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RC Marblehead

Update: We are currently updating to new tooling for our Katana M Class design.
The old moulds have been retired as we reached the point where the cumulative improvements identified warranted the change.
Work on the new high-temperature capable tooling is proceeding in parallel with our UAV and A Cat work.
Timing is tied to our ability to build capacity for work that will be carried out inhouse going forward. 
Also we have confirmed orders for the first few boats. 
Therefore deliveries are expected to start again in mid 2015.

Get in touch for more details and to book a build slot...

For the story of Katana, use the Katana tag on the front page of the Blog.
On this page you can read about our past proven designs and the story of our development journey.

The Story So Far...

Octave was a ‘clean sheet’ design incorporating a number of new ideas first tested in a series of prototypes specifically for our next production Marblehead Class boat.
Overall beam is moderately narrow at 150mm. 
The sections are semicircular amidships and 'U' shaped in the ends. 
Octave has a high prismatic coefficient achieved through pronounced rocker in the ends combined with volume in the tighter turn of the bilge in the forefoot and the run aft. 
At the prototype stage we tried emerging chines aft just above the static waterline as a way to push volume into the aft waterline end when heeled and to encourage clean separation of the water flow when appropriate. 
The next version had a similar volume distribution but the chines are softened to keep the flow clean.
The high prismatic coefficient and correspondingly reduced cross-sectional area result in better wave resistance characteristics at speed combined with enhanced pitch-damping. 
Above the water moderate flare amidships maintains trim and dynamic balance throughout the heel angle range. 
Particular attention has been paid to the aerodynamic design of the deck. A piercing bow smoothly transitions to a strongly chambered mid deck designed to interact with the all swing-rig sailplan. The included angle between hull and deck becomes more acute moving aft to aid clean separation of water flow when heeled.
The deck is clean and uncluttered in order to minimise aerodynamic drag and shed water when pressed downwind. The aim is to reduce hydrodynamic drag when the bow is immersed. Keeping drag low avoids a drop in boatspeed that would in turn increase apparent wind and hence trimming force, pushing the bow down further.
The structure of a modern Marblehead consists only of a stressed shell with panel stiffness dictated by hydrodynamic loads rather than mechanical ones. Since there is no additional internal structure, the only areas left where significant weight reductions can occur are the fin and mast interfaces. Our experiments in production methods were aimed at minimising the total amount of material used in these areas. 
The boat is configured for an integrated fin case but where practicality allows we always recommend to opt for bonding the fin into the boat directly.

Octave Development
The first prototype was completed in October 2009 and has been campaigned by Ray Joyce in Tasmania. The prototypes differ from the production boat in several significant respects - see recent tagged posts for more. 
In short the candidate features that we wanted to evaluate were pushed to extremes on the first boats. Feedback confirming the 'break-even' limits of the various design choices informed modifications to subsequent iterations. The lessons learned were incorporated into the calculations used to finalise the production boat. 
This approach allows us to learn, progress and validate our choices without subjecting the customer to excessive risk. 
By the time the investment is made in production tooling and the boat is offered to the market, confidence is high in the reliability of our performance expectations.
Only after completion of the prototyping programme were second generation CNC milled tools prepared to allow a different construction method incorporating a number of novel structural solutions in the production boat.
The following pictures show the key stages of construction of the prototypes.
Initially, for expediency and to minimise risk, hull and deck were laminated separately and joined by a moulded strip element running along each side.
The mast tubes and fin were glued into the hull then secured with secondary bonding (strips of carbon cloth laminated over the joint).
A plate was bonded to the top of the mast tubes and fin where they were to mate with the deck. This plate provides sufficient contact area to transfer the keel and rig loads to the deck. Finally the deck was glued on and the finishing work could begin.
This is a rather labour intensive construction method that requires precise cutting and fitting. It is acceptable for a prototype but needed to be improved upon for production boats.
Our investigation into construction methods has resulted in proprietary options for quick, light, strong and ‘seamless’ moulding and assembly.
In all other respects Octave is an evolution of our long line of Marblehead Class boats. Beam has remained similar since Nexus. Waterline beam is determined parametrically as the unique dimension that will allow a semicircular midsection for a given combination of displacement and prismatic coefficient. For any given displacement and prismatic coefficient there will be only one semicircle that gives the correct midsection area.
Above the water Octave is a bit fuller and more flared amidships than Nexus, continuing the direction explored by the Ajax family of designs. The centre of gravity has continued to move forward and the prismatic coefficient has increased progressively. The stern has become fuller just above the static waterline as part of the trend to higher prismatic coefficients.
Onboard systems are much simpler than on Ajax. This is mainly due to the return to an all swing-rig sail wardrobe. The winch system uses a captive loop housed entirely below deck, lowering the winch centre of gravity and maintaining the deck free of clutter.

