For the coastal cruiser, selecting an outboard for the dinghy is pretty straightforward — enough power and speed so the kids can play with the wake board and it doesn’t take all day to ferry supplies and guests out to the boat, minimum weight so the old man doesn’t get a hernia or drop it in the water while shipping the thing, and maximum reliability so it doesn’t cost a fortune in repairs and off-the-water time. That’s it, right? For the long-distance voyager there are some additional considerations.
Distance voyagers enjoy exploring interesting bays, coves and inlets in strange and far away places. That’s a big part of why they’re there. Just how far they can stray from the mother ship in their explorations depends on the speed and fuel economy of the outboard. They may have to return quickly if the weather suddenly turns or if the mother ship starts to drag. Also, the dinghy may be called upon to provide more than just transportation to and from shore and hauling groceries. Sometimes it may have to perform more like a harbor tug. Voyaging boats may find it necessary or desirable to seek anchorage in bays or inlets for which they have no charts, and not knowing the bottom, they may want to use the dinghy to move the big boat around. And despite their best efforts, voyagers may even find themselves aground and in need of a powerful little friend to pull them off.
Also, voyagers may find themselves in some remote island paradise when their trusty little outboard needs repairs and there’s no friendly marine mechanic nearby, which speaks to voyagers’ greater need for reliability and ease of maintenance.
As to whether the two-stroke or four-stroke engine is the better choice for the distance cruising boat, the two-strokes get the nod over the four-strokes. Why? Two-strokes are lighter weight, produce greater torque, and are easier to maintain and repair — all significant considerations for the distance cruiser.
But what about those &mdash hopefully rare &mdash times when you need a powerful little yawl more than you need a speedy little runabout? Nothing changes as to two-stroke vs. four-stroke or horsepower. What does change is prop dimensions. For those occasions when you need to push or pull the big boat around or pull it off a sandbar, a different prop is needed.
The prop that came with the motor was optimized in terms of diameter, pitch and material to give the best all-around performance for most conventional applications at lowest cost. Not the best prop for your little tugboat.
To quote my favorite propeller guru, naval architect Dave Gerr, in his excellent and informative compilation The Nature of Boats (International Marine), “There are more theories of propeller behavior and performance than you could put on a shelf full of books &mdash a sure sign that we don’t fully understand them.” However, while our knowledge may be lacking on the theoretical level, our understanding on a practical level is quite adequate to guide our selections.
While blade count, blade thickness, blade rake and cupping all influence the prop’s behavior, the major factors are diameter and pitch. Diameter is just that, the diameter (in inches) of the circle described by a spinning prop. Pitch, also measured in inches, is a theoretical measure of how far the propeller should move through the water in one revolution if there were no slippage.
Prop diameter does influence boat speed, but its greatest effect is on thrust. In other words, you get more thrust or push with a larger-diameter propeller for the same engine horsepower. There are limits as to how large a prop you can use, however. The blade must clear the underside of the ventilation plate on the lower unit by at least 15 percent of the prop’s diameter. Diameter also is limited by the load it imposes on the engine; too large a diameter or too small a pitch, or both, will overload the engine, shortening its life.
Propeller pitch acts like the gears in an automobile &mdash the higher gears (higher pitch) produce more speed but less thrust or pulling power, the lower gears (lower pitch) produce less speed but more thrust. So when the boat is to be used for towing or pushing, a lower-pitch prop is best.
Most outboards come with a three-blade prop. Increasing the number of blades will also increase thrust; it will also give you a smoother ride, less vibration and less load &mdash or stress &mdash on each blade.
Cupping, the slight curl on the trailing edge of the blade, is added to improve the prop’s bite on the water and reduce slippage. In addition to increasing efficiency, cupping also increases thrust somewhat. Most modern props are cupped to some extent.
So for those times when the dinghy must play tugboat, a larger-diameter lower-pitch four-bladed prop with cupped blades is preferred. Not all the time mind you, just when it’s time to play push-me-pull-me with the big boat. Leaving it on all the time will accelerate engine wear and shorten engine life.
