Rudder Design

[thinwater]

The other thing is that compared to a monohull with a deep spade rudder, a short cat skeg hung rudder has very little bending force on the shaft. You don't need the stiffness of carbon. Totally different beast.

 

[geem]

The problem with stainless is it can corrode and embedded in a foam core seems exactly the sort of place - if it went wrong - that oxygen depleted water would cause corrosion which simply can’t happen with composites

It’s the same reason my bronze through hull will be replaced with TruDesign and my stainless chainplates are being replaced with carbon I’m laminating in to the hull etc I’m just building a new toe rail to replace the aluminium one that having lasted 40 years is corroding so I’ll laminate in a grp one (which also seals all those pesky screw holes for the toe rail in to deck) and laminate in grp stanchions at the same time … if I can make it not corrode or rust or need maintenance I just think I might as well

Carbon rudders have a reputation for snapping suddenly. It may be less of an issue on a skeg supported rudder. The only failures I have seen were carbon spade rudders. It was interesting to see the construction of the failed rudder stock on a friends race boat. It was carbon rods wrapped in a bundle to form a larger diameter carbon shaft. Apparently this is the best possible and strongest way to build a carbon shaft. Even so, it still failed.
I would have thought a plain grp shaft would be sufficient if skeg supported. It will be as strong but less brittle/stiff. In reality, you don't need stiff since the support of the skeg will provide this.
I have attached a photo of of the drawing of my rudder. It may help with shape profiles. Mine is a 316 stainless shaft and super strong. We have only just removed it for the first time ever. 44 years old. No corrosion evident on the stainless steel shoe. I intend to drill a core to inspect a tang to see how they are doing but I don't expect a problem

 

[Trident]

I'll look at GRP shafts ... the carbon was simply for the ability to make cheaply (the boat is coming ashore next to the factory that makes some of the best carbon cats and I can use their rudder shaft mould and I have a ton of cloth as my company makes things in carbon) and to be inert and ably to be bonded in to the rudder blade . By bonding in to the skin and using laminated torque tabs from shaft to foam core and then skinning over, its almost impossible to ever have the parts leak or separate - whereas I have seen many rudders where the foam disintegrates and several where the tabs welded to the shaft to transfer load in to the core have separated from the shaft leaving it to twist in the ... water

I appreciate the comments about careful construction eliminating this but many well built rudders still fail and I've never done one before but have laminated in GRP and CF for 20 years or more making race bike parts etc so its very familiar. GRP shafts may well be good enough and even easier as there is no need to worry about the top and bottom fixings in stainless which would need a GRP layer over the carbon to ensure no reaction.

Construction aside, the plan is then to find a NACA profile that most closely matches the design curve of the skeg when dimensioned to the size of skeg and rudder as one , and then make the rudder blade to the profile of the "back end" of that . This then will make the rudder work better and lighter on the helm from what is said above. I assume that it will also reduce drag slightly so I can go faster ... whether any known log could measure the incremental increase is another matter

 

[thinwater]

I bent a shaft once on a log floating below the surface. Only a few degrees, but enough that it rubbed on the hull. I bent it back, which was a careful process, but took only a few hours and some careful application of force. With carbon it would have snapped and floated away. Much worse.

For me this is not a hypothetical.

 

[Trident]

Of course with a skeg this protects the shaft so not so much of a worry and very hard to bend anything with a top and bottom mount but I can see the issue on blade rudders could be very problematic

 

[rogerthebodger]

I bent a shaft once on a log floating below the surface. Only a few degrees, but enough that it rubbed on the hull. I bent it back, which was a careful process, but took only a few hours and some careful application of force. With carbon it would have snapped and floated away. Much worse.

For me this is not a hypothetical.

Straighten stainless steel shafts is easy with a garage press and some roller Vee blocks.

If you need accuracy a dial gauge will help.

Used to straighten shafts during my Engineering apprenticeship but today we throw it away and pay good money for a new one

 

[thinwater]

Straighten stainless steel shafts is easy with a garage press and some roller Vee blocks.

If you need accuracy a dial gauge will help.

Used to straighten shafts during my Engineering apprenticeship but today we throw it away and pay good money for a new one

I didn't even remove it from the boat. I built a V-shaped saddle to closely fit and support the trailing edge, so as not to damamge the foam and structure, calculated the required force and the amount of over-bend required to allow for spring-back, and gave it a slow pull (chain fall pulled by a winch, monitored with a strain gauge). Got it the first time. As it turns out, the force is not nearly enough to tax the bearings if carefully applied.

I understand the purpose of carbon on a 5-foot long spade rudder. Not on a stubby multihull rudder.

On one hand cat rudders are easier to fix. On the other hand, you're twice as likely to hit something.

 

[rogerthebodger]

A lot of people seem to be frightened of stainless steel due to crevious corrosion.

Crevious corrosion mainly takes place in oxygen deprived environments as the protection of stainless steel is due to a chromium oxide layer that like aluminum anodizing protects the underlying metal.

The oxygen in sea water will allow the oxide layer to reform if its damaged.

The classic is inside a foam/GRP rudder when sea water gets inside and caused crevious corrosion on the tangs due to the oxygen depleted sea water inside the rudder.

Corrosion is normally based on the combining of a non-stable metal and oxygen causing an oxide to form thus destroying the integrity of the base metal.

When stainless steel has access to oxygen t protective oxide layer will form thus protecting the underlying base metal.

