Re-Thinking FPB Prop Design

78 1 Prop One 1

You are looking at one of the two ZF propellers for FPB 78-1. Very high aspect ratio blades with relative low blade area. But maybe this isn’t the optimum.Almost all propeller design software is based on model tests which are set up to match maximum available power to the prop size. With most five bladed props, the design is oriented towards absorbing lots of power in a small diameter. This leads to much larger blade area than what you see here.

FPBs are so easily driven that we have the luxury of relatively large diameter props, with moderate blade area. This leads to a bit more efficiency.

In the past, we’ve added a little extra blade for rough water performance, paying a smooth water drag penalty. However, we are re-thinking this approach. Even fighting headwinds and seas, average fuel burn rarely increases more than 20%. So maybe we are oriented toward heavy going more than we need to be. As long as we maintain our upwind ability, maybe we are better off giving away a little efficiency here, in return for a larger gain where we operate most of the time?

Lifting bodies, like wings, sails, rudders and propeller blades, are governed by the same general rules in terms of lift-to-drag ratios. One of the most important factors is aspect ratio. Induced drag, the biggest prop drag component, is a function of aspect ratio squared. Small changes in disc area ratio yield big benefits, if you don’t need the extra surface area to absorb power. In the case of the FPB 78, the prop can absorb 330HP with the engine at M4 rating. But we will normally cruise between 70 and 90 HP at an M1 rating, so we have lots of extra area for rough water.

Significant prop cavitation, that you can feel or hear, is non-existent with our FPBs so far. This leads to the thought that maybe we have more area than we need. We are working with ZF Propellers to investigate this further. There is no database to which we can turn that addresses this issue, so ZF is doing a bit of lift line analysis using proprietary software. Both our instinct and that of Drew Ovieto at ZF is that there might be something substantial here.

We’ll let you know what we find.

Posted by Steve Dashew  (December 16, 2015)

6 Responses to “Re-Thinking FPB Prop Design”

  1. John Thomson Says:

    A higher blade count, thinner, with more skew? Noise abatement, cavitation concerns, and efficiency are similar goals shared with military and especially submarine propulsion engineers.

  2. Steve Dashew Says:

    Hi John:

    The issue of blade skew is under discussion. Given the shaft line to buttock angle, reasonable tip clearance, and low blade loading (even at our reduced disc area ratio) we are probably OK on the current skew angle.

  3. Virgulino Says:

    Great. Is this where you are heading? The cruising submarine??

  4. Steve Dashew Says:

    Have to admit the slight similarity to the sub approach is pleasing.

  5. Steve D Says:

    Curious how damage (impact) resistance factors into your thinking. Also how these highly efficient shapes tolerate fouling – I am reminded of the performance of the original laminar flow wings on the Rutan designs which saw dramatic performance changes in the rain or when contaminated with bugs. Entirely different environment at these blade speeds and fluids so may not have any relevance.

    Any recommendations for texts on the subject?

  6. Steve Dashew Says:

    We want the props to be the weak link and hopefully bend before doing damage to the tranny. A think layer of scale will cost you ten to fifteen percent in efficiency! But this is true with any prop.