Inboard or outboard?

An important decision in any powerboat project is the choice of exactly what powertrain to use. Our Starwind 860 is now almost at the point where that decision must be locked in.

Should it be an inboard or an outboard? Gasoline or diesel? What kind of propeller? And, importantly, why?

We have, so far, been using a 50 to 60 hp outboard for the reference design. That made sense in the early stages of the design, but we have been preparing the plans to also allow for a single inboard engine if the balance of factors shifted. Now that construction is underway, it's time to lock in the decision for good.

Single Gasoline Outboard

In our original, preliminary analysis for this boat, a single outboard was chosen for the reference design. Let's re-evaluate that choice.

The outboard has, in its favour:

  • Simple installation. Bolt it on, connect the controls, fuel line and battery cables, and you're ready to go.
  • Wide range of operating speeds. Most modern 60 hp outboards will run quite efficiently at any point in the 15 to 50 hp band, and can extend that band by 10 on each end with only a moderate decrease in fuel efficiency.
  • Ease of repair if we choose a popular brand. Parts are relatively cheap and, at least for the common motors, everyone has them in stock or on quick back-order.
  • Shallow draught can be easily achieved by tilting the engine up.
  • Handling with fully vectored thrust is considerably superior to single-screw inboards.
  • Capital cost. A Mercury 60 BigFoot with all required controls and accessories will come in comfortably under \$10,000.

We do, however, make some sacrifices:

  • Longevity is not nearly as good as a well-built diesel; the outboard will be a worn-out write-off long before a comparable inboard diesel goes in for its first major rebuild.
  • Heavy weather performance is relatively poor as the prop can ventilate if the boat starts pitching heavily in steep waves.

Inboard Diesel with Controllable-Pitch Propeller

One option being considered is an inboard diesel of 50 to 60 hp, such as a Yanmar 4JH5, coupled to a Sabb or Helseth CPP drivetrain. We'd have to add a large skeg to the centre hull to accommodate the propeller shaft, cut a slight tunnel into the underside of the hull, and move one of the fuel tanks aft; the engine would reside under the helm seats.

 

The 60 cm diameter CPP works by rotating its blades about their individual axes, thereby changing the pitch of the propeller, in response to commands from the helmsman. This allows the helmsman to bring the entire system to the engine's optimal operating point for any given set of conditions, rather than under-loading the engine at any speed except wide-open (as is the case with fixed pitch propellers.)

This setup would give us:

  • Significant improvment in average efficiency, relative to a fixed pitch propeller. This is particularly true on a boat like the Starwind 860 that has enormous variations in loading and in operating speed.
  • Considerably better range, on the order of 30% better than could be obtained with an outboard.
  • More thrust for towing or for bashing into heavy weather.

There are, however, many downsides:

  • This setup weighs 100 to 250 kg more than an outboard engine, depending on which diesel and gear we pick.
  • Longitudinal trim is much more sensitive to changes in fuel level, thanks to the fuel tank rearrangement that has to be done in the Starwind 860 to accommodate the big diesel. Not all boats would have this problem, of course.
  • Draught, which is a pretty big deal for us considering how many shallow and log-filled rivers we like to cruise, will always be worse than with the tiltable outboard.
  • Substantial extra capital cost. The diesel alone is a \$13,000 piece of equipment. Add the CPP, gear, shaft and all the little bits needed for installation and we'd be looking at something like \$20k to \$25k in drivetrain costs.

The diesel/CPP premium over the outboard will, at 2013 prices, pay for about ten to thirteen thousand litres of fuel. Keep in mind the outboard 860's expected fuel consumption - one nautical mile per litre at a 20-knot sprint, and four nautical miles per litre at a nice 7-knot cruise.

Inboard diesel with conventional drivetrain

This is essentially the same engine and geometry as the CPP case, but with a slightly smaller, fixed pitch propeller.

Advantages:

  • Simplicity and ease of repair, since the drivetrain is based on standard parts that yards everywhere can work with. (The engine, though, may be a different story.)
  • Seaworthiness advantages over the outboard, since the engine is better protected and the prop can't ventilate in choppy conditions. This does, of course, apply to the CPP as well.

Disadvantages:

  • Similar weight and trim penalty as for the CPP; the diesel inboard will always be heavier than the gasoline outboard.
  • Capital cost. While a conventional shaft setup can get by with cheaper materials than most CPPs are made from, there's still a lot of hardware and labour involved in rigging that \$13,000 engine.
  • Efficiency penalty relative to the CPP, and shorter maintenance intervals due to operating the engine at non-ideal conditions.

An outboard it shall be

I've swung these numbers every way I can think of, and the conclusion is clear. Given the way we plan to use this boat- as a weekend day-tripper, cottage workboat and occasional month-away cruiser, averaging perhaps eight hundred nautical miles a year- the outboard is almost always the cheapest option. Even when we factor in the complete replacement of the outboard after 18 years, when the diesels are still going strong, the outboard is still cost-competitive. When we factor in a rebuild of the diesels after 3000 to 4000 hours, the outboard wins by a long shot. It is only when we crank the fuel price north of $2 a litre (2013 dollars) and look at a 40 year horizon, by which time we're running a model year 2048 outboard, that either inboard shows an appreciable advantage. And, frankly, I don't trust my cost modelling that far out.

The outboard powered Starwind 860's range, with her anticipated 280 L of fuel tankage, is already 250 to 500 miles at full throttle, or 1500 miles if we take it easy. That's far more than we anticipate using in a single trip; the efficiency and range advantage of the inboards would rarely be noticed. While the extra thrust of the CPP setup might be nice on occasion, the improved handling of the vectored-thrust outboard will be appreciated on a daily basis.

Perhaps most importantly for us, the outboard gets us on the water sooner and for less money than either inboard option, and will have us back on the water sooner when it has to be repaired. And so the decision is confirmed, and construction of our prototype can proceed.

When the Starwind 860 plans are released for sale, though - after we are finished building ours - the skeg and engine bearers for the inboad will be included in the drawings, just in case you decide otherwise for your own boat.

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