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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Stability information
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Topic Review (Newest First)
10-08-2012 10:54 PM
Dean101
Re: Stability information

Barquito, one of your statements struck the root from which my curiosity springs. "If I where shopping, I would just generally look for boats that are seaworthy." But as a fairly inexperienced sailor, how do I know what is seaworthy? I don't think it would be smart on my part if I take the word of every seller. Sure, some might be telling the truth but how will I know if they are?

I get the feeling that seaworthiness is not a well-defined term. I read a lot of books and cruise through these forums looking for sound advice. The "offshore rules of thumb" (my term) stating that offshore boats should have capsize ratio's under 2.0, AVS of at least 140 degrees, ratings for offshore use, designed for offshore use, choose a seaworthy boat... well, those things sound reasonable. The only one of these things I can consistantly find on about every boat I've looked at is the capsize ratio, which doesn't seem to carry much weight. Finding AVS information and what any given older boat is designed or rated for is like looking for the proverbial needle in a haystack, and without some offshore experience on different boat designs how am I, the newbie, suppose to know what seaworthy is?

That's the view from my perspective. I don't have the money to buy, try, and then sell several boats till I figure it out. I'm trying to get a grip on what really makes a boat safe for offshore use so that when I get ready to buy, I'll know what to look for. I already know that the boat I buy will be from the used market, likely a few decades old. I know what I want to do with it. I'm simply trying to find a source for stability information on various older boats to help myself make an educated compromise when it comes time to buy.

@ Bob - I was using an online calculator. When I input the numbers, all of which were the same except the hull draft, I got greatly different numbers which taught me that even a small error in my input will return untrustworthy results. Me actually trying to calculate AVS by formula, even with totally accurate data, is laughable in the extreme! But seriously, your response comes from much knowledge and experience in your field. I respect that, but I do not have anything even close to your knowledge of how any given design will behave offshore. AVS information, if I can find a reliable and consistant source, for older boats will be one more thing I can put on a pro/con list for different boats I'm interested in. That, in turn, will paint a more accurate picture of each boat rather than focusing on just the "pretty" things.

I was a little long-winded there but I hope it helps to explain why I was asking about AVS.
10-07-2012 09:03 PM
Barquito
Re: Stability information

I'm not ready to be looking for world circling boats. However, if I where shopping, I would just generally look for boats that are seaworthy. After that, there would be other factors that would effect my final choice such as the interior layout, sailing characteristics, equipment included in the sale... I just think I would use a broader brush in evaluating boats.
10-07-2012 01:26 PM
seabreeze_97
Re: Stability information

A couple links to info from Ted Brewer that may help:
Good Old Boat - Is your boat stable? article
and
Ted Brewer Yacht Design
10-07-2012 01:23 PM
bobperry
Re: Stability information

Right. The simple capsize ratio is a bit too simplistic to give you any meaningful data. It's good for a quick look and if you did get a number under 2.00 then it might be worthwhile to investigate the stability issue further.

I'm not sure I understand this. If you are using a method that tells you the AVS for an Alberg 35 is above 150 degrees I'd throw that method out. I could be wrong but it seems too high to me. JonB is correct, that could be called the "garboard radius" although it's not a radius in most cases and it changes at each station. In a case where there are hollow garboards and you need to determine a "hull depth" it's going to be tricky. The way I would do it on paper is to draw a straight line tangent to and from the section to the centerline ignoring the garboards, i.e. pretending the keel is not there. But any simplistic approach to determining stability is going to be suspect.

If you contacted US Sailing I am certain they would have accurate stability data for the Alberg.

But why bother? The Alberg will have a high AVS. My guess would be around 138.17 degrees. That's a good number and there is nothing about that design the indicates it would have stability issues. Just look at it. Also keep in mind that critical to any stability study is the VCG. Without an accurate VCG the rest of the equasion is not going to give you an accurate number. If you have found a method that ignores an accurate VCG input then I would call it very suspect.

