DIY model fin/board test system

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

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 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.

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.

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.

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) ??

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 .

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.

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.

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