Iceberg has just tried out her new propellers, this time without “interceptor” strips, and she has pushed her smooth water maximum speed to 15.0672 knots,
the average of GPS readings in two directions (taken every second).
Although speed at wide open throttle is meaningless in a cruising context (aside from bragging rights), accurate performance data is essential for fine tuning the computer models we use to predict the impact of propeller changes on FPB 97-1, as well as other FPBs.
FPB 97-1 has now completed her first passage since changing propellers, averaging 13.2 knots over several hundred nautical miles, and burning just 60 liters/15.8 US gallons of diesel per hour. During this passage, her top surfing speed was 18.1 knots.
The FPBs are so lean on drag, and efficient with their propulsion packages, that there is little modeling data which relates. So we need to pick up data points in the real world when we can, and we appreciate the opportunity provided by the owner of FPB 97-1 to add to the database.
This is timely because we are about to release the propeller order for the first set of FPB 78 props. In terms of our historical data base, the FPB 83 Wind Horse and the FPB 97 are very close. The FPB 64 sits at the other end of the hydrostatic world (in FPB terms), while the FPB 78 is a little closer to the FPB 83/97. With this latest data set now in hand we are comfortable with the numbers for the FPB 78 propellers.
One of the tools we use to compare yachts under power is called the Froude number. This is a dimensionless ratio that takes into account a number of design factors. If the Froude numbers are close between designs, even though there may be major differences in size, you can have some confidence that the prediction formulae will produce consistent results with other yachts having similar ratios.
Coming around now to the props we are about to order from ZF: these will be specified so that if we find an efficiency band at the top end which we have not seen before, the props will have sufficient area and pitch to take advantage. We are being teased by data outside the Froude range. The sea trial props are specified to allow us to get into a Froude range higher than where we have been before, if it is possible. The odds of finding a new range of efficiency are not great, but if we don’ t try we won’t know the answer.
You are probably wondering what we are expecting. We are not going to divulge answers to that question anytime soon. Rather than speculate, we’ll let the FPB 78 speak for itself. However, we will tell you that at a Froude number that equals that of the FPB 97, we are only using half of the M 4 rated power from the John Deere 6068 AFM 75s.
Now a few words on the Veem props with interceptor strips about which we have written in the past. As sea trial propellers, the easily changed pitch that comes with the interceptor strips saves time and hassle. But these props are less efficient, noisier, and more prone to vibration than conventional propellers. Where the FPB 97 previously had a bit of vibration and noise with the interceptor equipped propellers, she is now vibration free and almost silent with her new props. We think the best approach is to use conventional props once the sea trial data has been accumulated.