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  #1  
Old 10-25-2004
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when does CSF make a difference?

How much significance should one allot to the CSF of a particular boat if no extreme sea conditions are to be encountered? Let''s say that you are primarily a coastal cruiser and will not be making any offshore passages. Should you then be concerned with a CSF of more than 2.0? After all, plenty of racing dinghys and catboats have CSFs in excess of 2.0 but sail happily and safely wthin harbors or even along coastal areas. I noticed that this was the subject of a debate based on an article by Don Casey. But again, outside of the context of offshore passagemaking, how important is this number?
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Old 10-25-2004
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when does CSF make a difference?

There is absolutely no useful imformation that can be gleaned from a boat''s Capsize Sreeen Factor. With the possible exception of beam, the capsize screen factor includes absolutely none of the factors that really control the stability of a boat. It does not include the vertical center of gravity, buoyancy and weight distributions, drag, sail area and sail area distribution, hull shape or windage.


The example that I typically give regarding the dangerously misleading nature of the Capsize Screen Formula is this one, if we had two identical sailboats except that one had a 1000 lb weight at its masthead, (and yes I know that no one would ever put a 1000 lb weight at the top of a mast) the capsize screen formula would predict that the boat with the weight at the top of its mast would be less likely to capsize while in reality it would be way more likely to capsize.

The reality is that heavier displacement boats appear to be more stable when in reality, they often lack stability relative to their drag making them less stable than lighter craft with more easily driven hulls and lower center of gravities especially as viewed relative to their centers of buoyancy.

A much more accurate indicator of stability is the CE''s STIX (Stability Index) but it is by far a more complex set of formulas to calculate.

Rod Johnstone had an interesting discussion of stability on the J-boats website that compared the Capsize Screen Factor with the actual stability of actual boats and pretty much showed the irrelevance of the Capsize screen factor.

Jeff
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Old 10-25-2004
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when does CSF make a difference?

Yes, there are definitely more sophisticated and more accurate ways of accessing a boat''s stability. My Folkboat had a very low (1.76) CSF. But that''s not where the boat''s stability came from . Fifty-eight percent of the boat''s weight was in the keel. My new boat is a Nonsuch 30. It is quite stiff due to its 12 foot beam, but in terms of reserve stability it has a CSF of 2.10. Yet people have sailed these boats in the nasty afternoon chop of Buzzards Bay, and in the often angry conditions of the Great Lakes (for which the boat was designed). By the way
is that discussion still on the J-boat site?
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Old 10-26-2004
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when does CSF make a difference?

I visited the J-boat site and the article it does not appear that the article is available. There is an article on performance cruising that is interesting and touches on some of the basic ideas. The folowing is the text of an earlier article on that looked at Motion Comfort. The data chart that went with this was also used for the later article on stability.


QUANTIFYING SEAKINDLINESS: THE SAILING COMFORT LENGTH INDEX
Copy right 1998 by Rodney S. Johnstone

How does one quantify the “seakindliness” or motion of sailboats in a meaningful way? Experienced sailors know that some sailboats feel more stable, safe, and have a more comfortable motion than others in turbulent sea conditions. A large boat is usually more comfortable than a small one - but not always. If two sailboats have the same length and displacement, the narrower one will usually have a smoother motion - but not necessarily. If two boats have the same length and beam, it is generally thought that the heavier one will be more comfortable and manageable - but frequently the reverse is true.

Speed is also a factor in seakindliness. Modern, relatively light sailboats tend to have greater speed potential than most traditional designs. Some authors in the last fifteen to twenty years have gone to great lengths to prove that speedy modern boats are somehow less seaworthy and seakindly than heavy, slow, traditional designs.. The message of this mindset, invariably, is that “heavy”, “slow”, and “traditional” are good and safe; and that “light”, “fast”, and “modern” are bad and dangerous. This age-old argument was answered succinctly by legendary English yacht designer, Uffa Fox, back in 1934 when he stated:
“Owners must praise their vessels, and owners of slow boats praise their comfortable motion in a seaway, quite forgetting that their vessels are comfortable in a sea because they are so slow….It is the speed of a fast yacht that makes her uncomfortable, but as her owner can, by shortening sail, reduce her speed, he has the choice between a fast uncomfortable passage, and a slow comfortable one, while the owner of a slow yacht has no choice.”

