Hello everyone! So today's post was made possible by @SnipeHunter and BrianK. I've been looking to optimize the acceleration and maneuverability of Idaho, and ran across a tweet from the USNI (the folks that published Tom Clancy's Books) explaining how prop blade count works for 1:1 scale ships, and the primary example given was the SoDaks (not the fantasy football SoDak, the SoDak that should be 24 seconds SoDak). The North Cackalackas and SoDaks both had vibration issues and took a long time to work up due to it. the physics is pretty neat. I'll spare you. the general jist is that in 1:1 ships, increased blade count 1: reduces the shockwaves that cause vibration. 2: decreases top speed 3: increases acceleration So, upon reading this, I looked at Idaho's new motors, realized that she has a ton of excess muscle (the boat is rated at 140 Shaft Squirrelpower) under the hood to handle any reduction in top speed, and I sent ChrisK an email. So after the new rudder rule, I wanted to go all-out and get the biggest prop that could fit on the installed stuffing tubes and still throw a thrust cone straight into the monster rudder. that turned out to be a 1.625" prop, which, as you know, are not available. Chris hooked me up back then with a 4 blade Kort prop of his own design and I went and had some cast in brass at Shapeways for about $70/pair after first testing some PA12 printed ones to see if they fit/work. Those props worked great at NATS 2022. they're the Control for this experiment. Chris sent me the files for two more test articles, a 5-blade and 6-blade version of the 4 blade props he had already sent me. same diameter, same blade shapes, just... MOAR. I had them printed in PA12, mounted them on spare shafts, and drove down to BrianK's house for some testing. Our Hypothesis was that the increased blade count would have the exact same effect as it does with 1:1 ships. METHODS so, for each, we ran Idaho on the speed course you will all use at NATS this year until we got her as close to 26 seconds as possible. we then did five dead-stop-full-throttle tests on a course that was approximately 12 feet, which it turns out is the distance it takes for Idaho to go from a dead stop to full speed with about a second of full speed at the end to act as wiggle room. testing was done in 24-36" deep water. if a test was impacted by wind or moss, we re-ran it. each test received five acceleration runs for statistical confidence. RESULTS It's impossible to get the same velocity every time, so the numbers to look at are acceleration rate and throttle setting. DISCUSSION 1:the boat was coming to speed at progressively lower throttle settings with increased blade count. 2: Acceleration was lower with increased blade count, however the differences between the accelerations (~5%) were statistically insignificant (P>0.05) when run through the ANOVA tables. just running a Shotgun Power Analysis of the issue, I would need *at least* thirty runs (I'm not kidding it could take 200 or more runs) to parse a 5% difference and even then, it is still unlikely to be significant. Our Hypotheses were absolutely smashed. if anything, increased blade count acts contrary to 1:1 scale Visual observations: Cavitation was not an issue at all. the boat performed handsomely. Maneuverability was tested, and found to increase with blade count, to the point where I can only, as a Scientist and professional, describe 4 blade Idaho as a Sad Potato, and 6 blade Idaho as an UNHOLY WEAPON. Idaho could likely hold her own in a turning battle now. Not quite to Baden levels of maneuverability, but enough to make NATS 2024 a very interesting proposition. Once the boat was taken to full power, the fun truly began. we did not bother to time the boat in 6 blade configuration as the results would have been completely absurd. the chittering fury of 140 squirrels unleashed is a glorious sight to behold. Future studies would likely test 2 and 3 blade props. In those tests I expect acceleration to increase, and maneuverability to decrease. it may be worthwhile to test this on a 2 rudder boat such as a 24 second SoDak, as this would confine and maximize any potential shockwave action, allowing us to see if excessive prop wave vibrations are a problem at our scale Thanks to Chris and Brian for helping out in this testing series!
What I plan to do with this data: I've ordered a pair of brass six-blade props, and per @bsgkid117 's input regarding throttle inputs, am going to switch over to a 2.3:1 gear ratio (12T Pinion and 28T drive) from the current 1.85:1 this should allow me to push the throttle endpoint to around 40% with the 6 blades
