Tried to append this to the old thread on last attempt at different way of going about this but thread is too old.

Since it is slow/expensive/have to wait for wind/not very scientific to try different board/fin designs I came up with what seems a fairly simple system to test out concepts. I initially built it to test board design ideas but in just testing the system saw it was good at also testing fins. Results seem to be measurable and repeatable and easily reproduced by anyone. I have no clue how to scale results from models back to real world but I am sure some out there can point me in the right direction.

Principle is just using a motor to spin a pole around the pool and drag a board/fin combo. A constant current power supply gives the motor constant torque which is in turn proportional to the force pulling the board around. This could be measured by putting an inline scale (like fish scale) on the tow rope but the main thing is for any particular current the force is almost the same. So if any modifications are done to board or fin that help or hinder performance this will be easily measured by the rpm.

In early testing I was more interested in getting the board to spin as fast as possible so I could then try Venturi concepts before I drilled holes in a real board. As the board spins around, centrifugal force pushes against the fin and at some point the fin cavitates/spins out. The moment of spinout can be seen on the power as it drops 11% immediately...the current/force on board stays the same.

Since Fangman has experimented with putting bulbs on fin and says they seem to make a fin more manageable, I super glued a tiny bulb on the bottom of the fin I was using. It was a rush job with super glue blobs everywhere and made no effort to sand fin or bulb smooth. I was staggered that the board speed could be increased 15% or more before it spun out with exactly the same increase in power. Since the board was travelling faster there was more ripples bouncing off the sides of the pool so in effect the board was in more chop than the bubbles design but still traveled quite a bit faster in control. All of these measurements on fin and bulbed fin were repeated at least 5 times each and almost exactly same results. Small variations I figured were caused by slight chop changes.

The next day I sanded the fin/bulb to remove all the surface defects thinking I would get further improvement but instead the spinout speed was much lower...nearly 30% lower. Trouble was the tow string length may not have been exactly the same as previous day. However, I tried multiple string lengths and could not improve above the bulbless design.

Next step is to make interchangeable fin system so can quickly change fins without changing rest of setup/board to see if can reproduce the above results. Need to play around where the tow point is on the board, try adding small weights near the fin etc. Video of progress so far below for those interested. My first attempt at voice over so don't be harsh...kept mixing knots and watts no matter how many takes I took. If anyone has ideas to try/improve let me know. I should have put a camera on the spin arm looking back at the board but will do this next. Edit, Boz has already suggested increasing current so get 21m/S (40kts) to test real fins and how they handle at various angles of attack

As usual bloody amazing Jim! You just have to work out how to put the "warts back, and if the bulb is really necessary if the warts do all the work.

Gourmet geek food! Very entertaining. It will be interesting to see the videos of various bulb structures with and without warts.

Maybe after that, adopt the system to optimize fins for heavy week (Coods, Albany, LG). Not sure how easy it will be to grow the weeds in your pool, or how well it will go over with your family .

I really do expect you to turn up in a DeLorean with your own flux capacitor one day, Flex. It will be really handy if you can get the speed increased and keep the angle of attack set. As the speed increases, the water inertia forces dominate over viscosity forces, so how a fin behaves at low speed versus high speed can be very different. Can't wait for the next instalment.

I mean , who does this ?
I appreciate this stuff . I love it . I'm just glad he uses his power for good . It would be too easy for him to hollow out a volcano and build an evil villain layer.

That is "Out of the box" thinking ! Love the simplicity of the setup. You could connect the board to the pole on a fixed angle (no rope to pull, but direct fixed with some kind of triangle), so the angle of attack would not change with the velocity. One variable less. But you need a rather stable pole then.

ya Jan, exactly that is on printer now. Both full scale and half scale. Pretty sure I will have to seriously upgrade power supply to push a real fin (and probably burn out the motor in process) but the 1/2 scale should be entertaining. Have indexed system to put fin at 0-14deg in 2 deg steps so always repeatable. Magnetic latch tuttle system for 1/2 scale to allow quick change of fins in the fin spinner and for board. Printing 2 each bulb and bulbless fins. One each will sand smooth other will leave as is and spin them all at various angles of attack and see what happens.

Include a load cell on the end and you can check the effects of drag as well

Yes Elmo, already order one 100g load cell but don't need to use it to measure drag as already have an indirect measure of drag from supplied current. This is held steady regardless what the fin is doing by the power supply. The load cell would measure the side force along the spin arm or vertical fin lift.

