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Old 09-15-2003
aflanigan aflanigan is offline
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Confused about overhangs...

I think Jeff H is in serious contention to capture the "resident amateur physicist" title here on the board (which I had aspired to)! ; > }

If you think about automobile ride comfort, inertia is a big component in perceived comfort, hence Jeff''s discussion of inertia is quite appropriate. In layman''s terms, a big boulevard cruiser weighing a couple of tons (think Cadillac) will ride over washboard and small potholes more comfortably than a little one ton econocar, because it''s inertia will resist the deflecting input fed to the car''s suspension (springs) by a bump. The other important component for that cushy "boulevard ride" that car enthusiast magazines sneered at back when I had time to read them is damping, as Jeff points out: Sooner or later, the Caddy will hit a bump or series of bumps that get its stiff springs bouncing; if not damped, you''ll be "hobby horsing". Hence shock absorbers connected between the wheels and the car frame.

So how is a monohull''s motion damped? Aside from the damping action of the sails motion during rolling or pitching, and the keel resistance to rolling that Jeff mentioned, it seems the primary job of damping pitch and roll falls to the hulls, a task for which they are not particularly well suited. There are probably two effective mechanisms that could potentially damp the energy of a pitching hull: vortex generation (turbulence generatation) and wave generation (pushing a hull section up and down in the water will generate waves, which requires energy). Sharp edges can generate turbulence, but tend to do it also when you don''t want it (moving through the water under sail). Tapered hull shapes probably generate waves better than flat sided hulls. In terms of damping, Jeff''s experience with "overhang" boats is reminiscent of attempts to damp pitching motions using bulbous formations on the bow. Overall, though, there''s probably not a lot of potential for motion damping in a monohull form, unless you''re designing a houseboat that''s not meant to go anywhere.

Another aspect of seakeeping/seakindliness which needs to be mentioned is resonance. Any harmonic system (like Jeff''s pendulum) can be driven to resonance with input at the right frequency. Like a parent pushing their child on a swing, if you have a boat whose natural pitch frequency is close to the typical frequency of waves you encounter in the waters you frequent, you are going to have a hard time of it. Various add-on systems (anti-roll tanks, bilge keels) have been designed to try and supplement the minimal resistance to rolling that a hydrodynamically efficient hull shape offers.

There''s a good basic discussion of seakeeping and various attempts to improve seakindliness by John Waterhouse at:

http://www.unols.org/SBCompendium/06Seakeeping.pdf

See also the preceding chapter on "stability" which contains the Figures Waterhouse refers to in his article:

http://www.unols.org/SBCompendium/05Stability.pdf

So what sort of design would be most comfortable for you? Ultimately, you are wading into a compromise situation when considering sailboat hull design no matter what direction you approach it from (sailing speed and efficiency, motion comfort, economy, cargo capacity, ergonomics, etc.) Long, slender hulls may be resistant to pitching, for example, but shorter, wider planform hulls will resist rolling better.

Regarding the Nonesuch, I haven''t sailed on one, but as Waterhouse says, "vessel mass is a key factor in the equation. For a given wave height a heavy vessel will have
lower accelerations, or move less, than a light weight vessel." Comparing the Nonesuch 26 to my Helms 25, for example, the waterline length is comparable, but the Nonesuch has more than twice the displacement, and a very substantial ballast keel weight of almost 3000 lbs., which will keep the vertical center of gravity low. I have no doubt that this design would be more seakindly than my lightweight, centerboard coastal cruiser in many conditions.

As Jeff notes, some people have more intolerance of acceleration (quick motion), others of amplitude (big, slow rolling or pitching). Perhaps the best thing to do would be to crew on some deliveries to get real world experience of how different hull shapes make you feel out on the open water.

Allen Flanigan
Alexandria, VA
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