Wavesize vs danger on a sailboat

[frozenhawaiian]

there's really no hard and fast answer to your question. height isn't the only factor, swell period makes a massive difference. lets say you have 10ft seas. a swell period of say 6 seconds is gonna make for same real steep swells and is gonna beat the snot out of you, whereas 10ft seas with a 30 seconds swell period can be a downright pleasant ride.

[thomm225]

Originally Posted by Nicks

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?

Forget the formula. You were just about there in your experience above:

"Not that it felt unsafe, but I could imagine that I wouldn't have been too comfortable if wave-size would increase much further."

When it feels unsafe to you, the Skipper, it's time to change what you are doing. In your situation, heading downwind if you could would have been much more comfortable and felt quite safe.

[mstrebe]

There's a lot of good information in this thread. The actual static testing done by the coastguard shows that breaking waves of 1/3 the boat's lenght, which is nearly always about equal to the beam width, are likely to capsize a boat when they are coming from abaft 100 degrees to about 150 degrees.

This happens because the moving water of a breaking wave pushes the highly buoyant aft quarter of the boat over the lateral resistance of the keel and is not sufficiently opposed by the opposite forward quarter of the bow, which has far less buoyancy than the aft quarter. Essentially, the boat is pushed up over it's keel and the bow lacks the buoyancy of the stern to resist it.

This is especially likely to happen in a broach, where following seas pick up the stern and push it sideways, rotating the boat until this angle of highest vulnerability is reached, which then rolls the boat.

Fin keeled boats are more likely to be able surf sideways in a breaking waves because their lateral resistance is dependent upon forward speed (which is good) but they're also far more likely to be broached because they rotate on their keels easily (which is bad). There's not enough data to determine whether those effects are better or worse in combination than a full keel, but my guess is that surfing doesn't mitigate as much risk as broaching causes, so I'd presume that a full keel under a competent skipper is safer.

In non-breaking waves, there's very little risk of roll-over at any wave height because the water itself isn't moving, and so the boat will rock with the wave surface (which can be extreme and can build momentum with sympathetic harmonics), but is never pushed over it's keel. This is what the OP experienced.

Drogues function by preventing the broaching rotation and keeping the most buoyant part of the boat (the stern) presented head-on to the waves. This increases the risk of being pooped, but that's a lot safer than being rolled. A robust storm hatch for the companionway and an open transom can completely mitigate the danger of pooping.

Many new production boats come with lexan or polycarbonate french-door companionway hatches, which are fine for most sailing, but these should be complimented with a set of fitted 1x8 hardwood slats that can be strapped in place over the closed companionway. Use flat straps that can be cut away by a rigging knife in the event of an emergency, and you've got a strong hatch that can take being pooped without risk. It needn't look good as nobody will ever see it; just boards cut to fit will do.

A breaking wave greater than the length of the boat can roll it no matter what you do.

Mid-ocean storm waves commonly reach 25 feet in height, and in extreme conditions can reach 50 feet. Singlet rogue waves are exceptionally rare but can reach 100 feet in height. This is why boats under 30 feet are unsafe in typical ocean storms, boats to 50 feet are safe except in extreme conditions, and boats over 100 feet are at very little risk under any circumstances (although rogue waves can cause significant damage even when they don't roll a boat).

[Adelie]

Originally Posted by mstrebe

There's a lot of good information in this thread. The actual static testing done by the coastguard shows that breaking waves of 1/3 the boat's lenght, which is nearly always about equal to the beam width, are likely to capsize a boat when they are coming from abaft 100 degrees to about 150 degrees.

Do you have a link for these tests? I'd like to read them.


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[mstrebe]

Originally Posted by Adelie

Do you have a link for these tests? I'd like to read them.


Sent from my iPhone using Cruisers Sailing Forum

I'm specifically talking about this report: Blume, P. 1993. On Capsize Model Testing, Proceedings of the U. S. Coast Guard
Vessel Stability Symposium, New London, CT, March 15-17, 1993.

Unfortunately I don't know of an online source for it.

Here's a link to the oft-quoted series drogue tests:
http://jordanseriesdrogue.com/pdf/dr...uardreport.pdf

This is a comprehensive capsize experiment:
Experimental studies of capsizing of intact ships in heavy seas | Stephen Schaffran - Academia.edu

The results begin on page 18.

