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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.

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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