Propulsion and the Outboard Revolution

In a few short years dozens of yachts, which previously would have been powered with diesel inboards with straight shafts or Volvo IPS drives, have been turning up with multiple outboards. Some have simply been adaptations of existing models, but many were purposely designed around multiple, large 4-stroke outboards. The outboard manufacturers have responded with larger, quieter, and more efficient engines. Volvo, which had developed the IPS drive, jumped intpo the mix by acquiring Seven Marine, builders of outboards to 700 plus horsepower.

So if you want dayboat or cruising boat up to 50 feet or so, what are the propulsion options and the advantages and disadvantages of each? For some boats there are no real options, like a 20’-30’ center consoles, which are outboard powered 90% of the time, with the exceptions being small sportfish boats whose owners demand the fishability of an uncluttered tramsom to work a fish. We also won’t deal with the extremes of size and speed such as 50’ plus boat with more than 3 outboards. I remember when twin outboards were condidered exotic.

Pros and Cons

35' to 40’ express cruising boats were typically powered by gas or diesel inboards. Gas inboards remain an option, but recent advances in the efficiency, weight, and noise levels make diesels even more preferable, except for the higher original cost. Gas inboard engine installations are far safer that they used to be but still have the inherent explosion danger of gas.

Outboards have jumped into this market segment with a vengeance. The big 4-strokes from the major manufacturers and the Evinrude Etec 2-strokes get all that machinery out of the bilge, offer higher speeds, and rival a well insulated inboard in sound levels. Plus they tip clear of the water when not in use, so there is no fouling on the running gear. Partially raised, outboards permit shoal operation, and will kick up when encountering an obstacle, minimizing damage. Also crab or lobster pot warps can be more easily cleared. There is still a gasoline tank under the deck, but that has only a few fitting between it and the engines.

The dependability of the new outboards rivals diesels, plus service is easier. There is nothing dripping oil in the bilge, except a generator, but that’s in a box.

So what's not to like? Fuel consumption is higher but fuel cost is lower. Range is less than a high efficiency drive like the Volvo IPS on the same boat, unless fuel capacity is inceased by a third. IPS drives are somewhat quieter.

Decisions, Decisions

Diesel Inboard/Outboard drives have a great appeal in some situations: you want the efficiency and range of diesels with the drive line parallel to the running surface like an IPS drive, but also want the shoal operation and ease of clearing a line like an outboard. Versus outboards the drawback is increased maintenance of the running gear since it stays in the water.

Outboards are faster, easier to maintain, cleaner, have shoal operation, and running surfaces don’t get fouled, and lines can be cleared. Drawback is miles per gallon and noise versus a well insulated diesel all the way aft as in an IPS drive.

Diesel IPS drives are highly efficient and quiet. Drawbacks are maintenance costs of the drives, and damage when hitting something since they are fixed.

Straight shaft diesels are not as efficient as IPS due to shaft angle, but simple and easy to maintain. The engines are more central in the boat and harder to insulate from the helm deck.

Choice depends on how you use the boat, where you keep it, how fast you want to go and so on. See my questions to ask yourself HERE under Selecting the Right Boat.



© Peter Bass 2013-19