As a stainless steel in a rudder will be open to oxygen the protective oxide layer will form. and as long as the inside is sealed any corrosion inside will use up any oxygen inside until all the oxygen is used up so no more oxidations can take place

I built in a shear pin into my spade self-steering rudder so if I hit anything the shear pin will break saving damage to the rudder

My main rudder has a bearing at the bottom of my skeg and the rudder shaft has a solid coupling at the to of the rudder middle of shaft to allow the rudder to be removed even when in the water

 

[Trident]

It is easy to stop water getting in with a cored and sheathed rudder as there are no joins unlike a moulded rudder, just the stock entry. On the rudder I made that was sealed with epoxy and was sound 25 years later when I sold the boat. Jefa use an O ring at this point on their rudders. Photo of your existing rudders would be useful and also how they are mounted on the skegs.

You should not need anodes on a stainless rudder and stock if it is only the stainless that is in the water.

Just wanted to come back to this if I may - how did you ensure a perfect seal on to a stainless stock ? I have seen lots of rudders with ingress from the stock - its one of my main concerns hence wanting to use a composite stock that gives a proper bond from stock to rudder. So I would be interested to hear how your seal was made please

I'm just weighing up the idea of blasting the original steel rudders clean , bonding foam over them and shaping to profile and then encapsulating in epoxy glass fibre and fairing with epoxy. As I said water ingress from the shaft is my prime worry but if that can be designed out ( a countersunk pocket top and bottom on the shaft with an o ring covered with thickened epoxy to fill or a "top hat" with seal inside glassed in top and bottom ) then the better shape combined with the stainless rudder underneath to still work if the GRP is bitten or damaged some other way does have an appeal for an ocean cruising boat

 

[noelex]

I assume that it will also reduce drag slightly so I can go faster ... whether any known log could measure the incremental increase is another matter

The rudders only form a small part of the vessel’s total drag, but it is amazing how much reduction in drag occurs with a streamlined shape. It is a much greater difference than you may intuitively think.

This diagram illustrates the difference. The drag coefficient of a streamlined shape is around a twentieth that of a flat plate of the same thickness.

This diagram should at least give you some incentive for all the hard work you have in store .

 

[Tranona]

Just wanted to come back to this if I may - how did you ensure a perfect seal on to a stainless stock ? I have seen lots of rudders with ingress from the stock - its one of my main concerns hence wanting to use a composite stock that gives a proper bond from stock to rudder. So I would be interested to hear how your seal was made please

I'm just weighing up the idea of blasting the original steel rudders clean , bonding foam over them and shaping to profile and then encapsulating in epoxy glass fibre and fairing with epoxy. As I said water ingress from the shaft is my prime worry but if that can be designed out ( a countersunk pocket top and bottom on the shaft with an o ring covered with thickened epoxy to fill or a "top hat" with seal inside glassed in top and bottom ) then the better shape combined with the stainless rudder underneath to still work if the GRP is bitten or damaged some other way does have an appeal for an ocean cruising boat

Welded a washer on the shaft top of the blade to form a pocket which then got filled with epoxy when I sheathed it. Jefa suggest filling the pocket on their rudders with Sikaflex. Some Bavaria owners have done what you suggest epoxy an O ring into a pocket. At the bottom a similar washer was left flush as a bearing surface for the bottom bearing which was a top hat Delrin insert in the bronze shoe. The blade construction was ply attached to the usual tangs and framed in solid wood then sheathed in epoxy glass cloth. Not a good profile as I had been seduced by a convincing argument from an owner of a sister ship that flat with a concave trailing edge was nearly as good as a foil. It was a vast improvement over the flat plate but was never convinced that the concave trailing edge did anything!

The usual source of water ingress in production rudders is failure of the bonding of the two clam shell mouldings and your design will eliminate that. Your proposal is more or less how I would do it if the existing plates and stock are sound. I assume the stock has 3 bearings - bottom of skeg, entry to hull and top so the stock does not have to be anything fancy. Mine was 1" 316 for a rudder similar size and proportions to yours except it had about 10% balance area - no skeg, traditional long keel and prop aperture.

 

[Trident]

Welded a washer on the shaft top of the blade to form a pocket which then got filled with epoxy when I sheathed it. Jefa suggest filling the pocket on their rudders with Sikaflex. Some Bavaria owners have done what you suggest epoxy an O ring into a pocket. At the bottom a similar washer was left flush as a bearing surface for the bottom bearing which was a top hat Delrin insert in the bronze shoe. The blade construction was ply attached to the usual tangs and framed in solid wood then sheathed in epoxy glass cloth. Not a good profile as I had been seduced by a convincing argument from an owner of a sister ship that flat with a concave trailing edge was nearly as good as a foil. It was a vast improvement over the flat plate but was never convinced that the concave trailing edge did anything!

The usual source of water ingress in production rudders is failure of the bonding of the two clam shell mouldings and your design will eliminate that. Your proposal is more or less how I would do it if the existing plates and stock are sound. I assume the stock has 3 bearings - bottom of skeg, entry to hull and top so the stock does not have to be anything fancy. Mine was 1" 316 for a rudder similar size and proportions to yours except it had about 10% balance area - no skeg, traditional long keel and prop aperture.

Thanks for that - something to look in to for me between now and spring when the haul out is booked

The shaft has a phosphor bronze bush in the shoe and the same glassed in to the hull at the top of the rudder and that's it as far as I can see - all very simply and of course it works. We've had no issues with steering and I've never heard of any on the Quasars except the usual hydraulic issues on those that chose that over wire and quadrant. Mine is wire which I much prefer and of course its easy to fix on the go with a length of dyneema . There is a steel (and rusty AF) bar to join the two stocks which is being replaced by a GRP tube (actually a ready made GRP scaffold pole that due to its intended use has all the test figures published and will be plenty strong enough in tension and compression to do the job ) so hopefully a very robust system and with the new rudder profiles rather more efficient

 

[Daydream believer]

deleted

 

[Tranona]

Just to add a bit more. If you do re-use the plates suggest you bore some holes in them so that the epoxy putty you bed the foam on form buttons linking one side to the other.