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Old 04-09-2011, 08:46   #1
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Capsize Factor ? I'm Confused !

From what I've read, a breaking wave equal in size to a boats beam will capsize the boat, if taken ahull, regardless of length or displacement, So why does the capsize factor equation have displacement in it?

Now I can see why beamier boats will be harder to initially capsize but on the other hand slower to self right, makes sense easily, kinda takes away the argument for greater avs through narrow beam, yeah your boat will self right much quicker, but hey it will be capsized by a small wave.

I'm focusing on capsize by waves here not by wind.

Can anyone put me right?
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Old 04-09-2011, 09:11   #2
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Re: capsize factor? im confused!!!

The adage was wave the size of the length of the boat.

No 13 foot wave will capsize a boat, but a 40 footer could!

Anyway thats 'old' technology. Now we talk about breaking waves. Not whitecaps but where the wave itself breaks at sea as it would onto shore.
This only happens in winter and/or in some definitive situations like wind against current in the Gulf Stream, Agulhas Current or in cyclones/hurricanes.

We would like to think that prudent weather watching and upto date weather information should keep us out of those areas at their difficult moments.





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Old 04-09-2011, 09:17   #3
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Re: capsize factor? im confused!!!

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Originally Posted by MarkJ View Post
The adage was wave the size of the length of the boat.

No 13 foot wave will capsize a boat, but a 40 footer could!

Anyway thats 'old' technology. Now we talk about breaking waves. Not whitecaps but where the wave itself breaks at sea as it would onto shore.
This only happens in winter and/or in some definitive situations like wind against current in the Gulf Stream, Agulhas Current or in cyclones/hurricanes.

We would like to think that prudent weather watching and upto date weather information should keep us out of those areas at their difficult moments.





Mark
But since "stuff happens," what if (assuming you are able to) you turn your boat into that wave, so it breaks over the bow or the forward quarter instead of abeam?

My boat is 31' long. Is it possible for my boat to survive a 30' breaking wave without capsizing? Yes, I'd do everything in my power to not be in such a situation. On the other hand, in a bad situation it isn't possible to know too much.

What if you have a drogue out, or some other significant drag mechanism? Can you turn the boat enough to avoid a beam wave?
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Old 04-09-2011, 09:22   #4
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Re: capsize factor? im confused!!!

Also, does having a full keel, modified full, or fin keel come into the equation??
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Old 04-09-2011, 09:23   #5
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Re: capsize factor? im confused!!!

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what if (assuming you are able to) you turn your boat into that wave, so it breaks over the bow or the forward quarter instead of abeam?
Yes. The principle of the parachute anchor is that. Thats what I have, and thats my thoughts for when trying to go upwind/stationary.

Or the other way turning down the wave which is the drogue principle, which I also have for downwind storms.

I have even only been in a boat once lying ahull and its was a 68 footer on the Grand Banks off Novia Scottia in a small (for the locals) gale and the peanut skipper decided to drop the sails and lie ahull. How damn stupid! Totally uncomfortable, does nothing apart from terrify crew and make everyone seasick and hungry.

Hove-to, sail, drift, whatever but dont lie ahull beam to the sea.


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Old 04-09-2011, 09:51   #6
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Re: capsize factor? im confused!!!

I did say "breaking wave" and beam on ie ahull , ie not the swell size, but the breaking wave part, loads of tank studies showed that beam on, if the breaking part of the wave was equal or greater than the beam the boat would capsize, it made no difference as to the length of the boat eg 10ft beam ,25 lwl or 35 lwl made no difference only the beam dimensions, weight again made no difference.

In the real world what is going to capsize you? loosing your keel, breaking wave or huge gust and being over canvassed, the first one you can do nothing about (other than make do with a full keeled boat), a smart cruiser should rarely be caught over canvassed, the breaking wave in a confused sea.... mmm harder to avoid.

my confusion was over people going on about high avs and large rm due to narrower hulls, yet that which makes it more stable on paper makes it more likely to capsize in the real world, 12ft of beam will capsize in a smaller wave than 14 ft of beam on a similar boat, the 12ft beam boat may have a far higher avs and greater rm on the paper but it will capsize first, yes it will certainly right first as well, but id rather not capsize in the first place...

Now i still don't get where weight comes into the equation in that capsize factor equation. now i could see the point that a freak gust could capsize an over canvased boat, but again if a boat is heavier they tend to carry a greater sail plan, so no winner there, the keel going makes no difference in weight and breaking waves beam on make no difference in weight again, so why does the capsize factor equation only consider weight and length?... beam plays a huge part in it...
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Old 04-09-2011, 10:11   #7
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Re: capsize factor? im confused!!!

