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-   -   How is capsize risk best understood? (http://www.sailnet.com/forums/learning-sail/77263-how-capsize-risk-best-understood.html)

 joelsanda 08-07-2011 12:18 AM

How is capsize risk best understood?

I'm in my second year of sailing, using the J22s at my local sailing club/school, where I took Basic Keelboat and Coastal Cruising courses last year. What I'd like to know is how to best understand the risk of a capsize?

I understand boat particulars weigh on this (hull type, keel type, onboard weight, etc), but I'm trying to understand how I can heel the J22 at 35/40 degrees and the darn thing just keeps going without capsizing. I saw a small catamaran capsize on the lake, and one day after I rushed back to the marina as a storm approached a 22-footer capsized during a race.

With a sailboat not overloaded is there a way to calculate the capsize risk? Do sailboats generally have a 'rating' or degree of heel that could lead to a capsize? During my practicals we had some stiff winds and I could feel a difference when we all sat on the windward side - I don't know if I could say the boat heeled less but it seemed to respond to tiller controls better.

Anyway ... having a hell of a time learning to sail and finally getting around to realizing a childhood dream. I'm sailing in Denver, on a small lake, where the average tack might last five minutes.

Thanks!

 WDS123 08-07-2011 12:38 AM

Welcome !

Righting Moment

Righting moment is the torque trying to return a boat into equilibrium, upright, when the boat is heeled.

This may be the most important single factor affecting the boat stability.

The magnitude of the torque can be calculated by multiplying the horizontal distance between the center of gravity of the boat and center of buoyancy of the underwater part of the boat by the boat displacement.

This sounds easy, but is not. To calculate the position of the center of buoyancy for all possible heeling angles and for all possible hull cross sections is not a trivial task.

Center of gravity sounds easier. You would think it lies on the center line of the boat, some distance above the bottom. This is more or less true for large boats. But for small boats, in the one sheeter or kayak category, it is not.

If the weight of the boat is 30 lbs and Your weight is 200 lbs, then the center of gravity of the boat plus You has very little to do with the boat, it's mostly about You. If You sit stiff in the middle of the boat, then the center of gravity is in line with the boat center line. But most of the time You don't sit stiff.

It seems, that when a small boat heels, most people "counter heel". If You model the center of gravity to lie on the boat centerline, the "counter heeling" has the same effect as lowering the center of gravity.

As long as the righting moment increases proportionally with the heeling angle, the boat acts like a theoretical torsional spring.

How to calculate and understand righting moment of a sailboat -

Articles Engineering

Plug and chug

http://www.wb-sails.fi/news/SailPowe...lPowerCalc.htm

 knuterikt 08-07-2011 05:30 AM

Here is a less technical description.:)
Stability @ sailtheory.com

 joelsanda 08-07-2011 02:51 PM

Thanks for the links, WDSchock and knuterikt. Bit to digest here, but I can see where a better question is stability and not necessarily capsize, since I'm interested in the former more than later :-)

 knuterikt 08-07-2011 05:01 PM

Quote:
 Originally Posted by joelsanda (Post 759321) I understand boat particulars weigh on this (hull type, keel type, onboard weight, etc), but I'm trying to understand how I can heel the J22 at 35/40 degrees and the darn thing just keeps going without capsizing. I saw a small catamaran capsize on the lake, and one day after I rushed back to the marina as a storm approached a 22-footer capsized during a race.
To describe at what heeling angle a boat will capsize there is a term called AWS (Angel of Vanishing Stability).

For keelboats this angel will typically be between 120 - 135 degrees. Meaning that the mast will be in the water before it capsizes. Keelboats have a combination of form and weight stability (the keel weight adding to the stability). So when sailing on flat (alomost) water an overpowered keel boat will in most cases turn into the wind before it reaches 90 degrees. If not it will stop heeling over when the wind pressure leaves the sail at 90 degrees heel.
Keelboats can capsize but that will be in situations where either
- surfing on a wave changes the form stability
- Large breaking waves throw the boat over past the AVS

All weights placed above the boats center of gravity (CG) will reduce the AVS.

The catamaran relies (primarily) on form stablity to avoid capsizing, so it will have an AVS of around 90 degrees. When a catamaran is overpowered it will capsize rather fast when it reaches its AVS.

 joelsanda 08-07-2011 09:17 PM

Quote:
 Originally Posted by knuterikt (Post 759516) To describe at what heeling angle a boat will capsize there is a term called AWS (Angel of Vanishing Stability). For keelboats this angel will typically be between 120 - 135 degrees.
Huh. I had assumed a heel not much greater than 90 degrees would mean a capsize, but now that I think about it none of the books I've read contained a definition of capsize. Thanks for the explanation.

 Barquito 08-08-2011 09:35 AM

To have a seat-of-your-pants understanding of stability, you need to get out in a bunch of conditions, and on a few different boats. You will find that hull shape effects how quickly the righting moment builds as the boat heels. ie: a boat that is beamy toward the stern will stay stiffer initially (form stability). A narrower boat, will seem tender at first, then when the heel gets greater, it will stiffen up. It really does help me to understand the forces that keep the boat upright in order to have confidence that it will stay that way in most conditions.

 tweitz 08-08-2011 10:25 AM

Also recognize that there are two forces at work -- righting moment, as has been discussed, and the reverse, which is the force that is tending to push the boat over. (we can call it wronging moment for amusement). Except in the breaking wave scenario, the wronging moment in a keel boat is primarily from the wind on the sails, and one of the key self corrections is that as the boat heels further, the sail presents less of a cross section to the wind, so the wronging moment diminishes and the boat tends to stabilize. This self correction affects every boat, with the wronging moment diminishing to zero either at 90 degrees of heel, or as the boat turns into the wind. The other key point is the position of crew, which, as has already been pointed out, has a greater proportionate influence the lighter the boat. If, as the boat heels, the crew moves to windward, even hiking out, the crew weight will tend to provide righting moment and keep the boat more upright. If the crew moves to leeward, the boat will heel even in light winds. In a lightweight boat with a crew that does not know what to do, the risk is higher, and the time to be most concerned is when coming about, because the windward and leeward sides of the boat reverse, and it is important to get the crew weight to the windward side in a timely manner.

 ccriders 08-08-2011 10:50 AM

[QUOTE=joelsanda;759321] Do sailboats generally have a 'rating' or degree of heel that could lead to a capsize? QUOTE]

In addition to the AVS, there is the capsize screening factor which you can find for many boats here: Sail Calculator Pro v3.53 - 2500+ boats.

While some people caution that this number is somewhat nebulous, it is atleast an attempt to put a bunch of calculations to work in evaluating many different designs.

As a practical matter, it will take a combination of wind and wave to capsize a keel boat, but then it may be difficult to stay aboard a boat that is finding its AVS.

Keep on having fun with your sailing,
John

 joelsanda 08-09-2011 12:19 AM

The most I've heeled was 40%, which felt very dramatic at the time - though I'm only in my second year and sailing J22s on a lake in Colorado. We kept pulling the main in to heel more so we could test how quickly a boat will right when the main is let out. Damn fast - just enough time for us to react. That was a great learning moment!

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