RE: Testing Sat in a fabulously-appointed Jacuzzi the likes of which makes me green, nay, JADE, with envy, and tested propulsion equipment?
RE: Testing Naw, the hot tub came with the house, I hardly ever use it for its intended purpose. The test equipment is to measure prop thrust. I have a brushless motor set up on platform that slides in and out, a fish scale to measure the thrust, and battery, wattmeter, ESC and controller to run the motor. I ran each prop at several speeds and recorded the thrust, wattage, and RPM. The good news is that the curves are in the right shape. Most props were low pitch and are noted by the number 25 after the prop name. The high pitch props are the 2.25" props and one of the 1.25" props. It is interesting to note that there is very little difference in thrust between 3 and 4 blade props of the same diameter. The results are somewhat surprising to me, it looks like I have been underestimating the motor Kv needed. I'll have to study the chart some more before I make any recommendations. After dunking my first scale (supposedly waterproof) and ruining it, I had to go buy a new scale. The new scale only had a resolution of 1 oz, so I'm going to re-run the 1" and 3/4" prop series when I get a scale with better resolution.
RE: Testing Here's a comparison of the power required to get thrust. Note that this is a measure of power in, not power seen by the prop. To know the power absorbed by the prop you have to subtract the power lost by the motor, battery, ESC, and wires. All these power losses will generally add up to 20% - 50% of the total power. The scatter on the chart is made even bigger because different props will produce thrust more efficiently than others. In general, it looks like it take 2 watts for every ounce of thrust +-10 watts. It also looks like the power requirements increase at the higher thrust levels - I think this may be because the larger props overloaded the motor I was using, so there was more power lost to heat.
You are some kind of something, Mark. Nice work Clearly, I need to buy more of your kits to fund more research
the power for thrust should be nonlinear on any single propeller... static thrust should trend with velocity squared while power consumption trends with the cube (assuming similar flow regime). One thing to keep in mind is that our efficiency in pumps (might be as bad in props but I have not looked into it deeply) will be on the order of 5-25% since we operate them in an inefficient regime.
This is the legitimate science done for the sake of battling boats! I mostly do mad science, which is more dangerous, but much less helpful
I re-ran the 3/4" prop tests today, using a better scale and higher RPM setup. I also was able to test a high pitch 1.25" 3 blade prop and add some additional points to the other prop curves. I updated the chart in my previous post with the new prop and points. The 3/4" prop results are below.
Looking at the first graph, I should look and see if I have room on Roma for 2.25" props is what you're saying... That's a huge leap in thrust delivered, all other things held equal. It looks like you got more thrust from the 3-blade 2.25" prop than the 4-blade, or is that just the icon locations making it look like that? I wonder what a 5-blade would change, if anything +/- tiny amount...
I'm still studying the 3 blade vs 4 blade graphs, but I have some thoughts. I read somewhere that going from 3 to 4 blades results in about 4% more thrust. But thrust also varies by diameter to the 4th power. In the case of the props I tested, the 3 blade was actually 2.22" while the 4 blade was 2.17". This seemingly small difference results in a 9% increase in thrust for the 3 blade prop. So 9% - 4% = 5% increase? It appears reasonable to me. Fortunately, the difference in thrust is made up by just a few RPM so I recommending picking the number of blades by what looks good to you. For RPM I used a laser tachometer. I put a small piece of reflective tape on the motor/shaft coupler and painted the rest of the coupler flat black. It worked great, as long as it wasn't in direct sunlight.
Glad to hear it, I already ordered a 4-blade 25 degree set of props for testing. I plan to see how well they fit on the big Balti, too.
Geeze Mark! Between you and Greg you guys are so ahead of the pack that it's almost painful for the rest of us! Keep it up! I love seeing what you've done, even if I only understand 3/4 of it
And I have been checking out those laser tachs , but wasn't sure if they would work accurately on our tiny shafts. Was the reflective material very tiny?
My shaft coupler has a 1/4" outside diameter and the reflective tape is about 0.2" x 0.4". I don't think you could put the tape directly on an 1/8" shaft because it's a little stiff and may have trouble sticking to that diameter. A wheel collar or the dogbone ends would be no problem.
Thanks Phil, that makes me feel better. I've was begining to think that you had stoped appreciating the engineering work I've been doing.
