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 -----Mensaje original----- De: [EMAIL PROTECTED] com [mailto:[EMAIL PROTECTED] mission.com] En nombre de [EMAIL PROTECTED] Enviado el: Monday, November 28, 2005 10:41 PM Para: For and about Montgomery Sailboats; Gilbert Landin CC: 'For and about Montgomery Sailboats' Asunto: RE: M_Boats: New Kick up rudder Idafailure story... In the Small Craft Advisor magazine (Same issue that reviewed the M15) Sept/Oct 2004 issue 29 on page 38 tells of a failure of the Idasailor rudder on the boat "Chubby" when off shore in heavy wind. He was north of the Golden Gate on his way up to Alaska... After mounting the original stock wooden rudder that he fortunatly brought along... he turned back toward the Berkely marina. He stated that he expected the the Idasailors rudder to be much more durable than the stock wooden rudder that had steered him to Hawaii. Apparently the failure was not the foil but one of the stainless steel straps on the lower pintle had torn off completely along the weld on one side of the pin. I would imagine that the IdaSailor folks took care of it and hopefully realize that they may have a bad batch of welded stainless. That could happen to any rudder maker.. It could have ended badly fortunatly it didn't. Would I buy and Idasailor Rudder Perhaps... This is just one story to ponder on and Consider what type of sailing I'd be doing . Small lake sailing is different than being up the North Coast in heavy seas and Heavy Wind. Lots of things can break on a boat in maximum conditions. There is a Murphys law in here somewhere... One other thing.. I noticed that the MSOG picture site listed under " M15 ocean racer" has a guy who is sailing an M15 highly modified around the world.. "Eye of Infinity" I believe is the name of his boat. I noticed He has a very heavy built wood rudder more like the original M17.. I wish you the best in your decision. Christian R20 Monty? someday... Quoting Gilbert Landin <[EMAIL PROTECTED]>: > Well guys, I was inspecting my keel bottom gudgeon and it is definitely > spider webbed. After all my groundings this season I am not surprised. My > old kickup rudder has fiberglass checkplates and a fiberglass sheath for > stability. It worked fine but was not very aerodynamic I could feel a > definite difference between it and the stock solid rudder. But I am sure if > the rudder works well for "How low can you go before being knocked down" Tom > then its got to be tuff. So we will see. > > Gilbert > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] > On Behalf Of Gilbert Landin > Sent: Monday, November 28, 2005 8:08 PM > To: 'For and about Montgomery Sailboats' > Subject: RE: M_Boats: New Kick up rudder > > Thanks Bill I did see Larrys pictures and comments and that certainly has my > attention. I will continue my quest. > > > Gilbert > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] > On Behalf Of [EMAIL PROTECTED] > Sent: Monday, November 28, 2005 10:15 AM > To: [email protected] > Subject: Re: M_Boats: New Kick up rudder > > In a message dated 11/27/2005 8:08:22 PM Pacific Standard Time, > [EMAIL PROTECTED] writes: > > I would like to hear from you guys that > bought the new lighter material rudders from ida sailor > > Hi Gilbert, > > Larry Yake with his early M17 was the first to try the Idasailor rudder. > There is a picture of the assembly on the Photo Site (link below). Larry > helped develop the shape of the rudder which is like the original designed > by Lyle > Hess. I can say after the two week NW Cruise, Larry was very happy with > performance and use. I think he may be the only M-Boater with one... > > My experience with them is from my past ownership of a Potter. Idasailor > is > the OEM rudder maker for all of the Potter craft. They may not put the > blade to a test like real race craft, but then again with the winds of our > SF > Bay, they do get to see use. I lurk on the Trailer Sailor Forum on > occasion > and they (Potters) seem to be quite happy with the product. > > Good luck on your quest. > > Bill > keeper of the Photo Site _www.msog.funtigo.com_ > (http://www.msog.funtigo.com) > > Rudder Link > _http://www.funtigo.com/MSOG?b=197153&c=1789789&p=start&cr=1_ > (http://www.funtigo.com/MSOG?b=197153&c=1789789&p=start&cr=1) > _______________________________________________ > http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats > > > _______________________________________________ > http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats > > > _______________________________________________ > http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats > _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats
