Seafury SF18 Surface Piercing Drive Unit
Seafury International Ltd produces a range of surface piercing drives and propellers for the marine industry, and have been involved in some extensive development of their drive units.
|These units have the top half of the propeller exposed during planing, compared to conventional installations where the entire propeller is submerged. As a result, the propeller and rudder carry significant bending and torsional loads. The advantages of this set up are, however, reduced drag forces, greater efficiencies and higher planing speeds.Motovated Design & Analysis carried out a range of stress and frequency analyses to check component strengths at 48Â knots, and accommodate future upsizing to 70Â knots.
The first step was the development of appropriate propeller and rudder loads (Figure 2). Test data was used to develop a realistic drag coefficient and extrapolate the results to different boat speeds. An analysis of the surface drive assembly was then undertaken, including a portion of the transom to check the transom flange bolt loads. These results were verified by individual analyses of the rudder and main casting, to ensure that the restraints and contacts were working correctly.
The rudder design was initially a stainless steel weldment of the rudder stock and blade. As a result of the stress analysis several changes were made, including casting the part in Aluminium Bronze to minimise fatigue issues, thickening the rudder blade and increasing the rudder stock diameter (Figure 3). This was largely due to the higher future speeds required, and as the rudder loads are a function of speed squared, the section sizes also had to increase.
Main Casting Analysis
The main casting analysis highlighted a couple of areas of relatively high stress around radii and section transitions. These were easily modified to improve the stress distribution giving a very sound design (Figure 3).
The final analysis performed was a frequency analysis of the surface drive assembly. The natural frequencies of the structure were compared against the driving frequencies of the engine and propeller to highlight where possible resonance issues may occur.
The SF18 unit was considered robust in the 48Â knot regime and adequate for prototype testing in the 70Â knot regime. The analyses performed before production will undoubtedly pay back dividends, and the results can also be used during the certification process.
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Figure 1. CAD model of SF18
surface piercing drive unit
Figure 2. Rudder load development
Figure 3. Stress plot of SF18
rudder and main casting