We are heartened by the resurgence of the M class worldwide after the recent slump and are working hard to remain at the forefront of development with Octave
Please enjoy the pictures and drop us an email with any comments or queries...

Previous Designs
Following is an outline of some of the stages in the journey toward our current Marblehead design philosophy. The text is partially a reworking of articles we have published at various times. 

Previous designs are listed from oldest to newest.

Our ongoing research started with a parametric study and has continued through incremental exploration punctuated by more ‘out there’ experiments. In the process we have amassed an enviable body of knowledge and a sophisticated arsenal of design tools.

Introduction to the challenge
The M is a development class. It is a test bed for ideas relevant to yacht design at large. Performance on short courses is the primary concern. Acceleration from down-speed manoeuvres is therefore vital. The boat must be well behaved and responsive in all weather conditions. Good communication with the skipper must be maintained, as feedback is essential to refinement. Paramount to our task is the notion that yacht design is a series of compromises, juggling conflicting requirements on many levels. The M Class rules restrict design space to a monohull with maximum values set for overall length and sail area. The boats have been evolving in competition since the 1930's. Essentially, the driving factor is the fixed ratio of length to sail area. The designer must ‘shrink’ the hull under the given rig, maximising efficiency.

Maverick, Design MDV01-01 Through MDV01-26
Maverick was the first Marblehead design by Dario Valenza.
It formed the baseline for a study undertaken with help from Henry Nehrybecki whose experience at the top end of the class was invaluable. The study set up a reliable matrix referencing the effects of parametric variations based on full size measurements.
As we worked our way through the range of beam, displacement, wetted surface area, prismatic coefficient and centre of gravity values we gained a reliable understanding of the effects of these values on performance.

This outlying light displacement 'skimming dish' design presented an unusual performance envelope. Acceleration and heavy weather downwind pace were outstanding but the boat was difficult to sail and decelerated too quickly in light airs. 
The aim of the exercise however was to fill in the matrix and gather data. With several hulls built as part of the study, different construction methods were also tried, varying cloth weight, resin content and joining methods.

Goose, MDV02-01 Through MDV03-06
Goose was the name given to a family of designs beginning with the first boat to incorporate the knowledge gained in the parametric study mentioned above.

The second hull was campaigned in Europe by a number of skippers and aroused interest from a number of Italian builders. This resulted in a co-operation with Cutuli, Furlanetto and Alberti. The final boats in the series ended up reflecting some trends that characterised the light air venues of Europe. Overhangs were introduced to reduce wetted area and decrease prismatic coefficient.

Construction was elaborate and elegant with some boats featuring curved Nomex cored decks. Great effort went into cleaning up the aerodynamic design of the topsides and deck. The sheeting system was relocated from the exterior to inside the boat, further cleaning up the deck and lowering the CG.

Foil area was gradually parred down to truly minimal values and a lot of work went into rig optimisation. The all swing rig platform used 'fat headed' mainsails in novel Mylar/aramid materials. Perhaps the most distinctive feature of this family of boats was the pioneering use of Waliki 'wing' masts on a swing-rig platform. 

Nexus, MDV04-01 Through MDV06-03
Having travelled as far as practically possible down the road of the previous design concept, Nexus was a 'clean sheet of paper' design.

