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.