A sailboat’s seakindliness, as well as its performance, depends primarily on its waterline length (L). Seakindliness is equally dependent on stability, or “stiffness”, or righting moment (RM): the ability of the boat to resist the heeling force of the sails. A boat’s seakindliness is directly proportional to how long it is and how “stiff” it is. The key to understanding the seakindliness of a particular design is to know the extent to which its “stiffness” is derived from its having a low center of gravity (CG), or from its having wide beam (B). To be more precise, seakindliness depends upon the ability of a boat’s displacement (the key element in RM), combined with the height of its CG, to overcome the waterplane inertia about the boat’s centerline(represented by B^3). Righting Moment RM=DISP*Righting Arm (RA). RA depends on hull form stability (represented by B) and CG height.

The most important prerequisite for a boat’s sailing comfort, or smooth motion in a seaway, is that its stiffness be derived more from its low center of gravity (CG) than from its Beam (B). This is indicated by a simple ratio of Righting Moment divided by Beam cubed (RM/B^3). The greater the number yielded by this ratio, the lower the center of gravity. This results in greater seakindliness, sail-carrying ability, and potential performance. (RM) is transverse righting force in foot-pounds when a boat is heeled to one degree. RM can be measured by a simple inclining test. (L) is length measured at the boat’s waterplane. (B) is maximum beam measured at the waterplane. If a boat’s stiffness comes from its wide B, its motion will be bouncy and abrupt when rolling in waves. This is indicated by a low value for RM/B^3. A seakindly boat which derives its “stiffness” primarily from its low Center of Gravity (CG) is indicated by a comparatively high value of RM/B^3 ratio.

A sample of 221 different designs of boats from 22 to 81’ LOA whose measurements are on file at US Sailing in Portsmouth RI is used to illustrate some physical characteristics related to relative values of RM/B^3. The median value of RM/B^3 for the “stiffest” fifty designs is 1.70. The median value of RM/B^3 for the “tippiest” fifty designs is 0.89. The average length/beam (LWL/B) ratio for the top group is 3.82, and only 2.96 for the bottom group.

A high or low rating on this index is independent of displacement/length ratio or DISP#/(2240*(0.01*LWL)^3). The highest 50 boats on the RM/B^3 scale have a displacement/length (D/L) ratio ranging from 55 (light) to 339 (heavy). A D/L ratio of less than 180 is light, 180-280 is moderate, and above 280 is heavy. By this definition, sixteen of the top 50 boats on the RM/B^3 scale are heavy, 16 are moderate, and 18 are light. At the bottom of the scale half of the 50 boats are heavy, 19 are moderate, and only 6 are light. The preponderance of heavy displacement boats at the “tippy” end of the scale reflects a modern trend toward increased accommodations and decreased ballast/displacement ratios in cruising sailboats, which results in higher center of gravity and decreased stability. In some circles these is refered to as modern “floating condos” with sails. If you choose the palatial accommodations combined with “low tech” solid fiberglass hull and deck construction, don’t expect much sailing performance, sailing comfort, or seaworthiness . 42 of the 50 “stiffest” boats on the RM/B^3 scale, but only 22 of the 50 least stiff boats, have sail ara to displacement (SA/DISP) ratios of over 16.0 - - what I consider to be a minimum for performance cruising-type speed under sail.

Another way of looking at it is to apply a constant to RM/B^3 to see how seakindly a particular design is for its length. If you multiply RM/B^3 by 14.85, the product should equal at least half of a boat’s load waterline length (L). Because (L) is the other major element of seakindliness and sailing comfort, a “Sailing Comfort Length” (SCL) can be determined for any boat for which L, RM and B are known. The greater the SCL, the more seakindly the boat. Whatsmore, all three of these elements can be measured by laymen with the boat afloat. The formula is:

SCL = 0.5*L + 14.85*RM/B^3

SCL should be equal to,or greater than, L. An SCL approaching or exceeding Length overall (LOA) is very favorable. SCL less than L indicates relatively less seakindliness.
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Old 10-26-2004
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when does CSF make a difference?

Jeff, thanks so much for sharing RJ''s article with us. A printed copy is saved for my reference book. You are so generous with your time and careful efforts on this web site ... thank you alone seems inadequate.
Mark L.
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when does CSF make a difference?

It is always my pleasure to try to be helpful. A lot of people have helped me over the years and I like having the chance to return the favor by trying to be helpful to others when I can.

Jeff
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when does CSF make a difference?

Is there any readily available data to easily compare various makes and models of boats readily available on the open market and what their respective numbers are?
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when does CSF make a difference?

There really are no easy sources. PHRF of Long Island Sound has a lot of information on their ratings page but you still have to go to Carl''s Calculator to get the numbers crunched. The IMS has a very detailed source of information about boats measured for the IMS but it is not all that easy to get that data. I understand that the CE has a summary of information on boats certified under the Small Craft Directive but I have not found a source of that summary either.

Jeff
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