The concept was driven by the realisation that minimum wetted area and a slender hull should be prioritised if ballast ratio could be maximised. A simple process was used to generate the prototype hull shape: displacement was chosen with reference to projected construction weight and optimum draught; A prismatic coefficient was decided; Waterline length was stipulated as overall length minus sufficient overhangs to account for trim, pitch damping and keeping the bow bumper clear of the upright waterplane; Finally the unique radius was calculated that would generate a semicircle with the correct area to constitute the 'midsection' of a hull with the prescribed geometric values. This semicircle was positioned at the LCG location and the hull lines constructed around it. This gave a very elegant slender piercing hull.
With the experienced gained to this point we could also confidently optimise freeboard and sheerline profile.

Construction technique was gradually refined to produce a monocoque, lightweight, stiff yacht with radiused gunwales. We played with different moulding techniques ranging from proven fore/aft split female tooling to more experimental arrangements. The radical technique of taking the hull directly off the plug yielded a seamless shell for all but the centre deck but required considerable finishing and care had to be taken in applying the final peel-ply layer.
The deck layout used a suspended ‘well’ arrangement for the radio gear. The centre deck moulding incorporated the radio gear tray, access hatch, mast tube and centreboard trunk. The majority of the ‘deck’ area was made of 'stickyback' sailcloth adhered to the side decks.

Nexus reflected thinking characteristic of the time with centres a long way aft, ultra fine entry with high forward freeboard and narrow flared aft sections.


Pi, MDV07
Pi proposed to dramatically improve M class yacht performance by attacking the fundamental issue of stability. Pi used the proven Nexus platform as a starting point to reduce variables and isolate the major item being tested: a canting rig.

Following is an abridged version of the academic report published at the time:
"A Marblehead can be 'powered up' with A-rig in fewer than three knots of breeze. Light weather performance is undermined by excessive stability. Matching fore-and-aft stability to athwartships stability is important for minimising drag. The maximum mast cant afforded (within the practical restrictions found through experimentation on the yacht) is of the order of 40 degrees. Resultant stability gains were close to 70%.With this newly available righting moment, our choice is between:
i) Having a boat with immense stability, able to carry A rig in, say, 10kn of wind.
ii)Reducing other forms of stability to bring the total RM back to sin35 x 0.55 x 4.2 kgm.
With option i)
- The boat would have the same displacement as the original.
- Heavy weather upwind performance would be close 20% greater in VMG.
- Fore/aft stability would become a problem as it would be roughly equal to the original and hence inadequate to handle the additional sail area carried upwind.
- The boat would be far too "stiff" making it unresponsive in light winds.
- The parasitic weight of the canting mechanism would be a dead loss in light air.
- All existing "gear" would be prone to breakages due to the additional loads imposed by the greater stability.
With option ii) we can
a) Reduce beam

- Further beam reduction would increase wetted surface area due to the departure from semicircular sections.
- Form stability is already at a practical minimum.
b) Reduce draft
- The penalty for draft is minimal (in the form of frontal area running) but is mitigated by possible fin area reduction due to fin efficiency. [It is worth noting that since then new research has been carried out on the effects of Reynolds Number on foil performance]
- A high-aspect ratio fin is desirable.
- Centre of lateral resistance would be closer to centre of buoyancy, reducing heeling moment.
- Launching, sailing close to shore and transport would be facilitated, but these are secondary considerations.
c) Reduce ballast mass
- Weight, the perennial enemy of speed, would be reduced.
- By reducing displacement an edge can be gained in light winds and running, when stability is not an issue.
- Existing gear can be used.
- Rigs can be changed at their intended wind velocity.
- The parasitic mass of the mechanics is offset by lower displacement.
The stand out choice was to reduce bulb weight. Calculations on stability indicated a required bulb weight of 1.3kg. This weight is impractical; it would make the boat prone to stalling when tacking in waves. Guessing at 2.1 kg, an IOM class bulb was adapted for the purpose. The test hull, though based on the Nexus lines, was scaled to take into account the decreased displacement."

