Treating Stainless Steel Rudder Blades

[Jon Hacking]

We’re having 2 new rudders built up here in Thailand for our 16 year old catamaran. It now looks like they’ll be made completely in 316L stainless steel instead of being SS/fiberglass composites like the originals.

They’ll also be a bit thinner, just thicker than the 1.5” SS shaft. I couldn’t see that thicker (fiberglass) blades would be any stronger, as most side loads will be transmitted to the shaft at the top of the blades anyway. Is there any reason that spade rudders should be thicker than 1.5”? Do I need a thicker airfoil shape?

My other question is about coating the blades. Normally, I’d epoxy coat the SS blade & then paint with antifouling. But apparently SS wants to “breathe” and removing the oxide layer that forms almost instantly on its surface can make it no longer stainless.

Can anyone explain why SS does that, and what can be done about it? Epoxy won’t stick to that oxide layer very well, so I usually paint on my epoxy & then sand the SS through the epoxy. Since there's no free oxygen, there's no oxidization. This gets me a good chemical bond (as well as providing a good mechanical key). But if this is going to cause corrosion, how should I treat the SS blades?

Any help much appreciated!

[allanpeda]

I'm no expert, but I read that stainless alloys are not actually inert; they rely on a microscopically thin oxide layer (passive film) to protect the surface from further oxidation. Consequently stainless must be exposed to a certain amount of oxygen to resist corrosion. If the surface becomes anoxic, the protective coating is depleted and it rusts. Honest, I saw the photos, and I do recall rusty stainless by tight fittings.

I'm sure people will argue up and down about painting or coating stainless. My gut says to leave it alone unless there is some compelling reason.

[daddle]

You can paint them, but they're pretty much just like steel rudders then, which is fine. Or leave them bare and clean them twice a month. They'll rust some if there's a portion that is at the rest waterline.

I wouldn't bother with the fancy epoxy work. I'd just prime and paint as one would any steel hull bottom.

The passivation of the chromium, that is the nice shiny layer, of s/s only persists at certain oxygen levels and pH conditions. It's designed for exposure to clean air. Other conditons favor the oxidation of the iron, which is rust.

[osirissail]

It sounds like you are having made Stainless Steel FLAT PLATE rudders, very much like those used by power boats.
- - These are very inefficient rudders especially on a sailing vessel. If you operate the boat as a "power boat" using the engines and differential thrust for steering then rudders perform only a minor function.
- - But operating as a sailing boat you should have the NACA airfoil - shaped rudders. The shape of the rudder is very important. The curve as you work aft from the leading edge of the rudder is determined by the NACA airfoil. If you look at other sailing vessel rudders, you will see how the rudder is shaped very similar to an airplane wing. This is necessary to be able to have good control of the vessel while underway without the engines.
- - Normally, replacement rudders for sailing vessels are made with a stainless steel rod - rudder shaft - and then a flat stainless plate or web is welded to the rudder stock. Finally a fiberglass "shaped" body of the rudder is formed by using foam to make the shape and fiberglass resin/cloth for a skin.
- - An easy way to make the shape is to find a "sister" vessel and get permission to make "mold" of their rudder.

[atoll]

apart from leaving the ss uncoated,which is not really an option due to fouling,suggest you treat them like cr10 steel and fit anodes direct to the metal to stop corrosion.

a foil shape,rather than a flat plate will help to stop vibration at higher speeds on the rudders,thickness shouldnt matter as long as they are strong enough overall.

