Advice needed:making carbon rudder stock

The aluminium stock of my 42ft racer / cruiser suffered severe fatigue and broke off - about 260 miles short of the small Caribbean island where I currently am. I initially considered making another rudder stock from alloy and even upsizing the lower bearing but decided against it because of all the accompanying corrosion problems. I now want to make a rudder stock myself from carbon and would appreciate advice with some of the calculations and specifications. I can provide all the rudder foil design specifications and dimensions to anyone that could possibly help me. There is a 4700kg loading onto the main lower bearing which is 88mm in diameter. I would like to commence the building process as soon as possible - it would be nice to sail out of here before the start of the hurricane season!

 

Carbon fiber rudders are usually built up from a pre made stock.

There are a number of mfg and conversion tables for the streignth required.

Carbon fiber needs to be transported frozen , and post cured after construction. Which is why its cheaper & far less risky to purchase the tube.

The past fre issues of Pro Boat Builder has had good rudder articles , might be on line?

FAST FRED

 

If you've never built complex carbon parts before, this is a really tricky one to start with. I'd look for a prefabricated tube of the correct dimensions, rather than build your own. This way you are assured of a solid layup with no voids. I'm afraid I can't help with those calculations at the moment, sorry.

 

I would like to differ from Fast Fred and Marshmat. Carbon fiber rudders are usually custom designed and built. You cannot rely on a pre-made tube to have anywhere near the strength and stiffness required for a rudder. If you have never worked with carbon fiber before, I would not use a rudder as a first-time experience. Only carbon pre-preg needs to be shipped frozen; dry carbon for vacuum bagged lay-up (room temp or heat cure) can be shipped in a normal cardboard box.

I can recommend a few builders for making your rudder, in no particular order:

Phil's Foils, in Ontario, Canada, near Ottawa.

GMT Composites in Bristol, Rhode Island.

Composite Solutions in Hingham, Massachusettes.

The latest article on rudder and keel design and construction that I wrote appeared in Professional Boatbuilder magazine, issue #96, page 72. The entire magazine is available on line at www.proboat.com.

I hope this helps.

Eric

 

Carbon rudder stocks are usually not round. They are rectangular shaped with the long sides forming part of the profile section. The easiest way is to start with a foam former and then laminate the unidirectional plies and plies at +-45 degrees for shear and torsion. You would have to know how thick the laminate ends up to be to shape the foam former correctly. Then you stick foam formers to the finished stock for the front and back of the rudder and overlaminate to build up the skins. I think the minimum equipment you need is a vacuum pump and laminating and vacuum bagging experience. Most of the custom stocks are made in pre-preg because you can apply all the plies at your leisure and then cook the whole thing when it's all done.

One show stopper might be your lower bearing. 88 mm is definitely on the small side to fit a rectangular stock through the hole. For a boat your size you would probably want a stock 100mm x 50mm roughly. The minimum bearing size is therefore 120 - 140mm but it depends alot on the profile thickness at the top of the rudder.

By the way carbon can also cause galvanic corrosion. You will have to insulate your bearing from the stock. Otherwise the bearing will slowly turn to dust.

Why don't you just get a stainless steel stock? Stronger than the alu stock that broke and also less problems with corrosion. If you want to do the whole carbon upgrade including larger bearings I would let some profissionals do the job. The materials are too expansive to waste and if the engineering is not right even a carbon stock will break. An experienced company will do the engineering in a day for you and come up with a laminate schedule including thickness deductions for the stock foam former.

 

That's IF you're using the prepreg.

Carbon fiber is also available as cloth without the resin.

 

Thank you all for responding and for your advice. My apologies for the delay in my response - since having read Eric Sponberg's article in the Professional Boat Builders magazine and Karsten's comments, I have further queries.

Firstly I must mention that I wish I could hand this problem over to the professionals and get them to make a new rudder and rudder stock but finances do not allow me this luxury. Fortunately I will be assisted by somebody with considerable epoxy experience.

It has been said that the rudder stock is either square or rectangular in shape. Why does it have to be like this? Is this for strenght reasons or for weight advantages? Why can the lay-up not be formed to form a solid stock in carbon - with the same shape and size of the previous alloy stock and then build the rudder foil around this with some suitable internal attachments? I attach hereto a drawing of the stock and rudder foil.

I would also highly appreciate advice with regards to the lay-up schedule of the unidirectional and the 45 degree laminates.

316 SS would not be option for me. It has the same strenght value as the alloy rudder stock I had that broke. I would therefore have to go for a much higher grade SS but this would be rather heavy for my boat which is 42ft but only 6000 kgs.

 

The stocks are hollow or filled with foam because a solid stock would be much heavyer and more expansive. Filling the void with carbon is a very expansive option if air costs nothing. The stocks are rectangular because this is the most efficient shape. You want to make the capping plies as far apart as possible. The unidirectional capping plies are interleaved with the shear plies. The forward and aft end of the stock only consists of the shear plies.

Because a thicker stock is much more efficient it is usually placed at the thickest section of the rudder. If possible the aft edge of the stock should be straight for easier construction. Sometimes it has to be kinked at the lower bearing depending on the rudder shape.

Attached is a sketch that shows a typical rudder.

The last rudder I did was for a 42' racing yacht with the folliwing data:
Displacement: 4.4t
Rudder chord @ upper end of blade: 335mm
Blade thickness @ upper end: 50mm
Blade length: 1760mm
Load on lower bearing (ABS): 6.7t
Bearing diameter: 160mm
Capping plies max 36 x 300 g/m2 approx 100mm wide
Shear plies max 8 x XC400 (400g/m2)
All carbon in prepreg and vacuum consolidated

Your rudder has a much lower aspect ratio and is thicker. The capping plies could be made much narrower for the same strength. Not quite sure if a hollow rectangular or nearly square stock would fit your 88mm bearing. Some rudders are also stiffness critical especially if they are thin with a high aspect ratio. The latest supermaxis used high modulus carbon for this reason. Most stocks have to be moved as far aft as possible at the lower bearing to increase the separation of the capping plies and to avoid the starboard and port capping plies overlapping at the forward edge because of the profile taper. This is why you end up with quite large bearing diameters.

Cheers,
Karsten

 

When using aluminium rudder stocks, I use an extra large diameter and machine the sides flat for the part inside the rudder. Much cheaper an easier than carbon.

 

I have built many carbon rudders and yes 100x50 is a pretty good start point.
Points to note are that it needs at least to be vacuum baged and also a post cure to cycle the resin to optimum loads. Not an easy introduction into carbon work.
What grade of alloy did you use?
It is probabbly (or should be) at least 6061.
We have made good alloy rudder stocks from 6082 alloy which is far better.
8000 series alloy is even better but cannot be welded and must be anodised.
It has been used but care must be taken.
see www.jefa.com for an interesting site??