CJR design strategy can be called holistic approach to the propulsion system. We don’t only make propellers, we analyse entire propulsion system of each boat starting from hull resistance prediction and hull propulsive factors. We select most efficient gear ratio, recommend changes if needed and analyse flow in every point under the hull using the most advanced CFD tools developed specifically to our requirements. This approach gives us the widest picture of possibilities we can use to run your boat more efficiently, quietly and fast or to any other specific, most demanding requirements.
PROPELLER AND STERNGEAR DESIGN PROCEDURE
Every propeller enquiry initiates estimation of effective power of the boat the needs to be propelled through the water at certain speed and resistance prediction plays a big part of it. As a first step CJR uses quick and reliable prediction software based on regression line developed by Southampton University on the basis of hundreds tested hulls. Much more sophisticated tool is Computational Fluid Dynamic software that can predict with a great detail and show flow around the hull, pressure field as well as running trim angle. The latter feature of the program can be used to find the most optimum trim angle for the boat.
OPTIMUM REDUCTION RATIO SELECTION
Having speed prediction of the hull natural step forward is to select optimum gear ratio that gives the most efficient propeller operation. Gear ratio and engine power govern almost all shaftline parts sizes including propellers, shafts, P-brackets and all associated parts. Gear ratio is a factor that has a huge influence on overall propeller performance.
Strictly connected to the reduction ratio, power of the engine and speed of the boat. CJR uses purpose built software based on number of different propeller series for quick selection of diameter, pitch and number of blades. Lifting line software is utilised for more detailed selection that allows checking blade area requirement due to cavitation. If you would like to read more about lifting surface software please click on the link
STRENGTH AND RULE CHECK
Safety at sea is the most crucial and paramount objective for CJR. Every component designed and built under classification society rules is sized to meet their requirements. CJR in-house software allows for checking of all parts of the sterngear to RINA, BV, LR and many other Societies rules. Additionally propellers blade thickness and stress level can be checked using software based on beam theory taking into consideration nonuniform loading over the blade. This method is approved by most IACS societies and can be used as an alternative to the rules to make propeller blade thinner and more efficient.
Besides Classification Society requirements, newly designed and fit for purpose components are analysed using Finite Element Method software ANSYS, which gives ultimate picture of component strength under most extreme loads. Please read more about FEA here.
HULL WAKE AND PROPELLER INFLOW
It is the most important information that shows the difference between good and bad propeller design. It can make even decent propeller noisy, vibrating and inefficient. Without knowledge about water inflow to the propeller, every propeller is a guess work. Using CFD software tailor made to suit planing and displacement crafts CJR introduced propeller wake calculation as a standard procedure. That allows CJR also to optimise propeller inflow by changing shaft brackets design and position or propeller tunnel shape and recommend these changes to boat designer.
FINAL PROPELLER DESIGN
The most advanced vortex-lattice code in propeller market is used to tune propeller final design. Propeller is run by the software in behind hull condition and every blade parameter like chord length, pitch, camber, profile shape can be separately adjusted to the flow to get highest possible efficiency at the propeller design point. Different type of cavitation can be detected on the blade and pressure pulses induced on the hull, the biggest source of noise, are calculated. Propeller model is generated together with pattern model and machine program.
PROPELLER SLIPSTREAM AND RUDDER DESIGN
Rudder area is calculated on the basis of boat size and type, and rudder sections are carefully selected, dependent on flow regime. Purpose built software that couples CFD and propeller code together allows CJR to replicate propeller slipstream under the hull. That is a great advantage in rudder design as detailed inflow velocities is known. Rudder sections will be aligned with the flow to generate minimum drag going on straight line and maximum lift force while turning.
SHAFT BRACKETS AND SHAFTLINE DESIGN
Knowledge about flow around the hull and sterngear allows designing shaft brackets with the least impact on water inflow to propeller as well generating lowest possible drag. Location of the bracket and other supporting bearings along the shaftline as well as entire shaftline can also be analysed using Shaft Lateral Vibration Calculation to find out if shaft stresses are within limits under all operation conditions and there is no thread of excessive vibration.