Well, kinda.

With high performance boosted diesels, the powerband is quite narrow. That is why truckers have so many gears in their trucks.

On the bigger high HP powerplants, one needs to run in the high turbo boost range, at the low RPM range, so its got the power and the ability to still burn the fuel and to use a long stroke without exceeding the tensile strength of the connecting rods and crankshaft.

Therefore, on racing diesels in Europe, multi-speed gearboxes are always used, and have been for well over a decade.

On gas engines, DonziJapan, you are closer to being correct: multi-speed gearboxes have pretty much been proven to be a waste of time in pleasure boat applications. However, they are not in gas powered offshore race boats because of rules, not ineffectiveness. They have been used offshore, and were then outlawed in the US circuits (not overseas). Plenty of flat bottom race boats use multi-speed transmissions.

And on commercially available diesels, multi-speed transmissions may be a waste of time because the companies try hard to have a very flat and wide powerband so they will work without multi-speed transmissions. Fuel efficiency is lost however: the new Yanmar BMW engines have disappointing HP per GPH numbers to support the high RPM needed to avoid gearboxes: they give up about 30% in the efficiency department over long stroke diesels.

But if designed for a 5 speed transmission, the high HP diesels can make a lot more power while still having a long stroke for high fuel efficiency.

That's a silly statement by a salesman. Those engines were HEAVY, and NOISY, and EXPENSIVE. Simply not a product that worked for the market.

My boat makes 60 knots with standard VolvoPenta D6-350/DPR engines/drives.No gearbox. 100 l/h at 50 kn. (Maybe someone can translate to US-numbers). The boat is a 36 ft Ocke Mannerfelt design twinstep RIB. Weighs about 4800 kilos empty.

If that much fuel efficiency is lost, then you might as well go back to a gasoline engine.

BTW, Volvo uses a blower on a few of their engine models that engages as low rpm, to get back some of the low speed torque.

Michael

I ran that package in a 12-meter (cat) patrol boat but the military was very nervous about the drives so no sale.

We also tested the Yamaha 420 HP diesel (20,000 Hour) on surface drives that was a decent running package but the engine weight quickly overcame any advantages to be had from switching over from the Sabre that was being used.

I had a 10 Meter catamaran gunboat in the works a single engine (Italian diesel) 65 MPH @ 6+ MPG but the Oil price back then dropped to $10.00 a Barrel so it was shelved.

We have secured our Manufacturer for the twin diesel powerplants, and will be one of the sponsors for the project, a multi speed transmission that has had great success will be used and a proven simple trimmable,steerable surface drive completes the package, will post when we are complete and tested.

I don't race so most of my time on the water is at cruise speed and not at top end.
I'd be happy cruising at 50 getting 6 mpg...or even 3 mpg. That's better than 1 mpg @ 40 now.

More details please

Give me a diesel making 875-900 #' at 2800-3000, a two speed tranny, low being like 1.3:1 and second being a overdrive of .7 to 8:1 going into a 1:1 outdrive overdrive running 33-34 pitch wheels and life would be pretty good.

33 x 9.5-foot Cat based on my race hull with a single 6 cylinder 320 Hp diesel (1010 pounds ) on a surface drive, I had two versions of the open style deck one with an enclosed walk around pilot house the other a Centre consol.