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Old 30-04-2011, 11:57   #16
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Re: Swell Size, Period and Boat Comfort?

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Originally Posted by boden36 View Post
I think each boat design has a natural harmonic and if wave period syncs with that you will be uncomfortable. A slight change of course will often offset this.
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Richard.
Exactly ……

When tank testing dynamic forces for new hull designs…. from yachts to freighters, the critical wave length is usually from 0.8 to 1.2 times the waterline length, when adjusted to speed.

That is why in some conditions, small yachts will fare better than large tankers experiencing the same long sets
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Old 12-05-2011, 17:50   #17
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Re: Swell Size, Period and Boat Comfort?

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Bora, I have experienced this ride lots of times whilst making that short trip. I believe that it is mostly due to backwash or reflected waves off of the cliffs that surround both entrances. The resulting chaotic wave patterns give rise to a horrid jerky and unpredictable motion. Yuck. One often experiences the same conditions around the entrance to Jervis bay... also surrounded by sheer cliffs. Down there, if just passing by, we try to stay at least 5 miles off to reduce this nastiness

The good news is that it only lasts a couple of hours...

Cheers,

.
Exactly. It's tempting to hug the coast on the trip from Sydney-Broken bay (or vice-versa) but the reflected waves from barrenjoey, long reef and North head can be pretty unpleasant, even in otherwise pretty good conditions.
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Old 12-05-2011, 18:16   #18
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Re: Swell Size, Period and Boat Comfort?

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Originally Posted by 44'cruisingcat View Post
Exactly. It's tempting to hug the coast on the trip from Sydney-Broken bay (or vice-versa) but the reflected waves from barrenjoey, long reef and North head can be pretty unpleasant, even in otherwise pretty good conditions.
Personaly I find that 10-12 k ofshore makes the run from broken bay to botany bay much more comfortable. Avoids all the reflected waves. Works for me!
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Old 13-05-2011, 11:34   #19
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Re: Swell Size, Period and Boat Comfort?

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Exactly ……

When tank testing dynamic forces for new hull designs…. from yachts to freighters, the critical wave length is usually from 0.8 to 1.2 times the waterline length, when adjusted to speed.

That is why in some conditions, small yachts will fare better than large tankers experiencing the same long sets
I'm not sure how to interpret the wave data in regard to sail-ability. Is there a rule of thumb that can be used? We sailed a J24 out to Boston Light yesterday in 15-20k winds and 1.5m swells. We were in a fairly narrow channel so met the waves head-on on a port tack. Was not a problem, although the hull did slam down fairly hard a time or two. Not sure what the wave period was, but the waves were steep. Is there a way to extract wave length , or some approximation thereof, from the buoy data.
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Old 14-05-2011, 13:03   #20
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Re: Swell Size, Period and Boat Comfort ?

Hi all:boden 36 posting above has it right regarding each boat having natural harmonics with different wave patterns and the necessity to change course(speed too?) when involved in these very uncomfortable wave trains.One such situation could arise when wave length =2x boat length and any course at right angles to these waves would put the bow in the trough while the stern is up on the crest and this bow/stern configuration would alternate with each passing wave.
With larger breaking waves a serious broach (pitchpole?) could occur when running as the bow drives down into the trough just as the stern is lifted high and slides sideways on the crest.
Any naval architects lurking out there? I'm sure they can help with this one.
Maybe I have slid sideways myself since thread seemed to be more about reflected waves off nearby cliffs;this will cause very chaotic sea conditions that will not be comfortable on any course or speed.
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Old 14-05-2011, 13:22   #21
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Re: Swell Size, Period and Boat Comfort ?

With regards to wave period and height I use the following rule of thumb; As the wave period in seconds approaches the wave height in feet it gets decidedly rougher. If the wave height in feet is greater than the wave period in seconds it is rough. If the wave period is 2x or more the wave height in feet it will be a very nice sail. This does not mean I won't go out in 5x5 or 6x6 but I won't expect a pleasant ride. Sorry this doesn't work so easily for you metric users.
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Old 14-05-2011, 13:34   #22
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Re: Swell Size, Period and Boat Comfort ?

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... If the wave period is 2x or more the wave height in feet it will be a very nice sail ...
Different boats will vary the "likely comfort zone" between Period (in seconds) = 1.4 x height (Ft) through Bill's Period = 2.0 x height.
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Old 15-05-2011, 12:56   #23
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Re: Swell Size, Period and Boat Comfort?

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Is there a way to extract wave length , or some approximation thereof, from the buoy data.
When the water depth is greater than the wavelength, the usual formula is: wavelength(in meters)=1.5*period^2 (in seconds)

So, the length of 10-second period waves is approx. 150 meters and their celerity (speed of crests propagation) is approx. 30 knots.

If the depth is less than the length obtained with this formula, the waves "feel the bottom" and become shorter and steeper.

Alain
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Old 15-05-2011, 13:11   #24
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Re: Swell Size, Period and Boat Comfort ?

There is a specific name for these reflected waves but i've forgotten it. It's bugging the hell out of me trying to remember it. I'll betcha Gord knows it.
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Old 15-05-2011, 13:49   #25
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Re: Swell Size, Period and Boat Comfort ?

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There is a specific name for these reflected waves but i've forgotten it. It's bugging the hell out of me trying to remember it. I'll betcha Gord knows it.
You might be thinking of "standing waves". There are several types of standing waves, some caused by the hydraulic effects of bars and other obstructions, but they are also caused by reflections.
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Old 05-06-2011, 04:56   #26
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Been caught in same way off too many headlands and steep fluffy coasts. Should stay 5 miles off if possible. You would think I would known now hey!
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Old 05-06-2011, 06:28   #27
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Re: Swell Size, Period and Boat Comfort?

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Originally Posted by Jim Cate View Post
in this case the cause of the unpleasant ride is the chaotic nature of the reflected waves from the sheer cliffs that abound in this particular bit of the Aussie coastline.
Jim is exactly right.

Sydney Harbour is 'my' harbour, my home and its the best harbour in the world for one million reasons, and the most beautiful.

But the slosh between the heads and off Barenjoey at Broken Bay can be a test.

The cliffs of the Heads are sandstone which fractures vertically. So the cliffs are sheer deep into the water. But the headlands round themselves, obviously, into the harbour. Inside the harbour but still in lne with the Heads are 2 other heads that are sheer: Dobroyd Point and Middle Head. These also reflect waves back out the Heads.

Its not dangerous but its a bit naughty with some types of boats and especially bad when there is little wind because you just hand around in it getting pushed up and down. It can be annoying because freinds on board start throwing up before the days adventure has really started

Mind you when there is a storm it gets big.

The video below is INSIDE Sydney Harbour from the bridge of a ferry between the City and a suburb near North Head. It looks like its in the ocean, but thats because you are looking out through the Heads to the ocean. In weather like this waves are reflected back 180 degrees inside the habour and surfers ride the waves at an inside beach into the wind.


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