Global warming? if you don’t believe it by now, why build a boat? If, as most of us do, we think it’s real, then we need to take tentative steps to play our part. When it comes to our playtime, however, energy consumption typically gets less attention. The same is not true in the commercial shipping industry, where the threat is very real. So how would you go about reducing the carbon footprint of your yachting operations?
At First Glance
At first glance, the picture is frankly bleak. The option of switching to alternative power sources is not there. That said, I’m assuming here that sails are out of the question? Firstly though, the reliable diesel engine is here for a long time yet. Also whilst it is fun to consider technology as a solution, the marine world is understandably conservative and slow to change. One stance here is to look at the big picture but more on that later. For now, let us consider the fuel options.
No single option will improve the engine efficiency, but there are alternatives out there with a lower carbon footprint than refined grades of distillate marine diesel fuel. The major issues are combustibility, storability, and energy density.
Comparison of energy density of alternative marine fuels – credit to DNV
GTL – Gas to Liquid – this is, in essence, synthetic diesel. It burns cleanly, having a much lower propensity to develop particulates in the exhaust stream. A direct drop-in alternative to typical diesel fuel, and the exhaust smells better too!
HVO – hydrotreated vegetable oil. This is going to cost a little more and be difficult to find. Should you be based in a single marina, then that’s probably less of an issue. Its made from plants, the carbon footprint is some 80% lower than the equivalent energy derived from diesel fuel. Again, it’s a shoo-in for distillate diesel; at a price and convenience penalty.
Moving beyond the obvious, we have the more exotic solutions of methanol and gas fuels. I could go on, but do you really want volatile, easily ignited fuels on your yacht? Fuels with a significantly lower density that require considerably more storage space than diesel? Perhaps not.
Then moving on again, we have batteries. For a few simple trips of a few miles through safe waters, electric propulsion is a good option. For silent operation and quick battery charging, the same applies. Clean, cheap, and silent. However, even with recent advances in Electric Vehicle (EV) technology, power density remains low. Hope is on the horizon in the form of solid-state battery technologies under development to rival “King Lithium.” Still, even with EVs’ advances, this technology remains 4 or 5 years off commercial visibility, maybe another ten years for marine adoption.
So what’s the best option here?
So, in summary, is this just a lost cause? Well, no, not really, I said we would circle back at some point. Now is where our class of yachts scores a significant victory! Hydrodynamic efficiency increases with an increasing ratio of waterline length to beam. By the same measure, for a given bunker volume, the required power decreases or maximum range increases. We recently undertook a quick comparison using a 65 foot traditional 2-deck semi-planning trawler yacht (A) and an 80-foot hull of similar internal volume but high L/B Ratio (B). The parameters were 10 knots average hull speed and 4000 miles of operation at that speed.
Traditional semi-planing hull range vs speed.
A – 2 by 800 bhp both engines running part load, $90,000 of fuel used for a 4000-mile voyage.
B – 2 by 160 bhp one running part load, the other prop feathered. $14,000
Difference in fuel consumption = 85%.
So, to my simple mind, it looks like we have two viable options if we really want to reduce our yachting carbon footprint and to assume we also select a motor yacht. 1 – slow down. Not always a viable option with a semi-planing hull form where sea-keeping ability often improves with speed. 2 – alter our purchasing expectations towards those yachts with a more hydrodynamic hull shape, though at the expense of internal volume.