Carderock Division
Naval Surface Warfare Center
CRDKNSWC-TM-20-98-23 Feb 1998
Total Ships Systems Directorate
Technical Memorandum
RESULTS OF THE PRELIMINARY TEST AND EVALUATION CONDUCTED ON A 42' DEEP VEE OPEN CRUISER INCORPORATING A "POWER VENT" TWIN FIXED SURFACING DRIVE SYSTEM
by
Richard F. Najarian
Combatant Craft Department (23)
Approved for public release, distribution is unlimited; Feb 1998, Requests for this document shall be referred to Director, Naval Surface Warfare Center, Carderock Division, Detachment Norfolk, 116 Lake View Parkway, Suite 200, Suffolk, VA 23435-2698
Abstract
A preliminary calm water test was conducted to evaluate the performance characteristics of the 42" "Power-Vent". The craft incorporates a unique, simple, high-speed propulsion system that may possess desirable characteristics and enhancements for operational craft. The data obtained from this test will be used for comparative purposes and to determine whether more detailed testing of the system should be conducted on a running test craft incorporating operational features.
This report discusses the performance characteristics of this particular craft,
outlines the test conducted, details the results of those tests and makes recommendations
for a follow-on evaluation.
1. Background
In response to recommendation from Naval Surface Warfare Development Group, Boat
Development Division (NSWDG BDD), a technical evaluation was preformed on the 42'
"Power-Vent" during the week of 26 Jan 98 in Pompano Bch, FL. This craft is the
next generation of a 22' craft that was tested by the Carderock Division Naval Surface
Warfare Center Detachment Norfolk, Combatant Craft Department (CDNSWCDN CCD) in Feb 87.
The test was funded by the Office of Special Technology (OST).
2. SYSTEM DESCRIPTION
"Power-Vent" is a propulsion concept that incorporates a supercavitating
surfacing propeller mounted on a fixed propeller shaft supported by a strut and stuffing
box enclosed in a vented tunnel. Steering is accomplished by a balanced rudder located aft
of the propeller. Since the propeller is located completely under the hull, air is
introduced to it by a mathematically calculated system of "vents". The 22' craft
is a single screw "Power-Vent" while the 42' craft is equipped with twin screws.
The 42' craft is equipped with twin Yanmar diesel engines rated at 350 hp at 3,350 rpm.
Reduction is through Twin-Disc gearboxes with a 1.533:1 ratio. The propellers installed
for the test were Rolla 19 1/2" x 26" 6-blade cleavers turning outboard.
3. TRIALS DESCRIPTION
The objective of the preliminary trials was to technically evaluate the powering
characteristics of the "Power-Vent" system in a calm water environment. The data
obtained from this test will be used to determine whether or not to proceed with a more
in-depth evaluation on a operationally configured craft. The measured test factors
included:
A. Weight (calibrated load cells)
B. Shaft RPM (optical pickup)
C. Torque (straingauged propulsion shaft)
D. Static and Dynamic Trim (bubble inclinometer)
E. Fuel Consumption (onboard Flowscan system)
F. Speed (calibrated radar gun)
G. Onboard Noise (B&K sound level meter)
The mechanical instrumentation was mounted on the starboard shaft.
4. TEST RESULTS
A. Weighing
The weighing trials were performed on 26 Jan 98 at the "4-Play Marine, Inc."
facility. To reduce the amount of free-surface effects, the aft tanks were topped off and
the forward tanks were verified to be empty. All anticipated outfit was installed on the
craft prior to weighing.
There was some difficulty in setting up a two-point lift using the facility's
"A-frames" so the decision was make to use the 20k load cells in compression on
either side of the hull on the chines. The cells were moved for and aft until the hull was
balanced. The craft weight and Longitudinal Center of Gravity (LCG) was measured at 17,143
lbs. @ 13' 9" LCG (fwd of the transom chine intersection).
B. Calm Water Trials
The calm water trials were conducted on the Intracostal Waterway on 27 Jan 98. The initial intent was to perform the trials offshore since the depth of the water in the waterway was only 15'. The standard rule of thumb for a planing craft is to use a depth equal to, or greater than, 3/4 the length of the craft being tested. The sea conditions offshore were considered too excessive to perform this particular test. Bottom effects caused by the shallow conditions of the Intracoastal Waterway may have contributed to some inaccuracies in the speed data.
Speed for each data point was obtained using a calibrated radar gun. Torque was measured on the starboard shaft utilizing a straingauage telemetry system. An optical pickup was used to measure the revolutions on the starboard shaft. Static and dynamic trim was obtained using a bubble inclinometer mounted on the hardtop framing structure using the craft's keel at the stern as the reference baseline.
The test was conducted with the craft at a displacement of 18,096 lbs with the LCG at 13'7 1/2" forward of the transom/chine intersection.
