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Old 28-05-2011, 13:42   #16
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Re: Treating Stainless Steel Rudder Blades

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Originally Posted by Hydra View Post
IMO, having half the thickness of the previous rudders will not change the effectiveness at low angles of attack. BUT this will reduce the stall angle: it might result in a dramatic loss of effectiveness at high angles.

Authors recommend 12% relative thickness (thickness/chord ratio) for rudders. See for example Amazon.com: Yacht Design Explained: A Boat Owner's Guide to the Principles and Practice of Design (9780393046465): Doug Hunter, Steve Killing: Books, p. 114

Then, the final answer depends on the rudder dimensions, chord and height.

Alain
This makes sense. I enjoy seeing postings which cite references, as opposed to blanket statements from a soapbox. And what you say squares with what I recall of of fluid mechanics.
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Old 28-05-2011, 20:27   #17
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Re: Treating Stainless Steel Rudder Blades

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Originally Posted by allanpeda View Post
... I enjoy seeing postings which cite references, as opposed to blanket statements from a soapbox...
I agree - Thanks Alain

OK, so at slow speeds &/or shallow angles of attack, we'd still have good lamilar flow over the rudders. But at high angles of attack the water on the back (far) side of the rudder wouldn't be able to follow the rudder, like the jib tell-tales fluttering or an airplane wing stalling.

But water's not like air - it can't expand. What happens to the water back there? Does it boil, like a prop cavitating? I'd think we'd have to be really moving to reach that point.

But 12% of our 27" rudder length is 3.1", which is about how thick our original rudders were.
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Old 28-05-2011, 20:59   #18
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Re: Treating Stainless Steel Rudder Blades

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But water's not like air - it can't expand. What happens to the water back there? Does it boil, like a prop cavitating? I'd think we'd have to be really moving to reach that point.
Turbulence. And a great reduction in the lift needed to turn the boat. Interestingly, neither air nor water is considered to be compressible (or expandable) in the conditions seen on sailboats (or low speed aircraft). Pressure but not compression. Not intuitive.

I'm not certain cavitation occurs on sailboat rudders, but I've heard they 'ventilate' buy sucking air down into the flow. Usually shortly before a sportboat performs a spectacular spinnaker run crash.
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Old 28-05-2011, 22:12   #19
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Re: Treating Stainless Steel Rudder Blades

So, if you are fabricating rudders from scratch, why must they be so thin in cross section? Seems like you could design them to any one of the NACA profiles... that's what I did many years ago when designing an auxiliary rudder wind vane. From memory I used a 0018 section, and it worked well at normal sailing speeds.

Cheers,
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Old 29-05-2011, 01:55   #20
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Re: Treating Stainless Steel Rudder Blades

For those wanting to get into more of the "nuts and bolts" of rudder design and NACA foils see:
SEDATION 4 - Rudder Design
Design and Construction Of Centerboards and Rudders
The Story Of The Sailboat Rudder - Naca 0012 and 0015 foil
Another Rudder Question (NACA) - Page 2 - Boat Design Forums
http://www.boat-links.com/foilfaq.html
http://books.google.com/books?id=euQ...page&q&f=false
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Old 29-05-2011, 07:42   #21
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Re: Treating Stainless Steel Rudder Blades

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Originally Posted by Jim Cate View Post
So, if you are fabricating rudders from scratch, why must they be so thin in cross section?
Good question, Jim. Mainly it's the practicalities of fabrication here in Phuket, Thailand, combined with my lack of knowledge (& thank you, everyone, for attempting to remedy that somewhat - up-hill battle, I know).

Basically, most rudders (including the first stab at ours by our machinist) have airfoil-shaped sections welded to the rudder stock & then a skin placed over those "stringers" (if you will). The problem with this design is that it requires welding around the shaft. Weld material is brittle & penetrates the shaft somewhat. My shafts are perhaps a bit small (1.5"), so I didn't want to weaken them at all if possible (we actually lost our mast in 1986 because the previous owner had welded across a cap-shroud chain-plate, so I'm a bit sensitive here).

