SC40

SAILBOAT DESIGNS

The Scandinavian Cruiser 40

The SC 40 Sport Cruiser sailing. This rendering was created from the original 3-D surface model from SYDI and rendered by Leo Wu at Tong Zhou 3d Design (TZ3D) Studios in Fuzhou, Fujian, China.

Design Scope and Style. Pangea Holdings Ltd. (PHL) provided the general concept of the design, an updated version of the Skerry Cruisers of Scandinavia from 100 years ago. The Skerry Cruisers were and are “Square Meter Boats” in various sizes as defined by the Skerry Cruiser (Square Meter) Rule. One of the current sizes still sailing in active fleets is the 30 Square Meter, and it is this version on which the SC 40 is based, being of similar length and weight. However, there is no regard taken at all to the Skerry Cruiser Rule in the design of the SC 40, or to any other design rule. The SC 40 will be a class unto itself.

The Skerry Cruisers’ principal design features are their long overall lengths, long overhangs, low freeboard, narrow beam, and small coach roof. They have fixed keels with a rudder attached on the keel trailing edge. The rigs are stayed sloop rigs. Propulsion power on the 30 Square Meter boats is by outboard motors.

Sail plan of the SC 40 Day Boat..

Deck plan of the SC 40 Day Boat.

Hull Form: The SC 40 is like the Skerry Cruisers in the basic hullform: long overall length, long overhangs, low freeboard, narrow beam, and small coach roof. Therein lies the sum total of the similarities--just about every other feature is different, although of true 21^st century types and styles. Most notable is the rig. The SC 40 is a sloop with mainsail, jib, and asymmetrical spinnaker, flying from a carbon fiber free-standing, rotating wing mast. The keel is a carbon fiber lifting keel with a T-configuration cast antimonial lead ballast bulb. The rudder is made of carbon fiber and is also lifting, housed within a composite rotating drum with steering by tiller affixed to the drum. The design accommodates three crew while sailing, and will support two sleeping people in port. Inboard propulsion is provided by a 4.8 kilowatt electric motor connected to a shaft and propeller and driven by two Lithium-ion 12-volt DC batteries. The SC 40 will be provided with a shipping/towing double-axle trailer. The complete boat sitting on its trailer will fit inside a standard 40-Ft. long shipping container. The mast is sized so that it, too, will fit inside the container, being built in two separate parts. The mast will be mounted into its place on the boat by a lifting mechanism attached to the trailer.

PARTICULARS

Length overall 11.957 M

Length on the waterline 7.663 M

Beam overall 2.040 M

Draft (canoe body), Design Waterline (Dwl) 0.321 M

Draft (keel) 1.750 M

Displacement (to Dwl) 2.115 Metric Tonnes

Ballast 1.000 Metric Tonnes

Sail Area: Main 32.500 Sq. M

Jib 10.810 Sq. M

Total 43.310 Sq. M

Prismatic coefficient (canoe body only) 0.5647

Displ./(.01Lwl)³ 131.0

Ballast/Displ. 0.473

SA/Vol.^2/3 26.72

SA/WS 2.993

The overall look of the SC 40 is quite unique, a blend of traditional as well as modern. The lines and surfaces are very clean and crisp. There is a minimum of deck hardware, and where possible, the metal hardware is polished bronze. There are no hatch or portlight frames above deck; all structural frames for openings are to be made of composites as much as possible and be positioned under deck. Joints between parts are to be tight and uniform.

Inboard profile of the SC 40 Day Boat.

Interior plan of the SC 40 Day Boat.

Rig Design: The SC 40 is equipped with a sloop rig with that includes a main sail, jib, and asymmetrical spinnaker which are flown from a free-standing, rotating wing mast. Although the mast is designed for strength and stiffness to be completely free-standing, it nevertheless is equipped with multi-part running backstays port and starboard which are used with the gennaker to keep the mast from pumping. The running backstays may also be used with the jib in heavy wind conditions.

The mast is comprised of two parts which include a fixed stub mast mounted on the inside cabin sole atop the Center Vertical Keel (CVK) and extends up through the coachroof. On the exposed portion of the stub mast above the coachroof two bearings, lower and upper, are mounted which accept the second part of the mast, the wing mast. The stub mast is connected and fastened to a stainless steel mast heel fitting which is fixed to the cabin sole and keel structure. It is also wedged at the coachroof boss with wedges and/or seals so that it does not leak water below.

The wing mast slips down over the stub mast and slides onto the bearings. The lower bearing is a combination thrust and journal (lateral load) bearing, and it is mounted directly against the boss molded into the top of the coachroof. The upper bearing is mounted at the top of the stub mast and is a simple journal bearing. Both bearings are sized and shaped to fit closely, but not necessarily tightly, to the inside surfaces of the wing mast, which may in turn be shaped with additional structure to make sure the fit is simple and easy to make. The weight of the wing mast and its running rigging keep it in place on the stub mast.

When the boat is sitting on the boat trailer, the complete mast is removed from the boat and its two parts separated and stowed on chocks on deck. Messengers are provided to make sure that the running rigging can be re-rove through the stub mast as necessary. A lifting mechanism, to be designed, is affixed to the trailer, and allows the crew to install the stub mast in place on the boat without the aid of a dockside crane. A crane may be used if desired.

Keel Design: The keel and bulb are built as a single unit. It incorporates lifting gear and attachments that allow it to move up and down. Usually, the keel is lowered into position for the day’s sailing and is retracted only when entering shallow harbors or being maneuvered onto the trailer. The total height of the keel blade and bulb is 1900 mm, and when fully retracted with the bulb against the hull, the top of the blade sticks out of the top of the coachroof. This height determines the full height of the boat when on the trailer.

