Bagatelle is a 44’ long lightweight sloop designed by Eric Sponberg for Tom Doyle. Bagatelle is noteworthy because she is an excellent example of lightweight engineering. Doyle wanted Bagatelle to be fast, light and easy to build out of wood and epoxy laminates. Doyle wanted a boat that would plane easily in light winds and he also wanted a boat that he could sail single handed for up to a week.
Hull Design
Bagatelle’s lines aren’t particularly earth shattering or revolutionary but instead are deeply conservative, stemming from the design requirements set forth by Doyle.
- Style- bagatelle has a long clear foredeck, low freeboard, a plumb stem and a “scoop” stern.
- Shallow draft- made necessary weight and speed requirements.
- Narrow beam- function of the intended weight and the desired ability to plane
- Transportable- The boat can be trucked from the marina to Doyle’s house with only a “wide load” sign and no special escorts.
The boat’s design borrows from the style of boats during the 1930’s with it’s “scoop” stern causing a the boat to have a long waterline with a shorter deck line. The teak paneled cabin house, also contribute to this sleek look. Bagatelle's design displacement was only 7130 lbs (3234.1 kg) at the waterline. Bagatelle features an 8’ keel that has 2500 lbs of cast steel in a bulb. The keel stays at 8’ for racing and cruising and raises to 5’ through a specially designed trunk for “gunkholing”.
Bagatelle has a very “hollow” waterline as a result of the CAD design process. For some reason CAD programs create hollows while traditional design methods using a pencil, splines and battens you have to work to create hollows and in CAD designs you have work to get rid of the waterline hollows.
Rig Design
Bagatelle was designed to have a high power to weight ratio, so the rig needed to be big and efficient. Bagatelle has a tall Bergstrom and Ridder (B&R) rig. The B&R has no backstays but instead has highly swept spreaders and standing rigging featuring both diagonal and reverse diagonal shrouds. The B&R rig is very well suited to heavily roached sails such as the elliptical mainsail that Bagatelle features. A mainsail with plenty of roach most closely resembles an elliptical planform; a shape with the most lift and the lowest drag or in other words the highest efficiency. Bagatelle features a carbon fiber mast to save weight and increase righting moment. The mast is fitted with aluminum spreaders to save costs. The headstay is set back 4 feet so the headsails can be tacked onto the deck making the rig easier to sail.
Hull Engineering and Construction
Bagatelle is built of 5/8” thick strip planking with two 1/8” thick mahogany veneers on the outside. This 3-layered hull laminate gives the hull very high sheer strength and allows the bottom to withstand in excess of 18 psi before it will stress crack.
This maybe a little weak for an offshore cruiser, but bear in mind that Bagatelle is not designed to be sailed offshore but as a buoy racer and short excursion boat. This “low” stress environment allows the hull to be lighter and simpler.
Deck and bulkhead engineering and construction
Bagatelle’s deck and bulkheads are made fro solid plywood laminates, foam cored panels could save some weight but not enough to justify the cost in most cases. Again the bulkheads are simple plywood and spaced to give maximum strength. Bagatelle’s keelbox is designed to be conveniently framed by bulkheads, thereby cutting the span distance in half.
Bagatelle’s deck is made of solid 1/4” plywood laminates. The deckhouse is balsa-cored fiberglass laminate to make it absolutely watertight and to save weight, allowing more weight to be placed in the keel.
Keel and bulb design
Since Bagatelle sails mostly in calm water, she has a thin 10% section foil to lessen the drag. The keel also employs an LSI-1 foil; LSI-1 foils have a maximum thickness at 40% of chord length instead of the more common 30% used in NACA 00-series foils. LSI-1 foils are also slightly hollow in the after sections creating a low wake drag shape.
Bagatelle’s keel is made out of a stainless steel frame cored with Douglas fir and carbon fiber skins. The keel also employs “crash” boxes fore and aft in the keelbox. Those crash boxes compress if the keel strikes an object preventing major damage from occurring.
Rudder design and engineering
Bagatelle uses a simple blade rudder with a thicker 15% LSI-1 foil section. Rudders that have thicker sections are more effective at high steering angles because the flow stays attached longer before cavitating. The blade rudder design allows the rudder to self-center and provides low loads to the tiller.
The rudder is built from a fiberglass G-10 core with foam coring to provide the shape, making for a light well-balanced rudder.
On water results
Bagatelle as launched on July 23, 2001; stability tests showed that she weighed 1639 lbs heaver than designed. Other tests showed the VCG was 5” lower than designed, making the boat more stable and possibly increasing her speed. The boat can tack through 85º upwind easily. Bagatelle cannot however sail directly downwind because the spreaders don’t allow the mainsail to set at the optimum angle making it jibe unexpectedly. Instead Bagatelle must tack or jibe downwind.
- Hollow waterline- Waterlines that are concave instead of covex live most boats. Hollow waterlines are generally undesirable Aesthetic elements.
- Gunkholing- a short cruise through rivers and sloughs
- NACA- National Advisory Committee of Aeronuatics
- VCG- Vertical Center of Gravity
- Bergstrom& Ridder rig-
/| |\
/ | | \
/ | | \
/ | | \
xxxxx| |xxxxx
\ /| |\ /
\ / | | \ /
/\ | | /\
/ \| |/ \
xxxxx| |xxxxx
\ /| |\ /
\ / | | \ /
/\ | | /\
/ \ | | / \
xxxxxx| |xxxxxx
\ /| |\ /
\ / | | \ /
/\ | | /\
/ \| |/ \
|___/____| |____\___|
\ Hull /