FAST BOATING WITH ARNESON SURFACE DRIVES
Fast boating With Arneson Surface Drives is much simpler than people usually think since the ASD behaves like a cross between an outdrive and a conventional shaft.
Just a few recommendations need to be given to “neophytes” on Arneson Surface Drives to exploit the best of the ASD system.
Surface propulsion has a reputation for very poor performance in reverse. A certain amount of this reputation is based on the fact that until very recently, nearly all surface propeller installations were on very high speed vessels using a “cleaver “ style propeller design. These propellers, due to the thick trailing edges, concave pressure face, and often heavy trailing edge cupping, are notoriously poor performers in reverse and this is true whether they are used as surface propellers or cavitating fully submerged propellers.
Over the last years new blade designs have been extensively studied by major propeller manufacturers to match pleasure as well as military vessel operating requirements.
New blade designs have improved reverse maneuverability in respect to the traditional “cleaver” type design and now harbor performance is very similar to conventional propeller applications, and thanks to this development together with an appropriate hull design and the use of the Arneson articulated Surface Drive system.
Unlike fixed shaft Surface systems that direct the whole slip stream against the vessel transom, Arneson Surface Drives, the first articulated system released into the market, can be trimmed down during docking maneuvering to improve reversing.
Dock maneuvering is made easier by trimming down drives in the red area (Picture 1) in a way that propellers can work in deeper water and water flow is not directed right against the transom. The drive angle depends on several factors, among them: hull deadrise,transom shape, propeller diameter, etc.
The optimum spot can be easily determinated by “playing” up and down the drive trim in the red area (Picture 1).
You will remain surprised that the ASD small rudders and the propellers produce the same positive grip on the water at low speed that helmsmen of shaft driven boats are used to having.
GETTING ON PLANE
Most planning hull designs, especially moderately low powered or heavy designs, are subject to problems getting through “hump” speed. High vessel resistance at pre planning speed, high propeller slip and reduced engine torque at less than full RPM can sometimes combine to make it impossible to reach the planning speed. With surface propulsion systems there is an additional factor which may make the situation worse. In fact the propeller is designed to operate with only half of the blade area immersed but at low speed, before the transom aerates or “dries outs”, the propeller must operate fully submerged. The result is that it takes much more torque to spin the propeller at a given engine RPM and sometimes the engine is not capable of providing the torque necessary to turn the propeller fast enough to get the boat up to the speed which allows the transom to aerate and unload the top half of the propeller.
To reduce this potential problem designers and boat builders in cooperation with Twin Disc Application Depts. and major propeller manufacturers, have to be particularly diligent in checking boat displacement, positioning the center of gravity and selecting the reduction ratio to be used at the gear box in order to obtain the best compromise between top speed and “get on plane” capability. Deeper ratios, therefore larger propeller diameters, may help in over coming this problem.
Arneson Surface Drives, having no geometric limitation in propeller diameter, free the designer from restrictions. There is virtually no technical limit to the size of propeller that will work. The designer is able to use a much deeper reduction ratio, and a larger lightly loaded and more efficient propeller.
Furthermore, one of the advantages of the Arneson “trimmability” is the improved capability to get on plane efficiently . Positioning the ASD down, in the red area shown in Picture 1, we obtain the effect called “bow down attitude” (blue arrow). The thrust forces generated by the propellers pass well above the dynamic center of pressure and the center of gravity causing the vessel bow to stay lower on the sea surface making it easier to get to planning speed. The drives remain in this position until the boat is over the hump (maximum hull resistance) then are trimmed up for optimum performance.
If the engine results overloaded (for the reasons specified before or due to occasional extra weight) it may be needed to trim Arneson Drives up in order to reduce propeller load (unloading the top half of the propeller). When engine starts increasing RPM, ASD can be trimmed down to obtain “bow down attitude” and get on plane quickly and easily.
Acceleration with Arneson Surface Drives is terrific!!! Try it to believe it
Once the planning condition is achieved trim position depends on different factors.
When the boat displacement matches the design displacement, trim position has to be about “zero” on the corresponding trim gauge (Picture 2). With this drive position engines have to reach the maximum rpm and the boats’ maximum performance.
We have noticed on a variety of applications that when the ASD is trimmed to its optimum high speed running position, and left in that position, the vessel never reaches its maximum potential speed.
However, if the operator trims the ASD up and allows the propeller to absorb the engine horsepower, adjusting the trim as the vessel speed increases, this will result in a speed increase of sometimes over seven to eight percent (7-8%).
This is mainly due the hull configuration which is particular for each single application.
In fact, hull configuration would require some trim angles up to reduce the wetted surface area corresponding to the top speed performance.
However, optimum performance is usually obtained with drives positioned in the yellow area Picture 2. Actual position is a boat feature, it depends on several factors and needs to be found by experience. However, normally it has to be within the yellow area.
It is also possible to reduce the boat speed and stay on plane by lowering the drives and increasing intermediate range speed. (ASD positions are still in the red area (Picture 1), lower at minimum planning speed, higher at higher planning speed).
When boat displacement is over the project displacement (beginning of a cruise with tanks full) ASD should be trimmed some degrees up to reduce the torque absorbed by the propeller. The ability to trim the ASD is somewhat analogous to adjusting the pitch on a controllable pitch propeller. Engine can work closer to its rated power curve.
Trim can be also used to suit different sea conditions. Arneson Surface Drives have always shown superior sea keeping ability thanks to the trimming capability.