POWER & PROPULSION
When it comes to speed and durability, it’s hard to beat Arneson.
No one would really argue with the assertion that the azimuthing pod drive—commonly referred to as IPS and Zeus (the Volvo Penta and Cummins versions, respectively)—has revolutionized pleasure boating. While some argue over the precise degree of improvements in speed and fuel efficiency, virtually no one disputes the fact that this drive system represents a signifcant improvement in performance over conventional inboard propulsion.
As with any engineering breakthrough, there are those who exaggerate the magnitude of the impact. I recently read an article in which the author stated that until pods came along pretty much nothing signifcant had occurred in the development of marine propulsion since Archimedes invented the screw. Obviously this individual had forgotten about the Arneson Drive.
When it was frst introduced in 1980 the Arneson was every bit as revolutionary as the pod, and many thought it would eventually supplant the conventional underwater propeller, and possibly even the stern drive, as the preferred means of propulsion in pleasure boats. Tat it did not in no way detracts from the significance of the invention.
Te Arneson Surface Drive (ASD) is actually a version of what is referred to as the surfacing-piercing drive, and the name derives from its eponymous inventor Howard Arneson, an engineer and inventor who earned some 45 patents. In the early 1960s he invented a device that automatically cleaned pools. He sold the Arneson Pool Sweep to Castle and Cook in 1969 and retired from there in 1981 to pursue his real passion, high-performance boating.
Te idea of a propeller drive that runs half-submerged had been around for some time, but it was Arneson who perfected a steerable, trimmable version of it. Arneson’s patent is for a “shroud-enclosed, inverted surface-piercing propeller outdrive.” Regardless of what you call it, the whole idea of a propeller running half out of the water seems counterintuitive. Wouldn’t you want it submerged so all the blades are in contact with the water?
In actuality, no. Te key to the success of any surface-piercing drive is that it reduces hydrodynamic drag. Since it mounts to the transom like a stern drive and extends aftward there are no underwater appendages like struts, shafts, and rudders to create drag. In the ASD, steering is efected by a relatively small skeg that protrudes beneath the propeller housing. Because the specially designed surfacing propellers affixed to these drives are designed to work half-submerged, there is virtually no loss in “bite” or prop efficiency compared to a fully submerged prop, and propeller drag is halved.
Another advantage of the Arneson Drive is that like stern and pod drives, the drive unit is laterally articulated so when you turn the wheel you change the direction of propeller thrust, instead of deflecting it of a rudder. Tis creates much greater steering accuracy and response as well as increasing efficiency in turns. Tere are also fixed surface drives with rudders mounted abaft the propeller, such as the Seafury, which eliminate the hydraulic rams and associated equipment that the Arneson Drive uses for steering and trimming.
Another advantage of surface-piercing drives is that they are mechanically simpler than stern or pod drives and that their propellers are less susceptible to cavitation. Te surface-piercing propeller is designed to operate at maximum efficiency when ventilated; that is, when the blades are exposed to the atmosphere. And because it has no underwater appendages a surface-drive boat can run in little more water than is required to float its hull.
So, how much more efficient is a surfacing drive? As with pods, there’s a range of estimates depending on whom you talk to, but most experts feel 15 percent is a fair average. Note that while efficiency typically translates into more speed, it can also mean increased fuel efficiency.
So why haven’t surface-piercing drives replaced inboards and stern drives? One reason is the presence of the propellers well aft of the transom, a problem that many boat designers address with large swim platforms. Another issue is that the efficiency advantages of this drive system increase as speed increases; the drive works best on deep-V and catamaran performance boats. Slow-speed steering response is also not as good as with stern drives, inboards, and pods.
Still, the ASD has proven itself in many of applications over the years and particularly since Twin Disc, the transmission manufacturer, bought Arneson in 1992. Today the drive continues to be the favorite of not only high-performance boatbuilders but military and law-enforcement agencies because of its unique combination of efficiency and durability.
In short, the ASD may not be as popular as the stern drive but there will always be a place for it.