Stability information

[Dean101]

How does someone go about finding stability information on various boats, some of which the manufacturer is out of business, while researching for a future purchase? I'm not looking for the vague capsize ratio. I would like to find specific values such as AVS. Is there any resource where this type of information is compiled?

I'm trying to narrow down my short list of preferred boats based on specific boat data rather than the glittering "she will take you around the world" statements that sellers and manufacturer's love to throw around.

[Stumble]

I don't know of any manufacturer that publishes this data. Depending on what boats you are looking at you could check the rolls of the classification societies and see if they have it on file, you could also hire a NA to do the calculations.

[Dean101]

Originally Posted by Stumble

I don't know of any manufacturer that publishes this data. Depending on what boats you are looking at you could check the rolls of the classification societies and see if they have it on file, you could also hire a NA to do the calculations.

Can you tell me the names of some of these classification societies? I'm still fairly new to sailing and am not familiar with them. Are they groups that evaluate racing boats and issue handicaps and such?

As far as hiring a NA, that could probably run into some serious money. I definitely want information from a source that has no potential monetary gain from promoting any certain brand. I was thinking that over the years, others have had this type of information calculated for just about every boat out there. Surely somebody, perhaps the classification societies you mentioned, would find this information important enough to assemble it into one database.

[SloopJonB]

Try this link.

Angle of Vanishing Stability

[LinekinBayCD]

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

[SloopJonB]

Originally Posted by LinekinBayCD

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.

[Dean101]

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?

[SloopJonB]

Originally Posted by Dean101

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?

[Dean101]

Originally Posted by SloopJonB

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.

[SloopJonB]

Originally Posted by Dean101

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.