Engineer’s Angle: Fuel for both yacht, tender attractive

Nov 4, 2019 by JD Anson

Engineer’s Angle: JD Anson

Whether taking the boss shopping in St. Tropez or dragging kids for miles on a banana in the Bahamas, tenders are indispensable to today’s yachting scene.

Gone are the days of a small inflatable taking care of the tender needs. Today’s tenders carried aboard are larger and more powerful than ever. Then there are the tow-behinds. Once 25-foot center consoles were deemed outrageous. Now these behemoths are the size of small yachts, some pushing over 45 feet with generators, air-conditioned cabins and sport quad, or even quint,  outboards of 300-400 hp and more. Each.

Besides the logistics of the gear required for towing, such as Spectra hawsers and specialized towing lights, these massively powered boats also require copious amounts of gasoline to keep the fun going. Since we travel to far-flung places, finding fuel can be difficult and, when found, can be of dubious quality. Many yachts have resorted to carrying on board large containers of gasoline, which end up stored in ways to make class societies cringe.

What if it were possible to safely carry as much fuel as the tenders could burn, with no additional tanks on board?

Those days have arrived.

After an accident in 1995, the U.S. Navy decided that carrying gasoline on board for tenders was a hazard it no longer wished to deal with. It directed Mercury Marine to come up with a diesel-powered outboard for its RIBs. Mercury gave the project to its racing division, whose engineers were used to pushing the edges on technology and innovation. These engineers came up with Direct Spark Ignition, diesel-powered outboard, a line of which are now rated up to 175 hp. This proved to the world the viability of diesel outboards. 

Because of fuel taxes and environmental restrictions in Europe, diesel outboards are very appealing. Diesel has proved cleaner-burning and more economical to operate. After a false start in small outboards, Yanmar, long a diesel engine manufacturer, has partnered with the German firm Neander to develop a 50 hp diesel outboard. Swedish OXE Diesel has a line of diesels ranging from 125 to 200 hp.

Previously, these had been aimed squarely at the commercial market, where economy and reliability are key to a successful business. These transportation, oil and fishing sectors are willing to pay the higher entry fee versus gasoline outboards for the eventual payoff of lower operating and maintenance costs and longer service life. Recently, pleasure craft, especially yachts, have been seen as a new market for diesel outboards.

But these 200 hp engines are still too small for the mega-tenders plying our waters these days. The Brits have joined the fray with the entrance of COX Marine. They have used F1 technology to engineer a diesel outboard of 300 hp. This eight-cylinder, 4.4-liter engine tops out at 4000 rpm, much lower than an equivalent gasoline engine’s 5000-6000 rpm.

Not to be outdone, OXE Diesel is planning on releasing their own 300 hp version in 2020, a twin turbo engine that consumes a third less fuel than a 300 hp gasoline engine and puts out an unheard of 680 Nm of torque at 1750 rpm.  The massive torque allows a larger propeller to be driven, thus allowing comparable boat speeds. This allows lower engine operating speeds, resulting in a quieter ride and lower fuel costs.

Because of the inherent strains of diesel’s high 16:1 compression ratio, previous engines have had to be quite robust. This has directly translated into a heavy engine, unsuited for outboard use. State-of-the-art advances in metallurgy technologies and computer aided design have allowed lighter materials to be used, resulting in a viable diesel alternative. 

The one-source fuel for both yacht and tender is becoming an attractive choice for both new-build and refits of existing large tenders. As the market develops and environmental concerns deepen, more manufacturers are expected to develop their own diesel versions of popular outboards.

The coming years should prove interesting, especially as cruising becomes more remote and tenders begin moving into sizes that qualify as yachts in their own right. Perhaps one day gasoline outboards will be relegated to the tender for the tender.

JD Anson has more than 20 years of experience as a chief engineer on megayachts. He is currently project manager at Fine Line Marine Electric ( in Fort Lauderdale. Comments are welcome below.