kubark42

How to properly design a rudder cassette?

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I've been looking around for information about how to (re)build a rudder cassette. I have an F27 trimaran and the aluminum cheek plates are showing their age. Too many years of salt in proximity of stainless have led to the rudder pivot point corroding enough that it needs some fixing. In fact, it's clear that this particular pivot point has already had replacement aluminum brazed on so it's clearly not a question of "it worked for 25 years, so don't fix a design which isn't broken". Plus, where's the fun in just redoing what was already done!

I CADed up a replacement made out of titanium, with carbon fiber stiffening reinforcement along principle stress axes. I have aggressively removed sheet metal material where I feel that there is little in the way of stress. 

The problem is that I have no idea what forces I'm designing against. When I consider how I've sailed the boat, and how little torque is actually transmitted through the rudder, it seems that the original design was massively overbuilt. However, I suspect Ian Farrier designed it for substantial abuse, like sliding down a steep wave in heavy seas, and so I've never come close to pushing the rudder to structural limits.

Here's the original (colored lines are for diagramming how the uphaul/downhaul lines run):

1158878753_RudderLines3Paint1.thumb.JPG.1bbe29a14e0fb51be6a40e9b0196b724.JPG

Here's what I've got so far, with a proposed 0.04" titanium sheet replacing the original 0.19" aluminum sheet: https://cad.onshape.com/documents/1c5a22f10a09417ce492546d/w/6205af308346e6f338843ca6/e/478becd53cd210a595543a50. That's a huge difference in thickness! Of course I'm making up for a lot of it with carbon fiber, but I'm still skeptical that I can just magically get away with using 6x less material. Still, it's easy to imagine that in a day and age of CNC water-jet cutting and (relatively) cheap carbon fiber we can do a lot better than 1989.

Does anyone know of a resource for spec'ing design loads? What is the rudder cassette's main job: to be super stiff for sailing feedback or to be super strong for taking that once-in-a-lifetime hit?

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1) Get mechanical engineering degree

2) Design rutter.

or 

1) Take a picture of one that hasn't broken and copy

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5 minutes ago, dash34 said:

1) Get mechanical engineering degree

Done.

Quote

2) Design rutter.

The rudder is still in good shape, so I just need to rebuild the cassette. Fortunately, I was able to get the original factory dimensions for the cassette cheeks and those are integrated into the CAD. It just seems like a waste of an ME degree to clone when we could challenge past assumptions.

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How about reverse engineering it to get the forces?  Take the existing cassette, and apply an increasingly larger deflection force until the cassette breaks.  Then multiply by 2 or 3 and calculate whether or not your design can handle it.  If your design isn't strong enough, decide whether a factor of 2 or 3 is really required.  

It will always be the dynamic loads that you have to worry about in this situation, IMHO.

Bob Perry can probably help with the estimates here. There is probably an algorithm in yacht design for estimating the rudder forces.  

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Here's a WAG.

Start with the maximum force you can pull on the tiller, balanced all by a force on the trailing edge of the foil, say at 2/3 of the immersed depth.

Multiply by some safety factor.. 3 or so.

 

I wouldn't worry about hits.. it's pivoting and all the impacts will be on the leading edge. Downhaul is a safety fuse.

 

Why not just rivet some more plate around the worn pivot holes, or drill the worn holes out and put some bushes in there?

 

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7 hours ago, kubark42 said:

The problem is that I have no idea what forces I'm designing against. When I consider how I've sailed the boat, and how little torque is actually transmitted through the rudder, it seems that the original design was massively overbuilt. However, I suspect Ian Farrier designed it for substantial abuse, like sliding down a steep wave in heavy seas,

Hm, not so sure that the design of the cassette has been analysed deeply in terms of mechanical strength. Many time the desiigner just take something that looks sturdy enough, which it sometimes is. Lasted 25 years, with some repairs. That's OK. 

In your case, such thin sheets you are considering - sure they will not start to wobble? Probably have sufficient strength but should also be stiff. 

//J

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Read this article. While it's about emergency rudders, it gives you the fundamentals for designing and building a simple rudder, rudder cassette and upper and lower pintles and gudgeons.

The only real challenge in the whole thing is choosing a maximum speed for the boat, which is necessary in order to calculate bending moments.

 

 

 

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10 hours ago, dash34 said:

 

2) Design rutter.

 

Don't need a stinking degree to design a rutter. Just get a consult from Hotrod...

