Hmmmm, for some reason, Carlos, your original post never made it to my mailbox. I didn't know how close the two centers were, having never looked into the topic. Imagine, someone on the list HAS! Amazing! :o) I suppose a plastic rudder could float w/o a cavity in it, depending on the sort of plastic (we all know polypropylene floats). The plastics link gave some densities, iirc. Fresh water is about 62.4 #/ft^3 or .0361 #/in^3. Gilbert, good idea with the camera! Wouldn't that be neat watching some of that footage?! Some web-savvy individual could put it on the internet, probably be the only rudder-cam in the world! (ever see the Lobster Cam? http://www.thelobstercam.com/) Tod M17 #408 BuscaBrisas <=shivering in the barn while her owner sips hot chocolate and dreams of warmer days. -----Original Message----- From: montgomery_boats-bounces+htmills=bright.net@mailman.xmission.com [mailto:montgomery_boats-bounces+htmills=bright.net@mailman.xmission.com] On Behalf Of IDCLLC@aol.com Sent: Monday, December 05, 2005 8:36 AM To: montgomery_boats@mailman.xmission.com Subject: Re: M_Boats: New Kick up rudder Idafailure story... Excellent treatise, Carlos! For composite strength, stiffness, ease of manufacture, and plain ole durability, a good piece of wood is hard to beat. Steve In a message dated 12/4/2005 8:38:21 PM Central Standard Time, carlos.albar-diaz@alumni.haas.org writes: Tod, I recall getting into this mix of aeronautical stuff and structural engineering when I was in (structural) Engineering school and was wording on designing a "go fast" centreboard and rudder for a Snipe dinghy. This shear center business is important for "thin walled beams" that are hollow (or not hollow but filled with a different material, like some plastic rudders filled with foam) and closed like an airplane wing. It is most important for thin walled beams that are open, like the L profile you mentioned. In those cases the shear center can be outside the beam. Any civil/mechanical engineering student should be able to calculate the shear center for your rudder given a drawing of the cross section. The aerodynamic center is more difficult. If the cross section of the rudder follows a NACA profile (this is a sign of a well-made rudder) then you can find this in a book. Look for "NACA profiles" in an engineering library. If the cross-section does not follow a NACA profile then you would need tank testing or a wind tunnel.. This shear center business is not as important for non-hollow beams made of a uniform material, such as a wooden boat rudder. A well designed rudder will have a cross section shaped like a "NACA profile". I recall that for every reasonable NACA profile that I looked at the shear center for a non-hollow section was relatively close to the aerodynamic center. With "relatively" I mean it was not far enough to create measurable twist, thanks to the thickness and non-hollowness of the rudder, which gives great rigidity to torsion/twist. To make a long story short, if we rule out hollow or open-section rudders (eg like the L profile in the example) we should not be worried about twist (this is around a vertical axis) of a good rudder. We should be concerned about plain bending to windward. I agree that plastics like HDPE are not stiff enough for this job until proven otherwise. In addition, they will only float if the section is made hollow, which creates other problems such as tearing out of the fittings and "shear center". All this before we get concerned about what happens when the plastic gets exposed to the sun. If a manufacturer of aftermarket rudders made of anything other than solid wood is serious about what they do they should be able to tell you all about shear center, aerodynamic center and the like. Have you asked them? Regards Carlos _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats