Bagatelle

SAILBOAT DESIGNS

BAGATELLE

Thomas Doyle of Waterford, CT, is the owner of this fresh new design from our board, which was launched on 23 July, 2001. An avid racer, Mr. Doyle wanted a lightweight boat that could plane in under 20 knots of wind, point really well and sail fast upwind, and yet still look like it belongs in good company sitting next to a Concordia yawl—in his words, “a modern classic.”

Specific features include:

Very lightweight construction (Douglas fir veneers over western red cedar strip-planking)
Chined hullform in the stern sections for planing
Lifting keel for easy trailerability
B&R rig (no backstay, triple spreaders) for high lift, large sail area, and high speed
Water ballast

PARTICULARS:

LOA 44’ 0”

LWL 40’ 4-3/4”

B 10’ 6-1/2”

D 5’ to 8’

Displacement 7,500 lbs. (approx. design lightship)

Sail Areas Main 669.1 sq.ft.

Jib 257.8 sq.ft.

Displ/(.01Lwl)³ 50.79 (at 7,500 lbs. displ.)

SA/Vol^2/3 38.71 (at 7,500 lbs. displ.)

We spent considerable time on this design studying Velocity Prediction Program (VPP) results in order to determine whether this boat will plane as desired. Practically all VPP programs operate with the boat in displacement sailing mode. However, since this boat intended to plane much of the time, we needed a special VPP. We found it in Peter Schwenn’s work at his company Velocity, Inc. Peter has modified the hydrodynamic code of his VPP to account for planing performance. The results of these studies confirmed our target values for hull weight, ballast weight, and sail area.

The VPP studies were conducted in three stages. The first stage was to determine the feasibility of the design and to test variations in beam and the addition of a chine. VPPs are not fluid dynamic codes, so they cannot show details of planing flow. Therefore, all we could look for were adverse drag effects that the volume of the chine might have, and we did not detect any. The second stage VPP study was to confirm our near-final design dimensions, the amount of ballast in the bulb, and the speed effects of water ballast (more ballast is usually better). The third VPP was with Peter Schwenn’s VPP code to confirm planing performance. The final comparison between non-planing and planing performance is shown in the figure below. Based on speed-length ratio, planing should be occuring at over 12 knots.

A rendering of Bagatelle’s final hullform is shown in the next picture below. Note the wide, flat after sections and the very narrow bow sections. The chine is very pronounced back aft, and as it goes forward, it blends into the bow and disappears forward of the keel.

Bagatelle’s general arrangement is shown in the next picture. This gives some idea of the structural layout as well—bulkheads, frames, and keel structure. Note that there is no standing headroom in the forward cabin. This is one of the compromises to exterior appearance and aesthetics. This design draws lots of compliments from the public, in part, I think, from its long, sleek look—the very long nose. The price for that look is no headroom. There is generous headroom in way of the coachroof.

The actual structure can be seen more detail in the next photo, taken when the hull was turned over. Note the ring frames and keel/keelson structure forward, and the two bulkheads at the fore and aft ends of the keel box. The very last frame near the stern is dummy frame to hold the hull in shape until the transom could be installed. The hull layup is on layer of 5/8” thick Western Red Cedar tongue-and-groove strip planking with two exterior layers of 1/8” Douglas Fir veneers running at 90° to the strip planking. The reason for running the veneers this way was to speed up laminating. The hull surface is pretty flat in any given area, which means there is very little compound curvature. With compound curvature, you need to lay up veneers at ±45°, and then place narrow spiles between each strip to make sure all strips abutt each properly. This is very time consuming. On Bagatelle, with 90° veneer strips, all strips can be pre-cut to standard widths with parallel edges, and butted one to the next all along the hull with no spiling. The veneers go on lickety-split. Of course, my engineering had to confirm that such a structure would be suitable for strength and stiffness. The whole process, and the final result, worked just fine.

The keel was a real naval architectural exercise because the blade had to have enough planform area but be very thin, lightweight, and deep in order to concentrate the minimum amount of ballast as low as possible. This meant building the blade out of solid Douglas Fir with a carbon fiber skin. In the photo below, which is a composite of three photos, builder Rick Waters is standing next to the 10’ 4” tall keel blade after the bulb has been attached, but before the blade has been laminated with its 5/16”-thick carbon fiber laminate skin. Note the rebate in the lead at the top of the bulb casting which is to allow the carbon fiber laminate to overlap onto the lead. There is also a stainless steel armature built into the bulb casting which extends up into the wood blade and this through-bolted thereto. The trailing edge of the keel is a solid piece of G-10 fiberglass about ½” thick.

The bulb is what I call a Beavertail/Swallowtail bulb, or rather, a BS Bulb (pun intended). The wide beavertail shape helps minimize the tip vortex by keeping the water flow running perpendicular to the span near the tip. The pointy swallowtail minimizes the vortices coming off the corners of what otherwise would be a square-tipped tail. In my experience, my BS Bulbs have worked very well, according to their owners, who have noted very nice performance with them.

One thing we noticed right away about the keel when sailing is that it hums, rather loudly in fact. This comes from the squared-off shape of the keel blade’s trailing edge. The flows on either side of the keel leave the blade in a perfect harmonious turbulence which vibrates the keel blade at its natural frequency. If we had planned to create a humming keel, we never would have made it—this is just one of those things that happens. To kill the sound, the trailing edge of the keel has to be chamfered off at 45° on one side so that the two flows leave the blade out of phase with each other.

The rudder is similar in foil section to the keel being the same mother foil shape, but thicker. The stock is a carbon fiber tube which is bonded into a foam and carbon fiber blade. Neck and carrier bearings support the rudder stock at the hull and deck respectively. Steering is by tiller from the rudder head. The shape has worked out very well, and the boat turns easily within its own length. Because the draft of the boat is 8’ with the the keel fully down, but 5’ in the upper sailing position, the rudder necessarily is not more than 5’ deep. If the boat is really heeling over far, most of the blade comes out of the water and you lose steering. Therefore, you don’t want to heel over too far.

For an even more in-depth review of this design, look at my cover-story article in Professional Boatbuilder magazine, issue #79, October/November 2002. This was my tenth article for PBB, and my first cover story. This was the first time PBB’s publication history in which the editors used a drawing for the cover instead of a photo. The drawing is of the construction detail of the stainless steel armature inside the bulb and keel blade.

In the end, Bagatelle ended up about 1,600 lbs. overweight, do to heavier-than-anticipated construction and the need for extra ballast forward to get the bow down close to her lines. She has hit 14 knots a number of times, but according to the owner, she seems to be achieving this in displacement or semi-displacement mode. That is, Bagatelle seems to not be fully planing. This is probably due to her heavier weight. Recently, we have been talking about another keel and bulb design that will improve stability and performance upwind.

The construction plans for Bagatelle are for sale in our stock plan list. Go to the stock plans page for more information. And please contact us by email or phone if you would like more information on this or other designs.