It includes specific tests in which 132 tests were run resulting in 22 capsizes, all of which happened in quartering or following seas, with no capsizes in direct beam seas.

[Adelie]

Thanks for the info you had, with the author and title hopefully I can track down the paper. Links to the other studies is appreciated.


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[Fishbreath]

Most of my sailing has been in the Pacific Ocean .. And we would joke about the fat lady in China,, who fell in.. There are rouge waves in the Pacific.. There are 3-4 times the rest of the swell.. And sometimes come for a different direction.. They seem to be during the day more than at night? I agree with what people have said about sets of three... Sometimes 4-5.. That's from my expire nice surfing,,. .. Anyway,..I've seen a lot of boats with nets around the bow and sides.. To keep sailors on board if the slip... God bless.. And wear a harness if you feel unsure.. ..

[thomm225]

All waves are so different though. There are no hard and fast rules as to what you and your boat can handle ......to a certain point.

Here are some pictures from Matt Rutherford's website. He sailed Solo Around the Americas and did so on the same type boat as the OP. His engine quit way before the trip was over also.

These waves look larger than 1/3 his boats length and some are breaking

Difficult Times at Sea | Solo Around the America's Under Sail

http://www.solotheamericas.org/?page_id=425

[Dbuettell]

Originally Posted by FamilyVan

As Pelagic and Snow have suggested, it is more complex than size alone. Capsize is far from the only risk of big waves. Getting slammed by a short steep 3 foot wave could cause you to fall down your companionway and bump your head, while a big round greeny of 15 feet might not do much of anything. There's risk of seasickness, things can break due to pounding.

If you're primarily concerned with a wave overwhelming a boat and capsizing it, I would think a boat like an Albin Vega could handle quite a bit more than 5'. There are so many other factors to consider though, including your comfort level and experience.

Maybe a good judge of wave height would be if you ask yourself "should I be out in this?", it might be a good time to consider finding a place to hide.

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I believe it was an Albin -Vega 27 that was singlehanded non stop around north and soputh america a few years ago by Matt Rutherford. That should give you some comfort, I knwo he had MAJOR weahter issues

[carstenb]

Originally Posted by Dbuettell

I believe it was an Albin -Vega 27 that was singlehanded non stop around north and soputh america a few years ago by Matt Rutherford. That should give you some comfort, I knwo he had MAJOR weahter issues

Indeed he did. The Albin is an extremely seaworthy boat in the right hands

[thomm225]

Originally Posted by Dbuettell

I believe it was an Albin -Vega 27 that was singlehanded non stop around north and soputh america a few years ago by Matt Rutherford. That should give you some comfort, I knwo he had MAJOR weahter issues

Yep, and those are the links I posted above of Matt's Sail Around the Americas. Check out the size of the waves in the photos.

[GordMay]

Capsize Resistance and Survivability When Smaller Vessels Encounter Extreme Waves
➥ http://www.ifremer.fr/web-com/molagnon/bv/johnson.pdf

More ➥ https://scholar.google.com/scholar?o...ar.google.com/

[robert sailor]

Originally Posted by rodrinn

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.

Excellent magazine article, well worth reading for those sailors thinking about offshore passages.

[chris95040]

I notice my NWS marine forecasts don't tell you the wave period.

Is it possible to estimate this using wind direction, current direction, wind speed, and wave height?

[thomm225]

Originally Posted by Don C L

Here is a video that some may find instructive: deep keel it looks like, pretty narrow beam... it may seem the keel hung rudder ( I am assuming) doesn't have too much effect on the boat's desire to pivot in these wave lengths...but is the boat in danger? No, I don't see that, I see a boat responding very nicely to the seas. But I do see a boat that is not gettting enough drive off the jib alone to have enough speed and effective rudder control. If it were me there I might put up MORE sail, or at least distribute the drive over both masts. Am I crazy? (Go ahead, say it, I can take it!)

The story on that video is that the crew is below decks working on a problem. (a massive leak I believe) So they may have been in a hurry and just did a quick set up with the jib and then got to working on the problem

The video was taken by the SAR Helo. The crew was able to repair the problem eventually and sailed the boat in without assistance. The Helo was unable to get in close enough to help due to the circumstances, and it wasn't to the point of abandoning ship