Diode 36 “Rampant” by Rodger Martin Yacht Design – a unique sportboat…with a twist! Conceived as a prototype for a future one-design class emphasizing speed, tactics, economy, and versatility.
Featured in January 2005 SEAHORSE
|Length Overall||36' / 10.97m|
|Length Waterline||33.75' / 10.29m|
|Beam||10.92' / 3.33m|
|Draft||9' / 2.74m|
|Displacement||4320# / 1959kg|
A sportboat set apart from other sportboats –
LIGHT & FAST – Stiff with 4-5 crew, slippery in light air, and sport-boat planing ability
FUN – No rules. This is a ‘One Design’ or PHRF speed machine for racing in protected waters.
TACTICAL – ROTATING KEEL – speed with pointing ability upwind
UNIQUE – A ‘32 foot boat with a 4 foot bow,’ from which to fly an asymmetrical spinnaker and code-0, thus avoiding the weight & complexity of a retracting sprit.
VERSATILE – Keel lifts to get to yacht club hoists and to go on a trailer. Beam under 11 feet (it is 10’ 11”) so it can be towed in the US without an escort. Lifting rudder for the same reason.
Ballast ~ 2240 lbs. (foil & bulb)
Sail Area ~ 700 sq.ft
S.A. / Disp Ratio ~ 43 (lightship)
S.A. / Disp Ratio ~ 37 (crewed)
D / L Ratio @ Lt. Ship ~ 48
D / L Ratio, crewed ~ 58
How do you make a planing hull go upwind better? Increase lift and reduce drag! An innovative feature of the Diode 36 is the rotating carbon keel. The ability to align the rotating keel closer to the longitudinal shape of the heeled waterlines, allowing for leeway, reduces drag for the lift provided from both keel and heeled hull. In addition the upwind heeled waterlines have been carefully sculpted to keep the heeled canoebody symmetrical, and no more than about 5 degrees off the centerline. The result: exceptional upwind speeds and angles for a superlight boat. Pointing with other IMS boats, Diode is sailing upwind at 7.2-7.3 knots, which is exceptional for a 36 foot sportboat of her type, and easily exceeding our own continuously refined VPP’s.
The Diode 36 is a “crossover” boat incorporating both dinghy and keelboat characteristics. Fine waterlines forward and deep forefoot provide wave-piercing and upwind efficiency. Waterline length is maximized for speed, hull sections are rounded for low wetted surface, and sections aft are wide for offwind planing. The waterplane is very narrow keeping wetted surface low, and there is great flare in the topsides to keep the limited crew as far outboard as the beam limit allows.
The hull has generous rocker for control over fore-and-aft trim, and to turn quickly. When the hull is upright, the ends are well out of the water; when the boat is heeled upwind, the forefoot ‘ rolls-in’ giving the bow enough bite to resist its being blown to leeward. Similarly, when the spinnaker is up, the bow rises keeping the forefoot well clear of the water, the prismatic high and the rudder well-immersed. The high rocker also allows the boat to slide more easily ‘off-axis’ off the wind with the keel down to leeward a couple of degrees. Off the wind, we have seen 15 in about 20 knots true, and these are the early days!
ROTATING / LIFTING CARBON KEEL
The global goal was to make our keel perform the same function as a gybing centerboard, and so we looked often at the 505 dinghy, which can be built with a gybing centerboard, for ideas and inspiration. A rotating inner drum contains the keel fin itself and is supported on two Ertalyte (PET-P plastic with excellent wear resistance, a low coefficient of friction, high strength) bearings within the fixed outer drum. The inner drum is retained by a flange resting on an Orkot (low friction composite) thrust bearing, and set up to be rotated by winch handles through port & starboard Antal line drivers leading to multiple purchase and then to the drum. The keel angle controls are located aft so that the skipper or main trimmer can play them. The keel turns easily and accurately when underway, and upwind we are seeing it perform exactly as anticipated, giving us speed with height.
The keel can be lifted with an A-frame and tackle that leads aft to the mainsheet winches. Once the keel is up, it can be pinned for motoring and hoisting, and for setting the boat on its trailer.
ENGINEERING & CONSTRUCTION
A major benefit of using concentric drums to fix the keel is load distribution – even with the 9’ draft, there is 25% bury to the fin cantilever which translates to manageable side loads taken to hull and cockpit sole, which did not have to be overly reinforced. Keel floors are also not required, keeping weight down. Forward of the mast is a delta bulkhead, spanning out to the composite chainplates – such structure is very efficient as it performs several functions; handling mast compression and inboard shroud tension, distributing the keel’s racking loads on the hull, and framing the forward edge of the cockpit and side tanks.
Keeping weight low and construction costs down emphasized the necessity for efficient use of materials and structure. In response we used a thick (1” A500 & A550 Core Cell) core in the hull (3/4” for deck and cockpit), with biaxial E-glass skins engineered to be as thin as possible while meeting ABS requirements for strength and stiffness. Internal structure of frames and stringers is minimized. There two frames forward of the delta bulkhead, the forwardmost aligned with the headstay. Aft of the keel drum are two frames. Due to the hull’s thick core the only longitudinal structure are panels framing the outboard engine storage well aft of the keel drum, and small stringers forward under deck. The hull shape also contributes to the boat’s efficiency. Soft form transitions everywhere eliminate stress concentrations. Unavoidable sharp transitions, such as the hull / deck joint, are radiused and cored to ease stress and maintain stiffness from panel to panel. Overall the boat is extremely stiff and plays upwind through waves with imperceptible shudder and zero fuss.
The Diode 36’s sailplan has moderate aspect ratio, designed to keep the boat on her feet even sailing with a small crew. The carbon mast is designed as a light, simple rig that’s easy to control and can be stepped and unstepped quickly. Long, swept spreaders give a stable rig and allow for a large non-overlapping headsail. Chainplates located out at the rail reduce rig loading and keep the sailplan light. Additional weight is saved using Equiplite halyard lead blocks, small Harken Slider system, and Vectran runners.
The first Diode 36, “Rampant” was built by New England Boatworks in Portsmouth, RI and launched September 17, 2004
See her acticle in Seahorse International Sailing magazine January 2005.