I'll bet you a dollar that when Carl Alberg designed the 35 he did not do a stability study. I'm sure he drew a shape that he knew was right and in line with other healthy designs of the day. It turns out that he was right and the 35 is a very good boat.

Stability is complicated to most sailors. I think a little understanding of how the elements work can be dangerous. I also think that way too much emphasis is placed on hard stability numbers. AVS numbers only tell a small, static part of the dynamics involved in a capsize. The rest is pretty hard to calculate. If you own a "normal" boat and not some radical design freak or extreme racing boat then your biggest assett in rough conditions will be your own ability to manage the boat in a seamanlike manner. I can't find the formula for that.

Look at Rod Johnstone's exhaustive study on actual, documented capsizes. His results were very interesting. In plotting numbers for the various cases he found that the boats with the best numbers were more prone to capsize. What does that say? Nothing to me. Too many variables to draw any conclusions as far as I am concerned.

I have this image of a couple on their boat in a storm. They get knocked down to 130 degrees. The husband turns to the terrified wife and says, "No worries dear. This boat is good till 132 degrees." I have been in the middle of the Pacific in a taiphoon. I have seen waves that would capsize any boat regardless of the numbers. In all the studies one thing becomes evident, the bigger the boat, the more resistant it is to being capsized.

Hope that helps.
10-07-2012 01:03 PM
Dean101
Re: Stability information

Quote:
Originally Posted by SloopJonB View Post
There's a design book called "Seaworthiness - The Forgotten Factor" by Marchaj that came out in the wake of the IOR stability games. I haven't read it but I've seen it highly recommended by authoritative voices. Perhaps it has what you're looking for.
Thanks! I'll check that out. In the meantime, I'm hoping the Google fairy will turn something up!
10-07-2012 12:49 PM
SloopJonB
Re: Stability information

Quote:
Originally Posted by Dean101 View Post
That all makes sense. I have spent the last 2 hours doing google searches, looking at ISO standards, and reading various PDF's trying to find some sort of database for stability information. I've found tantalizing hints and tidbits but they come up short and leave me even more frustrated. As important as this information is, my gut tells me that somebody somewhere at some time has made the effort to compile stability information on various boats. There are so many guidelines on minimum AVS for coastal vs. offshore use. There are even catagories for boats that define their intended use but I can't seem to track down any of that information. It seems like an information gap akin to nobody publishing towing capacities for vehicles.
There's a design book called "Seaworthiness - The Forgotten Factor" by Marchaj that came out in the wake of the IOR stability games. I haven't read it but I've seen it highly recommended by authoritative voices. Perhaps it has what you're looking for.
10-07-2012 12:29 PM
Dean101
Re: Stability information

Quote:
Originally Posted by SloopJonB View Post
I'm afraid not. The design aspect you refer to could be called "garboard radius" I guess and my last boat - Columbia 43 - had a fairly pronounced one. I ran into the same problem as you because of it. For practical purposes, if you get a satisfactory outcome using the lowest resultant, you are O/K. If you must have the absolutely accurate number you'll need to look further. Perhaps the tangent point of the radius?

Perhaps the maestro can weigh in on this?
That all makes sense. I have spent the last 2 hours doing google searches, looking at ISO standards, and reading various PDF's trying to find some sort of database for stability information. I've found tantalizing hints and tidbits but they come up short and leave me even more frustrated. As important as this information is, my gut tells me that somebody somewhere at some time has made the effort to compile stability information on various boats. There are so many guidelines on minimum AVS for coastal vs. offshore use. There are even catagories for boats that define their intended use but I can't seem to track down any of that information. It seems like an information gap akin to nobody publishing towing capacities for vehicles.
10-07-2012 11:56 AM
SloopJonB
Re: Stability information

Quote:
Originally Posted by Dean101 View Post
Sail Calculator pro is a pretty neat site. I like the ability to select a boat from a list and being able to compare two different boats. Sloop was right though, it only showed me a capsize ratio. Maybe it is just a lack of understanding on my part but the capsize ratio really doesn't tell me anything. I read that an offshore boat should have a capsize ratio of less than 2.0 but that number doesn't really tell me anything important.