And from another article explaining the basics for a broader audience:
"To approach the problem laterally, we break it down to its natural components. The sail force is applied above the water and it forms a couple with its underwater opposite. Sailing craft performance is directly related to the ability to provide an opposing couple with minimum penalty (energy redirection).
Multihulls use the leverage afforded by massive beam. Skiffs use crew weight on trapeze. Windsurfers use body and rig weight. Open 60's use some combination of the above in addition to water ballast or a canting keel (some have experimented with canting rigs albeit using cantilever of the heel. The class promptly banned canting rigs). Conventional yachts must use heel.
M class r-c yachts are symmetrical and hence restricted to form and mass stability derived through heel.
To form a couple some distance must displace the forces of gravity (at CG) and buoyancy (at CB). Heel reduces the efficiency of keel and sails.
Within this restriction boats have been refined to exquisite levels, but they must face the above limitations. Trading displacement for beam does not result in a net gain due to the scale. Furthermore, since the class rules restrict sail area, the ratio of wetted area to sail area would increase prohibitively.
Varying geometry means the necessary moment can exist before heel. It also means that the boats equilibrium will not be the upright position. Stability will increase in a different manner with applied heeling moment.
Canting keels have been proven in open60 and 50 class yachts. They offer a marginal advantage in some conditions to those classes albeit with some drawbacks. Canting keels are prohibited in the M class rules but could be included once their merits are proven. Providing lateral resistance would require an additional appendage.
The chosen mechanical solution was to pivot the rig at the hull, sweeping it bodily to windward within a watertight 'well'. This maximizes the stability effect desired. It also presents the sails to undisturbed wind. Options considered for mast control included rack and pinion type mechanical arrangements, cogs, miniature hydraulics and chain-drives. All the above options were discarded on account of parasitic weight. A 3:1 purchase with a standard winch drum and continuos loop was selected. This purchase system also moved the sheet lead ring with cant."

Looking back now, it is clear that the concept had merit but the practical issues to resolve are significant. The principal technical requirement is finding a solution to address the change in helm balance with mast cant. This was anticipated on the experimental boats and addressed by positioning the mast further aft than would otherwise have been called for. A more comprehensive solution was found to be required since the reduced 'lead' made tacking difficult. Perhaps varying rake with cant or shifting the CLR with adjustable foils would address the problem. From the point of view of the skipper, managing mast cant is an additional challenge but the performance gains may justify the effort. It remains to be seen whether the new class rules, with their modified wording specifically addressing the trail blazed by Pi, would still sanction such a configuration. This is a path that demands a renewed foray when the opportunity arises.

Ajax 1, MDV08
Ajax Mk1 incorporated the lessons from Nexus etc. The general look of the boat was more angular, with sharp gunwales and a different deck patch geometry. Cerberus (Design MDV06-05) marked the culmination of the Nexus family of hull shapes. It is worth noting that with Cerberus we reached a turning point as it marked the beginning of the move toward fuller forebodies. This trend was followed by boats such as the 2006 Waliki Skalpel and the V5 IACC fleet. Our move was rooted in a better understanding of wave interference in displacement hulls. The effects of the study on 'blunt' bows were interesting and surprisingly far-reaching.

Cheron (Design MDV06-11) drew from these lessons, attempting to tie in the positive dynamic effects with a longer wavetrain. Comparison with internationally proven boats went something like this: whilst beam was still on the narrow side (15mm wider than Cerberus on deck - BMAX located at about 60% LOA), hull proportions were more conservative than Bantock's contemporary Rock. His wide stern, heavy displacement and powerful hull point to a boat where the compromise is shifted more toward blasting out of the tacks, powering through in waves and being able to foot well. The Berlioz, with dished hull and large section rigs, showed impressive top speeds on the reaches but seemed to struggle upwind when pressed and on all points of sail in light airs.

The Skalpels are the most similar to Ajax Mk1 in concept (with a notable difference being the extra forward beam at deck level, necessary to provide an adequate shroud base) but are more critical downwind. Ajax Mk1 likes to sail with a bit more heel, winding up tight and high. Sydney offers a wide cross-section of weather conditions, so a bias toward stronger breezes (e.g. Nordic boats) or toward the light stuff (e.g. US and Italy) really shows over a season.

