AutoProp   
								By Wayne Strausbaugh
							wstraus@cconnect.net,  (252) 637-9327
    Since I have experimented with 4 or 5 variations of 3-blade props, a discussion of my Autoprop is best preceded by a history of performance with other propellers.  I have a V-42 center cockpit with a Perkins 4-108 and a Hurth 150 transmission running a 1.95:1 drive ratio.  I have repitched the original prop 3 times, tried a prop of a different brand and finally gone to an Autoprop. What I have learned probably applies to all heavy cruising boats in general but the following is more germane to those with a transmission ratio of around 2:1.
    The original factory prop was 18X10 and gave a max speed of 6.3 to 6.5 Kts. Cruise speed was 5.8 at around 2000 RPM. I repitched it 3 times from 18X10 to 17 1/2X11 (no change in performance), 17X12 (picked up a few tenths of a knot and lost a bit of "thrust" at low speed), & finally 16X13. The smaller prop is a "speed" prop. That is, it is optimized for speed in calm water. Speed went to 5.8 at 1800 RPM with about 6.3 Kts at 2000. Top speed was 7.6 but the engine would immediately start to increase engine
water temp. Seven knots was sustainable at 2300 RPM. Low end thrust suffered greatly with the small prop. Acceleration was cut in half and the boat had a hard time maintaining speed with any wave action on the bow. Moderate pitching (3 to 5 feet) resulted in 4 knots or less. But the boat cruised well in the ICW and motor sailed well. The bottom line here is that a smaller prop with more pitch equals better speed but less thrust. I tried another brand of 3-blade prop (maybe a Michigan?) with a pitch of 16X13 but it had a slightly different shaping of the blades and the engine could hardly pull 2000 RPM.  I repitched it to 16X12 and it was satisfactory but never equaled the performance of the original Tayana prop. The Tayana prop had wider and thicker blades.
    Then I got an Autoprop. The Autoprop was easy to install on a V-42. It is a 18 1/4 inch prop. I simply dove under the boat, placed it on the shaft and tightened the nut (note: I had previously replaced my original Tayana shaft with a standard taper U. S. shaft). Speeds are now 6.3 at 1800 and 6.6 at 2000 RPM. The engine only has to produce 75 to 80% of its previous horsepower to achieve RPM and fuel savings are at least 20%. The engine never labors.  Max RPM available (momentary throttle increase to full power) is 3600 when stopped and 2800 at 7 knots. Top speed is 7.8 knots (at a little over 2600 RPM) but the engine still runs warm when exceeding hull speed. Thrust is better than any previous prop I have tried as well as acceleration. The boat has stopped in 50 feet from 5 1/2 knots! Head seas are no longer a problem. Any loss of speed from pitching immediately results in the prop repitching and speed being recovered rather quickly. Lots of thrust is available when needed.  But the biggest test was when I took another V-42 in tow and made 5.8 knots at 2300 RPM with only a 4-degree rise in engine temp (on a 92-degree day in warm water!). The Autoprop is both a "speed" prop and a "power" or "high thrust" prop. It is reportedly considerably faster than a MaxProp. It is also expensive. But if an owner thinks that his boat requires a larger engine and would also appreciate a 20% increase in fuel range, this type of prop could prove to be a reasonable and money-saving alternative.
    My V-42 is a 1982. I have lived aboard for 17 years and cruised almost 42,000 miles. I have nearly 6000 hours on my Perkins so have lots of experience with motoring and how the boat handles. I have cruised over 7500 NM's with the AutoProp. 
    I bought the AutoProp from Steve Armiage at RTL, Inc in Newport via mailorder for about $1950. The Autoprop just slid on the shaft and did not interfere with the "Spurs" in front of it. A zinc goes on the hub of the prop, costs $20 each and I go through 2 or 3 of them a year.  They don’t last long.  A propshaft zinc might help considerably. 
    Performance is rather spectacular but with a few "quirks". When first put on, I backed out of the slip and ran the throttle to 2000 RPM in forward gear. Everybody aboard suspected that the prop had fallen off! The boat was still backing and there was no "propwash" behind the boat. But after I added a little more RPM, we noticed the boat responding slowly and away we went. The boat accelerates slow at first but then accelerates faster the faster you go. This seems to be because the prop pitches infinitely based on speed versus horsepower applied. It never, ever caivtates. It can't because pitch varies widely and is always optimum. It also does not vibrate. When tied in a slip, I have often applied cruise power (1800 RPM) and it takes 20 seconds before the water behind the boat is disturbed at all. And then it is only ripples, never bubbles, about 20 to 30 feet behind the boat.  I mention this because one of my only concerns with the prop are the wide-eyed dock attendants when I pull into a transient marina.  It takes almost 1 1/2 to 2 times the RPM you normally use to stop.  Plus there are no bubbles or prop wash.  The engine sounds very much like the captain has it in neutral.  As a single-handed, I usually have to tell the dock hands that I will stop the boat with the engine and then keep them from reacting when they don't see the normal cues of burbling water from a boat backing-down.  Likewise when maneuvering in marina fairways - lots of RPM and no prop wash.  It gets people's attention... All a minor inconvenience.  Handling is improved. "Propwalk" is almost gone completely.  The boat backs neither right nor left when the rudder is amidships.  It will answer a port rudder in reverse but never a starboard rudder due to the skeg-hung rudder.  When a sharp turn is started for a "back-and-fill" turn, the boats continues to swing nicely when backing so I can turn the boat around much better than before.  But a V-42 still will not back in a straight line - ever!  It will always back around into the wind - even 2 or 3 knots of wind.  Once you realize this, you learn to take advantage of it. 
    Underway, the prop is amazing.  In general, it takes 200 to 300 less RPM to hold the same speed as before.  But the engine is not loaded down.  Quite the opposite.  It is obvious to me from the sound that it is very lightly loaded.  Hence the dramatic increase in fuel efficiency. 
    Tests have shown that you have at least 20% more thrust at full power if you run aground.  That may be because full engine RPM is available if speed is zero (3600 RPM on my Perkins). My old prop would only turn 2450 RPM if the boat was stopped. 
    The other anomaly is motorsailing at sea. When the seas are over 5 feet and from the stern quadrant, the up-and-down motion of the boat can set up a very slight vibration if RPM's are 1400 or below. This seems to be because the prop now has to repitch to compensate for vertical velocities as well as horizontal. At 1500 RPM or above, the slight vibration is gone.  This is not unique to the Autoprop as I have experienced the same phenomenon with a fixed prop.  Also, the prop spins slightly if in neutral above 4 knots. It doesn't spin when the boat is shut down and left in gear. A shaft brake is not needed with a Hurth transmission. 
    Bottom line: a great prop!  Its like having an engine with 20% more horsepower and an additional 20% more fuel when on a passage. I can now motor all the way from Norfolk to Miami on one tank of fuel... Maintenance has consisted of replacing zincs and rotating the shaft monthly to clear fouling from the bearings. The greatest benefit will be to those boats with a prop that is poorly matched.  Each AutoProp is custom-made for each boat based on the engine horsepower/transmission drive ratio's.