Outboard engines are usually a complete waste of money for commercial and government vessels.
Quite simply, they are not designed for anything other than recreational, or occasionally, racing use.
Outboard engines are usually merely petrol car engines turned vertically and connected to a modified automobile transmission. They were designed to provide the torque that an automobile requires in order to operate efficiently. The problems start to arise with respect to torque: Boats are under far higher levels of resistance than automobiles, particular at lower rpm ranges. So, many companies have tried to use diesel engines in outboards, rather than gasoline engines. What was the problem? Simple. The gearbox and drive systems were not designed to withstand the amount of torque produced by a diesel.
There is an old saying in the marine industry: “Horsepower sells boats, but torque actually moves them”. A quick glance at the marketing materials produced by outboard manufacturers quickly confirms that fact: Virtually none of them mention torque or show torque curves.
Some advantages of a diesel inboard over gasoline outboards:
- Diesel fuel is normally cheaper than gasoline in most markets. For example, in Australia, August 2015, retail price of diesel is A$1.29 per liter average, petrol A$1.34. In some countries the differential exceeds 20%. For instance, during the same period in Thailand, diesel averaged US$0.64 per liter and petrol US$0.93.
- Diesel engines always consume less fuel than gasoline engines. For instance, a Mercury Verado 350 consumes a whopping 353 g/kWh at full throttle, compared to 221 g/kWh on the Marinediesel VGT300. This equates to over 1/3 higher consumption. At cruising speed of 5,000 rpm on the outboard, the fuel consumption drops to 190 g/kWh, but the power produced decreases by over 1/3, netting just over 220 hp. The VGT300’s cruising speed of 2,800 rpm yields a fuel consumption of 195 g/kWh, but still producing nearly 280 hp.
- As far as torque is concerned, virtually no comparison is possible. The outboard produces barely 400 Nm of torque, under a drastic bell curve, whereas the VGT 300 produces nearly 600 Nm of torque along the majority of the rpm range. A huge difference in actual work performed by the engine.
- Gasoline outboards have a MTBO of under 400 hours, light recreational use (all of them). A diesel inboard will last up to 3,000 hours MTBO. You could overhaul an outboard five times before needing to overhaul the diesel.
- The outboards are lighter. 303 kg versus 515 kg. This is reflected in their lower torque produced.
- However, space is less of a constraint. The Mercury Verado 350 is less than 100 mm smaller on all dimensions. A quick look at the photo above shows a transom width of about 1 meter. The VGT engines will require approximately 300 mm more length, and no extra width. A mere 300 mm extra height is necessary. In other words, on the boat picture, two VGT engines WILL fit.
- Outboards cost less to purchase. MSRP on the Mercury Verado 350 was US$32,000 in August 2015. The VGT 350 MSRP was US$37,000. However, given the fuel cost and consumption differentials, this small difference in acquisition cost quickly disappears.
- Gasoline is far more explosive than diesel. The inboards are much safer.
- Diesel engines have higher maintenance costs, but require much less maintenance.
As is evident in the points above, a strong case can be made for equipping your vessel with the Marinediesel VGT Series instead of outboards. The cost differential at acquisition is minimal, and the diesels far outperform outboards in nearly every instance. In particular, on the higher horsepower outboards, the price differential starts becoming significantly less.
Finally, in the picture above, twin Yamaha 200 hp outboards are pictured. A single VGT 400 or VGT 450 will outperform those two engines, lessening the price differential, even after gearboxes and drives are taken into account on the VGT engines.