We have stated previously that Propeller design is very much an optimization process. There is lots of empirical theory on how best to increase speed, decrease drag, improve reverse, decrease cost, decrease complexity, etc. the real trick is balancing all of the factors so that it appeals to the largest segment of the sailing population.
We thought that a published example might be illustrative so that the informed buyer could realize how simple it is to ‘play’ with test results.
Voile Magazine performed one of the more comprehensive comparative tests using a Jeanneau Sun Odyssey test bed and a good cross section of the propellers available at the time (2009). There have not been a lot of epiphanies in propeller or design theory since, so it remains one of the best collections of real world data.
It is normal to have your eyes drawn to the speed at 2200 rpm line of the chart as a good measure of what your real world experience might be with each prop. What will my vessel speed be at a reasonable cruising rpm? If you poke at the data a bit, however, you will realize that this line is almost meaningless.
One of the most important services that we provide as the sales agent for KiwiProps is matching our propeller range with your vessel and powertrain. The objective is to setup a propeller with a load curve that is matched to your engines power curve so that the engine can reach its maximum rpm and no further. This is critical to the long term life of your engine, and improper loading will actually void most manufacturers warranties.
So go back to the chart and look at the line above the 2200 rpm speeds and take a look at the max rpm ratings. THESE SHOULD ALL BE 3600 RPM. Not one is over 3600, and 4 of them highlighted in red are significantly under. Every manufacturer knows this should be 3600 and they supplied the preconfigured propeller SO WHAT IS GOING ON HERE? Since propellers out of the box are a ‘best guess’ your fixed propeller remains a best guess for the rest of its life, but virtually all feathering and folding propellers offer the option of fine tuning the pitch to match up the max rpm. SO WHY WAS THAT NOT DONE?
The simple answer is that several of the manufacturers realized this was primarily a speed test so they supplied overpitched props to produce higher speeds at lower rpm. The fact that this would ultimately overload your engine for its entire cruising life was not factored in nor highlighted. I graphed the data Vessel speed vs inverse max rpm and this approach is clearly visible in the KiwiProps commentary on the website:
Scroll about halfway down to the Cruise Speed vs Max RPM graph and you will see that far from measuring motoring efficiency differences between props we are actually measuring differences between pitch/dia or load settings. We have superimposed a best fit line onto the XY plot. To put this in perspective, it is entirely possible that we could provide 10 different KiwiProps and produce the same range of results.
To be fair, there is definitely something to be learned with regards to overall efficiency, fuel consumption, vessel speed @ rpm and KiwiProps, as mentioned in a previous post, is actively working on modifications to our blade designs to chase some of these efficiencies. Really interesting stuff, but unfortunately the Voile article doesn’t go into enough depth. Contact us if you’d like to know more!!!