However for starters, don't really need strain gauges as just going to compare fins...i.e. Applying the same current to a variety of identical fins with only one thing different between them (surface roughness, bulb, airfoil etc) at the same angle of attack should result in different powers shown on the power supply for any particular current. Power = torque x rpm and torque is constant. So fins with higher power (for same applied current) = higher speed and thus must have lower drag. Electrical losses will be the same between fins at the same applied current so can ignore. However, without measuring the radial force can't get any indication of lift (I think). In addition to pure drag, the experiment should be able to determine fin attributes that are more stable if they spin out at higher powers for the same angle of attack. Will be interesting how drag vs spinout power/speed interact... Attached graph of possible outcome of experiment? So far have printed a nice quiver of fins in various layer heights to simulate various surface roughness and will sand some of the fine detail ones further. Most in Naca009 but one so far with Naca0024. (Each fin is around 40cents)

In fullness of time really need 3 or 4 strain gauges depending where you place them. 3 orthogonal on the arm to measure both components of fin lift (vertical and horizontal) the another on the angle of attack arm...these projects grow legs very quickly.

That's amazing R&D. Good work Flex

Thought bubble and trigonometry in my head which is a bit dodgy at best - For any given velocity - Lift calculation using sine (Angle of attack) x power used to move the fin in the zero condition = power generated along the perpendicular vector (lift) ??

hmmmm...bit of a head spinner...just to clarify Fangman....power in zero condition =?? = power required to move fin at zero angle of attack at velocity X??

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Flex2 said..
hmmmm...bit of a head spinner...just to clarify Fangman....power in zero condition =?? = power required to move fin at zero angle of attack at velocity X??

yes Flex. Sorry, I should have been clearer - thought bubble when I was trying to clean up dishes, and I defo don't multitask that well. I should have sat down and done a drawing, but I did draw it in the detergent bubbles in the sink, if that helps.

Still trying to get my head around your statement Fangman as it implies only need one measurement of drag at zero AOA and can then determine lift at any angle for same velocity without further measurement. Will take your word for it so first load sensor will measure vertical lift as is only unknown. Mini Fin spinner slightly modified to fit the load cell as per pic.

Gave this baby a quick spin yesterday with both a GoPro and Jan's DIY GPS on the spin arm to measure velocity. Trouble was the extra mass made it impossible to keep the fin at constant height so need to add counter balance and upgrade to steel spin arm as this thing is starting to become frightening at speed.

One interesting find was Jan's GPS worked well at low speeds but once speed got above 5kts the error bar starts to increase dramatically, the velocity becomes erratic and completely wrong and the position gets progressively wildly wrong (like 50+m or so out). Even though I was spinning about 18kts the GPS maxed out at about 9knots. This is approved unit. Second pic is screenshot from GPSSpeedreader. You can see the correct 2.8m circular track at low speed diverge dramatically as spin speed increases. Maybe this is a way of cloaking yourself if you know someone is tracking your phone by simply spinning it?

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Flex2 said..
Still trying to get my head around your statement Fangman as it implies only need one measurement of drag at zero AOA and can then determine lift at any angle for same velocity without further measurement. Will take your word for it so first load sensor will measure vertical lift as is only unknown. Mini Fin spinner slightly modified to fit the load cell as per pic.

Gave this baby a quick spin yesterday with both a GoPro and Jan's DIY GPS on the spin arm to measure velocity. Trouble was the extra mass made it impossible to keep the fin at constant height so need to add counter balance and upgrade to steel spin arm as this thing is starting to become frightening at speed.

One interesting find was Jan's GPS worked well at low speeds but once speed got above 5kts the error bar starts to increase dramatically, the velocity becomes erratic and completely wrong and the position gets progressively wildly wrong (like 50+m or so out). Even though I was spinning about 18kts the GPS maxed out at about 9knots. This is approved unit. Second pic is screenshot from GPSSpeedreader. You can see the correct 2.8m circular track at low speed diverge dramatically as spin speed increases. Maybe this is a way of cloaking yourself if you know someone is tracking your phone by simply spinning it?

Hey Flex.

I was speaking to Slowy about why GPS's can never be used for official speed records and he mentioned something techo about aliasing and the way to prove it (which someone did) was to run it circles and it will go out of whack.

Seems you have also proved that to be correct.

Don't know all the details but he had lots of big words I don't understand on the theory behind it :)

Cheers
Al

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fangman said..

Flex2 said..
hmmmm...bit of a head spinner...just to clarify Fangman....power in zero condition =?? = power required to move fin at zero angle of attack at velocity X??

yes Flex. Sorry, I should have been clearer - thought bubble when I was trying to clean up dishes, and I defo don't multitask that well. I should have sat down and done a drawing, but I did draw it in the detergent bubbles in the sink, if that helps.

