Rudder shaft

Hoping to get some feedback from the group on rudder shaft sizing.

Looking at changing from transom hung rudders to spade rudders. Simply to free up the transoms as the boat is a cruiser.

Thinking of using 25mm OD with a 3mm wall shaft and utilising PVC conduit glassed against a bulkhead with a tiller on top to be joined in the usual way.

The bulkhead is 800mm high so there will be plenty of support, the aim of the PVC is to assist with load bearing.

 

Hi Cookie,
You will hardly get a sensible reply if you don't tell us what boat are you talking about.
Boat type, size, weight, speed and rudder area - things like that.
Cheers

 

This is a representation of the forces, moments and efforts to act on a spade rudder. Which means that while you have a bulkhead 800 mm high, which you should concern more is the resistance at point "A" and bulkhead, there can not do much.
The second image shows a possible solution, but there are others.

 

Sorry, internet is playing up and I somehow managed to overtype a paragraph!

The boat is a 25ft cruising cat weight is 1000kg (looking Ok so far....)

Rudder height 410 High, 330 wide with the usual profile. 40mm thick at the widest, balanced spade style

(Original design is transom hung 700x220x30)

Height is limited by mini keel depth.

The bottom point will have a bushing for load.

Hope this helps!

 

25x3 tube is in my opinion too small to carry the loads of a spade rudder on a 25 ft cat. Spade rudders are more loaded that transom rudders, hence the shaft has to be beefed up with respect to the original design.
Can you tell us what are the typical max. speeds you have attained on that boat, what is the diameter of the original rudder, and what will be the approximate immersion H1 indicated in the attached pic?

 

Yes 25mm is too small. I'd expect at least 32mm, but without knowing more about the boat I cannot say anything more

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks all for the input so far, as it is a new boat I am guessing but would expect arpund 10 knots with brief excursions to 15

 

As TANSL pointed out, the highest load is located just above the rudder. If you don't want to use a solid shaft because of the extra weight, fill that part of the tube with a tight fitting steel plug or weld a piece of solid shaft to your tube.

The PVC tube glasses to the transom is good to keep the water out, but as a bearing for the shaft it doesn't qualify.

 

I'm thinking of going to solid, weight isn't overly critical.

I should have explained the pic is more to act as a tube to avoid stuffing boxes etc, the rudder will have a bushing to run on

 

With a 40mm wide rudder section I would be looking at what I could get at just less than this size - like Richard said about 32mm. I got 50mm wide 6mm wall seamless pipe (or tube) for my 38ft cat. If you could get something close to 32 - 35 with a reasonable wall thickness from somewhere close and easily sourced I would go this route.

If you cannot then there is another way to go. Just near you in Launceston is the designer of my cat Robin Chamberlin. On Big Wave Rider ( a super fast Chamberlin ocean crosser) Robin and the owner got some cedar, put it on a lathe, wrapped carbon around the turned cedar to oversize and then put it back on the lathe with very sharp tools and turned the carbon perfectly round again. The good thing about this is that you don't have stainless underwater (stainless really needs Oxygen and can get pitting underwater) and you can easily attach the webs to the still square section below the rudder tube. You can also taper the laminate. Of course you don't have to make the round sections out of carbon - E glass would be fine for the turned round sections as it is just a bearing. Add graphite to the resin mix for extra slip.

You don't even really need the lathe if you want. Make up a square rudder stock out of cedar and uni. Then get overly large inner bearings made to be round on the outside and fit your square section on the inside. French Vendee boats do it this way. Again easy to make all your own rudder and webs if like me you are fine with composite and can't weld at all. Still need rudder tube bearings too. I like Tufnol rather than Nylon as Tufnol epoxies well and doesn't expand in water like Nylon.

To make the rudder tubes I just waxed a tube of the correct size. Glassed it with a thin layer of glass, slit the glass with a single knife cut, removed the tube and then glassed away again until I was happy. Big tube and big bearings work better with less pressure for same force.

Some quick calcs

Second moment of area for 35x35mm square section= 125 000 mm^4
Second moment of area for 40x40mm square section= 213 000mm^4
Second moment of area for 32mm tube 4mm wall = 35 000mm^4

E cedar - 8 GPa
E stainless - 200

EI for tube rudder stock = 7 000 000
EI for 40 mm square = 1.7 000 000

So you need to add lots of stiffness to the wood stock to be equivalent. Add enough carbon/glass and the wood won't be more than a core.

E glass = 75 GPa
4mm glass on sides of cedar with 2mm front and back I = 115 000mm^4
EI = 8.6 000 000

Even less laminate required with carbon. Caveat - above calcs done late at night and I am a physics teacher/sailor/part time boatbuilder - not a composite engineer

cheers

Phil

 

Thanks Phil,

Some great info. Big wave rider is certainly an impressive boat!

The cedar (I have plenty of Oregon I could make a nice sized shaft from, also what I was planning to make the rudder blades out of) with uni certainly sounds within my capabilities. Any suggestions on the uni? I have plenty of 460gsm already (it was cheaper with freight to buy complete rolls than the lengths I needed)

Never having used Carbon I certainly feel more comfortable working woth epoxy and uni. (Obviously not that different but a big expense for something to learn on with such a critical part)

 

I agree with Richard Woods. According to my quick calcs, a 32x3 AISI 316L tube will allow you to pull the tiller hard over at the max speed (15 kts) without having to worry about bending the shaft.
A 25x3 tube will let you do that maneuver up to 10 kts approximately, but above that speed you will have to be more gentle with the tiller.
At this point, you have several solutions to choose from, so choose wisely.
Cheers

 

Cookie as I said before I am not a composite engineer although I have designed and built some cats (up to 7.3m)

If the boat were mine I would go ahead with composite shafts because I am getting right into everything composite. My folding cats have all composite hinges, chainplates etc. My 38ft cat has composite chainplates, composite genoa tracks and even the motor nacelle is held on by just two composite hinges that are in essence pretty lightweight. My solar panel mount is made from in part composite surf boat oars. Light strong and homogenous.

As you are close to Robin I would try to track him down and have a chat. I love the boat he designed for me and Robin is a seaman of the first order. There are very few designers with anywhere near his record or number of sea miles and he will probably sort you out for a very small fee - or a few cups of tea.

That being said if I was to go composite I would work out the largest in hull tube I could do and go square section with round outside bearings. As I have calculated above with 4mm of uni on the sides of the square and 2mm front and back you should exceed the strength of the stainless tube. The problem is that you need to really pack the uni on tightly so I would pull it along the sections and then wrap it with peel ply to consolidate it. Also I may have the E of the glass incorrect. I haven't done the experiment to check. A friend who is building a composite mast has to send the designer some samples so the designer can check the properties of their laminate. This can be more work.

Rudders are something you really need so doing something different is a bit way out. I bent one of my rudders last year. If it was composite it would have stayed straight or broken. If I lost my rudders I would make them totally composite but that is for me. As Dacquiri says tread carefully

Cheers

Phil

 

You could use an oval composite as it would match the rudder profile better. You can also make stocks from aluminium, but they break rather than bend

A solid stock is a waste of weight, the middle isn't doing anything. Your rudders are quite fat. Try to avoid mixing materials, especially below the WL. If you put carbon, stainless and aluminium together you can run your nav lights from the electrolysis

Richard Woods