[minaret]

Originally Posted by osirissail

It sounds like you are having made Stainless Steel FLAT PLATE rudders, very much like those used by power boats.
- - These are very inefficient rudders especially on a sailing vessel. If you operate the boat as a "power boat" using the engines and differential thrust for steering then rudders perform only a minor function.
- - But operating as a sailing boat you should have the NACA airfoil - shaped rudders. The shape of the rudder is very important. The curve as you work aft from the leading edge of the rudder is determined by the NACA airfoil. If you look at other sailing vessel rudders, you will see how the rudder is shaped very similar to an airplane wing. This is necessary to be able to have good control of the vessel while underway without the engines.
- - Normally, replacement rudders for sailing vessels are made with a stainless steel rod - rudder shaft - and then a flat stainless plate or web is welded to the rudder stock. Finally a fiberglass "shaped" body of the rudder is formed by using foam to make the shape and fiberglass resin/cloth for a skin.
- - An easy way to make the shape is to find a "sister" vessel and get permission to make "mold" of their rudder.

I doubt they are plate rudders as they are 1 1/2 thick. Those rudders would weigh a ridiculous amount if they were plate! Sounds like they welded a SS skin onto an armature. Not sure why they went with SS below the waterline, since it is designed for exposure to air. Steel would have been a lot cheaper and just as good. If you leave them bare you'll have problems with fouling. So you gotta paint. Make sure you acid etch before priming. Makes a big difference. Also never coat cold steel, surface condensation will cause moisture to accumulate faster than you can wipe it off. In our yard we've had good luck using Interlux 2000 on steel below the waterline. You're not trying to prevent osmosis on a glass hull so you dont need as many coats, 2-3 will do it. Barrier coat is good for steel too...

[goboatingnow]

Stainless is really an appalling underwater metal. In the absence of oxygen it's actually worse them mild steel. Either leave it bare or go for a composite rudder which will be better by far.

In fact ordinary galvanised mild steel then enclosed in good paint layer is actually better then activated SS

I've never seen SS rudders in general for the above reasons.

Dave

[David M]

If you leave it bare then there is no question that the entire rudder will get oxygen. Paint it and after one ding in the paint there will be a question of whether or not there is an area that is exposed that is not getting sufficient oxygen. Also, the less painting, the less of a PITA is the maintenance of your boat.

The other alternative is to put 6 or more layers of an epoxy barrier coat (like I did with my aluminum research vessel) and hope you never hit something large and metallic which could damage all six layers. You also need to consider fouling on a bare stainless steel rudder. In that respect, having an epoxy barrier coat finish would be a better choice.

Overall, I would opt for multiple coats of an epoxy barrier coat...like Interlux 2000....so you can put an anti-fouling coat over it.

[osirissail]

I would agree that if the new rudders are made of metal skin with the proper shape for a rudder then stainless would be worst choice and marine rated steel would be best. Then you would use standard metal hull coating techniques to protect the steel.
- - But I would suspect that metal rudders would be awfully heavy when compared to composite/FRG rudders.

[minaret]

I've seen SS rudders built just like this before, skin welded on armature. But only little spade rudders on trailered power boats that live out of the water. They like shiny rudders...
Changing the foil shape of your rudders is bad IMHO. NA carefully designed them the way they were for many reasons...

[Jim Cate]

I won't comment on the design of the replacement rudder... not enough data presented. But all the bashing of s/s used underwater reminded me of the thousands of s/s prop shafts in use. I've had a few myself, some left bare, some anti-fouled directly, some primed with various you-beaut concoctions. They mostly grew stuff on them eventually, and none of them seemed to suffer from all the afflictions mentioned above -- no visible rust, no pitting (on the exposed portions of the shaft).

So why would a rudder be different?

Cheers,

Jim

[allanpeda]

Originally Posted by Jim Cate

... They mostly grew stuff on them eventually, and none of them seemed to suffer from all the afflictions mentioned above -- no visible rust, no pitting (on the exposed portions of the shaft).

So why would a rudder be different?

Cheers,

Jim

Somehow, no matter how simple the problem, I find that posting a question on a forum such as this inflates the complexity exponentially. It's almost like negative information.

[Jon Hacking]

Excellent answers everyone - Thank you! The expertise on CF is awesome!

Originally Posted by osirissail

It sounds like you are having made Stainless Steel FLAT PLATE rudders, very much like those used by power boats.

