Wake fraction?????

Hi

Is wake fraction hugely important when designing a propeller? My propeller must produce a thrust of 10Nm at speed of 200Rpm. Vessel speed is 2.86 knots and will be submersed in the water(submarine). The motor has 2.67 hp available. How would i calculate an accurate wake fraction from this?

 

Saunders

As in all things hydrodynamic, start with Saunders to estimate wake fraction, then seacrh the literature further. Go to www.sname.org

The wake fraction changes the speed of the water into the prop, which changes everything. One important exercise in prop design is to run the design at a range of wake fractions to see just what the effect is, then see what you need to do about it.

This is why computer based approaches are a good idea.

 

Wake fraction refers to the ratio of the difference of the speed of the water passing the prop and that over the whole hull.

To figure it out you'll have to do model tests, however some good estimates can be made if the hull shape is known... A big issue is the wake distribution. More water tends to be pulled along behind the boat at the top of the propeller than at the bottom, causing the propeller loading to vary, leading to vibration.

Good references are available by Harvald and another is Principles of Naval Architecture by Lewis, or also Ship Theory by Tupper/

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