Forums > Sailing General

Wave Height vs Period Rule of Thumb

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Created by Toph > 9 months ago, 7 Jul 2020
Toph
WA, 1797 posts
7 Jul 2020 9:39PM
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When it comes to "going out" most of us have our rules of thumbs, 15kts and 1.5m swell, wind or tide direction or what have you.

For me I don't choose to go out in >15-20kts and/or 2m swell. I'm starting to think that that might be a bit restrictive as it doesn't take into account the swell period. A large swell might well be comfortable (I'm talking comfort here and not tolerated or safe if caught out) with a longer period between peaks.

Is there a good rule of thumb when determining a comfortable wave height versus period.

Ramona
NSW, 7400 posts
8 Jul 2020 8:13AM
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No. Too many other factors effect comfort. 2m swell offshore is fine but can be unpleasant in shallow water. 20 knots of wind over shallow water and 2m of swell with 3 knots of current against the wind can be downright dangerous!

Poodle
WA, 865 posts
8 Jul 2020 11:20AM
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Yeh what Ramona said. Wind is your best guide.

"Swell" consists of 2 parts - Swell - Which is the underlying wave action, generated from far away - Typically a longer priod. Then sea, the localised wave action generated by wind (typically shorter period than swell). This is perceived as chop in more modest breeze. And of course the overriding factor here is height of sea + swell.

Theres an old sailing truism - The best time to put in a reef is when you first think about doing it. Similarly, if you have to think about whether to go out or not, probably best not to go out.

boty
QLD, 685 posts
8 Jul 2020 5:22PM
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all i care about is sea state dont care how strong it is dont care how big the swell is its the chop on top i hate and the noise when it it really blows

Achernar
QLD, 369 posts
8 Jul 2020 5:55PM
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There is a mathematical response to this question (I'm a water engineer specializing in flooding and know something about ocean waves), but you are, presumably, talking about how waves affect comfort and other perceptions.

Technically, our term "wave" covers a multitude of phenomena, from surface ripples to tsunamis, from waves that transfer mass (e.g. tides) to waves that transfer energy (e.g. ocean swells) and every combination in between.

Something of interest, however, is how waves slow down and "pile up" as they approach the shore and the water gets shallower. They also diffract. I've seen some cool (and scary) simulations of what happens when a 10m tsunami hits Moreton Bay. So, a wave offshore would be much more comfortable than the same wave inshore.

Madmouse
377 posts
8 Jul 2020 5:18PM
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What about light winds off somewhere like Pt Perpendicular!

Toph
WA, 1797 posts
8 Jul 2020 5:50PM
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I'm a bit conservative by nature and I have 2 family members who I need to be a bit careful with if I want them to keep enjoying sailing.
We have experienced some crappy weather and big sharp seas. We are not novices to sailing but I can't help think we sometimes miss out based on 2m swell mindset. And I'm talking about choosing to go out in and not what you'll be confident in being in..

I was going to sail from Fremantle to Busselton tonight with my family and a mate who has some small sailing experience but no long passage and no overnight experience. I was put off by the 2.5 to 2.8 metre forecast swell even though the winds were 5-15kts. The swell period was 13 seconds..

MorningBird
NSW, 2644 posts
8 Jul 2020 8:41PM
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Swell is irrelevant to comfort in that the boat moves predictably over the swell, it can be quite relaxing. Wind and sea and current make it uncomfortable.
I get Seasick so understand.

r13
NSW, 1427 posts
8 Jul 2020 8:58PM
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See here a good summary;

www.bom.gov.au/marine/knowledge-centre/reference/waves.shtml

FreeRadical
WA, 855 posts
8 Jul 2020 10:41PM
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Swell can be a contributor to seasickness. Anytime your eyes can't grasp what your ears are feeling, you have the potential to get sick. Plenty of people get sick riding over swell in calm conditions.

Good call Toph, you might have ended up pulling a very long watch.

Ramona
NSW, 7400 posts
9 Jul 2020 8:27AM
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Select to expand quote
Madmouse said..
What about light winds off somewhere like Pt Perpendicular!


Keep at least half a mile off the cliffs and it's fine. Even when the current is fresh down.

AUS126
NSW, 192 posts
9 Jul 2020 12:38PM
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The surf starts getting good when the period gets to about 12. So I would think you would start having fun if 12 second waves were hitting tide or intersecting from different directions. It also amazes me how quickly large period waves travel.

boty
QLD, 685 posts
9 Jul 2020 4:31PM
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Select to expand quote
Ramona said..

Madmouse said..
What about light winds off somewhere like Pt Perpendicular!



Keep at least half a mile off the cliffs and it's fine. Even when the current is fresh down.


or go in super tight and your in the washing machine for less time and exposed to the adverse current for less time

nswsailor
NSW, 1423 posts
9 Jul 2020 4:38PM
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So nobody will be putting this into practice next Tuesday!

44 knot winds and 8m plus seas off Newcastle.
Even up at the Camden Haven it will be 42 knot winds and 7m seas.

Ramona
NSW, 7400 posts
9 Jul 2020 5:37PM
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Select to expand quote
nswsailor said..
So nobody will be putting this into practice next Tuesday!

44 knot winds and 8m plus seas off Newcastle.
Even up at the Camden Haven it will be 42 knot winds and 7m seas.



48 knots for Shoalhaven Bight. Anytime the wind is in the Westerly quadrant in the Shoalhaven add 10 knots to the forecast! I think I will be finding something to do in the shed!

Toph
WA, 1797 posts
9 Jul 2020 6:34PM
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Select to expand quote
FreeRadical said..
Good call Toph, you might have ended up pulling a very long watch.


Spoken like someone who's experienced it

boty
QLD, 685 posts
10 Jul 2020 7:31AM
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Select to expand quote
Ramona said..

nswsailor said..
So nobody will be putting this into practice next Tuesday!

44 knot winds and 8m plus seas off Newcastle.
Even up at the Camden Haven it will be 42 knot winds and 7m seas.




48 knots for Shoalhaven Bight. Anytime the wind is in the Westerly quadrant in the Shoalhaven add 10 knots to the forecast! I think I will be finding something to do in the shed!


sounds wise i dont think i would have enough thermal wear for that low



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"Wave Height vs Period Rule of Thumb" started by Toph