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  #1  
Old 08-09-2010
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What constitutes .......

What constitutes the good righting ability and sea kindly motion when comes to looking for a blue water boat. What really matter besides the captain and the crews in the a heavy sea?
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Old 08-09-2010
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Old 08-10-2010
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I had to look that one up. But can you use it in a sentence?
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Old 08-10-2010
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such an opportunity for discussion

Surely we can get some sort of technical discussion of capsize screening formulas, angle of vanishing stability, reserve stability in bow entries, chines, weight aloft, center of gravity, cockpit volume and drainage, and such. I like to hear all that naval architect talk.

In other words, all the stuff about why it's good to have a boat that stays more or less right side up, doesn't let water in, keeps you warm and dry and secure, doesn't fall apart, isn't a beast to keep on course, and doesn't try to hurt you or make you puke.
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Old 08-10-2010
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sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice sailingdog is just really nice
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Originally Posted by jerryrlitton View Post
I had to look that one up. But can you use it in a sentence?
It would help if he could spell it properly first.
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Old 08-10-2010
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I apologize in advance that this is a long answer and it includes material that were portions of draft articles which I had written for other purposes, but hopefully it begins to answer the questions on the table.

Righting ability is just one aspect of a vessel's stability. While useful in understanding the vessel's likelihood to re-right itself after a roll over or extreme knockdown, it does not provide a complete picture of whether a vessel is likely survive since down flooding and structural issues also come into play.

That said, there seems to be a general agreement within the literature that minimally a boat intended for offshore use should have a Limit of Positive Stability (LPS also called AVS) that exceeds 125 degrees with something greater 135 degrees being even more ideal. The problem with LPS is that it is very loading dependent and complicated to calculate, so designers and manufacturers rarely publish the LPS of their vessels, especially with the vessels fully loaded for cruising or at half load which can be extremely reduced from the vessel empty LPS.

In the absense of a published LPS, there are some indicators which can be helpful in making a relative judgement between similar boats. Ballast to displacement ratios can be helpful in comparing boats of similar draft and configuration. But this can be misleading since factors like the density ballast (i.e. Cast lead vs lead pigs vs iron vs iron in matrix) or bolt on vs encapsulated keels can impact the center of the vertical height of the ballast and the boats ultimate stability. Hull form and the height of the hull and cabin come into play. A narrow boat with a lot of freeboard and a high cabin will in theory have a higher LPS, but all else being equal it will require less force to heel it to that limit.

Like all things in yacht design, LPS vs usable (stability within the range of normal sailing) stability requires moderation. Too much of a focus usable stability can result in a boat which lacks ultimate stability, but by the same token, too much of an emphasis on ultimate stability, can produce a boat that is not great at normal angles of heel.

The issue with seakindliness is even more complex than righting ability. There really isn't a perfect way to numerically define a seakindly motion. In a general sense, seakindliness is a balancing act. Ideally, you do not want a boat that is too rolly or pitchy or too stiff. Boats that roll and pitch through a wide angle tend be hard on crews in terms of requiring crew members to constantly shift thier balance, but (and this is an important but) in a general sense, they have a gentler, less violent motion which is often seen as a good thing. By the same token a boat which has too much resistance to roll and pitch, will tend to have a quicker and more violent motion, which is very hard on crews. The factors that most directly impact motion such as weight and displacement distribution, dampening, roll moments of inertia, and the like are hard to define numerically. There has been a lot of research on understanding and improving the motion characteristics of sailing vessels and if you are really interested in this topic I would see if you can get your hands on that research.

In a very general sense, to make some broad generalizations. heavy displacement, traditional working watercraft evolved designs tend to have gentler motions, although they can often be more rolly and have poor LPS's. CCA designs tend to also have gentler motions but through much wider roll and pitch angles. Depending on the specific design, they can have very high LPS angle but at the price of less stability in the normally used range of heel. IOR boats tend to be the worst of all worlds, having a tendancy to roll and pitch through wide angles but to fetch up with a quick jerk at the ends of the roll, which is also combined with a poor range of stability across the entire spectrum. IMS (the rule that followed IOR) boats vary widely, but the early and mid-period boats tended to have small roll and pitch angles and good dampening which kept them from having uncomfortable accelerations (except in heave), and they have very high stability throughout the entire heel range up to comparatively large LPS angles. I mention these racing rules only since within any particular era the design of production coastal cruising boats often followed the design trends in race boat design.

Lastly, you will often see recommendations for two surrogate formulas (by which I mean formulas which try to give a quick approximation without detailed calculation without actually using the detailed calculations needed to actually predict the behavior of a vessel) the Capsize Screen Formula and the Motion Comfort Index. But as I have explained here many times in the past, (and I am about to explain yet again much to the consternation of some who disagree with me) these surrogate formulas tell almost nothing about the reality of a boat's likelihood of capsize or its motion comfort. In fact in my opinion and most of the yeacht designers that I have discussed this with, they provide so little indication of a boat's behavior that to rely on them in any way borders on the dangerous.
Both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.
I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 1000 pound weight at the top of the mast. (Yes, I know that no one would install a 1000 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.
And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and its resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.