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I'm not sure that adding that carbon fiber to re-inforce the titanium is a valid concept. They are have such different characteristics that load sharing will probably not take place in the manner you assume. The Ti is far more flexible and will be nowhere near its yield before the very stiff and brittle carbon fiber has reached its limits. There is a great description of this which uses the example of lifting a heavy weight using a length of nylon rope in conjunction with a very low stretch rope such as Kevlar. Neither can lift the load independently and you would think together they could but the nylon stretches before it bears its expected share of the load and the Kevlar one still gets overloaded and breaks after which the nylon one fails as well. I wish I could find that example as it was better worded but I think I got the point across.

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1994 ABS classification for yachts (yes, I know it is considered obsolete now) has a section on rudder loads in Part 3 (pg 215) which may be of some help: https://ww2.eagle.org/content/dam/eagle/rules-and-guides/current/special_service/62_yachts_2018/yacht-part-3-oct18.pdf

You can also try "Principles of Yacht Design" (Larsson & Eliasson).  They have a lot of stuff and probably have a section on rudders, but I can't recall for certain.  It's a good read regardless.

While what Rasper says about paralleling different materials is true, it is mainly true for materials with widely differing Elastic Modulus.  So  E-Glass and carbon don't work together well mainly because the E of carbon is so much greater that the carbon bears somewhere around 80-85% of the load.   IIRC, the E of Titanium is about 110 Gpa while the E for standard UD carbon is about 135 GPa.  http://www.performance-composites.com/carbonfibre/mechanicalproperties_2.asp  Note the numbers provided on this chart are for epoxy composites cured at 120F.   In this case, while the carbon still bears more, it is only about 55%.  Not too bad in my estimation.

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- The old offshore yacht ABS guide is notorious for underpredicting rudder forces that design rudder stocks. Multiply x3 to get something realistic for a fast boat like a F27. It's acceptable (barely) for older displacement hulls that never surfed. But did give torque and B.M. for design. You can use them as a starting point for loads.

- don't mix Ti and carbon unless you know what you are doing and the particular element is designed to solely carry the load

- the 0.04" (1mm) Ti sounds like local buckling will be your friend :)

- are you aware of the challenges of welding Ti?

- if I was starting from scratch I'd do an all carbon cassette trunk. Or all Ti.

<OK - went and looked at your design. You're a metal guy aren't you, without a lot of composite experience? The carbon used in that manner (bonded to Ti??) is not a good use of it.

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5 hours ago, Zonker said:

- The old offshore yacht ABS guide is notorious for underpredicting rudder forces that design rudder stocks. Multiply x3 to get something realistic for a fast boat like a F27. It's acceptable (barely) for older displacement hulls that never surfed. But did give torque and B.M. for design. You can use them as a starting point for loads.

- don't mix Ti and carbon unless you know what you are doing and the particular element is designed to solely carry the load

- the 0.04" (1mm) Ti sounds like local buckling will be your friend :)

- are you aware of the challenges of welding Ti?

- if I was starting from scratch I'd do an all carbon cassette trunk. Or all Ti.

<OK - went and looked at your design. You're a metal guy aren't you, without a lot of composite experience? The carbon used in that manner (bonded to Ti??) is not a good use of it.

No titanium welding required, this is a pure sheet with holes cut via water jet. The allure of this project is that I can experiment with titanium for (almost) the same amount of machining effort as aluminum.

I wouldn't call myself a metal or composite guy, I've got some experience with both but not enough experience with either to optimize a design. I've got some materials at hand and am working with them. If after analysis they won't work for the job, then I'll have to find a different solution. Tough to tell without the design requirements.

The role of the titanium in this structure is 1) to give me a very flat surface to work against, so I can experiment with CF layups and not have to worry about putting holes in the wrong place and 2) to have something which won't explosively and irrevocably fail. If the titanium gets wrenched out of alignment, I can probably work something out to get back home.

I'll read up on the ABS guide @12 metre linked to, and keep in mind that it needs some SF for the fast boats. I'm almost scared to calculate what the forces are for fear of discovering that even the original two aluminum plates are completely underspec'ed!

I'm still hazy on whether the principle design feature is to be stiff enough or strong enough. I'm leaning toward strong enough based on what people have said here, but @Jaramazasks a fair question about wobble.

