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Old 03-09-2015, 08:56   #16
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Re: Wavesize vs danger on a sailboat

By the way Nick - the albin is very seaworthy boat. Keep sailing and you'll get used to it.

Sailed mu old 22 footer across Køge Bugt one time in a gale (actually 3 times across inthe same day - don't ask me why)


That was an interesting ride...........................
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Old 03-09-2015, 10:46   #17
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Re: Wavesize vs danger on a sailboat

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Originally Posted by Nicks View Post
Hi,

I was recently out in some light-heavy weather where we encountered a couple of larger than usual (short) waves. Nothing special, perhaps around 3-5 feet which is more than what we're usually sailing in.

We were broad-reaching and we got a pretty good roll from it. (Small sailboat 27 foot). Not that it felt unsafe, but I could imagine that I wouldn't have been too comfortable if wave-size would increase much further.

Is there a formular to calculate when it's better to go stern-to-waves instead of broad reaching, or will a sailboat just power through most of it?

How big do waves need to get before its considered unsafe?
No! It is an academic question with far to many variables of boat design, as mentioned wave height, period and type of wave. And maybe more important the seamanship of the Captain.
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Old 03-09-2015, 12:33   #18
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Re: Wavesize vs danger on a sailboat

I think a lot has to do with righting moment.

In a previous boat, a Contessa 32 which has an incredibly high righting moment, I was broad reaching at night trying to stay off Tuamotu archipelago which were (in pre sat-nav days) somewhere to leeward; when a breaking wave (that sounded like a train) picked her up and while hanging from a stanchion, I saw the tricolour at the top of the mast illuminating the water it was submerged in. It was only momentarily and she popped straight back up.

This article in Yachting Monthly Understand your boat and her statistics has some good points and the difference in vanishing angle of stability between a Contessa 32 and a Bavaria 32 is well illustrated in a diagram

A great boat will compensate for asinine seamanship.
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Old 03-09-2015, 12:37   #19
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Re: Wavesize vs danger on a sailboat

Oh Boy!

The most dangerous wave in my humble 40 year experience, aside from a rogue wave that I have never encountered, is a high rolling sea shoaling up in a relatively shallow inlet.


Poop and or broach.
Come in under full sail and full power to keep her straight.

I would prefer not to do it again.
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Old 03-09-2015, 12:49   #20
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Re: Wavesize vs danger on a sailboat

Stern to waves gives them a chance to poop you, fill the cockpit which usually has inadequate drains, and flood into the cabin. A lot of that depends on boat design and whether you've kept the hatch boards in place and lazarettes secured.


Stern to waves also give them the chance to steal your rudder control and gybe you. Some boats, especially "flat" wide hulls, may be more likely to lose steerage when waves push them from behind. A more traditional hull like your Vega wouldn't suffer as much as the flat ones will.
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Old 03-09-2015, 13:01   #21
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Re: Wavesize vs danger on a sailboat

I am in the process buying my first full keel boat (Rafiki 37) How does full keel vs fin keel fit in here?
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Old 03-09-2015, 13:08   #22
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Re: Wavesize vs danger on a sailboat

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I am in the process buying my first full keel boat (Rafiki 37) How does full keel vs fin keel fit in here?
Uh oh, you're going to start a fight

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Old 03-09-2015, 13:13   #23
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Re: Wavesize vs danger on a sailboat

Quote:
Originally Posted by Nicks View Post
Hi,

I was recently out in some light-heavy weather where we encountered a couple of larger than usual (short) waves. Nothing special, perhaps around 3-5 feet which is more than what we're usually sailing in.

We were broad-reaching and we got a pretty good roll from it. (Small sailboat 27 foot). Not that it felt unsafe, but I could imagine that I wouldn't have been too comfortable if wave-size would increase much further.

Is there a formular to calculate when it's better to go stern-to-waves instead of broad reaching, or will a sailboat just power through most of it?

How big do waves need to get before its considered unsafe?
There is no simple formula for what you are asking. There are some rules of thumb that others have posted.

In engineering terms you are interested in the wave vector force component that can exceed your vessels righting moment.

Its relatively simple to calculate what that static force threshold is for different heights above your boats center of gravity / bouyancy.

What is not simple is using that information while sailing or calculating the dynamic values in real time.

In practice you want to avoid beam oriented waves that induce a rolling moment that exceeds your vessels instantaneous righting moment.