Osiris, these are being made as foil rudders, not flat. They'll be the same size & shape but where the original composite rudders were 3" thick, these will be only 1.75" thick. I was wondering how this would affect their steering capabilities?

Originally Posted by osirissail

Normally, replacement rudders for sailing vessels are made with a stainless steel rudder shaft and then a flat stainless plate or web is welded to the rudder stock. Finally a fiberglass "shaped" body of the rudder is formed by using foam to make the shape and fiberglass resin/cloth for a skin.

Agreed, and this is how the originals were made. But this puts the stainless that is welded to the shaft in Bending when the shaft turns, which is not the best thing for steel. By welding skins to the left & right edges of the shaft (instead of a web in the middle), turning the shaft will put those skins in Tension (for one skin) & Compression (for the other), which steel is very good at.

But even building up a composite rudder, the epoxy/glass/foam etc would need to be bonded to whatever SS "web" is welded to the shaft. So the SS inside the rudder would still not have oxygen around it. Would this make it corrode? How is this any better than coating the rudder blades & painting them?

As David mentions, we have found that coating underwater metal surfaces (saildrives, props, etc) in several layers of epoxy really helps the antifouling stay on. Even our props are clean (with our normal antifouling paint still on!) after more than a year. Rudders are a high-growth area for us, so I think we need to put antifouling on them. The question is how to do that & still preserve the steel as much as we can?

But I would suspect that metal rudders would be awfully heavy when compared to composite/FRG rudders.

I'd think so too, but our original rudders had no foam in them (solid resin mixture? with a glass skin). The SS ones will be hollow, so possibly lighter.

Minaret, thanks for the advice but we don't have to worry about condensation on the steel here! It's hot ALL the time

Atoll: ...fit anodes direct to the metal to stop corrosion.

Good point - I'd forgotten about that since our last rudders didn't need them.

BTW, we only lost 1 rudder so we have the other to use as a mold, but its shaft had some bad pitting (16 years old) so we're replacing both rudders.

[osirissail]

Originally Posted by Jon Hacking

. . . Osiris, these are being made as foil rudders, not flat. They'll be the same size & shape but where the original composite rudders were 3" thick, these will be only 1.75" thick. I was wondering how this would affect their steering capabilities?. . .

If the new rudders are only half the thickness - then unless they are also only half the physical size - you are changing dramatically the effectiveness of the rudder.
- - As an example, if you have seen how "fat" the wing of a small low speed airplane is compared to that of a high speed fighter - you can see how the "airfoil" shape makes a serious difference in whether the craft flies or not.
- - In the water, the same principles generally apply which is why sailboat keels are "shaped" and rudders are also shaped. And especially "Spade" style rudders. There are established ratio dimensions to any style rudder be it "barn door," "skeg hung" or "spade." They can be found in nautical design books. And you can then "shape" the SS skin to conform to the proper dimensions. This is normally done by cutting and welding to the rudder stock "ribs" that are cut to conform to the proper dimensions. Then the skin is laid over the ribs and welded together.
- - But the easiest way to "borrow" the proper shape is by using a fairly rigid cardboard and a knife and cut a pattern that mimics an already existing OEM rudder on another boat of the same size as the one your boat needs. Do both sides of the "example" rudder. Then cut the ribs that will used to "shape" the metal skin into the proper shape.
- - Since just about every FRG boat in existence - which add up to many thousands, if not tens of thousands - have foam cored FRG rudders, there is little worry about things like "tension" or "compression." Properly laid FRG over a properly shaped foam core is the easiest way to construct a rudder. Remember foam coring comes in many different "densities" from light weight filler to structural.
- - Installing a rudder that is "half" the thickness or worst, the wrong shape, will most probably result in half or less of turning performance. That translates to having to carry significantly more rudder deflection when sailing. That translates into generating significantly more "drag" due to rudder deflection which will slow the boat. Not a big deal in a power boat but a definite big deal in a sailing vessel.

[Hydra]

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