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COMPOSITE YACHT RUDDERS

Application: This article applies to well-engineered and built carbon fibre “spade” rudders hung independently of any skeg or the stern.

Advantages of carbon fibre composite rudders

Composite rudders and rudder stocks are constructed from a “composite” structure consisting of resin, reinforcing matrix and core. The main structural reinforcing matrix is often carbon fibre (carbon) although there are some heavier non carbon examples sometimes found.

The main structural shaft part of the rudder is the stock. Stainless steel (316 grade), or alloy (aluminium based alloy) rudder stocks have many disadvantages compared to carbon. They are typically much heavier, often twice the weight, corrosion prone and are difficult to seal at the penetration of the rudder stock into the rudder blade.

On the other hand, carbon composite structure for rudders has many advantages: The strength of carbon fibre offers design/engineering possibilities not possible with stainless or alloy. Same size like for like rudders can be much stronger/more durable. Thinner higher aspect ratio lower drag performance shapes can be used. The modern America’s cup boats showcase this attribute astonishingly, flying on rudders and foils over the water at 50 knots plus. Carbon rudder stocks are typically rectangular in sectional shape. The stock structures can be easily built with fibre layers tapered to the match the varying loads in different parts of the rudder. For example the greatest wall thickness will typically be at the highest load point, at the bottom bearing where the stock exits the hull. The laminate thickness in the rudder stock walls will taper towards the top bearing and bottom of the stock buried in the blade.

Carbon rudders are reliable for all vessels if engineered and built appropriately. There are few water ingress issues because the composite blade/stock becomes a one-piece structure when bonded together . Even if damaged, the materials – such as closed cell PVC foam and resin impregnated carbon fibre – are not water absorbing so can usually be dried and repaired if damaged.

Note: carbon should be isolated from the other vessel structure, and this can be done by using plastic type bearings.

What’s required to order a custom carbon fibre rudder?

If the rudder is a replacement for a previous stainless steel or alloy stock it may be beneficial to reconsider the blade profile (side on) shape and chord shape as carbon presents new opportunities. Other technological developments are likely to have occurred such as in rudder bearings. Any performance issues identified may be addressed such as blade balance, weight on the helm etc.

Steering systems related to the rudder stock and blade will also need to be replaced or systems modified which again is an opportunity to update/optimise as part of the process.

  • The bearings and rudder tube
  • Gland or hull penetration sealing system
  • The tiller head system (if fitted)
  • The attachment systems on the stock for the steering quadrant or tiller and autopilot tiller (if fitted)

Carbon rudder design/specification process

Information gathering

  • If possible, obtain original vessel drawings including rudder and steering system
  • Check the as built detail onboard and take photos
  • Note any issues with the existing/prior rudder/steering system/vessel directional behaviour
  • Note any preferences for bearings, gland sealing type

Initial specification

  • Discuss plan with suitably experienced designer/Naval Architect and obtain price for design estimate to do the work excluding the structural laminate specification. Note that the laminate specification may be provided to the designer by a specialist composite engineering company, depending on whether the designer is also a structural engineer or not. The design work may or may not include specifying the bearings.
  • Brief the designer to carry out the following initial design work to enable discussions to take place with a builder.
    • review information
    • prepare preliminary drawing of profile/planform of rudder and approximate top and bottom chord lengths, widths and section and position of rudder stock. This would include amount of blade balance, blade area in square meters old vs new.
    • Bearings
    • Approx. /sketch level detail of tiller head and steering quadrant/tiller clamp details and requirements.

Builder engagement

  • When prelim work is complete, discuss with suitably qualified builder/s to obtain estimates for construction of blade and stock.
  • Discuss modifications to steering system with builder and builder preference regarding composite laminate engineering, bearings, glands/gaitors, sealing system, and if possible estimate for the work required in addition to the blade and stock construction in order to remove old rudder and bearings, fit the new bearings and rudder, steering and commission on board.

Finalising project spec

  • Designer/engineer complete working drawings and specification incorporating any builder/owner input, to agreed specification and fee.
  • Builder and owner agree price estimate, time for completion and tolerances/standard of finished product and scope (i.e. is it complete turn key or just the components for owner fitting).

Note: engineering could be in builder or engineer contracts.