Let’s say you’ve got a 9.9-hp motor on a 12-foot RIB. Chances are the prop is three-bladed, 9 inches (in diameter) by 9 inches (pitch), or thereabouts. Your tugboat should probably have a four-bladed 9.75-by-6.5-inch prop.
The right stuff
Outboard motors usually come with aluminum propellers because they’re light and relatively inexpensive. An aluminum prop for a 10- to 15-hp outboard will cost on the order of $75. Their lighter weight is easier on the lower drive, and that’s good. Other than that, there’s not a lot to recommend them. Aluminum is highly active galvanically, so it is susceptible to erosion, cavitation damage, galvanic corrosion and stray currents. It is soft and easily damaged by solid impact, resulting in imbalance, vibration and possibly damage to the lower unit. Significant damage will require welding, reshaping and balancing by a qualified prop shop. This can take time and probably cost in the neighborhood of $60 to $100, which is about the cost of new replacement or spare &mdash which, incidentally, you should always carry.
Stainless-steel propellers are much stronger and heavier than aluminum, and while they are electrically conductive and subject to galvanic corrosion and stray currents, they are not nearly as susceptible to environmental corrosion, erosion, cavitation or impact damage. Their superior strength is both a blessing and a curse. It allows for blades that are 40 to 50 percent thinner than aluminum and thus more efficient, producing 3 to 4 percent improvement in boat speed at wide-open throttle. However, forces from severe impacts will be more readily transmitted to the shaft and lower drive unit, possibly resulting in the need to repair the prop, the hub and the lower unit. Minor repairs such as dings and dents can take longer to repair and cost more than aluminum props. Costs of lower unit repairs, if necessary, are painful to contemplate, possibly running as high as several thousand dollars. Also, a stainless prop for a 10- to 15-hp engine is about $225, a high price to pay for a couple more knots of speed.
Composite propellers, typically made of various proprietary carbon-fiber or glass-reinforced-polymer formulations, have some very attractive features. They are approximately half the weight of an aluminum prop and about one-sixth the weight of stainless, and thus much easier on the shaft and the lower unit. They are stronger than aluminum by as much as 17 percent in some cases. Some are electrically nonconductive and thus are not susceptible to galvanic corrosion or stray currents (carbon fiber, on the other hand, will conduct electrical currents). Composite props are not susceptible to general corrosion, and they are highly resistant to erosion and cavitation damage. In case of solid impact, the composite material will absorb most of the impact energy, not transmitting it to the hub, shaft and gearbox. That’s the good news. The bad news is that the blade (or blades) will likely be destroyed. However, some composite props (Piranha, ProPulse) have easily replaceable blades, a five-minute $15 to $20 on-the-spot job! Manufacturers claim that performance is superior to aluminum, and purchase price is about the same or slightly less. Sounds like a good candidate for your tugboat prop, just be sure to get a good-quality composite.
Reliability and maintenance
Over the past decade or so, the design and production quality &mdash and thus the reliability &mdash of outboard engines has come a long way. Witness Evinrude’s E-Tec engines: no scheduled maintenance for the first three years. Alas, as with all mechanical contrivances, some routine maintenance is still required on most outboards. However, with small two-stroke engines, the tasks are simple and well within the capabilities of the offshore voyaging sailor. Basically, maintenance involves coolant-system flushing as frequently as practical, replacement of gearbox oil at least annually, inspection and replacement of the sacrificial zincs (internal and external) and spark plug replacement. Every couple of months it’s a good idea to remove the cover, wipe engine surfaces down, service grease points according to the manual, and spray engine surfaces with a moisture-displacing anticorrosive spray (Boeshield T-9, CorrosionX, etc.). The point is it’s not a big deal.
A good setup for an offshore voyager might be a 10- to 12-foot inflatable RIB, a 10-hp, two-stroke motor, a spare aluminum replacement prop, a spare set of spark plugs, a composite large-diameter low-pitch alternative prop with a set of replacement blades, and a firm resolve to read and follow the manufacturer’s manual religiously.