Since breaking my rudder (see here www.seabreeze.com.au/forums/Sailing/General/What-did-you-do-to-your-sailing-boat-today?page=161), I have been thinking about rudder design and rudder theory.
To recap, the "new" rudder that broke on my Cavalier 28 was fitted two or three owners ago to provide better downwind performance. The boat came with the "original" rudder, which is smaller. It is a transom-mounted rudder, and the transom has a rake on it. The "new" (broken) rudder is about 200mm longer, 150mm wider, with pre-balance (the leading edge is in front of the axis of rotation). I understand what the pre-balance does. However, I have some other questions ...
Should the rudder be heavy or light? Light might be better, because you have less weight at the ends of the boat. But, if light also means highly buoyant, it will also put an upthrust on the rudder pintles and gudgeons, and I'm not sure my pintles and gudgeons are designed to take such a force. Maybe it should be neutrally buoyant, when the bottom is submerged up to the waterline?
Should the lower end be square or pointed (per a Spitfire wing)? This is to do with vortex-shedding along the bottom edge. Maybe it doesn't make a noticeable difference?
Should the trailing edge be knife-edge sharp, or squared off to, say 5mm? This is to do with vortex-shedding, again, and I have heard that a little squaring off improves it.
Other than timber, what material(s) could I use to make up the core? If you've seen the photo of the rotten timber in my broken rudder, you'll understand why I'm asking this question.
The Cav 28 moulds are still around including the rudder moulds. Cav 28 Association may be a good starting point.
I would make up a quick one first and use the profile the designer drew. Glue up two pieces of structural ply to a thickness of about 36mm. Shape with a disc sander. Just one lay up of cloth and epoxy till you try it out. I would add some depth and make the tip pointed. Round leading edge and square of the trailing edge so the paint sticks better. Best if the rudder floats for that occasion you let it go!
Once you're satisfied with the shape then add some extra reinforcing, Carbon fibre etc and finish off the layup. With this rudder set up it's easy to make a rudder blade and experiment at little expense. Have some fun!
Select to expand quoteAchernar said..
Other than timber, what material(s) could I use to make up the core? If you've seen the photo of the rotten timber in my broken rudder, you'll understand why I'm asking this question.
My rudder is very similar to yours except that it has a narrow skeg in front of it. See pics.
Due to not being properly sealed it became worm infested which required much of it being rebuilt. I believe some of the timber may have been oregon but I used selected pieces of meranti (the reddest bits come from the centre of the tree) for the repair.
The skeg and rudder are laminated timber about 40 mm thick. It wasn't difficult but it was time consuming. I glued the laminates with epoxy glue, shaped it and then sheathed it with fibre glass to keep the worms out.
It is fairly light but I did not test it to see if it floats.
When I bought my Lotus 9.2 I had looked at a Cav 28 prior. They are a great yacht but the Lotus 9.2 is just did it better for me.
Transom hung rudders just make so much sense.
You should get some ideas here, 60 years of rudder research.
www.tandfonline.com/action/cookieAbsent
Select to expand quoteCharriot said..
Make sure its semibalance , if you make it.
Hi Charriot,
Can you expand on this a bit? I'm not familiar with the the term, aside from a literal translation.
Thanks!
G'day Archenar,
I don't have skills in making rudders sorry, but I would like to have a go at it one day, it's interesting. Musing out loud about some general thoughts..
I note Lydia beat me to it, but I was wondering what was the issue with the original design, was it not enough bite?
If so, I would be leery tapering the rudder tip too much, as when you're heeled you want surface area to still bite on the water, so makiing a nice triangle shape (exaggerated) kinda defeats the purpose, you would lessen the effectiveness from both the heeling and the top of the rudder being further out of the water.
Off cambering the trailing edge to make it asymmetrical definitely helps cut down on cavitation and drag, but this is more a function at higher speeds, I am unsure if the benefit is realised at cruising speeds,
Looking at my rudders, I notice two things, (aside from they are short :) the leading edge has a slight taper from shaft to tip and is quite blunt (the chord is fat too). The trailing edge by comparison is vertical and a lot thinner. The helm is weightless across the wind range, in actual fact the quicker we go the lighter the helm. At high speeds it's a go-kart, twitch the helm and the response is immediate, The stubby draft becomes evident with too much sail up, but that isn't the fault of the rudders per se, that's me being lazy and not downsizing or trimming to suit. No rudder should make up for a lazy or over powered trim in my simple mind. 
It was interesting to read up on the loss of Shockwave recently, where a well engineered new keel let go. I know this was a keel and not a rudder, but it has some interesting parallels.