The keel blade is made of carbon fiber over a solid core made of higher density closed cell foam or wood. The blade aerofoil section is a modified GA(W) aerofoil of 10% thickness. The basic blade chord length is 600 mm, therefore the blade width is 60 mm.

The ballast bulb is an antimonial lead casting with a stainless steel armature built into it for affixing it to the keel blade. The bulb shape is a flattened bulb with a scalloped and flattened tail. It is positioned such that the nose of the bulb is ahead of the keel blade leading edge. The position of the keel in the boat and the position of the bulb on the blade are absolutely critical to the flotation and the proper sailing performance of the boat. The estimated weight of the ballast bulb is 1,000 kgs.

The keel slides up and down inside the keel casing which is made of composites and bonded into the boat. The casing inside shape is rectangular, but the keel blade is aerofoil shaped. The bottom end of the keel casing is closed off between the casing and the keel blade by a hard rubber plug that serves three purposes: 1) It closes the gap to the keel to minimize hydrodynamic drag; 2) It provides a soft contact support of the bulb against the hull when the keel is fully retracted; and 3) It absorbs the shock of the keel blade against the hull in grounding. Complete details will be provided in the final design.

The keel is lifted by rope rove to a 4500-lb-pull (2.0 tonnes) marine trailer winch that is mounted on the centerline bulkhead ahead of the keel.

Rudder Design: The rudder is a carbon fiber blade that is housed in a rotating composite drum that is supported in the hull by upper and lower bearings. Complete details will be forthcoming in the final design. Steering is by a tiller which is affixed to the drum. The rudder slides up and down by hand and is pinned in place at any of a number of discrete heights with a stainless steel locking pin.

The rudder blade is made of carbon fiber over foam core, molded in a female mold. Its overall length is 1550 mm, and the chord length is 300 mm. The aerofoil section is, like the keel, a modified GA(W) aerofoil of 10% thickness, therefore its width is 30 mm.

The rudder slides up and down in a composite rudder drum, OD 350 mm x approx. 710 mm tall. The lower end of the drum will be slightly convex to ensure low hydrodynamic drag when the rudder is articulated. The hull is very flat in section in way of the rudder, so drag should be minimal. The drum sits in a casing that is made out of composite in separate tooling and bonded permanently into the boat, connecting the hull and deck together. There are rabbeted surfaces top and bottom that accept the bearings for a flush fit.

The bearings top and bottom are the diameter of the drum and are made out of plastic or composite and equipped with plastic ball bearings. Both bearings are fastened to the rudder drum with fasteners and caulking. The lower bearing has a tight slip fit into the rudder drum casing, and the upper bearing is fastened to the deck by a flange. The flange incorporates a locking device that prevents the buoyancy of the drum from pushing it up out of the casing. The joint between the drum and the casing is close but not tight; therefore, water is allowed through the lower bearing and up into the space between the drum and the casing. This amount of water is minimal as the lower end of the drum is only just below the waterline.

The tiller is a custom molded carbon fiber tiller that is pinned to a gooseneck on top of the rudder drum. It is painted the same color as the deck, and be affixed with wood veneer either side in the style of the hull, wing mast, and boom. Similarly, this wood veneer is sealed with woven fiberglass cloth and UV inhibiting resin. The tiller also has a tiller extension so that the helmsman may control the tiller while sitting on the cockpit coamings or standing forward.

Transverse sections of the SC 40 Day Boat.

Mechanical Systems: The main mechanical system on the SC 40 is the propulsion motor, propeller shafting, and propeller. It is supplied as a complete system by OZ Marine in Sweden. The motor and shafting will be fixed in place on the hull centerline and underneath the cockpit. Access to the motor and shaft seal is through the main companionway step which is removable. Bulkhead panels port and starboard of the main companionway, and the fixed seat to port in the main cabin, are permanently fixed in place, but have removeable access covers. The space directly underneath the cockpit sole is closed off by bulkheads both sides and back to protect the motor space from water intrusion. Access openings in the side bulkheads allow service access to the motor and shaft stern tube.

Electrical Systems: The electrical system is 12VDC supplied by two 12V Lithium-ion batteries connected in parallel. These are kept charged by either two solar panels or a portable generator. The DC control panel is located in the main companionway step and should comprise only the few necessary circuit breakers and the master battery switch. Shore power is optional.

Weights and Stability: The weight estimate is quite detailed at the preliminary design phase as of this date, and it will be further refined and updated as the final design and construction progress. The SC 40 has a short and narrow waterplane, so accurate weight and center of gravity calculations are particularly important at this early stage to ensure the correct draft, displacement and trim in the final design. Little changes in weight and CG have big effects on displacement and trim, and these have to be kept under control.

But as a result of the light weight, low center of gravity, and narrow hull form, the SC 40 has a surprizing range of stability--180! That's right, there is no negative area under the righting arm curve, as can be seen in the righting arm chart below:

The SC 40 stability curve has no negative area.

Conclusion: The Scandinavian Cruiser 40 promises to be a remarkable sailing yacht that is both easy to sail and affordable. There are many modern daysailers around the world in the 9M – 12M (30’-40’) range, but they don’t have quite this style—retro/modern we may call it—and most are more expensive than what the SC 40 is trying to achieve. The big reasons for the SC 40s low price (which unfortunately cannot be disclosed at this time because it is a work in progress, but which is expected to come in well under the competition) come from quality construction in China and ease of shipping in a standard 40’ shipping container direct from the factory. Finally, there is not one other production sailing yacht of this class that is equipped with a free-standing wingmast rig. All of these features make the SC 40 truly unique.

The SC 40 Day Boat at anchor, 3-D rendering from TZ3D Studio..

For more information on the SC 40 design, please contact us through the Contact Us link below. You may also visit the Scandinavian Cruisers website at www.scandinaviancruisers.com.