This is my thought bubble. There are a lot of assumptions and simplifications going here that may make this set up invalid, so I am more than happy to stand corrected on this one.

Just expanding my pea brain..
The way I see a working fin , is in its character drifting sideways . Head on drag , not so much . So pulling the fin through the water at 1 deg or so would be more realistic. The fatter the fin the grippier it is doing this .?
The grippiest fin I have ever used is the FF 28. Better than a 55 cm pointer . Umongst us who use a 28 cm FF there is no argument. I believe it's because it's an inch thick . It can handle chop and big back foot pressure . Amazing for a 28 cm fin . But it won't lift the board and you can feel some drag . Having said that , my fastest time was on a smaller FF .
If experiments in pools can give a direction , I'm all for it , because I believe a short ,perfect , fast all round fin is possible .

Wow! Amazing. A little suggestion for the video. I am not sure what video editing program you are using but get Shotcut, it is very easy to use and you can easily remove the original audio (with the background hum) and put a clearer one. It would make the video nicer (but keep the background violin!). It is free at shotcut.org/

Otherwise, please hurry up developing the next generation fin!

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duzzi said..
Wow! Amazing. A little suggestion for the video. I am not sure what video editing program you are using but get Shotcut, it is very easy to use and you can easily remove the original audio (with the background hum) and put a clearer one. It would make the video nicer (but keep the background violin!). It is free at shotcut.org/

Otherwise, please hurry up developing the next generation fin!

Thanks for the feedback duzzi. I'm using iMovie. Can reduce the video audio to zero no prob and add music/voice over separate. The aim was a little audio of the action. Trouble was this was my first voice over attempt and there are 3 separate edits/exports to get the multiple picture in picture which each take quite a while. Didn't consider audio while doing this so the multiple picture in picture audio built on each other to make the annoying hum. Next vid will reduce audio to zero if I talk..

Fangman, ya, that makes more sense...

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Hey Flex.

I was speaking to Slowy about why GPS's can never be used for official speed records and he mentioned something techo about aliasing and the way to prove it (which someone did) was to run it circles and it will go out of whack.

Seems you have also proved that to be correct.

Don't know all the details but he had lots of big words I don't understand on the theory behind it :)

Cheers
Al

hi Al. some nuance must be lost from this conversation because 'GPS' is indeed used for 'official' speed records by both WSSRC and WGPSSRC. Not to mention speed records in many other fields of endeavour. It all depends on what sort of gps is used and how it is used. Aliasing is not normally an issue for straight point A to B measurement.

That is a really good example of the aliasing effect (in 2 dimensions). You can see the higher frequencies folding back into DC - Why the track wanders off.

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sailquik said..

Hey Flex.

I was speaking to Slowy about why GPS's can never be used for official speed records and he mentioned something techo about aliasing and the way to prove it (which someone did) was to run it circles and it will go out of whack.

Seems you have also proved that to be correct.

Don't know all the details but he had lots of big words I don't understand on the theory behind it :)

Cheers
Al

hi Al. some nuance must be lost from this conversation because 'GPS' is indeed used for 'official' speed records by both WSSRC and WGPSSRC. Not to mention speed records in many other fields of endeavour. It all depends on what sort of gps is used and how it is used. Aliasing is not normally an issue for straight point A to B measurement.

Thanks for the info. I still just a learner ??

Bloody hell Jim...now I see what you do with all that spare time! What about the Pantera? You could always donate it to me.
I see another video of yours about the Tesla EV across Australia...were you involved with that? Jurgen and Helen are good friends of mine.

ha ha Nauli, ya, Pantera project has been pushed back due to many other projects. Ya, I was heavily involved with racing the EV across Oz. Helen and Jurgen had the easy job just driving the car....off thread topic so can discuss further when I see you next.

Meant to post an update earlier but the sudden death in Albany of my main sailing buddy Steve "Reg" Brown from a suspected heart attack put an end to that. However, to keep occupied, and for sure "Reg" appreciated this project, and would have done the same, I progressed experiments much faster than my ability to edit and post videos.

I'm a bit torn whether to post this now or wait till I have edited all the results into watchable form. Since the results are intriguing I thought it better to show at least my progress so far. The results are repeatable by anyone with access to a pool (maybe could shrink this to bathtub sized tester?) Anyone considering mind altering substances should try this first as the results really are pretty trippy.

Reminds me of when Beatty Park Pool had a cable set up to pull water skiers 50m.