They had gone for a deeper draft, and to offset this they had removed weight from both the keel and the fin. These mods actually lessened the RM (righting moment), so theoretically this should be less load, it all sort of makes sense on paper. Then the keel fell off. No concrete evidence as the boat is still floating around out there, but the preliminary thoughts appeared to believe the larger surface area from the deeper draft negated the lighter weight and still overloaded the keel joint, resulting in the keel departing from the boat. One race a few years back saw three new rudders fail (and not from collisions), so not going too extreme seems a sensible and logical approach.
I was recently getting educated on tubercles, where they serrate and add protrusions to the leading edge of the rudders. This improves the efficiency of the rudder stall angle, allowing grip for longer period. The idea is copied from humpback whales, which are known for their maneuverability (apparently!)
Us mere mortals don't need to worry about tubercles I think, but the science is interesting !
It does sound like fun, could you keep us posted on what creation you come up with?
Cheers,
SB
Select to expand quoteRamona said..
I call it pre balance but it is covered well in Trek's link page 5:2
Thanks Ramona,
So it's referring to the position of the ruddder shaft fore and aft in the rudder. I'm surprised mine aren't more balanced as they feel really neutral. I wonder if that has something to do with how fat the leading edge is?
That's a great article , thanks Trek!
Select to expand quoteshaggybaxter said..
It was interesting to read up on the loss of Shockwave recently, where a well engineered new keel let go. I know this was a keel and not a rudder, but it has some interesting parallels.
They had gone for a deeper draft, and to offset this they had removed weight from both the keel and the fin. These mods actually lessened the RM (righting moment), so theoretically this should be less load, it all sort of makes sense on paper. Then the keel fell off. No concrete evidence as the boat is still floating around out there, but the preliminary thoughts appeared to believe the larger surface area from the deeper draft negated the lighter weight and still overloaded the keel joint, resulting in the keel departing from the boat.
Careful, Shaggy. I think it might be Showtime you are referring to, not Shockwave.
And if so, the yacht designer is very keen to reassure us that the replacement keel itself failed, presumably somewhere along the chord length and the hull/keel joint was left intact. This is quite different to 'the keel fell off'.
And, yes, no concrete evidence yet.
Select to expand quoteGravy7 said..shaggybaxter said..
It was interesting to read up on the loss of Shockwave recently, where a well engineered new keel let go. I know this was a keel and not a rudder, but it has some interesting parallels.
They had gone for a deeper draft, and to offset this they had removed weight from both the keel and the fin. These mods actually lessened the RM (righting moment), so theoretically this should be less load, it all sort of makes sense on paper. Then the keel fell off. No concrete evidence as the boat is still floating around out there, but the preliminary thoughts appeared to believe the larger surface area from the deeper draft negated the lighter weight and still overloaded the keel joint, resulting in the keel departing from the boat.
Careful, Shaggy. I think it might be Showtime you are referring to, not Shockwave.
And if so, the yacht designer is very keen to reassure us that the replacement keel itself failed, presumably somewhere along the chord length and the hull/keel joint was left intact. This is quite different to 'the keel fell off'.
And, yes, no concrete evidence yet.
Thanks Gravy,
I appreciate the correction. This sort of thing still makes me lose sleep at night, the keel designer and fabrication appear as professional as the original build and we can still end up going swimming.
Scary thoughts when its early am and the crew are below and your miles from the nearest land... I'm doing my own ram repair atm that looks like a factory issue (from a factory that has a zero defect history, so I'm the first..) , so maybe I am more conscious of it than normal!
Cheers!
SB
Select to expand quotelydia said..
So what was wrong with the original rudder design
AFAIK, we are dealing with three rudders;
* Laurie Davidson's original, which looks like a Spitfire wing, but I have yet to see a photo or get confirmation
* The replacement rudder, which was fitted to the boat when I bought it last year. I have been told by the previous owner that the owner before him fitted it with his Dad to provide more grip downwind, especially reaching under spinnaker. Apparently, it occasionally needed two on the tiller to hold course. It might have had its leading edge extended, and a break was repaired by fitting stainless steel cheek plates below the lower pintle. This is the rudder that broke. The holes for the pintles and cheek plates appear to have allowed water to get in and rot the timber core.
* The "original" rudder, which is smaller than the replacement rudder and also came with the boat as a separate item. This is the rudder I am re-fitting to the boat.
Until I test-drive the "original" extensively, I can't answer the question directly. I expect that it will have more to do with which set-up would be better for my style of sailing, i.e. leisurely cruising or hard racing. To start with, I'll certainly tend towards the leisurely end of the spectrum, but the comparison will be interesting.
The original Cav 28 rudder should do the job. There's 100 or so 28's out there sailing just fine.
Select to expand quotetroubadour said..
