CMG aboard all sailing yachts
Welcome back everyone!
Something we get a lot of response to relates to our claims that CMG can benefit them on their yacht. Our claim of unlimited range and faster passage times is challenged as unreasonable because the demonstration vessel Whio II is a ‘performance’ catamaran, and such energy would not be available to a heavy displacement monohull.
A common engineering rule-of-thumb is ‘there’s no free lunch’ and it refers to systems that appear to give you more energy that they consume. For example, how can a heat pump that uses 1kW of electricity five 4.5kW of heat? The answer is that it uses 1kW of electricity and 3.75kW of heat from the air or water tank, and is only really about 90% efficient or so.
The difference between the ‘energy in’ and the ‘work out’ is calculated as efficiency. A traditional yacht has terrible efficiency because the burning of diesel in a generator or engine is only about 25% efficient. The energy content in the fuel is mostly spent producing heat, and this efficiency can be improved by using utilising that heat to heat shower water for example.
A traditional engineering approach is to make everything on the yacht use as little energy as possible in order to make the limited amount of energy stored in a fuel tank last as long as possible. At sea, what you take with you is all you have, therefore rationing water and use of washers, driers, and engine hours need to be managed as part of the safety factor of any passage. Getting to a marina for fresh water and charging the batteries is a key way to use less fuel and makes the vessel less flexible for exploring or island hopping.
The CMG system offers you the opportunity to refill your energy storage on the way, and in good wind conditions, everyone understands the trade off of excess speed to recharge the batteries. The CMG system converts the vessel’s momentum into electricity. It has nothing to do with the kinetic energy of water passing the propeller. A heavy yacht will have more momentum at lower speed than a light one, so a heavy yacht’s speed will be less affected by increasing the hull drag with the propeller to generate more power than a light and sporty one. Therefore, if you are sailing a 40 foot monohull with a huge kite in light winds, trickle charging the batteries at a speed of 5kn will be justifiably effective.
Here’s a description of a modelling scenario, let’s assume your yacht’s speed log is going from 4.5k to 5.5k depending on which side of the swell you are on, in an easy rolling sea, on a Beneteau Oceanis 42CC (dry displacement 9,400kg). The idea is that on first principles, momentum is simply mass times velocity, (measured in kg.m/s). Let’s say the total displacement of this cruiser is 11,000kg.
The CMG systems allows you to set your maximum speed, so if the captain set it for 4,5kn (9m/s) then the propeller will slow you down each time the swell pushes the boat forward and be neutral when the natural speed of the boat equals that. The available power is then half the difference between these two momentum results because it is roughly a sinusoidal movement. This means your average speed will lower from 5kn to 4.5kn however this also equals 8.25kW available energy to be harvested by the propellers, at their minimal efficiency this could easily trickle charge the batteries at up to 2.5kW. There are no hydro-generating technologies which can generate anything at below 8kn currently, so this is where the vast amount of energy will come from. In good wind conditions slowing the same boat from 8kn boatspeed to 5kn (propellers will start to cavitate at a 3kn boatspeed reduction) offers a total maximum available energy for harvest of 33kW. Meaning that this boat might supercharge at something like 15 -20 kW allowing a battery pack of 55kWh a recharge time under 4 hours from 10%. Incidentally, this is far faster than plugging into a marina supply (16A is only 3.5kW)
This is a first principles approach, so don’t get caught up in the numbers, as each specific case will be different. This is just a walk through of the first principles of a theoretical 40 foot mono. The propellers, cables, motors, and control equipment all affect these numbers, so will the sea state and weather conditions.
Next time we will discuss how CMG copes with long term high pressure zones where there is not a breath of wind for a week or more.