amount of interest as it spans 6 days and averages 100 miles a day of having the sh*t kicked out of you. I did it back in 1987 and broke the transom of our rib on the third day forcing us to retire - I was quite pleased as my body felt it had gone 15 rounds with Mike Tyson!! A friend of mine from Jersey is attempting it at the end of this year in a Mako. He has trained all year and I wish him well.
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Old 16 November 2002, 12:56   #6
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Matiboy, I stuffed a hard nosed Osprey 5.2m about 10 years ago in the North Channel (between Scotland and NI) with interesting results. We had planned to go diving and the forecast was F7. However when we lauched it was flat calm. An hour later F5 and shortly after yep F7. We sacked the dive and turned home. Shortly after whilst surfing down some very big waves we dropped into a big hole. The bows went straight under like a dart and all hell broke loose. As the driver I took the full blast in the face and chest. Guy sitting behind me didn't know what happened other than lots of green water everywhere. Diver on the Stbd tube saw what was coming and dived on top of the kit to stop it washing out the back of the boat (top bloke!) Diver on the Port tube was looking aft and also took the full impact of green water. The only reason he wasn't washed out the back of the boat was he hit the A frame with his shoulder and bent it!

After a few seconds calm decended on the scene and yes the waves didn't seem to be very big anymore, but the only parts of the rib showing above the sea level were the GPS mounted on top of the dash and the top of the a frame. Even the steering wheel in my hands was under water. After what seemed like an iternity she started to move up, bows first at 45 degrees. Think nuclear sub in Red October). No need for elephant trunks water just cascaded over the stern and outboard. She levelled off with the hull full and the top of the transom just clear of the sea level. The Merc 60 hp 2 stoke started first time (brilliant little engine) and we chugged away. the rib adopted a bow up stance and more water poored over the transom and out through the trunk. We got home shortly after having been at sea for 3 hours in horrible conditions. Lessons learnt.
1. The Coastguard might be right when he says F7.
2. When going down waves use the trottle with bottle. You must avoid stuffing the bow by using lots of power and have a bow up attitude.
3. Make dam sure your tubes are secure. Flatacraft used to add every webbing straps at the bow just to make sure. Osprey used a bow locker which had a huge surface area to glue the tubes on.
Pete
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Old 16 November 2002, 14:10   #7
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Reakon this skipper was a RIBster?
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Old 16 November 2002, 17:00   #8
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They cannot be that big, you have all lived to tell the tell.
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Old 17 November 2002, 02:44   #9
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OK Alan, I wondered how long it would be before you would chip in on this thread! There's no-one with more experience than you of all types of heavy weather ribbing, so I would be interested to know three things more from you...

1 Given adequate sea room, what d'you reckon are the most dangerous sea conditions for a 6 to 10m RIB?

2 Given that most RIBs are open and therefore (as has been demonstrated on this thread) liable to complete green water flooding, how, in these circumstances, important is the need for additional bouyancy in the form of covers or cabins?

3 Following your experience with green water that broke backward sloping front cabin windows, would you now change your mind and have a forward sloping screen on any future cabin RIB?

Over to you...
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Old 17 November 2002, 06:31   #10
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Hi Mike

Without question, any breaking wave's are the one that cause the biggest threat. You can have huge waves and apart from being scary the concept of the rib is that it will just bob up and down. In most cases that I hear of people getting into problems is when they expect the boat to do someting that it will never do. A good example of this would be to try and make headway in 30ft seas in a 7 metre rib. When we were faced with this problem back in 1997 on the Atlantic Challenge all we did was to go to sleep for 18 hours untill the storm / waves became acceptable. This is where a Sat Com is extreamly usefull re other thread.

If you are in a good quality rib and it h as been designed and built correctly it actualy becomes more sea worthy when it is filled with water! the boat then develops an inforced keel weight and will become part of the sea bobbing up and down until you are ready to make way again. Apart from Spirt, I have always filled my boats with water to see how they float, Do any other Rib builders do this? You do nothave to try this as I have proved that if Spirit doesnt sing neither will Magallen Alpha. ( along with all of the Ribtec range)

Upon reflextion the only thing I would change is the size of the front screens and reduce them down to either "eye Slits" or have some sort of shut system that would blank them off completly. The seas that broke the windows on Around the World would have broken what ever way the slope was. They where bloody massive!!

Hope this helps Alan P
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