The original Cav 28 rudder should do the job. There's 100 or so 28's out there sailing just fine.
cant agree more Laurie Davidson's designs are always right .make sure you use a proper naca section probably 0013and that it is accurate
Select to expand quoteboty said..
cant agree more Laurie Davidson's designs are always right .make sure you use a proper naca section probably 0013and that it is accurate
Just for context this is a NACA 0013 foil which is a turbulent flow blade and should work well at relatively high angles of attack without stalling
I would build the new rudder out of western red cedar - not plywood. Laminate (using epoxy glue - clamp - DO NOT USE FASTENINGS) cedar pieces milled to be as wide as the section required to make a nice blank. Then make a few NACA profiles for different parts of the foil - up top, halfway and nearer the bottom. The elliptical profile shown by the original looks like a nice planform so copy it. If you want some extra balance you can extend the top pintle so that it cocks the rudder a bit underneath. You can use a power planer to get the meat off and then reduce the depth setting as you get closer to the NACA profile. Then switch to the long bed hand plane.
When you get it all nice you should then cut a small rebate down the maximum thickness of about 3mm and about 150mm wide. Into this rebate place 2 layers of 400gm uni and wet out with epoxy. Then grind over and get smooth. Glass all over with 400gm double bias using only epoxy.
If you want to you can re-use the old pintles. Drill out holes for the bolts and then oversize them by about 5 mm all around. Fill with sloppy epoxy glue (plug one side with tape) and re drill. You won't get water ingress if you do this.
Smear with microballoons and fair with 60 grit paper on a long board. Repeat and change paper to 120, then paint with epoxy high build and then top coat. A rudder is a nice job that gets you a lot of high visual return for not much effort. Take it steady and it will be a thing of joy.
(I would caution you on using any ply in the rudder. Ply is rater weak in bending as half the plies are going the wrong way. Also, it is not as effective against water ingress if you hit something and the glass gets penetrated.
cheers
Phil
I would build the new rudder out of western red cedar - not plywood. Laminate (using epoxy glue - clamp - DO NOT USE FASTENINGS) cedar pieces milled to be as wide as the section required to make a nice blank. Then make a few NACA profiles for different parts of the foil - up top, halfway and nearer the bottom. The elliptical profile shown by the original looks like a nice planform so copy it. If you want some extra balance you can extend the top pintle so that it cocks the rudder a bit underneath. You can use a power planer to get the meat off and then reduce the depth setting as you get closer to the NACA profile. Then switch to the long bed hand plane.
When you get it all nice you should then cut a small rebate down the maximum thickness of about 3mm and about 150mm wide. Into this rebate place 2 layers of 400gm uni and wet out with epoxy. Then grind over and get smooth. Glass all over with 400gm double bias using only epoxy.
If you want to you can re-use the old pintles. Drill out holes for the bolts and then oversize them by about 5 mm all around. Fill with sloppy epoxy glue (plug one side with tape) and re drill. You won't get water ingress if you do this.
Smear with microballoons and fair with 60 grit paper on a long board. Repeat and change paper to 120, then paint with epoxy high build and then top coat. A rudder is a nice job that gets you a lot of high visual return for not much effort. Take it steady and it will be a thing of joy.
(I would caution you on using any ply in the rudder. Ply is rater weak in bending as half the plies are going the wrong way. Also, it is not as effective against water ingress if you hit something and the glass gets penetrated.
cheers
Phil
I would build one to the original profile too. Looks like the right amount of prebalance, I suggested using plywood laminated simply because it's easier to get the Naca profile by using the glue lines as you plane or sand down the ply. Far easier to get both sides the same. By the time you finish the laminate the material in the core wont matter much. Transom hung rudders have the advantage of no SS shaft running down into the core. The temperature changes and different expansion rates will always let water into the core where the shaft enters the blade.
Troubadour - thanks for the photos.
Does anyone know where or how I could get the drawings or major dimensions?
Kankama - thanks for the detailed guidance on construction.
The cross section data of the naca 0013 can be scaled up from the 0012 as per below link by multiplying the thickness by 13/12.
airfoiltools.com/airfoil/details?airfoil=n0012-il
Agree the 0013 - it is a good rudder cross sectional shape, and that the chord length of the rudder down its depth can be reasonably scaled off the photo.
For skiff rudders I have built, I've made thin ply templates of the 1/2 cross section to check one side then the other, to shape the timber core prismatic section. Allow for the thickness of the grp you are going to put on top of the timber. Prismatic meaning same cross section all the way down. So if you want an elliptical section do this prismatic shape all the way down first, then cut the elliptical section into the timber core after you have the prismatic section all shaped, and then plane the section in down low in the elliptical section with electric plane and hand plane by eye - it should finish up well.
If you wanted to stay with the prismatic section all the way - our Ross 930 has this but I can't work out how to add the photo sorry - I expect this would work ok with (say) a 50-75mm radius on the leading edge lower corner, suitably faired in. Our rudder is very stable, doesn't ventilate or cavitate easily, we have only had a few round-ups in nearly 4 seasons of racing and these were when over-pressed with kites to make marks or shallow rocks or land.