There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and lace w:st="on">Hulllace> Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

There is a convenient calculator at http://www.sailingusa.info/cal__avs.htm

But as I said above It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return.

Respectfully,
Jeff
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  #7  
Old 08-10-2010
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The sails, mast, and rig also have a big role to play in making the boat stable and sea-kindly. The wind pushing against the sails is generally a good, stabilizing force; in general a sailboat in a good breeze is going to roll less and be more comfortable than a motorboat or the sailboat with its sails down, to give a simple example. With the sails up, some weight aloft can be a good thing. But, as in the example Jeff gave above, we know that too much weight aloft can be bad.

Sail trim and efficient rig set up can also be vital to comfort. For example, some boats are very poorly set up for, say, long-distance downwind work in light air and their sails slat back and forth, gear is subject to terrible chafe, the poor trim calls for lots of work from the helmsperson or autopilot, and the poor boat barely moves. But, with the rig and sails properly set up and trimmed, many of these difficulties could go away and the crew could be much more comfortable.

Another very basic notion that racing sailors and cruisers try to practice is getting weight into the center of the boat to reduce violent pitching motion and help the boat go more easily through the waves.

One other bit of knowledge that might be helpful to newer sailors in this discussion is a refresher on the axes of motion through which a boat can move. East coast artist "Bowsprite" did a very lovely and fun blog post called
"Six Degrees of Freedom and the Drunken Sailor"

( six degrees of freedom and the drunken sailor « Bowsprite: A New York Harbor Sketchbook )
that illustrates:

Heaving - moving up and down (linear, along y-axis),
Swaying - moving left and right (linear, along x-axis),
Surging - moving forward and backward (linear, fore and aft),
Pitching - tilting forward and backward (rotational, along y-axis about the center point),
Yawing - turning left and right (rotational, change heading side to side), and
Rolling - tilting side to side (rotational, about horizontal fore and aft center line)

Describing and providing for all of these motions is part of a good designer's job, IMHO.

Jeff, are Hull Draft and Displacement Volume in the formula measured in meters and cubic meters? I think my computer cut off or mangled the end of your post. When I made my post the first time SailNet got extremely confused and treated it as an EDIT of YOUR post. Strange!
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Old 08-10-2010
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Let's see if my post is posted more or less properly now.

Jeff -- Sorry, something very strange happened above with SailNet treating my post as an edit to yours! I've no idea what's happening but let me try to get my whole post in.

The sails, mast, and rig also have a big role to play in making the boat stable and sea-kindly. The wind pushing against the sails is generally a good, stabilizing force; in general a sailboat in a good breeze is going to roll less and be more comfortable than a motorboat or the sailboat with its sails down, to give a simple example. With the sails up, some weight aloft can be a good thing. But, as in the example Jeff gave above, we know that too much weight aloft can be bad.

Sail trim and efficient rig set up can also be vital to comfort. For example, some boats are very poorly set up for, say, long-distance downwind work in light air and their sails slat back and forth, gear is subject to terrible chafe, the poor trim calls for lots of work from the helmsperson or autopilot, and the poor boat barely moves. But, with the rig and sails properly set up and trimmed, many of these difficulties could go away and the crew could be much more comfortable.

Another very basic notion that racing sailors and cruisers try to practice is getting weight into the center of the boat to reduce violent pitching motion and help the boat go more easily through the waves.

One other bit of knowledge that might be helpful to newer sailors in this discussion is a refresher on the axes of motion through which a boat can move. East coast artist "Bowsprite" did a very lovely and fun blog post called
"Six Degrees of Freedom and the Drunken Sailor"

( six degrees of freedom and the drunken sailor « Bowsprite: A New York Harbor Sketchbook )
that illustrates:

Heaving - moving up and down (linear, along y-axis),
Swaying - moving left and right (linear, along x-axis),
Surging - moving forward and backward (linear, fore and aft),
Pitching - tilting forward and backward (rotational, along y-axis about the center point),
Yawing - turning left and right (rotational, change heading side to side), and
Rolling - tilting side to side (rotational, about horizontal fore and aft center line)

Describing and providing for all of these motions is part of a good designer's job, IMHO.

Jeff, are Hull Draft and Displacement Volume in the formula measured in meters and cubic meters? I think my computer cut off or mangled the end of your post.
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Old 08-10-2010
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Moderator, HELP!

note above -- this discussion got so technical that it has even confused SailNet's posting system.
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Old 08-10-2010
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See Bob Perry's book:

Amazon.com: Yacht Design According to Perry eBook: Robert H. Perry: Kindle Store


There's a metric ton(ne) of information out there on the subject if you do a little research and reading on yacht design. Bob's book is great because it covers design and production issues as well as an interesting narrative of how his designs came to be, i.e., it's very readable!

For day sailing, what's important? In cold water climates (think most of the west coast), really good foul weather gear is key! For coastal and offshore sailing, that same foul weather gear, a good wet locker, and cabin heating... dry and warm is tops on the list, and most boats made all of the above!

Last edited by puddinlegs; 08-10-2010 at 03:13 PM.
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