Rudder Materials & Construction

The area of the Help & Advice forums where requests for help about the centreboard & rudder of an Osprey can be posted. Additionally, if owners or others have hints and tips that might be of benefit to others they can be posted in the forum as well.

Rudder Materials & Construction

Postby No Idea » Thu Jan 17, 2008 4:20 pm

The material of construction and all design details for a rudder, rudder stock, tiller and tiller extension are completely at the discretion of the owner, i.e. the owner has free reign as to the shape, lifting (pivot or daggerboard) or fixed, wood, GRP, FRP or carbon fibre (fiber for our North American readers ;) ).
No Idea

Re: Rudder Materials & Construction

Postby No Idea » Sat Nov 21, 2009 12:31 pm

Had a good look at my rudder stock and my rudder blade.

I dont think its possible to modify them so the blade will go vertical and stay there.


Can anyone post some dimensions for a good rudder stock and blade made of wood or fibreglass?

Many thanks
No Idea

Re: Rudder Materials & Construction

Postby No Idea » Fri Nov 27, 2009 8:11 pm

Im struggling.

Ive got some free time as I cant fix my damaged hull because it keeps raining on it.

I would really like to make a new rudder in the next couple of weeks so it can have a chance to harden before the rain stops, the hull gets fixed and I next end up sailing it.

To do this, I need the sizes to cut it.

Please, Is anyone sitting within reach of a wooden mk 2 or 3 vertical setting wooden rudder?

Could you measure it for me?

This is what I will need:-

rudder dims.jpg

I will be happy with sizes in metres, yards, or even inches or centimetres.

Many thanks
No Idea

Re: Rudder Materials & Construction

Postby Graham Thumwood » Sun Nov 29, 2009 6:16 pm

I'll take some measurements tomorrow at the workshop (I realised I have Mk II, III and IV Ospreys in at the moment) but are you sure the existing rudder cannot be adapted? Normally if a rudder refuses to go down vertical simply removing the pivot bolt, lining the blade up vertical and then drilling a new pivot hole through the blade will sort it. By the way surly the thickness measurement you are asking for all depends on your rudder stock?
Graham Thumwood
Posts: 90
Joined: Sun Jun 22, 2008 4:24 pm

Re: Rudder Materials & Construction

Postby No Idea » Sun Nov 29, 2009 10:58 pm

Many thanks.

I would take a guess from your earlier comments that the blade I have is an origional. It is also cut to be flat at the bottom like your oldest blade. As not having the blade exactly upright upsets the balance of the boat somehow, I would surmise that it must create a little lift at the stern when it kicks up, which adversely affects stability. I am guessing here, as everyone agreed a kicked up blade would do this, but no one actually told me why lol.

Following on from that, if I were to stand my blade up, the bottom of the blade would then be facing the front of the boat and would be offering a 45deg 'foil' with its old bottom edge.

My Rudder stock is coming apart, a bit like the rest of the boat, so if the later ones have thicker blades, I will build a new stock to go with the new blade.

I intend to use 2 or 3 thicknesses of marine ply and sheathe it in layup resin and matt.

Looking forward to having the dimensions so I can start building and posting some more pics!
No Idea

Re: Rudder Materials & Construction

Postby Graham Thumwood » Mon Nov 30, 2009 9:50 pm

O.K. - measured a couple of blades today. One was 830mm long from pivot hole to tip and 230 wide, parallel from top to bottom. The other was 910 long and tapered from 260 wide just below the stock to 170 wide at the tip. The alloy stock had a 25mm wide slot and the blade appeared to be about 23mm thick but I was measuring with a grotty tape as I am in mid workshop move and couldn't find my callipers...

As to the section through them I suggest you spend a happy hour or twelve googling NACA foil sections - I know I have spent far too long doing this in search of the perfect foil shape! (I'm erring towards a NACA 10 or 12 at the moment if that helps!) Good advice on foil construction can be found on the Cherub dinghy website - I think I have posted a link before when someone was asking about foil construction.

Hope this helps
Graham Thumwood
Posts: 90
Joined: Sun Jun 22, 2008 4:24 pm

Re: Rudder Materials & Construction

Postby No Idea » Wed Dec 02, 2009 9:11 am

Thank you Graham, that makes it soo much easier thatn guessing!

I would have instinctively gone for about half the length and an extra 100mm in width!

The gap in my rudder stock is 19mm, which suggests to me that the blade probably needs the extra thickness around the pin to cope with the extra laods caused by the extra length.

Time for a couple of days of frever pitch activity.

Took the stock apart and cut another piece of marine ply to make the extra width.

Surprised at the little sheave built inside the stock for the rope.

Putting paper over where I need the ply to fit and pricking it along the edges with a pin

Paper is cut out, now Im checking to make sure it fits before putting it on the ply and drawing round it, and cutting it out.

I then cut 4 lengths of 6mm marine ply that had been stored outside and due to all the rain, shoved it in the kitchen on a radiator to thoroughly dry.
No Idea

Re: Rudder Materials & Construction

Postby No Idea » Sat Dec 12, 2009 5:16 pm

Cut out 4 sheets of marine ply for the rudder blade.

Mixed fibreglass up and added microfibres.

I coated the boards with resin mix, then coated them again with microfibre mix and stuck them together.

I put them all in a polythene bin liner to stop it leaking fibreglass everywhere and then clamped the boards together through the bag.


When it had set, I put it on top of a radiator to sort of bake it rock hard.


WHen I took the blade blank out of the bag, it had the green writing from the bag all over the blade.

Not quite what I had in mind.

No Idea

Re: Rudder Materials & Construction

Postby No Idea » Sun Dec 13, 2009 12:09 pm

I put a brick under each end of the the blank rudder blade and stood on it.

Ive been losing weight since I got pig flu and sort of stopped, but I think that was a reasonable test of my laminating.

It passed.

Carefully cleaned up the edges where the fibreglass had leaked out.




While doing this I realised I had made a blunder.

When I laminated and clamped the ply together, I forgot to clamp it to a solid, true piece of 25mm thick ply - which would have stopped it setting with a twist.

My kids took notes while I practiced all my swearwords.

Having thought about it a bit more, I think I will plane one corner/ edge until its straight, then machine it until its flat again.

Not quite sure how Im going to achieve that though.

Going to have to use some more inginuity or is that spelt bodgery?

Going to look at my electric plane, my bench and my router and see if I can work out how to machine it.
No Idea

Re: Rudder Materials & Construction

Postby No Idea » Sun Dec 13, 2009 12:24 pm

Still havent worked out how to machine the blank.

SO in the meantime while Ive been thinking.....

I spent some time looking at the NACA 12 or 10 that Mr Thumwood suggested.

Typed it into google.

Spent some time rummaging through the results.

Found that there are free programmes that allow you to plot the shape of these curves.

Managed to download and install a programme called the NACA Generator that would calculate the points of the curve but would not print them, ot the curve unless you paid.

Jotted down the numbers, so I could plot it myself.

I use Serif Draw Plus to do all the line drawings I add to this site. It is an easy job to plot co ordinates on it.
The earlier versions of this programme are available for free download direct from Serif if you find yourself needing to use one.

Having plotted it , I printed itand measured it to make sure it is actually the size it should be. Then I glued it to a sheet of ply.


I think Im going to cut it out with a fretsaw and use it as a pattern to set the router to contour the blank.

I think!

Still havent worked out how to cut it out with the router yet. lol
No Idea


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