For the timber core to use I would lean to clear grained oregon - having more strength than western red cedar for not much more weight penalty. I realise the grp laminate as suggested above will take all the loads due to its greater stiffness than the timber - but for a Cav28 suggest the oregon core would be the go.
Think some rudders have been built from (say) 12mm ply then oregon strips glued down each side. The West System people should have a white paper onon this?
Shaggy, you probably seen it
Semi balanced rudder. A rudder with a small part of its area, less than 20%, forward of the turning axis. At no angle rudder is balanced. Axis of rotation lies less than 0.2 L.
Select to expand quotetroubadour said..
The Cav 28 moulds are still around including the rudder moulds. Cav 28 Association may be a good starting point.
Why not start back with the original? Laurie was pretty fussy about getting foils right, as others have mentioned
Select to expand quoteRamona said..
I would make up a quick one first and use the profile the designer drew. Glue up two pieces of structural ply to a thickness of about 36mm. Shape with a disc sander. Just one lay up of cloth and epoxy till you try it out. I would add some depth and make the tip pointed. Round leading edge and square of the trailing edge so the paint sticks better. Best if the rudder floats for that occasion you let it go!
Once you're satisfied with the shape then add some extra reinforcing, Carbon fibre etc and finish off the layup. With this rudder set up it's easy to make a rudder blade and experiment at little expense. Have some fun!
I'd also glue 2-3 pieces of heavy cloth (600-900gsm quad ) in between the 2 pieces of ply to get a sharp and solid trailing edge as well as adding extra stiffness to the blade.
Select to expand quoteHarb said..Ramona said..
I'd also glue 2-3 pieces of heavy cloth (600-900gsm quad ) in between the 2 pieces of ply to get a sharp and solid trailing edge as well as adding extra stiffness to the blade.
Theoretically, the trailing edge being sharp is the go but with a transom hung rudder a squared-off trailing edge may be more practical. Especially if the yacht is on a mooring and there are amateur fishermen drifting about in tinnies and they love banging into yachts! The sharp edge may chip easily.
Never had an issue with the edges chipping but then again its usually me who is doing the drifting and banging into tinnies. 
Have had fishos in tinnies drift down and bump / crash into my swing moored yacht when I was on board trying to fix something - ok the 2 of them were concentrating on landing a good sized tailor which they did so a good result - but the load case described by Ramona is certainly more than valid.
So the heavy cloth between the ply pieces will strengthen the leading and trailing edges to impact loads onto them.
But the leading edge will have the glass wrapped around it and will continue back for a fair % of the rudder blade chord - the heavy cloth between the plies will assist the impact strength of the leading edge for sure. The leading edge has a defined radius in all NACA profiles.
We tested the load case of snaring the course leeward mark chain with our rudder last season due to my steering stupidity and came off second best.......I had to dive over to clear it.......some sizeable bruises which were impacted to the rudder leading edge were fixed the next slipping - these bruises did not penetrate such as to allow water ingress.
For the trailing edge most rudders will have a similar glass wrap around it but the radius will be a lot tighter and the fabric a lot lighter and will need specific tensioning to avoid bubbles in the laminate.
The heavy cloth in between the 2 ply pieces will not add much bending stiffness as it is on and close to the neutral axis of the rudder in bending....but agree it is a good idea so as to increase the leading and trailing edges resistance to impact damage straight onto them.
For these 
I used 2 layers of 900gsm quadaxial to cover my ply so no way I could wrap that around the trailing edge, barely managed to get it around the leading edge without any bubbles. Yes, I know it was a bit of an overkill but I mostly sail in shallow waters so I wanted something strong enough to handle scrapping the blades on rocks and the weight of the boat when I ran aground.
Ideally I would have vacuum bagged the blades but didn't have the gear so I placed clear polyethylene over the glass and used the plastic spreader to get rid of the bubbles this way, next best thing to vacuum bag method.
On the trailing edge the combined thickness of the 6 layers of 900gsm cloth was enough to form a long trailing edge out of just glass. On the ply side I used epoxy+ 403 bog to build the wedge instead of getting the ply thinned down too much so the trailing edge is only glass /epoxy for at least 40-50mm. I left it to set before I shaped it and then wrapped it with 2 layers of 225gsm twill. On the the trailing edge it needed a bit of convincing to form the knife edge but I got the solution from looking at the hammock. I stapled 2 sides to pine slats and suspended the cloth then dropped the blades in using their weight to remove the wrinkles and tension the cloth. Gave them another 3 coats of epoxy while hanging in the "hammock",.
I've done the drawing with the laptop trackpoint so a bit rough but gives you the idea, the blue is the 403 bog and the red is the twill cloth.
Top effort right there Harb - rudders and keels are where overkill is needed................
