Authored by: Captain Todd Shuster and Dr. Scott Veirs
A new type of boat will study the orcas and their environment this fall. The Gato Verde is a 13-meter (42-foot) sailing catamaran that recently became the first biodiesel-electric charter vessel on the West Coast. Last winter, dual 27-horsepower diesel engines were replaced with two electric motors, extra batteries, and a 14-KW genset burning biodiesel. The re-powered Gato Verde will serve ecotourists through Gato Verde Adventure Sailing out of Bellingham, Washington, this summer and the Beam Reach marine science and sustainability school this fall when students will study orcas and acoustics.
We’re confident that ecotourists will appreciate the changes. Sea trials indicate that propulsion noise and vibration is dramatically reduced in the cockpit and hulls. Diesel fumes and exhaust are gone and the increased propeller pitch and extra blade have enhanced maneuverability in close quarters. The total mass of the propulsion and power system increased only ~100-kg, and distributing the extra batteries in the bows re-balanced the load and improved hull performance.
Teachers in boat-based programs like Beam Reach and researchers who study orca acoustics will also value the quieter system. Class discussions in the cockpit will benefit from the ability to transit quietly under electric power. Fumes and exhaust from the biodiesel genset will be less distracting than fumes from petroleum diesel and its combustion. Acoustic observations will be much easier to make while moving under power; last fall, extended, continuous recordings were only acquired when the wind and currents allowed us to sail parallel to the traveling orcas.
Noise reduction in engine compartment and underwater
The modifications significantly decreased the in-air sound pressure levels (SPL) in the engine compartments. Using a sound meter from Radio Shack held horizontally in different parts of the port engine compartment, we measured sound pressure levels before and after the re-powering of Gato Verde. In comparing the conventional diesel propulsion system with the electric one (powered by batteries only, no generator), sound pressure levels (C-averaging setting) were reduced at all measurement locations:
|Sound pressure level in decibels (dB)|
|upper compartment||lower compartment|
For reference, a -6 dB shift is SPL is generally perceived as a halving of loudness. All measurements were made on the horizontal centerline of the compartments, except the loudest point measurements which were at (~1 cm from) the the air intake on the diesel engines and at the outboard base of the sail drive in the hybrid system.
Preliminary, qualitative observations indicate that underwater propulsion noise is reduced, as well. Quantification of this improvement will have to await re-occupation this fall of the site where the diesel engine noise was measured. If the noise reduction is substantial and the economic benefits are made clear, then Gato Verde may provide other commercial and private vessels with an inspiring example of technology that can reduce underwater noise in orca habitat.
Engineering and performance
Gato Verde is a 1995 Fountaine Pajot Venezia 42 Catamara (LOA 42’, LWL 40’+, beam 23’, dry weight 19000lbs, full capacity weight 23500lbs). The previous propulsion system consisted of dual Yanmar 27hp (3gm30) engines with saildrives. Each Yanmar was replaced with an “off-the-shelf” Thoosa 9000
system. The muscle of each electric system is a Lynch motor and the brains are a 4 quadrant (regen) Navitas controller. There is a Link 10 Battery monitor on each system. Each of the two battery packs consist of four 12V Group 31 AGM batteries providing 105AH @ 48V to each motor. The motors are mounted to the old Yanmar SD20 sail drives and are turning new 3 blade 17”X15” props. The saildrive reduction is 2.6:1 and was used without modification by mounting the motor to the existing power input shaft with a custom fitting.
The new biodiesel-electric-sail power system allows Gato Verde to match (or better) previous
motoring performance while decreasing fuel & lubricating oil consumption by up to 50%. Additionally, the battery pack enables Gato Verde to motor silently for up to 3 hours. When extended motoring is required, the on-board biodiesel generator will provide enough electricity to power the electric
motors continously. Based on the volume of the fuel tank, we estimate an endurance of 125 hours/tank or about 625 miles. Finally, the propulsion motors will be able to re-charge the battery bank when under sail by letting the props spin turning the motors into generators.
The Thoosa 9000W system was chosen for several reasons. The system is simple and uses an efficient 4-quadrant regeneration controller. Given the risks of early adoption, it was comforting that the Thoosa system can be upgraded with increased voltage if extra power is ever needed. It was convenient that the Thoosa importer (NGC Marine in Racine, WI) could provide the two systems within the desired installation window. Finally, Hank at NGC Marine provided compelling performance projections (16×16 propeller; 8 batteries [Group 31 AGM] totaling 210AH) with and without the gen-set running:
|Without gen-set||With 12KW DC gen-set|
|speed (knots)||Endurance (min)||speed (knots)||% battery assist|
For comparison, the Yanmar engines propelled Gato Verde at 7.4 knots in calm conditions @ 3400 RPM.
The bio/diesel gen-set consists of an eCycle DC generator built on a 3-cylinder 23 HP Kubota D902. The water-cooled motor/generator puts out over 12KW @ 58V DC. The buck-boost regulated system is more expensive than diode charge regulation, but it will put out the full charging voltage for the 48V battery pack no matter the RPM of the engine. Changing RPM changes current, not voltage. Thus, the generator speed can be reduced or increased to provide the exact amount of energy needed for any given conditions. For battery charging or boosting, the generator can be run at low speeds using less fuel and creating less noise. In a situation where maximum power is needed the generator speed can be increased to meet the demand.
Sea trials were conducted on April 6, 2006. The following performance data was taken in calm conditions the running times are estimates with around 20% reserve:
(Note: the last 2 data points are unevenly plotted in the range.)
Gato Verde’s first charter with the electric drive system was a great success. Since the DC generator parts had still not arrived and a small gasoline AC generator was used for battery charging. The regeneration under sail worked whenever we were sailing over 6.4 knots. We don’t have detailed data on speed vs. current output yet but measured as much as 11 amps at one point when our boat speed was approximately 7.8 knots. We’re looking forward to getting out in a good blow to do some serious data collection.