Rolla's main product is its uncontested leadership in marine technology. This gets translated into the Rolla physical product, which is in house production of:

Fully-submerged propellers - manufactured in NibrAl from 28" up to 75" diameter with a power range from 600 Hp up to 4000 Hp.

Surface piercing propellers - manufactured in
- NibrAl from 28" up to 65" diameter, with a power range from 600 hp up to 5500 hp.
- Stainless Steel from 16" up to 32.5" diameter with a power range from 250 hp up to 1400 hp.

Technology

Submerged Propellers are designed with a computer panel code method allowing the entire 3-D study and optimization of the geometry. To properly interface the propeller with its intended application the hull lines of any design of particular importance can be preliminarily analyzed to predict resistance, trim and wake field at the propeller disc. Once the velocity field on the propeller has been computed, it is used as "inflow" for the design of the propeller with the panel method and CFD (Computational Fluid Dynamic) analysis.

Surface piercing propellers are designed from systematic series based on tunnel testing and permit ROLLA to calculate torque - thrust coefficients, efficiency and horizontal - vertical force figures for different shaft inclinations and propeller immersions.

The technology is an integral part of the product, and neither could exist without the other. ROLLA services could best be described as the performance at time of sea trails which is the result of working with the shipyard through the whole propulsion process which in an ideal situation would also include the CFD analysis of the hull.

For each specific application the propeller has a dedicated design and a dedicated pattern. All the design codes are proprietary and have been developed in house. The propellers can be designed to meet any Classification Register rules such as Lloyd's Register, ABS, RINa, etc

All ROLLA propellers are dynamically balanced and geometrically comply with the "S" Class of ISO 484/2 standards. A full CMM (Computerized Measurement Machine) measurement report for Register purposes can be supplied for each propeller set. back to top

CFD and Consultancy

Analysis of propellers

Submerged propellers are designed using programs developed in-house and based on the panel method. Every possible shape is analyzed and the best are used to generate modified versions, until the perfect propeller shape is found at the end of the iterative process. The panel method is fast and sufficiently accurate for most conventional propellers, but since it is based on "potential flow theory", the viscous effects are neglected.

As the "roll up" effect of the trailing wake is difficult to be computed due to loaded propellers or because of design conditions, every propeller, designed with the panel method, is retested, using the CFD code. This allows the propeller load to be fine-tuned (thrust and torque) and cure local flow phenomena (i.e. at the blade-hub junction) which can be detected and cured.

Almost every submerged propeller is subject to cavitation
- when it is operating in the ship's wake
- when it is subject to the effect of shaft inclination.

For many years, ROLLA has been a member of the "Consortium on Cavitation", a team formed by MIT Boston, and transferred to the UT of Austin . The Consortium aims at developing codes, that are able to analyze the cavitation on marine propellers. Of the many things achieved, two programs have been developed based on the Potential Flow Theory

Flow Theory
- the PUF (Lifting Surface Method)
- the PROPCAV (Panel Method).
Rolla has developed a CFD method, that is able to solve the Navier-Stokes equations in 2-D (profile) and 3-D (propeller) conditions, also under conditions of non-stationary cavitation. back to top

Surface piercing propellers

At present Rolla is developing an instrument that shows the details of the flow around the propeller. The blade is studied as it enters and exits the water, not only optimizing thrust, torque and secondary forces, but also minimizing the pressure fluctuations that cause vibrations and structural damage. back to top

Resistance Prediction for hulls

Planing Craft

In order to design the propeller the boat resistance must be known. The problem is with hard-chine planing craft of small and medium dimensions where tank tests are not usually carried out and where the available resistance prediction methods, such as the Savitsky theory, are not applicable to the present hulls (because of warped shape, chine, spray rails, tunnels and flaps).

The flow is usually characterized by a thin spray that emerges at the bow from the stagnation line and becomes detached or reattached at the chine and the spray rails. There is also a complex flow at the transom whose nature depends on the speed and any breaking wave. These sorts of problems have been successfully solved using the High Resolution Interface Capturing (HRIC) a treatment of free surface flows. Rolla is now able to test planing hulls at any speed, and using greater accuracy than that which was used when testing with a towing tank. Thanks to HRIC Rolla can predict hull resistance, trim, and also local flow details that was not possible while using the towing tank. Flow visualization gives a better understanding of both overall and specific aspects of the behavior of the boat. Rolla uses HRIC not only in order to design propellers but also to be able to offer shipyards the analysis and optimization of their boats. back to top

Displacement Craft

Displacement craft generate regular waves that produce wave resistance. Towing tanks are mainly used to measure this resistance in the model scale and then to transpose the value to the full scale. The numerical methods, based on panel methods, can give useful qualitative information about the flow, but fail when viscous effects are relevant (i.e. at the stern) or when the wave is too steep or breaking (i.e. at the bow). Using the CFD method the viscous flow around the hull is established, and the free surface of whatever complexity is also accurately determined using the HRIC method. The level of accuracy of the simulation is the same as that of a towing tank, and the flow visualization gives a much clearer understanding of the flow phenomenon. Pressure distribution, dynamic wetted surface, wave pattern, streamlines and wake field at the propeller disk can be easily and effectively controlled. back to top

Sea Keeping

At Rolla, sea keeping simulations are routinely performed on fast yachts. The code used solves the Navier Stokes equations moving in time, and the VOF method is used to model the deformation of the free surfaces. Regular or irregular incoming waves with any length and height can be modelled that test extreme movements of the hull. The results of particular interest are concerned with the added resistance to waves, vertical accelerations onboard (for comfort analysis) and pressure impacts (fundamental for the correct hull structure). back to top