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Old 24-07-2022, 17:11   #1
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Wave interval

For those who have crossed oceans:
I had an interesting conversation with a sailor today, who seemed to believe that a vessel smaller than 60 feet wasn’t (more or less) ideal to cross oceans with. He had a 35 foot boat in the harbor that he did quite a bit of racing with/ something on the order of a J boat.

I had (read) somewhere that “the perfect length of boat” for most wave intervals was 37 feet? Pacific Seacraft comes to mind. He continue to be suggestable, saying in reference to crossing oceans and a 60’ boat: “you want something that’ll get you there”. *possibly a reference to speed?

Anyway…. my head is saying “60 feet? Really?”

You tell me. I thought that generally, anything beyond 30 feet would do the trick.

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Old 24-07-2022, 17:45   #2
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Re: Wave interval

I did 2 crossings on a 30 footer. The lentgh does not matter much, more of the design, boat construction and handling. In my opinion wave length has nothing to do with seaworthyness.
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Old 24-07-2022, 17:45   #3
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pirate Re: Wave interval

People have done it on 21ftrs.. circumnavigating even.
Pay no attention to the 'big boat' advocates, they are just as prone to sinking as a 30ftr.
Personally I've solo'd a Bene 321, a 331 and a Hunter 37 across the Atlantic and a 30ftr across the Australian Bight and down to Tasmania.
If you can handle it the boat certainly can.
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Old 24-07-2022, 18:21   #4
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Re: Wave interval

I call bullsh*t on that 60 foot thing....there must have been alcohol involved to come up with such a brash statement.

Many, many more people have successfully circumnavigated in 40 footers or less, than 60 footers..60 foot is kinda on the extreme end of people doing a RTW trip...very rare in my humble opinion.

That is not to say, it likely provides a more comfortable trip, considering it's size, but comfort cannot be equated to success....
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Old 24-07-2022, 18:25   #5
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Re: Wave interval

Wave speed in open water in trade wind conditions is generally 12-15 kts.

When the wave period reaches 15 seconds wave speed is around 30 kts.
Wind will be 40 kts + and wave height 8-10 mts e.g. Southern Ocean on a good day

Lots of factors including depth, wind speed, fetch, duration of wind contribute to the wave speed.

To get on a wave and stay there boat speed is needed.
Longer waterlines help get there.
Low wetted area and a flatter rocker aft keep you there.

A longer WL creates a wider sweet spot for fast, comfortable, safe downwind passages.
Once outside the sweet spot you need to trade off parts of fast & comfortable to maintain safe.

Make up your own mind about optimum length.
There are more factors then waves to consider when making a choice
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Old 24-07-2022, 18:47   #6
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Re: Wave interval

Humble is the man who works with his vessel instead of against her. When the sea gets angry don't fight back. Instead, praise her power and ask for safe passage.


It is all about responding to the sea you are in. I have been in 12 foot rollers that were actually comfortable to ride with a 25 foot open cockpit sloop and 10 foot chop that required constant steering changes to prevent pitch poling in a 50 foot ketch.
When a 38 foot boat is sailing at 6 knots in waves with a period of 6 seconds and the wave period changes to 4 seconds you must adjust your speed to match or suffer constant rolls and surges.
Which brings up tacking to a set course. You have to consider the options of spending an extra 3 or 4 hours to make your 50 miles or lower sail and motor using your limited fuel reserves. These are the decisions that are based on the experience you went through to get a license in the first place. (If you went through the required sea time instead of Captain in a box)
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Old 25-07-2022, 12:13   #7
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Re: Wave interval

the best sailing curriculum out there is the " school of hard knocks"....some people manage to pass in one year, others take several years, others quit after only a few months and yet others never make it to end not matter how many years they try...
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Old 26-07-2022, 08:47   #8
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Re: Wave interval

One of the old definitions for dangerous conditions was when the wave height > 30% of boat length (duh). Obviously using formulas like that would imply that a 60' boat can travel safely in a wider range of conditions than a 40' boat. However, with the right amount of planning and crew skill, you may have a pretty low chance of ever encountering those conditions out in the open even with a smaller boat.
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Old 26-07-2022, 09:18   #9
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Re: Wave interval

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Originally Posted by NPCampbell View Post
One of the old definitions for dangerous conditions was when the wave height > 30% of boat length (duh).
That would be breaking wave over 30% boat length. Big difference between a 12-foot swell and a 12-foot crashing over the decks breaking wave.
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Old 26-07-2022, 13:41   #10
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Wave interval

Having just read of a couple killed by the boom on their 60 footer, in a manner similar to an equally tragic event off NZ a few years ago, I seriously question if the big boat is actually safer. Yes, getting to the next harbour before bad weather is certainly an advantage of long waterlines, but being dependent on electrical or hydraulic systems to control the boat seems an eventual recipe for disaster. And although I concede the boom of a 20 footer can still kill you, I’d say you’ve got a better chance of surviving that than the boom of a 60 footer.

So no, the safety of a boat for ocean crossing is a sum of too many factors to simply put it down to length.
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Old 26-07-2022, 15:24   #11
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Re: Wave interval

Pitching resonance frequency and wave frequency. Even a ship can have problems when they match or come close. So really, what LWL is best depends on the conditions.

Safety is mostly a matter of what is found between the skippers ears.
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Old 26-07-2022, 18:04   #12
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Re: Wave interval

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Originally Posted by David M View Post
Pitching resonance frequency and wave frequency. Even a ship can have problems when they match or come close. So really, what LWL is best depends on the conditions.

Safety is mostly a matter of what is found between the skippers ears.


This response makes a great deal of sense to me: it makes me think of an oscilloscope and the dynamic intervals between two seperate wave forms…. something like having them both move to a dissonant harmonic. That application makes it easier to understand for any length vessel, any interval.
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Old 26-07-2022, 19:41   #13
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Re: Wave interval

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That would be breaking wave over 30% boat length. Big difference between a 12-foot swell and a 12-foot crashing over the decks breaking wave.
Thanks, I forgot that part. If your boat is both less than 30% of wave height and less than roughly 50% of wave length then the waves will typically be breaking and the wave height will pose a threat to your vessel.
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Old 26-07-2022, 22:05   #14
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Re: Wave interval

Quote:
Originally Posted by NPCampbell View Post
Thanks, I forgot that part. If your boat is both less than 30% of wave height and less than roughly 50% of wave length then the waves will typically be breaking and the wave height will pose a threat to your vessel.
Your boat length has nothing to do with whether a wave is breaking.
The commonly accepted figure is waves break if their height exceeds 1/7th of their wave length.
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Old 27-07-2022, 04:31   #15
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Re: Wave interval

I've been in some pretty gnarly seas......15-18'...there is no rhyme or reason to their wave size, some are bigger than others, some are cresting....others come from different directions...some times one wave rides on top of another, interval between waves vary, etc, etc, etc/
I was in a 40 footer...there is no sailing in those conditions...with wind speeds at 50-60 knots...
wouldn't have made any difference if I was in a 60 footer or 30 footer...
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