Rudder design. How?

[nathanlee]

The full explanation of my situation is here: http://onkudu.com/fitting-out-kudu/rudder-trouble/

But if you don't want to read all that, then the basic gist of it is...

I need to build a new rudder. The old rudder was a flat sheet of ply, which is of course very inefficient. I took a template off a fin keel Corribee's rudder, but it's too deep for my bilge keels. The design was also totally unbalanced, which I fear might make the steering too heavy.

I don't know what to do. Obviously I could just reduce the "draft" of the rudder, but I have no idea how this will affect the handling, or indeed where to go to find the information to work it out (if that's even possible for a layman).

Any help would be very much appreciated since this is the only remaining problem I have to overcome and if I don't start making progress with it soon, I'm in danger of delaying my launch date.

Nathan

 

[catalac08]

A good starting point for rudder proportions and dimensions is the vetus catalogue of stock spade rudders that they sell. When I built a balanced rudder in steel to replace the original on a Macwester 26 (which has an awful unbalanced and particularly low aspect shape) I used this as a guide and it worked out extremely well and the helm was feather light wheras previously you needed arms like a gorilla,. Although there was still weather helm the balance section rendered that only a very slight pull on the tiller in the worst case. Perhaps I was lucky but I do not think it is rocket science. I used 18% balance area forward of the pivot line. The vetus catalogue also used to show section diagrams for good flow over the rudder. also to help you with the profile.

 

[caiman]

I too have rudder problems.There is a lot of information on a site called Duckworks,and also Rudder Design & Centreboards by Gougeon Brothers.Sorry I can't post a link.
It's quite easy to get the offsets required to make a foil,just google NACA.
During my reading,I've seen the figure 18% ahead of the axis mentioned previously,I've yet to decide on mine but thats not far off.I have also read that a longer span(length) is more efficient than a wider chord(width).
You can get a lot of advice from multihull forums.Lots of people build their own boats and so building the foils is standard issue.I have had a lot of concise accurate help from one in particular.
My own rudder will be foam/timber sections around the original stainless steel stock to get the foil shape.Then laminated using epoxy,glass and carbon fibre.Over the Christmas holidays ,I experimented with vacum bagging using those bags you get from the Factory Shop for storing your duvets in,and a domestic hoover.The results where more than worth the extra effort.
Hope this helps.
Cheers

 

[William_H]

Rudder design

Rudder design is fairly simple. The best rudder is of an appropriate size with an aspect ratio (length to chord) of at least 5 to one. With a nice airfoil section. Fatter toward the front gives more resistance to stalling while thinner gives less drag.
However when you fit it into the boat more priorities dictate compromise.
You indicate limit on depth. either you fit a frangible section (able to be smashed off without damaging main part or pintles) on the bottom or you have a shallow rudder. A frangible section would be good if you do not go aground often.
Once the depth is determined then the chord dimension is dictated by overall rudder area. Just get an idea from the original or from other boats.
The figure of 18% will probably be fine for the balance. That means 18% area forward of the shaft. This will balance out 36% of the force needed to turn the rudder.

If the old rudder is firmly attached to the shaft you could use it as a basis for the new rudder. Add foam or filler to the sides front and back to get a nice foil shape then cover with polyester resin and f/glass.
You need to put a lot of fibreglass on so that it becomes a fibreglass rudder with wooden internal mold. Rather than a wooden rudder with f/g sheath. Make the f/g at least 6mm thick. delamination is not a problem then. So long as the shaft remains attached the wood can disappear and f/g is still strong enough.
if you want to try a frangible extension on the bottom. Stick foam to the bottom and carve to shape. Then sheath with a lot thinner f/glass. If it gets scrunched up on grounding only the extension will break not the rudder itself. Depending on how strong you feel the rudder and shaft are you might make the extension a bit stronger to handle a gentle grounding.

Without doubt the deeper the rudder the better the control of the boat. I added an extra 75 mm to the bottom of my already deep rudder. It made a huge difference in controlling the boat with a shy spinnacker.
Which brings me to my next comment from your blog. It is much more the total sail area and the heel of the boat that causes most weather helm. So a large jib (or spin) forward of the mast will not counteract weather helm as you might think but simply heels the boat causing more weather helm. Reduce the main will aleviate WH but so will reducing jib.
The WH is caused when you heel with the drive forward (sails) being way to leeward while drag (keel) goes to windward while at the same time the shape of the hull in the water becomes asymetric with varying amounts of effectt on WH. Hence some hulls are more susceptible to WH at a given heel angle than others.
The gap between the rudder top and hull should be minimised. This kind of rudder benefits a lot by being close to the under hull to stop eddies however a gap will allow eddies across the top giving drag and loss of rudder power.