The site that SloopJon linked did give me an angle of vanishing stability, which is much more important to me. Somebody correct me if I'm wrong but it's my understanding that the capsize ratio is a general indication of a given boats resistance to being capsized where the AVS is the more accurate measurement of angle of heel at which the boat will no longer right itself. In the event of a 90 degree knockdown, knowing the actual AVS, 140 degrees for example, would tell me that I have another 50 degrees to go before some other righting force must be applied to bring the boat back. Again, the capsize ratio tells me nothing.

SloopJon, a couple of questions. On that site one of the parameters it asks for is Hull draft not including the keel. How is that determined? In the list of specs I've looked at, the draft given is the total draft of the boat. Also, some boats have a pretty apparent point at which the hull ends and the keel starts but others have more of a wineglass profile that makes that point a little less apparent. I tried estimating draft by looking for that point in the pictures but it takes very little difference in the value entered to change the AVS drastically. Here are the results of what I entered for an Alberg 35;

Beam 9.75 9.75
Weight 12600 12600
Ballast 5300 5300
Hull draft 2.5 2.0

AVS 182.22 152.45

The AVS given on my second try seems much more reasonable but the variance in the results are great enough that I'm skeptical of trying to estimate the hull draft. Only 6 inches of error produced a difference of 30 degrees. Do you have any suggestions to help me get a more accurate draft entry?
I'm afraid not. The design aspect you refer to could be called "garboard radius" I guess and my last boat - Columbia 43 - had a fairly pronounced one. I ran into the same problem as you because of it. For practical purposes, if you get a satisfactory outcome using the lowest resultant, you are O/K. If you must have the absolutely accurate number you'll need to look further. Perhaps the tangent point of the radius?

Perhaps the maestro can weigh in on this?
10-07-2012 09:34 AM
Dean101
Re: Stability information

Sail Calculator pro is a pretty neat site. I like the ability to select a boat from a list and being able to compare two different boats. Sloop was right though, it only showed me a capsize ratio. Maybe it is just a lack of understanding on my part but the capsize ratio really doesn't tell me anything. I read that an offshore boat should have a capsize ratio of less than 2.0 but that number doesn't really tell me anything important.

The site that SloopJon linked did give me an angle of vanishing stability, which is much more important to me. Somebody correct me if I'm wrong but it's my understanding that the capsize ratio is a general indication of a given boats resistance to being capsized where the AVS is the more accurate measurement of angle of heel at which the boat will no longer right itself. In the event of a 90 degree knockdown, knowing the actual AVS, 140 degrees for example, would tell me that I have another 50 degrees to go before some other righting force must be applied to bring the boat back. Again, the capsize ratio tells me nothing.

SloopJon, a couple of questions. On that site one of the parameters it asks for is Hull draft not including the keel. How is that determined? In the list of specs I've looked at, the draft given is the total draft of the boat. Also, some boats have a pretty apparent point at which the hull ends and the keel starts but others have more of a wineglass profile that makes that point a little less apparent. I tried estimating draft by looking for that point in the pictures but it takes very little difference in the value entered to change the AVS drastically. Here are the results of what I entered for an Alberg 35;

Beam 9.75 9.75
Weight 12600 12600
Ballast 5300 5300
Hull draft 2.5 2.0

AVS 182.22 152.45

The AVS given on my second try seems much more reasonable but the variance in the results are great enough that I'm skeptical of trying to estimate the hull draft. Only 6 inches of error produced a difference of 30 degrees. Do you have any suggestions to help me get a more accurate draft entry?
10-06-2012 08:18 PM
SloopJonB
Re: Stability information

Quote:
Originally Posted by LinekinBayCD View Post
Check this web site out. The data base has tons of boats data pre entered and you can compare in a chart two different boats or enter in your own data.

Sail Calculator Pro v3.53 - 2500+ boats
That's a good & useful site but it only has capsize ratio calculated, not AVS.
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