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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 10-30-2008
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Quote:
Originally Posted by billyruffn View Post
If you're at sea and bad weather is going to overtake you (i.e. you can't avoid it) -- make sure you do everything you can long before it arrives.
Informative post - good one.

Just two things that come to mind:
  • Try and identify where the centre of the storm is and which direction it's heading and steer a course that doesn't keep you in it's path. Read this Buys Ballot's law - Wikipedia, the free encyclopedia
  • If you have the time and you haven't got a bunch of sickies aboard (or maybe even if you have), get some food down everyone before the weather arrives because you never know when next conditions will be conducive to cooking. Hungry people don't function very well.
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  #22  
Old 10-31-2008
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Omatako - you've got to be a physicist or something. Throwing down Black Box Theory and Buys Ballot's Law. Impressive.

Personally, I sail by the Second Law of Thermodynamics - specifically Entropy: It's just a matter of time before it all goes to hell anyway, so have a drink and enjoy it.

And of course, avoid the storm if you can. I mean there's that.
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  #23  
Old 11-06-2008
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Is there any sort of physics formula or general rule of thumb for calculating what it will take to capsize a boat? I'm talking about a 25'+ keelboat here, not a sunfish, which everyone knows can capsize easily.

I'm pretty new to sailing and I'd like to know what it would take to knock my boat over!
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  #24  
Old 11-06-2008
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No, there's no general rule... there are far too many variables, like wave height and period, wind strength, whether your fuel and water tanks are full, etc... the list goes on.
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Originally Posted by southernsmoke View Post
Is there any sort of physics formula or general rule of thumb for calculating what it will take to capsize a boat? I'm talking about a 25'+ keelboat here, not a sunfish, which everyone knows can capsize easily.

I'm pretty new to sailing and I'd like to know what it would take to knock my boat over!
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Old 11-06-2008
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This site attempted to come up with a capsize ratio, but I'd use it to just compare 2 boats to each other. As in, one boat is more prone than another. But trying to come up with a wind speed or wave height is impossible.

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  #26  
Old 11-06-2008
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If you want a sailboat comparator, use SAILCALC instead. Has the measurements of a lot of boats pre-loaded.
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  #27  
Old 11-06-2008
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General rule, sure. Anything that puts your, what is it? Center of mass above your center of gravity? then the boat will continue to roll until it comes to rest in the stable inverted position. Look for those phrases, look for numbers relating to force and inertia from wind and waves--the numbers are out there. I think Marchaj's Seaworthiness the Forgotten Factor is one of many texts that go into the details that are to be considered.

Whether that is caused by wind, waves, flooding, interior ballast or supplies shifting--that's where it can get complicated and for that you can look at each circumstance that concerns you, to see where the upset happens.

Many (most?) hulls have enough weather helm that they'll round up and point into the wind before wind alone can roll them further than a broach, although a good spinaker broach can push harder and faster.

Usually the weak point is the human onboard, who hasn't secured things, reefed things, or lost control of things. Usually you will be far too busy, far too quickly, to keep track of fine details in the "how much further till we capsize?" considerations.
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Old 11-06-2008
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While I haven't capsized, I've had a full keel boat past 90* and it while it wasn't fun, it wasn't as dramatic as I thought it would be. Sure the cabin was a mess from flying objects but it stayed dry, even with the companionway hatch open. Rolling completely would be a different story though. That introduces the risk of being swept off, the rig coming down and maybe holing the boat, oh, the minor issue of a few thousand gallons of seawater inside the boat as well as likely crew injuries, hypothermia etc ad naseum.
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Old 11-06-2008
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Quote:
Originally Posted by southernsmoke View Post
Is there any sort of physics formula or general rule of thumb for calculating what it will take to capsize a boat? I'm talking about a 25'+ keelboat here, not a sunfish, which everyone knows can capsize easily.

I'm pretty new to sailing and I'd like to know what it would take to knock my boat over!
Sup Southernsmoke? Welcome. This is one I can finally answer...kind of. It so happens I'm currently reading the book "Heavy Weather Sailing" (go figure) and it says that

"The simple answer to avoid capsizing is to avoid breaking waves."

It then goes on to say that model tests they carried out showed that

"when the breaking wave was 30% of the hull length high, from trough to crest, it could capsize some of the yachts, while waves to a height of 60% of the hull length would comfortably overwhelm all of the boats we tested. In real terms this means that for a 33' boat, when the breaking wave is 10', this presents a capsize risk; and when the breaking wave is 20' high, this appears to be a capsize certainty in any shape of boat."

It goes on to discuss the variations of full keel, fin, etc. and do all the math on the various configurations (that's where you can get your physics). And it discusses techniques to keep the wind/waves off the beam (drogues, warps, anchors, etc.). It also stresses that the issue is breaking waves - saying that the waves can be extremely high/steep and not necessarily cause a capsize if they aren't breaking. But the above seems to be the rule of thumb.

It's a good book so far.
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  #30  
Old 11-11-2008
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I posted this question in BFS - but thought it might get a better shot here...

Iin reading "Heavy Weather Sailing" they go into a pretty detailed discussion of warps/drogues versus parachutes - and the pros and cons between the two. They seem to advocate the parachute technique off the bow (either directly into the oncoming seas - or 15-20 degrees off using a bridle as the Pardys do) - VERSUS the warp/drogue approach (stern-to-seas) as the safest way to ride out a storm.

Would you guys concur with that bow-on parachute approach (especially those of you who've actually tried both)? I'm curious about this because I remember Skip Allen's trip where he was using a drogue and was very worried about getting his hatchboards blown out (and was getting repeatedly pooped) - and I wondered by he wouldn't have gone bow-to-seas. Just curious.
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