I posted video of a sports boat regata at our club last weekend. It was interesting that these boats seem to have rudder and keel designed for hull speed of +10 knots. At lower speeds they seemed to be lacking in rudder power to counteract the huge spinnacker. and lacking keel area when they round up and slow down so leeway is huge. In this particular race style (short legs) though not obvious from the video I think they would have been better to sacrifice hull speed by increasing drag of rudder and keel by making them bigger to get more control.

One thing for sure if you get a savage WH you need a good rudder to counteract it. good luck olewill

 

[Ubergeekian]

I need to build a new rudder. The old rudder was a flat sheet of ply, which is of course very inefficient.

Could the Corribee Owners' Association help? They have a whole page on rudders here. Ron Lunney seems to be the chap to talk to.

 

[oldsaltoz]

Having built and repaired several rudders I can assure you many fail due to poor design and construction.

The most common failures include:
Foam or timber core with solid link between the outer skins and the tangs.

So when the core fails the tangs are free to move inside the rudder. Better to include pads by removing some core material and filling with Micro fibres so the skins can be glassed to them, if the core fails the tangs will be still be held.

Poor welding of the tangs or failure to pickle the stainless welds resulting in weld failure.

I have often had to get tangs re welded and properly pickled and on some have added gussets.

Failure to include sacrificial top and bottom sections resulting damaged shaft, tangs and even the hull.

The bottom section should be at least 400 mm deep on an average rubber for a 30 footer, the top should also have at least 50 mm to avoid problems of ropes getting between hull and rudder on close fitted rudders.

Failure to properly seal the stock to rudder at the point of entry.

My method to seal the shaft is to add 'O' rings with a little Vaseline under and between them before glassing them in.

Last but not least is just bad design and construction.

I am amazed at way some people build rudders that are destined to fail because the build is just not strong enough in the right places.

PS. I have 15% of the total rudder area in front of the point of axis to provide good feedback and make steering a whole lot better, 18% tends to reduce the feedback too much for my liking.

Good luck.

 

[caiman]

I reckon the link provided by Ubergeekian is the way to go.Reading the link has certainly given me more food for thought.Nice one.
William_H,18% area ahead of the shaft will balance out 36% of the force needed on the tiller.This is the first time I've seen a direct figure quoted for 'lead' area.Is this ratio just 'double'?ie does 15% area 'lead' give 30% 'balance'?Knowing this will help me in deciding the final shape for my rudder.One of the old rudder problems was too much 'lead' which caused the tiller to snatch and 'self servo',so I want to get it right.
Cheers

 

[Seajet]

Oldsaltoz is right, beware too much balance, especially with a twin keeler which will tend to have a much more neutral feel to the helm; it would be easy to design out all the feedback !

I don't think a narrow low boat like a Corribee is going to suffer heeling weather helm, but I am certain she will suffer rig induced WH, and the point of CE, centre of effort of the rig, means you do indeed need to keep an eye on the main trim in every sense.

In your case I think good control of the main, with a more powerful and easily adjusted kicker than the token efforts one often sees would help; add a clew outhaul with enough advantage to actually pull the sail flat when desired and you're a lot of the way there.

For the clew outhaul I'd suggest something simple but effective, say take the clew line to a ball bearing ( probably cheek ) block at the boom end, if difficult to fit central then have the line tied to an eye, forward through the clew, aft to a BALL BEARING cheek block the other side then forward to a clamcleat with fairlead, so the line is always held and it's a good angle of pull.

With this, the kicker and a careful eye on mast rake ie not significantly aft - allowing for trim with crew weight in cockpit - you should be about ready for as decent a rudder as possible.

 

[nathanlee]

Guys, thank you very much. These replies have been brilliant. Somebody made a post the other day about general sweeping thank you statements rather than individual responses, but since the above are essentially in agreement, it seems rather pointless. Suffice to say, I am truly thankful for anybody who has taken the time to offer advice.

So, based on the above, I've come up with this...
http://onkudu.com/fitting-out-kudu/rudder-calculations/

(there's pictures, otherwise I'd have posted it here).

It doesn't seem like I can get the shape I want (need) without reducing the area of the rudder considerably.

Does what I've done make sense?

 

[Seajet]

Nathan,

had a look at your proposal, ( the relatively deep rectangle ) looks fine to me; one thing, as you know twin keelers can sometimes dry out bows down, so don't make the rudder too deep to promote this.

If you can make it strong at 44mm thick that sounds good to me, remember frontal area drag probably has more effect than having the last word in NACA sections.

