This is a continuation of another thread first posted here:
http://www.sailnet.com/forums/general-discussion-sailing-related/37733-shoal-draft-vs-deep-keel.html
NOTE: IF YOU WANT TO TALK ABOUT GENERAL PROS AND CONS OF SHOAL VERSE DEEP DRAFT BOATS, such as the pros and cons of shoal draft vis a vis getting through the Intracoastal Waterway, or cruising the Bahamas, or visiting certain Pacific atolls with your family and the implications thereof, etc, GO TO THE LINK ABOVE.
THIS THREAD IS INTENDED TO BE A TECHNICAL DISCUSSION CONCERNING DESIGN ATTRIBUTES AND CHARACTERISTICS OF SHOAL AND DEEP DRAFT BOATS. THIS IS THE THREAD TO DISCUSS CONCEPTS SUCH AS MOMENTS OF INERTIA, RIGHTING MOMENTS, METACENTERS, ETC.
Posts #41 and #42 of the other thread raised the discussion to a higher technical level. I have taken the liberty of quoting them here so as to continue the technical discussion in the new Design and Construction forum.
Diva 27 said:
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Originally Posted by Diva27
There's a lot of fuzzy thinking about sailboat stability out there. As coauthor of Yacht Design Explained (shameless plug: buy it, read it, memorize it), I'll touch on the basics.
Keel draft matters with respect to how it affects how deep the boat's centre of gravity is located. It's important because as a boat heels, its stability depends on the horizontal separation between the centre of buoyancy and the centre of gravity. That separation is the lever arm that describes the righting moment. Generally speaking, the deeper a keel is, the lower the boat's centre of gravity becomes, and you can achieve the same righting moment in the same hull with a shallower but heavier keel. Keel draft is critical to windward performance. Deep and narrow is more efficient than short and long. So with a shoal draft, you're generally accepting reduced windward performance in pointing ability, as well as an overall heavier (and slower) boat. But draft is critical to determining where you can anchor, of course. I have a vintage C&C 27 that draws 4 feet 3 inches, and I'm glad, given water levels on Georgian Bay in recent years.
Bottom line is that a shoal draft design can be as stable as a deeper draft design, provided the designer has done his/her homework. Where it's a matter of putting a different keel on the same hull, the shallow draft keel is going to need enough volume to hold more ballast, or else the additional ballast has to be placed in a much less efficient location, up in the bilge. Shortening the rig also helps in this case, as this lowers the centre of sail force. The distance between the centre of sail force and centre of keel lift, measured down through the mast and keel, determines the heeling arm. The longer the heeling arm is, the greater the potential heeling moment (force times distance), and that requires a larger righting moment to oppose. I've been grossly simple here, but maybe you get the idea of what's involved.
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To which Robert Gainer replied:
Quote:
Originally Posted by Tartan34C
Nice short and concise description. But because the distance term in the polar moment of inertia is squared and the distance term in righting moment is liner the amount of energy to flip a shoal draft boat that has the same righting moment as a deep draft boat is less. Also as the draft increases the horizontal separation between the center of buoyancy and gravity increases more rapidly at a greater draft for a given heel ((depth of CG )X(tangent of heel)) so you really need a higher righting moment on the shoal draft boat to get a similar feel as the deep draft boat. Look at an ice skater in a spin to see the effect of changing the polar moment of inertia. Bringing in the arms increases the speed of rotation. Everything is a tradeoff and nothing is straight forward.
All the best,
Robert Gainer
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