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Originally Posted by shadow View Post
Also, does having a full keel, modified full, or fin keel come into the equation??
I'd say yes if the wave is taken from abeam.
I think the more underwater resistance (lateral) the better the possibility of the boat sort of "tripping" over its keel.
I think I've read somewhere that some boats with retractable/swing keels pull them in when the possibility of a broach is exists.
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Old 04-09-2011, 10:13   #8
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I was under the impression that capsize factor was the boats ability to right itself after a capsize. Not predicting the likelyhood of a capsize as the name implies.
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Old 04-09-2011, 10:18   #9
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Re: capsize factor? im confused!!!

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Originally Posted by stevensuf View Post
In the real world what is going to capsize you? l .


In the real world you will not capsize.

So instead of worrying about stats I feel folks should buy the boat most comfortable for them.

I just lost a post where I described how the 5 yachts I personally know sank. None of them were by capsizing at sea. All were driver error, one thru-hull failure; 2 solo sailors slept through alarms with wind vane steering and put onto beachs/reefs; one following a buddy boat into a channel instead of navigating himself.

5 boats lost and NONE by what everyone always worries about when buying a boat
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Old 04-09-2011, 10:21   #10
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Re: capsize factor? im confused!!!

A vessels ability to resist capsize is the positive area under its stability curve.
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Old 04-09-2011, 10:56   #11
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Re: capsize factor? im confused!!!

the positive area in the stability curve is its upwards positive range of stability nothing to do with capsizing as such, ie the greater the top part the more stable it is the correct way up, the greater the negative area the more stable it is upside down, neither tell you its resistance to capsize .only really are a guide as to how quickly it will self right, the greater the positive area the quicker it should right.
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Old 04-09-2011, 11:07   #12
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Re: capsize factor? im confused!!!

Then the people who taught my Stability and Trim class and Naval Architecture class must all be wrong. The guy who taught my naval Architecture Class owned a naval architecture firm that designed ships. Perhaps all the ships they designed are capsized by now because they were all wrong?

The positive area below the stability curve IS a ships ability to resist capsize...period.
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Old 04-09-2011, 11:51   #13
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Re: capsize factor? im confused!!!

Interesting discussion. I suspect you two are arguing over semantics.

Stevensuf : - Does not the phrase "the more stable it is the correct way up" also mean its resistance to roll and therefore by extension its resistance to capsize? After all if you can resist any rolling moment, you will never capsize. (pitch poling is a whole other topic)

CA Marchaj has written about stability in "Seaworthiness the forgotten factor". A dry read, but well worth the effort. In the end he argues for a long keeled heavy displacement boat as being the most likely to be seaworthy. Curiously, there are few builders of such boats these days.

Market forces and the "need for speed" rule.

Back to your original query - I think any discussion is theoretical at best - tank testing may or may not reflect real world experience. You're losing too much sleep worrying about the possibility of capsize in one narrow scenario when there are many other factors that come into play. I seem to recall a catamaran capsizing while attempting a river bar crossing in Australia, its beam was around 22ft and the wave was around 10 feet. I'm hazy about the facts, someone please correct me - it was a brand new Seawind (?). It would seem that there are breaking waves and then there are Breaking Waves!.
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Old 04-09-2011, 12:02   #14
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Re: capsize factor? im confused!!!

Quote:
Originally Posted by stevensuf View Post
the positive area in the stability curve is its upwards positive range of stability nothing to do with capsizing as such, ie the greater the top part the more stable it is the correct way up, the greater the negative area the more stable it is upside down, neither tell you its resistance to capsize .only really are a guide as to how quickly it will self right, the greater the positive area the quicker it should right.
I think Ted Brewer could help with your question..he invented the or a formula...did he not ?
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Old 04-09-2011, 12:09   #15
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Re: capsize factor? im confused!!!

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I'd say yes if the wave is taken from abeam.
I think the more underwater resistance (lateral) the better the possibility of the boat sort of "tripping" over its keel.
I think I've read somewhere that some boats with retractable/swing keels pull them in when the possibility of a broach is exists.
would the longer keel make it roll more?.... the water on the front of the wave is moving up, so more lateral area would push the keel up the face of the wave right? interesting situation, dont know what is true. regarding narrow and wider beam... most older style narrow beam boats have a high ballast ratio to counteract the tendency to to heel fast. A wider beam has great initial stability, but reaches a point of no return...
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