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The trawler crowd may
not be into speed, but most other boat owners like to occasionally open
up their engines and skip across the water like a flyingfish. I know I
do. There’s a certain machismo about speed, and I enjoy it in short
bursts. On a good day, my 31-footer hits 40 mph. The speed doesn’t
last long, though, as the fuel consumption races equally fast from 25
gph to 60 gph.
While many of us enjoy
brief interludes of high-speed boating, there are adrenalin aficionados
who dig big speed (try 60-, 70-, and even 150-plus mph) all the time.
Instead of using house-size powerplants that burn fuel like lemmings eat
grass, many performance enthusiasts are running faster and more efficiently
with surface drive technology. And, thanks to the surface-drive’s
trademark rooster tail, with more flair, too.
At first glance, a surface-drive
system doesn’t look like any propulsion technology you’ve seen.
It has a short shaft off the transom and most closely resembles a stern
drive. But unlike the stern drive, it doesn’t hang down, the shaft
shoots straight back. In fact, when a boat is underway, almost the entire
drive unit is out of the water. In addition, the propeller on a surface-drive
unit is different (see “Piercing Props,” this story). It has
to be designed—unlike a completely submerged straight-inboard or
stern-drive system—to work in both air and water. When running at
high speed, only about 40 to 50 percent of a surface prop is in the water.
There are two types
of surface drives, fixed and articulated. The fixed system utilizes rudders
and works great for straightaways, but it also reduces maneuverability
and it cannot be trimmed, so the boat’s running attitude is constant.
The articulated system, developed by Howard Arneson and known as the Arneson
Surface Drive (ASD), allows the driver to change the amount of prop in
or out of the water, thus affecting the boat’s attitude. The ASD’s
articulated joint also allows the propeller to be directed and provide
better steerage than a fixed system. This is not the only way to steer
with surface drives; other methods include utilizing a rudder aft of the
propeller that’s cantilevered off the surface-drive support structure,
or a transom-mounted rudder ahead and “transversely offset”
to the side of the propeller. In an article from Professional Boatbuilder,
naval architect Paul Kamen says that the articulated drive offers the
same effect as being able to change the diameter of a fully submerged
prop.
To optimize a surface
drive’s ability, your boat should be built from the get-go to accommodate
this technology. For instance, a squared-transom boat could deflect water
when in reverse and reduce effective slow-speed handling. It could even
give the effect of forward thrust, according to New Zealand-based surface-drive
manufacturer Seafury Propulsion. Magnum Marine, one of several performance-oriented
boatbuilders (some others include Pershing and Baia) that use surface-drive
systems, says its boats have drives installed on a vertical transom surface,
which enables the drives and thrust to work as a hull extension. Magnum’s
president, Katrin Theodoli, says that her company puts a small wedge between
the transom and the drive plate so the drive is square. The result provides
uninterrupted water flow through the drive, optimizing the performance
of Magnum Marine’s boats.
Next page >
Part
2: Like any propulsion
system, there are cons, too. > Page 1, 2,
3
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