Results of experiments with sharp gunwale profiles led to the abandonment of the radical moulding technique perfected for Nexus.
The trade-off here is between windage to windward and separation of the water flow to leeward. Once again we are reminded that wetted area is everything on these little boats. Short of a peaked foredeck, sharp gunwales are the most effective compromise solution.

As far as hull geometry is concerned, centres gradually moved forward, prismatic moved toward the higher range and draft was reduced slightly from the rule maximum. The whole package was parred down, eliminating fittings and reducing the total number of parts in the perennial battle with weight. The emphasis on simplicity lowered costs as a side benefit.
There were also simpler and lighter swing rigs. They used Walicki spars spliced into cantilevered 12 and 14 mm tube just above the boom/yard junction. Each spar was 'tied' together at four points using carbon tow to give the largely unidirectional tubes better 'hoop' strength.

With new carbon booms, the rigs were incredibly light and mainsail leech control had improved. Nexus had successfully married a stayed rotating wing with a swing-rig. Compression adjustment had allowed the rigs to 'fall off' in light air and ‘fall forward’ on the run.
The new masts were not over-rotating, but the aerodynamics were still acceptable and still superior to a round tube. Beyond the weight saving and simplicity, the main improvement was in gust response. Instead of having a tendency to invert (as they did with the pivot point on the leading edge), now the spars bent athwartships, spilling the leech of the main and flattening the sails when necessary.

Be sure to check out the race reports in the In the Press page featuring contemporary accounts of development on the water.

Ajax 2, MDV09
Ajax 2 was an all new production standard design that aimed to be competitive across all conditions. This meant a careful compromise between wetted area and wave drag with painstaking treatment of the transition from entry, through midsection, to stern. The fin and LCB were placed fairly centrally though slightly further forward than most contemporary designs. Foil sections were highly optimised and foil areas were based on the results of a long evolution.

Crosspollination with the IOM class had taught us precision. This was reflected in the deck layout and adjustment modes. ‘Bowsies’ were completely eliminated in favour of 'hooks and holes' adjustors and turnbuckles.
The winch could be ordered through-deck or below deck according to owner preference. The deck detailing included the option of sophisticated soft ‘padeyes’ for standing rigging and an elegant moulded solution for the sheet lead to the winch when this was mounted below deck.

Since the deck was flat and sat within the sheerline, it made sense to use foam-sandwich in its construction. The top skin was carbon. We used Corecell in the middle because it has the right compromise of density, rigidity and resilience. Nomex was considered but with so little deck area (large lightning and access holes were standard) the advantages would have been utterly marginal compared to the cost. The lower deck skin was of very thin S-glass scrim.
With the deck assuring rigidity, the hull consisted of a layer of 120g/m^2 carbon cloth and an internal glass scrim for resilience/watertightness. Paul Armstrong invested heavily in the plug and female mould. The time and effort he poured into fairing are immediately visible in every Ajax 2hull. The centreboard lived in a carbon case glued through the hull and through the lower deck skin.

The rudder tube was supported in the hull and deck with an intermediate section cut out of it so the tiller could capture the shaft. The fin had a foam-core that tapered to almost nothing toward the thin tip. The rudder used a lower density core and a carbon shaft.
Perhaps the single most distinctive feature of this design was the use of conventional shroudless rigs for all suits with the exception of A-rig.
Overall Ajax 2 has proven fast, reliable and rugged. The design represents the culmination of a long evolution that is now taking another new turn with Octave.


  1. This is an awesome resource for sailors. I was looking ot get into RC sailing from: and was considering the megatech nirvana II. What is your expert opinon on this craft? Cheers

  2. Bonjour, Vos classe M sont-il à vendre si oui où peut-on en trouver en France. Merci.


  3. Hi Patrick, Tooling for Katana Mk2 is well underway. Keep an eye on the front page for updates. Contact us on " info at carbonicboats dot com " for more details about ordering and delivery.

  4. Hello Dario thank you for the answer. See you.
    Regards Patrick