Wow, Tod, nice bit of research work. I didn't have to go to the 4th website to get the headache. Not being an engineer, and not even playing one on TV, it's mostly over my head, but I follow your examples of forces somewhat. I'm sure I don't understand all the forces at work on a boat rudder, knowing only what I feel and the results I see. As you mentioned, the "feel" can be deceiving. It would be interesting to be able to look down the rudder under various conditions to see if there were some mysterious things happening. Nice thread here. Good discussion. Larry On Tue, 29 Nov 2005 00:04:17 -0500 <htmills@bright.net> writes:
Larry,
I wasn't thinking about torque from weather helm, but instead a twisting of the blade due to the force on the blade from the water not being in line with what structural engineers call the "shear center". You might not even particularly feel any weather helm, depending on how well balanced the rudder blade is. Sometimes you get funny things like that going on. For example, if you have a 10' long piece of angle iron sticking out of a wall, looking like an "L" and you put a sandbag on the end of it, it won't just bend down a bit. It will also bend to one side. Similarly, if you have a 10' long piece of channel sticking out of a wall like so: [ and you push down on it at the centroid of the cross-sectional area, it will not only bend down a bit but it will also twist. To get it to bend straight down, you actually have to put a little bracket on the backside of the [ away from the horizontal legs (flanges) and push down there (to the left of the vertical web as I typed it) or else counter twist as you push down.
I was curious so I googled the modulus of elasticity of some plastics like HDPE and UHMW and Starboard and it looks like they are quite a bit (about 10 times) more bendy than the same section built of mahogany.
http://www.machinist-materials.com/comparison_table_for_plastics.htm
http://www.righteouswoods.net/mahogany_african.html (probably our rudders are Honduras Mahogany, I'd guess, but this gets us in the ball park)
However, that still doesn't mean that the rudder couldn't give quite satisfactory performance. It could be that in both cases the deflections are not especially significant. If I were racing, though, I'd choose the wood any day over an unreinforced plastic, unless the course were through some skinny water! :o)
I'm still wondering about the location of the shear center of a foil, though. Didn't see anything really juicy. Here is a brief mention of the subject:
http://www.eng.auburn.edu/~drmac/elasticaxis.html
If you wanna give yourself a headache, look at this one. Sure gives me one:
http://www.ae.msstate.edu/~masoud/Teaching/exp/A15.7_8_ex1.html
The thing is, your typical foil such as an airplane wing isn't made of a homogenous (sp?) material the way our rudders are.
Anyway, not knocking them; for some places they make much better sense than the standard non kick-up rudder.
Tod M17 #408 BuscaBrisas
-----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 Larry E Yake Sent: Monday, November 28, 2005 8:19 PM To: montgomery_boats@mailman.xmission.com Subject: Re: M_Boats: New Kick up rudder
Regarding the Ida Sailor kick-up rudder, I would think you would be experiencing a huge amount of weatherhelm before you had enough torque to make the rudder blade twist. The cheek plates are pretty substantial. I really doubt we're getting any twist or flex. The tiller doesn't have any of the flex feel that I've noticed on the stock M15 kick up rudder. I've only been using mine one season so I can't speak to the long term durability issue other than to say the quality and strength of construction look very good. The pointing ability doesn't seem to be any different than the stock rudder, which is a good thing. My stock rudder was one of the earlier, balanced models and I was reluctant to replace it, but since it couldn't even be raised vertically in shallow water I needed to do something. The foil shape is very good on the stock 17 rudder, so I was pleased that the kick-up rudder seems equal in performance. I have no vested interest in Ida Sailor, and get nothing from them for endorsements. I did get a discount on my rudder for helping with the design. I just believe it's a good product. The rudder breaking on the Santana is somewhat worrisome. I have also heard of a corrosion problem on the aluminum cheekplates on one of the east coast boats, which was also blamed on substandard material. There are about a dozen Ida Sailor M17 rudders out there last I heard. Anyone else having any problems?
Larry
On Mon, 28 Nov 2005 19:32:55 -0500 <htmills@bright.net> writes:
While those plastics no doubt offer some advantages, I would suspect that rigidity of the overall blade would be less than the factory rudder, meaning that when viewed from the top the tip of the rudder would want to
bow to weather with weather helm and might well twist, putting the tip at
a different angle of attack than the upper part of the blade if the
shear center of the foil doesn't coincide with the center of lift, which
could offset some of the gains of the "good" foil shape. Still, for some people it's probably a very good option, certainly better than getting your transom bolts popped.
Tod M17 #408 BuscaBrisas (<= who's transom bolts got stretched significantly once)
-----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 Tom Smith Sent: Monday, November 28, 2005 12:34 PM To: For and about Montgomery Sailboats Subject: RE: M_Boats: New Kick up rudder
Larry has an Idasailor rudder, Randy Graves has one, and I have one that's not yet installed. I'll let those guys speak regarding durability
over time and performance, but I'm impressed with the quality of the build. I wasn't aware of a new, lighter version, or if the Montgomery blades can be ordered in a different hdpe material than what I got or not. It sure seems industrial strength to me, and in my opinion it's a superior solution to grounding than the original slide-up design.
t
/)) Tom Smith & Jane VanWinkle M15/345--Chukar M17/496--Unnamed
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