Stable High Pressure Zones
Welcome back everyone!
This week we explain our modelling in periods of little or no wind. How does the system cope with stable high pressure zones, the Variables, and extended periods of calm? Planning a passage on any yacht involves predicting a weather window when you can travel in suitable winds to drive the boat to its destination, minimising engine use because of the cost, noise, and reduced safety factor (having less fuel onboard).
Our demonstration yacht, Whio II, will be able to do any passage arriving in a harbour on a predetermined date, with its energy use managed through CMG to enable this tour. In contrast, the maximum cruising range provided by a tank of diesel is determined by the size of the fuel tank and the speed you choose to go. On most cruising yachts, motoring for 3-4 days is considered safe for almost any passage, this cannot be replicated by batteries currently which lead most to conclude, rather incorrectly, that electric yachts need a back-up system.
This is one of our planned tours for Whio II in the far north, a circuit of Kodiak Island and a visit to Anchorage. Modeling is based on 3 years of summer weather data from the region. With the 6kW of solar on board, all domestic use was covered however, one years data included a long foggy period which both reduced this and also killed the adiabatic breeze of cold heavy air advancing down fjords or afternoon sea breezes as the land warmed up.
Slow moving high pressure systems can be seen in advance of their arrival and this meant that passage planning was started several days in advance of the planned departure date. If the forecast showed <3kn, this means that Whio II was motoring-only, and the CMG battery state started to predict a level below ‘safe’. To mitigate this, supercharging (sailing for charging purposes, not en route) opportunities were looked for. If a sea breeze did pipe up, Whio II slipped anchor and did a few hours of supercharging to prepare the batteries for the passage.
Operationally, when the wind was at 4-5kn then Whio II would use its motor-sailing capabilities to travel further by optimising apparent wind, so that a minimum speed (5kn) could be maintained while we tacked downwind, for example. This meant that the normal use of 7.5 kW to maintain 5kn was reduced to 1-3kW, effectively extending the range enough to complete the journey with the safety reserve maintained.
The CMG system predicts the battery state at the destination using existing forecasting and route planning algorithms. CMG also updates this prediction live multiple times per second when sailing. The CMG kWh number goes up and down like your VMG kn log, enabling you to trim the sails and alter course to optimise the regeneration or motor-sailing energy use. In this way you can easily maintain the prediction by sailing the boat, getting it into the groove and treating a period of unexpected calm something you have to get through as you would in any yacht race. This is a sailor’s solution; the motor is always running and your job is not to decide when to pack it in and motor as it is now, but to continue at 5kn. The boat is designed to never go slower than this, and work your way through it to the next predicted puff.
One feature of inter-tropical convergence zones, is having periods of no-wind. These are called the Variables or doldrums depending on where you are, and it can cover several hundred nm, or move with you, trapping you in no-wind for weeks. Another feature of these zones are squalls, which occur when downdrafts associated with heavy rain occur in very localised regions. These are not accurately forecast, but the regional conditions are. Squalls are clearly locally on radar systems.
Whio II is capable of maneuvering to take advantage of squall conditions in supercharging mode and has the capability of generating 50 nm of range in the short duration of heavy wind. To really take advantage of this, Whio II is designed with furling systems meaning the main and jib can be optimised quickly without going outside. On approach, plus setting the CMG speed for supercharging at 10kn can eliminate the shock of a largely stationary yacht with flagging sails being hit. At 10kn, apparent wind will have the vessel more tightly trimmed with reefs in when the gust hits and all excess energy will go directly to supercharging. While this all sounds too easy, it signals an alert watch, top of the range doppler radar or LIDAR and a yacht-racing approach to navigating, supercharging, and passage planning.
Again, this is the sailor’s solution. Remember, if there is an option, Whio II will be routed around such weather, passages planned just as cruisers do today, to avoid this, but the true test of unlimited range will be to cross the variables using such techniques to maintain 5kn at all times and let the world know that fossil fuels are no longer necessary on yachts.