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Old 03-09-2015, 15:31   #24
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Re: Wavesize vs danger on a sailboat

Ah this is interesting stuff. 2 things here, waves and how much danger you are in. First I enjoy having the discussion of the variation is wave heights and where they come from, and yes every once in a while you will see a larger than average wave, and usually they are somewhere else, not underneath you. There is the rare "rogue" wave and then there are the others less huge but still unusual and if they are breaking and you happen to be broad reaching in front of one, yes, you'll probably get tossed on the beam. And I was just reading Olin Stephens' commentary in "Heavy Weather Sailing" where he talks about the hazard is not to the hull so much but the cabin taking the hit when you are thrown on your side at the bottom of a wave. So I would guess that what is "too big" depends on your boat size, displacement, lateral resistance, center of gravity, etc. vs what you are willing to tolerate, which is above my pay grade. And it is the size of the breaking part of the wave, not the entire wave, that is of most concern to me. That is a function of wind speed and fetch, wave height, length, currents and probably water depth... but this I'd like to be corrected on if I am wrong. Anyway, it is a never ending source of fascination to me trying to figure out why and where the larger waves are occurring. But then, I am easily amused. By the way, this discussion is good to remind us to be sure heavy items like batteries cannot move if the boat were upside down for some reason.
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Old 03-09-2015, 15:39   #25
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Re: Wavesize vs danger on a sailboat

60 % of your boat length. roughly.
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Old 03-09-2015, 15:49   #26
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Re: Wavesize vs danger on a sailboat

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I'm a little sceptical as that implies that the area under the curve from 0 to 90 degrees has no effect on capsize risk.

Here's how I see it : the breaking wave imparts a certain impulse to the boat. How far the boat heels in response is proportional to the area under the righting moment curve. If you run out of righting moment before the boat stops heeling, it capsizes.
I think the logic they where using was that knockdowns when the vessel quickly returns upright are not as dangerous as a complete inversion past the AVS where the vessel can remain upside down for a long while. On this basis the area under the curve from 90 to AVS seems logical, and tests seem to correlate.



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Old 03-09-2015, 15:58   #27
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Re: Wavesize vs danger on a sailboat

I was reading an interesting TSB report yesterday, where a 7 meter deep v RIB was station keeping in against an ebb tide and was actually capsized by a standing wave, that developed as the ebb increased. Supposedly the wave developed to port while the operator was looking at a point of land to starboard. Very odd, a tidal standing wave capsizing a deep V-Rib.

I know current driven waves can be bad, especially where wind is counter to the current, but the boat was stationary and was overwhelmed by a rapidly developing standing wave?

I haven't really figured out links yet, but if some one was curious to Google try "Marine investigation report M12W0070".

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Old 03-09-2015, 16:07   #28
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Re: Wavesize vs danger on a sailboat

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Originally Posted by coolboat30 View Post
I am in the process buying my first full keel boat (Rafiki 37) How does full keel vs fin keel fit in here?
I am guessing most folks would rather be in the Rafiki in rough seas!
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Old 03-09-2015, 16:21   #29
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Re: Wavesize vs danger on a sailboat

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I am guessing most folks would rather be in the Rafiki in rough seas!
I've been pondering this much of the afternoon (in between working hard of course). I've been considering the hydrodynamics and is a full keel better in big beam seas? Or is lateral resistance actually a disadvantage in a beam sea? Could it cause the boat to trip like a buried gunwhale or leeward dagger board on a cat? Is this a point of sail where a fin might outperform?

I don't know the answer, this is a question, not an answer.

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Old 03-09-2015, 16:25   #30
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Re: Wavesize vs danger on a sailboat

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Originally Posted by FamilyVan View Post
I was reading an interesting TSB report yesterday, where a 7 meter deep v RIB was station keeping in against an ebb tide and was actually capsized by a standing wave, that developed as the ebb increased. Supposedly the wave developed to port while the operator was looking at a point of land to starboard. Very odd, a tidal standing wave capsizing a deep V-Rib.

I know current driven waves can be bad, especially where wind is counter to the current, but the boat was stationary and was overwhelmed by a rapidly developing standing wave?

I haven't really figured out links yet, but if some one was curious to Google try "Marine investigation report M12W0070".

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If the wave was standing, he must have been moving, with the current. In that case not only does the wave lift the downstream rail, it will catch the upstream rail, push it, and continue the boat's roll all the way over.
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