Re: [Amel Yacht Owners] Re: Flexofold Prop


greatketch@...
 

I read the test report in YM, and like most of their work, it is a useful and well thought out piece of work.  

But (you knew that was coming, right?) they use bollard pull at full throttle as the primary measure of the prop's performance. I certainly understand the appeal of the measurement.  It is easy to measure with precision.  It is repeatable. It eliminates a lot of other variables. It is easy to report and rank. It has a lot going for it.  

But... it is a not a good measure of the real world performance of the prop in use (unless you are a tugboat).  For example, slight differences in pitch can result in fairly significant difference in engine RPM at full throttle.  If the engine is running at a different RPM it is producing different horsepower, resulting numbers would be skewed quit a bit in ways that have nothing to do with the design of the prop.  Our props typically run at about 30% slip, this test measures the thrust at 100% slip, a very different flow regime.

I especially think that measuring "side pull" at full throttle while tied to the dock  is a very poor stand-in for prop walk, to the point of being almost worthless.  Especially when you consider that props don't really produce side thrust on their own, it is the interaction of the slip stream with the hull that makes the boat go sideways.  A prop that had low "slip" (that's good) when the boat was moving would come out much poorer in ranking on this test than a prop with higher "slip" even though it would produce less prop walk when actually moving the boat.

It's easy to throw rocks at a published test like this and really, really hard to come up with a simple measure of prop performance. It is almost as bad as testing anchors! To be fair to YM they do acknowledge the limitations of the testing at the end of the article.

My take-home story is that all feathering and folding props have so much lower drag than a fixed prop that whatever is left is totally insignificant on a cruising boat.  On our Amels there is no reason to discriminate between them based on this measurement.

Performance under power is really, really hard to measure in a way that you can actually compare apples to apples.  The interactions between engine, transmission, mounting geometry, gear ratios, etc, etc. will totally overwhelm the differences from one prop to another. Then there is the question of what performance measure is really important. Do you care about maximum speed?  Highest speed at cruising RPM?  Match of prop power curve to engine power output?  Maneuvering?  Motorsailing?  Fuel efficiency at cruising speed?  Overall fuel efficiency?  Different boat owners come up with different things of highest value to them.

 On one boat Prop A will be best, on another it will be Prop B. Without actual, concrete testing data on your exact hull with your exact drive train, the differences reported not really useful tools to discriminate between them.

Best to select based on whatever weighted combination of cost, robustness, maintenance needs, reputation, and advertising claims that feels right to you.  Certainly, do include published testing results in your evaluation, but understand if they used your boat in the testing the rankings might be very different in ways that would be unpredictable and, almost certainly, of very minor impact on the performance of your boat in the real world.

Bill Kinney
SM160, Harmonie
Fort Lauderdale, FL

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