 

UPDATE: I'm sailing tomorrow and there's some good wind so I'll be able to get the boat in stride. I quickly went over the ABS guide and calculated .132*1.33*1.1*1*.3*10^2 ~= 6kN at 10kts, and .132*1.33*1.1*1*.3*17^2 ~=17kN at 17kts.  I suspect that these numbers are the worst case scenario for deflection at peak lift. I wonder if it makes sense to design around that, since the boat is very much not going to go that speed if the rudder is deflected over like that. The whole boat weighs something around 1500kg, so a 17kN force would be over 1G of negative acceleration. Ooof.

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22 minutes ago, kubark42 said:

I wonder if it makes sense to design around that, since the boat is very much not going to go that speed if the rudder is deflected over like that. 

bzzz.

Consider the worst scenario -- that's what you're designing to.

Trying to save a broach during a fast surf down a wave, for example.

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Hi Kubark,

 I recently got to look closely at an F-27 rudder and I think that the geometry and case are quite well thought out. I have quite a bit of kick-up rudder design experience and I just built the set shown in the photo. Aluminum plate walls, bolts, and spacers make quite a bit of sense for the purpose. There are a few different loads in kick-up rudder cases and some of them are difficult to address with composites. Cassette style rudders less so, because it's easier to keep the walls of the case from being forced apart.

DSC_1699.jpg

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I understand the point about worst-case scenarios, but you'll never be going fast anywhere if you have 1.1G of decceleration. That's only 2 seconds from 17kts to a full stop. That kind of deceleration would throw people off the boat.

The assumptions which led to this calculation are probably dependent on things such as the rudder staying fully submersed and the boat speed not changing much due to the drag, like a big heavy keelboat going down a steep wave. 

Not saying that the forces aren't high, just that IMO the original cassette design likely wasn't for anything approaching 17kN. The aluminum plate -- 3/16" with 100% of the lateral load going through the pivot point-- can only hold 2.5kN or so of shear before failing. There are two plates, so we get to 5kN if the load is perfectly distributed and the aluminum has undergone no corrosion. 5kN doesn't even get to a design speed of 10kts, a speed we exceed on almost every sail.

Considering that F-boats have a reputation of being extremely well built and sea-worthy, and that Ian Farrier designed them to sail in coastal waters not deep bluewater, it seems fair to say that the ABS standard is not appropriate for a light trimaran. It also seems fair to say in light of comments here that the aluminum plate was not particularly overbuilt.

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Just make it and go and see if it works. Itll likely bend before it breaks so you can nurse it home if it fails. 

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First off, the L/D coefficients that classic texts show for aerofoils are steady state. Transient Cl just before/after stall can get a lot higher. ABS doesn't think or know about this. And they used to underpredict boat speeds.

Yes, the worst case (I think) is a big surfing broach on the top of a wave where rudder is hard over, boat is going one way, wave is going another and the rudder is at stall. Don't have the paper on hand but I've think it was Cl was approaching 1.5 - 1.75 or something. Don't assume you're decelerating the whole boat - you're just loading the rudder up sideways.

So I don't think a 17 knot velocity is out of the question.

And no I don't think the cassette is overbuilt. Ian was a bright guy.

How's your insurance if your rudder cassette design fails? (semi-serious here, you don't seem like you've done enough of this stuff to safely design it, and mixing Ti and CF in the way you are considering is just bad practice.)

I'm a M.Eng/NA with about 30 years experience. Designed a lot of stuff in metals and composites...

 

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That's my understanding, too - that the maximum bending moment is based on some maximum speed and occurs when the rudder blade is dragged sideways through the water. That's the case analyzed in the article I quoted, above - and the approach I've seen used by several naval architects. That said, multi hulls are different beasts and I'm unsure of how applicable it might be.

Why not give Melvin and Morrelli a call, they'd be happy to give you a few rules of thumb.

 

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plus a bit or quite a bit for hitting stuff and or the bottom

as a rudder built to just hydo force limits is real world weak

what if you back into a sand bank so it can't pop up

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Actually it was problematic: post too small, broke, doubled in D and carbon. Also fuse was overloaded constantly, looking to upsize to a 3/8 or + steel pin; total absence of bearings so changed in to a graphite/glass lower and delrin upper; linkage doesn't really like to be tilted even with a couple of universal links: tiller/1/2 quadrant was glued on so couldn't be disassembled/serviced, changed to bolt and key; just replaced the whole thing, hope it works. But conceptually being able to raise it is nice, just think the Farrier design is less problematic and cleaner though I've fixed those, too. Not sure the underslung rudder is worth it, I like the French designs.

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