Presumably the upper edge will be upswept to follow the hull rocker ? If so the next thing to ponder is the inviting gap for lines to get into...think I'd make it as small as possible so that keeping the tiller lashed central you'd know you have a small gap, hopefully keeping lines etc out.

Then again one could say 'if the boat runs into anything or settles with the rudder on a hard lump and bends/ shifts the shaft, the rudder blade might foul on the hull' - and so it goes on; all I can think to answer that is, have the shaft strong, and you're unlikely to have the rudder pefectly fore & aft on hitting something ? How about a small fragible upper section ? Note the clever way I've turned what seemed like a solution into another few ponderables - pure British Aerospace training, that is !

 

[nathanlee]

Nathan,

had a look at your proposal, ( the relatively deep rectangle ) looks fine to me; one thing, as you know twin keelers can sometimes dry out bows down, so don't make the rudder too deep to promote this.

If you can make it strong at 44mm thick that sounds good to me, remember frontal area drag probably has more effect than having the last word in NACA sections.

Presumably the upper edge will be upswept to follow the hull rocker ? If so the next thing to ponder is the inviting gap for lines to get into...think I'd make it as small as possible so that keeping the tiller lashed central you'd know you have a small gap, hopefully keeping lines etc out.

Then again one could say 'if the boat runs into anything or settles with the rudder on a hard lump and bends/ shifts the shaft, the rudder blade might foul on the hull' - and so it goes on; all I can think to answer that is, have the shaft strong, and you're unlikely to have the rudder pefectly fore & aft on hitting something ? How about a small fragible upper section ? Note the clever way I've turned what seemed like a solution into another few ponderables - pure British Aerospace training, that is !

Sorry, that second picture may have been misleading. In order to get a similar surface area to the old rudder, I'd need to make the new one a rectangle of 53cm x 44cm. The overall thickness would be 5.3cm maximum (for NACA 0012). This seems way too thick, and ideally I'd like no more than two bits of 18mm ply, so 3.6cm, which would be (I'm guessing) NACA 0008? Would that suffice?

Impact is something I hadn't considered, so I may make the area forward of the shaft an impact zone, using foam and GRP for the leading edge. Hopefully this would limit bent shafts and such, especially on a boat weighing less than a ton.

 

[Little Rascal]

Nathan,

NACA 0008 should be fine.

You might have already found this but here is a comparison of NACA 0008 and NACA 0012 lift and drag curves.

www.worldofkrauss.com/foils/1726
www.worldofkrauss.com/foils/1718

They are shown for aircraft but the principles are clear.

The 0012 stalls at a higher angle of attack.
The 0012 has a higher drag coefficent (0.03 as opposed to the 0008s 0.02)

When you think of it you are sacrificing the ability to turn a degree or so tighter or sacrificing a bit of speed. I know what I'd choose!

Bear in mind these graphs are computed for wings of infinite span - in reality, with such a low aspect ratio the rudder will not stall until a much greater angle.

By increasing the aspect ratio of the rudder it should become more effective too - although this is difficult to assess if you also change the balance point.

Jon

 

[nathanlee]

That's it then. Rudder design sorted

Thanks all. I shall post progress to the blog.

 

[caiman]

What's the design that you've settled on?
Cheers

 

[electrosys]

Is there any reason not to make a copy of an existing rudder blade from a Corribee/ Coromandel bilge-keeler ?

 

[nathanlee]

What's the design that you've settled on?
Cheers

53cm x 44cm, with a NACA 0008 hydrofoil profile. That's a maximum thickness of 8% of the chord (44cm), so give or take a bit, 3.5cm, which is a double layer of 18mm ply.

I'll cut a half profile of the hydrofoil out of ply, then use a belt sander to shape the ply, use a bit of filler for my inevitable mistakes, then laminate in roving with epoxy.

 

[nathanlee]

Is there any reason not to make a copy of an existing rudder blade from a Corribee/ Coromandel bilge-keeler ?

The later corribee's had skegs, and I can't find one that's not.

 

[Little Rascal]

Nathan,

Sorry this is off topic but:

Can you remind me of the name of that Swedish(?) guy who went off cruising with various ladyfriends in unfeasibly small boats? I've forgotten his name...

Ta

Jon

 

[nathanlee]

Was it Sven Yrvind?

 

[caiman]

Sucking eggs and all that,after you've got a rough shape with the belt sander,try using a long piece of flat timber the span of the rudder,covered with sanding paper to get to the final shape.(torture board)It will take a bit longer,and is harder work but I got good results by doing this,and so used less filler to fair the Dagger Board I made.But there again,you might be better with a belt sander than I am !!
Good luck with the build.I spent yesterday afternoon spoiling some perfectly good foam with a router,so it's plan B for me.
Cheers