I was recently asked, “What is better for my motor yacht, jet propulsion or traditional twin screw diesels?”
My answer was, “That depends on whether you want performance (speed) or cruising economy, and that is further affected by the size of your yacht.”
In the early 1990’s water jet propulsion, with its increased speed derived from the same horsepower, was the new model for performance motor yachts. But, jet drives have recently fallen out of favor as the optimal propulsion for cruising yachts over 100 feet.
On the other hand they are still an enticing choice for mid-sized performance yachts in the 70 to 90 foot range (like the Riva 86 domino or the Pershing 74) especially when the increased engine life which a jet driving diesel will have is factored into the buying decision. When the boat will be spending 85% of its time operating at full speed the jet drive, and only operated at the less efficient slower speeds near the dock, then a jet drive is simply the optimal choice.
But, for cruising yachts carrying large fuel loads, which are normally operated at slow and medium speeds jet drives are not a good choice. This is because a jets fuel economy at high speed or slow speed is nearly identical to their high speed consumption. By contrast, a traditional twin screw diesel installation can increase its fuel economy by a factor of three at lower speeds. Consequently, for owners of large motor yachts jets no longer have the same allure as 20 years ago.
JET DRIVE BASICS: Here is what you can expect from a jet drive:
Jet drives need to spend the majority of their operating time at 30+ knots.
Jet boats need to have weight conscious interior designs, where traditional diesels are more forgiving of added weight.
Optimal performance requires light fuel loads.
Jet drive engine life is longer. Engine life between major overhauls for a jet drive may be 30,000 hours, where a twin screw diesel propulsion system will normally need a major overhaul at 12,000 to 16,000 hours.
The jet drive will add 10 to 20% more speed for the same horsepower. For a hull which does 30 knots with a given horsepower, the same hull will do 36 knots with a jet drive of the same horsepower.
Because they do not extend below the hull, jet drives can operate in very shallow waters. This is advantages in areas like the Bahamas.
Jet drives are more difficult to maneuver. Backing down is difficult with fixed jet drives, and takes even more skill during opposing wind and current conditions. If the jets are steerable (repositionable) this problem is solved.
For an 85 ft motor yacht, where the standard M.O. is to run out to the Bahamas from Miami, and both docks are alongside with no backing down required, and fuel available at both ends of the run so that fuel loads will be light; jet drives are the only way to go. But for a 130 foot, tri-deck world cruising motor yacht, with heavy equipment and fuel loads, a twin diesel simply makes much more sense.
I have been involved with the design of jet drive installations from 20 foot runabouts to the Navy’s 400 foot Littoral Combat Ships (LCS), with steerable jet drives, which solve all the maneuverability problems.