Composite construction process

Different processing methods can be used depending on budget and time. If cost is no barrier, then female composite moulds can be made from computer cut (CNC) male plugs. This way accuracy can be assured to a fraction of a millimetre as the finished dimensions will be as per the moulded surfaces. Alternatively female moulds can be milled directly in a stable board type material and the rudder blade and stock shells made from the sealed and painted moulds. Americas cup foils would be likely to use this type of technology when possible, to assure the dimensional accuracy where at very high speed any inaccuracy may be significant and can be amplified.

However, a more cost-effective processing method, and my usual preference for one off custom rudders is to male mould the stock on a stiffened foam mandrel and hand plane the blade to shape out of closed cell foam. The rudder stock and blade core are usually constructed separately. Then the stock is fitted and glued into the blade before the blade shell is formed by glassing over the stock and foam blade assembly. Then the blade is coated in fairing filler and planed/faired with the aid of templates. Accuracy of less than 1mm can be achieved by a skilled builder.

Composite engineers should specify the laminate stacks including thicknesses so the mandrel and foam blade dimensions can be calculated/checked by the builder.

It is useful to also fit and mill the built-up areas (build ups) where needle roller bearings will run on the stock on before bonding the stock into the blade. The milling can be done on a lathe. The build ups can be done later using stainless steel or custom GRP shells glued over but alignment will be more difficult and less precise. Solid GRP bearing journal shells/bearing finish will be better than stainless steel as they will not be prone to pitting/crevice corrosion. 

Note that self-aligning bearings are common such as those supplied by JEFA that we use and prefer. The system enables the plastic bearing race to be removed and replaced. However, it is important that the bearing shells on the stock are absolutely concentric as any difference will cause jamming/damage to the steering system. Self-aligning bearings will not help if the shaft journals/shells are not concentric.

Long term case study

I have built several composite rudders beginning with non carbon ones in the IMS Rule era and using various techniques.

The most recent examples completed were in 2012 for the Davidson 42 “Spitfire” to my planform, chord geometry, selected NACA 30% max thickness section profile, balance and steering system and bearing specification. The geometry also included a clamp on under deck composite tiller for autopilot and drag link steering connection. Engineering was specified by Gurit NZ office for the blade, stock and composite tiller.

We also built a similar rudder for the Young 11 “Force 11” in 2014 but was specified by the engineers who crew in her. We used JEFA bearings glued into a single tube from deck to hull.

Spitfire’s rudder is now almost 10 years old and has performed very well.

Processing summary

The processing for Spitfire and Force 11 consisted of:

  • a male moulded stock using a foam mandrel vacuum bagged and cured in about 3 stages. The uni directional 0 degree carbon was interspersed with +/-45deg double bias carbon that was easy to lay and the consolidated thicknesses were well within specification.
  • a male moulded closed cell PVC foam blade vacuum bagged
  • Wet layup slow cure epoxies were used from Adhesive Technologies that were post cured to the resin specification.
  • Carbon composites fibres were provided by NZ Composites.

The above processing was fast and cost effective and resulted in a very light/strong structure.

Bearings

Spitfire and Force 11 used JEFA plastic needle roller bearings and matched stainless steel sleeves for fitting on the stock. The bottom bearings were supplied by JEFA bonded into a GRP rudder tubes that were bonded in to the hulls. We had a custom made neoprene gaitor made to seal the rudder tube to the stock which is almost due for replacement after 10 years. An under deck nke autopilot is fitted on Spitfire.

Spitfire had wheel steering that has recently been removed and the vessel restored to tiller steering as the helm is light enough with the re designed rudder not to need the wheel.  The change necessitated a seal at deck level and another under deck gaitor has been fitted with a drain tube to the lower gaitor. This allows space for the composite tiller and autopilot ram to be attached and for the whole system to be watertight.

I designed and developed a new design tiller head system when the Spitfire rudder was converted to tiller. This was built using composite plate that can be supplied CNC cut and scaled to suit the tiller head size. Force 11’s tiller head has been simply done using carbon plate by her composite engineer crew members.

Lessons learnt

Spitfire was sold when her rudder was approx. 1 year old in 2013 and purchased back in 2020. Her rudder was removed in late 2020 for the first time to our knowledge, and the following was noted:

  • Some pitting in the stainless steel bearing shells
  • Some slight play in the bottom needle roller bearing, enough to rattle slightly when under motor with the 2 blade prop.
  • Bottom gaitor slightly worn but serviceable