54 SEAHORSE

When designing a lightweight cruisingcatamaran we are faced with some trickyproblems to solve: conventional propul-sion or hybrid, how much solar and hydrogeneration, how big to go on alternatorsand battery banks… and where do weanticipate the next jump in efficiency, sizeand weight savings? Then there’s the issueof how we accurately spec for powerdemands on a luxury, large performancecatamaran, with all of the amenities, keep-ing weight-sensitivity and maintenance inmind, with the appropriate number ofredundancies and amount of reliability…and decide that the system is ‘optimised’?

Only a real understanding of the use-cases can properly inform the conceptualdesign of truly optimised systems. For this

purpose our design team flew around theworld meeting some of the most experi-enced Gunboat skippers onboard theirboats. We have placed our team on all themodels in the fleet during a regatta, gonecruising, joined in on yard talk duringrefits, and listened to successes and innova-tions as well as areas for improvement. Themessages were clear: no internal combus-tion engines under berths; less is more; usethe very best components; consider mainte-nance at every decision; and always withtop performance and low weight in mind.

So for the latest Gunboat 68 we made apivotal decision to put the engine roomamidships; offering more internal floor areaand centralising weight, which along withthe flat hull rocker greatly reduces pitching,

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further increasing performance. Nearbyspaces were allocated for the battery com-partment, inverters/chargers and switchgear– managing to nearly halve the wire weighton the Gunboat 68 by placing theswitchgear (Mastervolt’s CZone system) inthe closest practical proximity to the loads.

Having carefully analysed scenarios thatrequired supplemental power sources weworked with Yanmar to utilise excessmain engine power with high-efficiency,high-output alternators and with Solbianto optimise an efficient 3KW solar arrayalong with an optimised core power system.(Why two fossil fuel main engines? TheGunboat 68 relies on two Yanmar 80s, butwe’re watching closely as hybrid systemsare moving from early adopters to massmarket – for suitable reliability, cost andweight for this project. And it should benoted that a Gunboat 68 is anticipated tomotor only 3% of the time in its lifespan.We placed the focus of our engineeringefforts on 40% of the energy requirement,which is during sailing and anchoring.)

With the spaces and major componentsproperly arranged we then focused on real-world optimisation. The traditional exer-cise is to add up all the consumers over 24hours and match them with sufficient suppliers, to arrive at the size of the alter-nators, genset, solar and battery bank.

To me this always seemed silly. The loadconsumers aren’t spread equally over 24hours and neither are the producers, espe-cially the renewable sources – but these

systems had always leant on the presence ofa genset as a base load producer that isalways available, and didn’t warrant acloser evaluation of this question. The morethe other power components evolved themore that gensets seemed like a very heavyget-out-of-jail-free card that, no matter thebrand, also seemed to be a pain for every-one in the fleet (noise, cost, maintenance,poor access for maintenance, weight,weight allocation/location on the boat, lossof storage…). Could we get rid of it?

To find out we created a tool for evalu-ating real-world scenarios (such as a‘Caribbean day’ with some motoring,anchoring and sailing; 24 hours sailing during a passage; and 24 hours at anchor),and tracked the state of charge of the batterybank hour by hour as virtual loads wereturned on and off, while making best andworst-case assumptions for solar input.

For the 24-hour sailing scenario withmany loads running and no engines, ourtool highlighted a small deficiency in thecurve that could be remedied by a Watt &Sea hydro-generator (with the option toadd a second). One unit produces up to600W of constant power while sailingabove 7kt – and it’s a safe bet the Gunboat68 is doing more than 7kt the majority ofthe time under sail. Below 7kt it is likelythe owner will choose to run the motorswhich produce huge amounts of power. Soin essence you always have an availablepower source.

However, the most stress you’re going to

put on the electrical system is air-condition-ing, so we showed our first two owners howmany hours they would have to run theengines if they wanted to use aircon whenunplugged from shore power.

Gunboat 6801 Condor went with thestandard battery bank – they were onlyinterested in using aircon at the dock. Gun-boat 6802 Dash wanted to be able to run afew cabins through the night at anchor. Weadded two additional Mastervolt batteriesto the system and ran the simulation again,showing that it can be done, and also clearlyshowing worst-case scenarios for filling thebatteries again the next day. As a result thefirst four owners became fully committed tothe compromises and keeping gensets offtheir boats became a priority for them.

Beyond its utility for our design team thetool we have created allows owners to bewell-informed and make decisions basedon real-world expectations from the start.

The 68 and her systems ‘evolve’ appro-priately on each new boat we build. Withthe energy density of batteries increasingdaily and the efficiency of the loads and theproducers consistently improving, we maynot see generators on this Gunboat seriesuntil they become the producers for propul-sion too, as in a serial hybrid system.

Right now the innovation to be cele-brated is an ever-refined approach to build-ing more efficient systems that ownersunderstand and appreciate during their sail-ing adventures onboard Gunboats. We’veput the balance of power in their hands. �

Top, left to right: before designing a holistic power system for the new Gunboat 68s different usage models were explored in detail,the results illustrated here in pie charts: sailing consumption; anchoring consumption; Caribbean day consumption. A hybrid mainpower system was considered but rejected on weight, performance and reliability grounds (though this situation is fast improving).Different forms of fossil-free power generation are employed including solar cells (opposite) and hydro-generators (right) which haveproved efficient even at low speeds. Major components are all sited (above) amidships to concentrate mass and minimise cable runs