Rudder Post in Carbon or Kevlar

This thread is to demystify the design and fabrication of composite rudders. Hopefully, we can come up with some norms and a basic design that is adaptable for anyone and any boat. And, a process that is simple as scantlings are for hull design. Also, a way of building this that is doable by talent amateurs and professionals.

To start here is a design that meets the space parameters for my project. I'll post the rudder post now and the pivoting box and support box this afternoon.

This rudder will be under the boat not on the transom ... In a way, it is on a bulkhead with the hull wrapped around it.

The UHMWPE (plastic bearings) are 3.5" O.D. and 3" I.D. This is the largest size I can find available.

The boat weight is 8,000 KG fully loaded. The overall thickness of the rudder foil is 60 mm. I'll post other information next time current files are in the wrong format.

 

More images

The rudder can swing up 78 degrees before it contacts the back cutout.

The pivot point is not shown in these cut away view. The pivots will be plywood circles supporting 3.5 inch UHMWPE bearings. They will be screwed on to the side of the rudder post housing. This will allow the entire housing to be pulled out. Not sure how the shear and cyclic wear will effect the screws hold in plywood.

Note that the foam and fiberglass covering of the rudder is not shown.

The funny looking cowling is the foam stern impact protection of my catamaran.

Is it necessary to for the post/shaft to be solid? Anyone with ideas on how to layup and bag the carbon fiber shaft? The winglets can be added later.

Regards and thanks to all who participate.w the shear and cyclic wear will effect the screws hold in plywood.

Note that the foam and fiberglass covering of the rudder is not shown.

Is it neccessary to for the post/shaft to be solid? Anyone with ideas on how to layup and bag the carbon fiber shaft? The winglets can be added later.

Regards and thanks to all who participate.

 

G'day,

Certainly not solid as the torque needs to be taken by fibres wrapped axially. This can be done by wrapping a cedar or foam core, but this is not ideal. Best is a pressure mould, but this is not for the faint hearted and the risk of damage is too high for me to tell you how. We do build them in Holland and have just tested a similar size shaft. A 450 mm length deflected 2 mm under 10 tonnes load. We can also do the engineering if your designer can't/won't.

The ribs are not really doing much. You get a bigger bonding area and a lighter rudder by glassing pieces of glassed on the inside foam or ply onto the aft edge of the rudder and gluing the traiuling edge together, then wrapping double bias around the shaft and blade. Then add polystyrene (impact resistance) to the nose and glass round that and back to the trailling edge. Remarkably close to NACA 0012.

UMPWE bearings can be machined to any size and section, which gives more options for ensuring they are properly bonded in than buying off the shelf round ones.

Love the proa on your gallery page! ;-)

Any questions, let me know.

regards,

Rob

 

Thanks Rob for the excellent advice and direction. I am not ready to throw in the towel ... yet. smile

So as I get slightly more educated; I have found a formula that calculates the rudder diameter needed in bronze. Must have done something wrong because it came out to 110 to 90 mm. Can not be this big and heavy.

As for the carbon rudder post idea. Rob is dead on that they are hollow. I found a few sights that show completed rudders or how to wrap up the tube after the carbon has been laid. The wrapping technique seems a good way to go. The links below are to the techniques on the internet.

http://fossfoam.com/rudders.htm#top
http://www.proboat-digital.com/proboat/200508/ (go to Keels & Rudder)
http://www.grafil.com/grafilproducts.html
http://www.tohotenaxamerica.com/products/pls017.php (engr data)
http://www.solarcomposites.com/MakingShafts.html (heat shrink tube method)
can't find the spiral tape method. will post the link later.

 

G'day,

Wasn't trying to get you to throw in the towel, a carbon shaft will be a lot better than any other material. Scantlings are tricky. I use the thickness of the blade at 20% of the chord to decide the width of the shaft and put as much material into the mould as will fit. Little bit heavier, little bit more expensive (cheaper than paying for the engineering), but never broken one.

Wrapping is tricky. The fibres tend to get out of alignment and if you start with off axis fibres around a shaft, then reduce the diameter of the wrap, the fibres invariably kink, rendering them useless. You also need a layer of off axis between every 2 mms of lengthwise foibre, as well as in the first and last layers. If you do wrap it, make sure the off axis is the first layer, then put the 2 mms of unis on top, then wrap it. Then repeat for the next layer. Whatever you do, make a sample first, then test it, weigh it and cut it up and look for voids and out of alignment fibres. Send me some photos for criticism if you like.

regards,

Rob

 

Advancements

After a lot of surfing and with the sound advice of Rob, I have these files to post. The files are in .pdf so that the details of the drawing can be read.

The "rudder calcs. pdf" file has the front and profile view of the rudder. The file has all information including rudder bearing info and NACA 0012 offsets.

The "rudder stock DIM.pdf has the cross sections of the rudder post. I need to list the surface areas and the wall thickness of each cross section. Also need to add the offsets for the rudder stock mold and foam core.

To build the rudders, the carbon fiber tow and glass biaxel will be wet layed on the core that is mounted on a rotating jig. After the lay-up is done, it will go into a mold and vacuum bagged. Then the foam can be cut with templates drawn from the cross sections. These can be glued to the rudder stock and then draped in glass and then wet out.

Still much work to do.

Thanks again Rob.

 

I'm just wondering how you would make the layer of tow to be a bit even, not thick here and thin there?

And how will you put on the biax on the shaft, spiralling or straight down with a seam?

 

layup

I was planning on using 10 oz bi axel fiberglass for the off axis. Will try laying it over and around the shaft with an overlap and reversing the side of the overlap each layer. The final layer to be a sock of biax.

Like the idea of spiraling on tow over the shaft. It would most likely take two talent people ... which is one and a half more than what I have available. smile

From what I have seen. Tow is a flat ribbon shape of fibers. I played with some from West Systems several years ago. Kinda like pulling tape off of a roll. When it comes off it is straight and even ... the more I mess with it the more unorderly it becomes.

Do not remember how easily it cuts with shears ... after it is wet out. Rob will probably provide wisdom for us. (thanks in advance)

 

G'day,

Be careful about crimping the off axis, otherwise your proposal should work well. set up the shaft so it can be turned and spiralling works, but the biax (+/-45) is easier.

Our tow is much narrower and thicjer than the Gougeons. Wet it and lay it on. Squeegee it from the middle if you have to, but preferably do it right to start with. Cuts easily enough, but it is much less messy to cut the dry lengths, unless you have a wet out machine.

regards,

Rob

 

Nero,

Is there any update on your proposed rudder build? I'd like to discuss this with you but no contact info given in your profile. Drop me an e-mail if possible.

Regards,

Doug

 

update 2009

I have not been keeping up with boatdesign.net. I was loosing too much time on the internet.

I will be back in the states in 3 weeks. The carbon shafts are my first priority. I will post a few more picts then. The picts I took at the end of 2008 season were all dark and worthless.

regards

 

This been tried? :

Expanding mandrel:
Use a pvc mandrel - split it lengthwise. (Grease it and) cover with release film. Stick a bike tube in there. Wet out and pump up the tube. Stretches the tangential fibers.
Alternatively to putting it in a vac bag, if you need a perfectly round outside, cover outside of laminate with release film and "shade cloth". Drill lots of holes in a larger tube and split it. Slip it over the shade cloth. Wind lots of string around it to keep it together. Pump.
The laminate is squeezed between the two pvc tubes and the resin gets pushed out of the holes.