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Old 29-07-2022, 20:02   #16
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Re: Wave interval

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Originally Posted by StuM View Post
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.
No, but boat length does determine if a wave is both breaking and dangerous.

breaking wave height > boat length * ratio > (1/7) * wave length
ratio >= 30%

boat length * ratio > (1/7) * wave length
boat length > (1/7) * wave length / ratio

If wave height is 30% of your boat length and your boat length is greater than 47.6% of wave length then you will have both breaking and dangerous waves.
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Old 30-07-2022, 00:17   #17
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Re: Wave interval

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Originally Posted by NPCampbell View Post

If wave height is 30% of your boat length and your boat length is greater than 47.6% of wave length then you will have both breaking and dangerous waves.
To put some numbers into this
10mt vessel
3mt wave height
18mt wave length

Dangerous breaking wave

15mt vessel
3mt wave eight
18mt wave length

Benign breaking wave


For a 3 mt wave to be stacking up to a wavelength as short as 18 mts the water would be shoaling i.e. you are in the surf zone facing dire peril what ever length vessel you command.
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Old 30-07-2022, 03:26   #18
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Re: Wave interval

If you look at results of the rallies across oceans, the results indicate the longer boats travel faster on average. These rallies are a mix of mild racers and cruisers so of some reliance here. Many boats in the rallies travel at speeds that are comfortable rather than maximum speed i.e they will slow the boat or change course to make the crossing more comfortable.
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Old 30-07-2022, 04:35   #19
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Old 05-08-2022, 14:56   #20
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Re: Wave interval

Read Trekka by John Guzwell. He built his own 21 foot boat in timber and circumnavigated. Never an issue with the siz of the boat.
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Old 05-08-2022, 16:58   #21
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Re: Wave interval

Vendee Globe suggests smaller boats are much more at risk in the Southern Ocean than the 15m LOA boats. Even when sailed by professionals the 10m to 12m boats are at risk.

Down there wave intervals are a little longer and normal wind a little stronger.
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Old 05-08-2022, 20:32   #22
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Re: Wave interval

I’ve sailed 5’ on Great Lakes more times than I can count. Yet once 5’ waves sent us back to port because of short intervals. And I sailed 15’ waves off Australia that was absolutely perfect sleigh ride (even though a beat)

The boat also makes a difference. Beating in 5’ our old Catalina 30 would almost come to a stop with each direct hit. I’d often overpower the headsail and fight the rudder so I had the speed to punch through. Heavier boats (those heavy slow Taiwanese boats) had sufficient weight to punch through more comfortably.
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