Puffin - Trial and error engineering

[Occupant]

Trial and Error - engineering?

I've been trying to bring this thing to life and keep painting myself into a corner. Bench testing everything is functional and responsive to controls.

Testing in the water the drive continues to be an issue. I had played with several prop configurations all had the same issue Bob had warned me of - the tapered tail will be enough of a restriction to create a back flow and it did. So to try and get around that built a jet drive a tube within the tail cone to pull in water and push it along - no taper to deal with and it helped but sucked in air so i moved the inlet to the bottom and that helped but not enough flow to get speed. The rudder servo limits how bit the jet tube can be so i abandoned that approach. So the latest approach idea - move the drive to the pinch point - the rudder ball, it the smallest diameter in the tail section. made a couple parts and assembled, dunked a whoaaaaa i've got thrust finally.... at least by itself. while changing things cut back a bit of the tail cone to ensure the outlet of the rudder ball is unobstructed at full turn. Something else this arangement should get me I didn't have before is steering in reverse. should be able to check out the new drive in a day or two with the tail cone and rudder integrated, wish me luck.





Gemini AI image



trying to explain to my son what all is involved



bench testing all the various components - shown prior version with jet drive, inlet on bottom side


rudder ball with integrated drive

[MFR1964]

Had the same issue with the spirou, ended up low tech, placed the prop just outside of the tapered end and added a kurtnozzle for steering, works fine, going reverse was done by placing a cone near the inlet slits.

Manfred.

[Occupant]

Was going down a rabbit hole with material questions for vacuum forming and it evolved into a design review of sorts using AI. Eventually got around to this area - the prop and the dialog ended up with this after stating the design i had would overheat the motor / ESC:

Adding winglets (bending the blade tips into small 90-degree flanges) is a brilliant compromise that gives you the best of both worlds. [1]

In advanced marine hydrodynamics, a propeller blade is essentially a rotating wing underwater. By adding winglets to your 3D-printed prop, you get the cavitation-suppressing benefits of a full ring without the crushing hydrodynamic drag penalties. [1, 2, 3, 4]

Winglet vs. Full Ring: The Direct Trade-offs

Characteristic [1, 2, 3]	Full Ring Propeller	Winglet Propeller (Winner)
Tip Vortex Cavitation	Completely eliminated	Significantly Reduced (Up to 68% suppression)
Rotational Drag	Extreme (massive battery drain)	Very Low (releases motor power)
Gyroscopic Effect	Heavy (fights your rudder servo)	Negligible (keeps steering snappy)
3D Printing Ease	Requires zero supports	Requires precise printing/supports

How to Correctly Design the Winglets for a Submarine Prop

If you want to test a 3D-printed winglet design, you cannot just bend the tips arbitrarily. Marine research dictates highly specific parameters for maximizing thrust and suppressing tip vortex cavitation (TVC): [1, 2]

• The Bend Direction (Crucial): Bend the winglets downward toward the high-pressure side of the blade (the aft face that pushes water backward). Bending them backward (-90°) is statistically proven to suppress TVC by up to 68%, outperforming upward (+90°) or 45-degree bends. [1, 2]
• The Scale: The height of the winglet should be exactly 10% of the blade's total length. If your blade is 20mm from center-hub to tip, the bent winglet flange should only stand 2mm tall. Anything larger creates excess drag; anything smaller won't block the pressure leak. [1]
• The Curvature: Do not print a sharp, hard 90-degree corner. Use a smooth radius fillet at the bend. A sharp corner creates a structural weak point where the 3D-printed layers will easily shear off under high-RPM water resistance. [1]

Why the Winglet Beats the Ring for an Acrobatic Diver

For your specific 1000KV motor on a 3S LiPo battery, the winglet wins because it keeps the motor operating at its optimal efficiency curve. [1]

A full ring acts like a giant spinning brake drum in the water, dragging down your motor's maximum RPM and turning battery capacity into pure motor heat. The winglet only adds a tiny bit of wetted surface area at the very tip, reducing fluid resistance while redirecting energy back into axial forward thrust. Your sub will stay incredibly punchy, exit loops faster, and keep your electronic speed controller (ESC) cool. [1, 2]

So i've modeled up the Prop and its got tons of power, quick dip in the pool and not sure how its going to behave yet, just had the WTC and drive end in the water with a cheap radio to drive the throttle and rudder. without adequate weight it sits high and wants to pull air thru. Also the prop seems too hot, get too much out of it - how do you know when you've got too much thrust? I suppose can limit it somewhat by changing the performance curves in the transmitter but is that really the way to go?

On the plus side the WTC with silicone pieces seal well. All connectors to the WTC are sealed and function electrically! The peristaltic pump moves water in/out

While i was in AI wonderland gave it the the model radio i'm using and described the failsafe condition i wanted and it generated a step by step set of instructions to set up the failsafe. went thru and programmed the radio and it got about half of it right, if i lose the TX signal it immediately purges the ballast and then cuts off the pump. that beig said once the pump is off its off, i need to add some more logic to it.

All in all was impressed with the assistance AI offered.



WTC with drive and rudder




on the bench testing the fail safe