The tests consisted of twelve data points starting at the maximum rpm (3,230 indicated), then 3,150 rpm, followed by 150 rpm reductions to 1,650 rpm. Each data point was obtained by operating the craft on opposite directions to eliminate wind and current effects. A minimum of six readings were taken for each data point during each run and then averaged.
The maximum speed obtained during the trials was 40.5kts (46.6 mph)
Data for this test is presented in table 1 and shown graphically in Appendix A, Figures
1 and 3. Comparative data is presented in Appendix A, Figures 2 and 4. C. Acceleration
Trials The acceleration trials were performed on 27 Jan 98. The object of the test was to
measure the time for the craft to obtain a speed of 25-mph (21.7 kts) from idle in gear.
This data is considered qualitative and is used for comparative purposes. Speed was
measured using a calibrated radar gun and time was obtained using a stopwatch. Three runs
were made in each direction to cancel wind and current effects. The obtained times were
then averaged. The craft displacement and LCG were the same as indicated in the calm water
trials. The average acceleration from 0-25 mph was 12.02 seconds. Data for this test is
presented in Table 2. D. Onboard noise Trials The onboard noise trials were conducted on
27 Jan 98. Data was obtained using a hand held sound level meter. Noise levels were
measured in decibels on the 'A' Weighted scale (closely represents the human ear). The
reference point for the levels was established at 20 Upa. Readings were taken at engine
speeds of idle, cruise (3,100 rpm) and maximum (3,230 rpm) at the transom, coxswain flat
and forward deck areas. Because of the open design of the craft, there was quite a bit of
wind noise that most likely was introduced into the readings. The maximum level measured
was 97.0 dBA at the transom and maximum speed. Data for this test is presented in table 3
and shown graphically in Appendix B, Figure 1.
5. DISCUSSION AND RECOMMENDATIONS
While it is the intent of this technical evaluation to present data that is accurate and repeatable, it must be mentioned here that part of the evaluation involved objective determination by unit operators. In addition of the Calm Water Trials, the craft was operated by NSWDG drivers and engineers dockside and offshore to evaluate its maneuverability, and capability to perform certain tactical maneuvers. These included reverse operation, docking, high-speed turns, and close aboard formations. Other factors the operators evaluated were the extent of reparability and maintainability of the propulsion system. Opinions and recommendations of the operators were presented in References (a) and (b)
Evaluation of the performance data of this craft indicated it compares favorable with other "Deep-V" Hulls with outdrive propulsion. However, other similar craft equipped with various surfacing drives show slightly better performance (see Enclosure 1). The measured LCG is located quite a bit forward of other similar craft that have been tested (by approximately 20%). This was indicated by the static trim of only +0.5 deg (similar craft are usually around +3.0 deg). This forward LCG can contribute to a certain amount of added drag. As mentioned previously, the testing was performed in shallow water so there was probably some effect on performance due to bottom effects. Discussions with the designer and manufacturer indicated they are in the process of determining the optimum propeller/ration combination so the craft might not be exhibiting its maximum potential. With the results of this test the designer has at his disposal the actual horsepower data that can be used for correct propeller and gearbox ratio determination.
One interesting characteristic of the "Power Vent" surfacing drive system is its inherent ability to provide the correct about of ventilation to the supercavitating propellers with out operator input. Other surfacing systems require the units to constantly be trimmed up or down while the craft gets on step. If this is not done properly, the propellers tend to overload the engines to the point where the engines are well below their torque range. They create a situation where the craft cannot steadily operate at any speed in the range from just below the "hump" to the fully planing phase. Typically, this rang is between 12 to 20 kts for craft of this size. Either the craft operates in the displacement mode or in the planing mode and has trouble in the transition phase between these two. The "Power Vent" concept is a fixed system that provides the correct amount of air to the propellers through specially design vents and baffles. What this ability allows is the craft and the propulsion units to work together as a system. The craft exhibited the ability to produce a given speed for a given engine rpm. As shown by the data, the craft was able to remain at any of the transitional speeds while properly loading the engines with them being overloaded. The NSWDG naval architect stated he was concerned that since the drives are untrimmable, the craft may have some problem lifting its bow under certain circumstances. Since this craft has a naturally forward LCG and appears to carry it well, maybe the follow-on craft could be designed with LCG further aft to start and then use tabs to bring it to the proper attitude.
One of the benefits this system has over other surfacing and outdrive units is that it is contained completely under the hull. While this reduces the amount of buoyancy available at the stern for the same length craft, it allows the overall length to be increased because there is very little extending behind the transom. Additionally, there is a large reduction in the amount of control devices that are normally associated with trimmable, steerable surfacing units.
Taking into account the existing performance of the craft (even with it not being optimized), the operators objective evaluation of arrangement possibilities and the low maintenance/low tech design of the propulsion system, it is recommended that a running test vehicle be obtained that incorporates these factors into an operational arrangement.
References
(a) CDNSWCDN Trip Repot, 42' Power Vent, dtd 2 Feb 98, Najarian
(b) OST Trip Report, 42' Power Vent, dtd 5 Feb 98, McCuchen