I don't want welding around the shaft, I want all welding to be longitudinally along the shaft. I also feel that butt-welding plate(s) perpendicular to the shaft puts those plate(s) in bending, which is inherantly weak (even though this is fairly standard for rudders these days).

So I was exploring the possibility of welding the skins of the rudder blade to the outer edges of the shaft, which would make the blade only slightly wider than the shaft. But now I realize why we need a thicker chord.

Now the problem will be to find a way to attach the (wider) skins to the shaft in such a way that it's mechanically strong as well as practically doable. I've got an appointment with my machinist tomorrow, so I'll discuss the matter with him then. Except that I don't speak Thai & he doesn't speak English...

Ah, the joys of cruising!

Osiris, thanks for the many links. Some good info in there! (but 1 & 5 actually go to the same place).
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Old 29-05-2011, 07:54   #22
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Re: Treating Stainless Steel Rudder Blades

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...
I don't want welding around the shaft, I want all welding to be longitudinally along the shaft. I also feel that butt-welding plate(s) perpendicular to the shaft puts those plate(s) in bending, which is inherantly weak (even though this is fairly standard for rudders these days).

So I was exploring the possibility of welding the skins of the rudder blade to the outer edges of the shaft, which would make the blade only slightly wider than the shaft. But now I realize why we need a thicker chord.
I took a welding course a while back, and based on what they covered, it might be useful to look at this from an (unbiased fabricators) perspective. There are ways to make good solid weld without running the risk of stress cracks and embrittlement. I find that using ones own experience is useful, but lets not jump past the initial problem entirely. You want to be sure the replacement rudder is strong; the rudder is going to be welded stainless, so you want to be sure it is string enough. I have read that the gold standard are TIG welds, that they can be stronger than the material itself, and that stress forms at tight corners, so sometimes there is a need to radius bends, often with building up areas. MIG is pretty good too. Also welding technique can contribute to failure due to effects such as thermal expansion/contraction. The design may not be poor, maybe the technique was.

So you are right to talk to the machinist, perhaps you should post to a welding forum?
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Old 29-05-2011, 07:59   #23
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Re: Treating Stainless Steel Rudder Blades

instead of doing a full weld around the shaft on the foil sections .

spot weld in 4/6 places around the shaft,prevents heat build up ,leeching of chrome and distortion
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Old 29-05-2011, 09:21   #24
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Re: Treating Stainless Steel Rudder Blades

One of the problems with welding exotic metals in exotic locations is the that they do not have access to the proper or best alloy for the job. When you mention Stainless Steel, they only normally know of the version used in restaurants instead of the multitude of actual different alloys that fit under the general category of Stainless Steel.
- - This can lead to problems in getting a durable long lasting weld when fabricating something like a rudder. So it is best to keep the welding and design to the simplest and least involved construction.
- - One suggestion though, if you are looking for a way to "widen" the 1.5 inch diameter shaft is to attach long strips to the rudder stock that run vertically or along the long axis of the rudder shaft. And then attach the skin to these attached strips. These strips can be machine screwed onto the rudder stock rather than welded to it.
- - Another fabrication possibility is to construct the skin form using the vertical strips independent of the rudder shaft itself. Then mill two "keyways" down the length of the rudder stock. The final assembly would see the skin and strips sliding up the keyways with final welding only at the top and bottom of the shaft when the rudder stock enters and exits the pre-made airfoil skin assembly. I am sure that there are a myriad of other possible fabrication possibilities that minimize actual welding.
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Old 29-05-2011, 09:42   #25
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Re: Treating Stainless Steel Rudder Blades

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instead of doing a full weld around the shaft on the foil sections .

spot weld in 4/6 places around the shaft,prevents heat build up ,leeching of chrome and distortion
Yes, I recall that was one issue with welds. Also, as noted above, the alloy chosen. It seems that you may be fighting uphill to get exactly the level of quality you would expect in the US or the UK. Might want to keep in mind "good enough" until you get somewhere near a machinist you trust.
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