First trial of horizontal fin drive

[Skip Johnson]

32" long 3.1 # craft is early in development but looks good so far. Approximately 1.4 mph on less than a watt.

The foils shown below top to bottom

Wider 6310 foil not yet trialed
6310 foil currently on craft
Crescent plan foil not quite as effective as 6310
Flexible fin was even less effective

Next trials will try a faster geared drive and will relocate the batteries to hulls to move CG aft then perhaps the higher A/R foil.

Ultimate goal is to repower the craft with paddle system currently used by MR340 racers using same motor and compare results.

 

[tropostudio]

Pretty cool, Skip.

Do you have an electronic speed controller, or is speed fixed based on battery voltage? It would allow you to verify power consumption and boat speed across a range of motor speeds with one fin before you change gear ratios and fin configuration. How are you measuring power consumption and boat speed? Looks like great fun!

 

[Skip Johnson]

Speed is fixed, 5- 1.2v nimh AA batteries. Current for the little n20 gearmotors runs 150-250 milliamps depending on load. For initial run speed was estimated by stopwatch timing between two points on shore and then pacing off the distance. Further trials will include putting a couple of markers down spaced by a measured distance. Initial run used a gearmotor with a 90/1 ratio ~75rpm, next trial will be with a virtually identical gearmotor with a 45/1 ratio ~150 rpm. Those are the two choices powerwise. This is an experiment largely based on existing stuff; gearmotors are leftovers from making a working Fallout door; most of the mechanical bits are from FRP panels left over from making a removable motor mount for the proa; crossbeams are acm panel scrap collected when I was a practicing Architect. The only thing bought was a ball bearing linear slide initially and then an inexpensive RC rig when it looked like it was worth pursuing. 3D printing greatly expedited the fin end of things initial fins were determined primarily by the size of the bed 320mm x 320mm.

It is an interesting endeavor ;-) I haven't messed around with RC model boats for a very long time, the little red pushrod clip on the rudder horn could easily be over 50 years old. I'm not really interested in getting back to model boats per se but am really interested in finding the best way to propel a human powered craft. I've tried several different things over the years but nothing previously showed this much promise.

 

[mc_rash]

Interesting!

Would have expected that the circular shaped foils wouldn't perform as well

 

[tropostudio]

I'd also imagine there is no intrinsic advantage to the crescent planform, especially with the tip chord going to zero. Looks like it would essentially be a triangular wing, with undesirable twisting moment around the root. And a 3D print isn't going to be very rigid to start with.

@Skip Johnson - PWM ESC's for brushed or brushless motors and digital wattmeters are quite cheap from the usual online places. I likely have extra lying about, and am happy to look if you would find them useful. Data logging would be another project though - you'd need to use an Arduino or similar. I get it if you want to keep it simple.

 

[Skip Johnson]

I'd also imagine there is no intrinsic advantage to the crescent planform, especially with the tip chord going to zero. Looks like it would essentially be a triangular wing, with undesirable twisting moment around the root. And a 3D print isn't going to be very rigid to start with.

@Skip Johnson - PWM ESC's for brushed or brushless motors and digital wattmeters are quite cheap from the usual online places. I likely have extra lying about, and am happy to look if you would find them useful. Data logging would be another project though - you'd need to use an Arduino or similar. I get it if you want to keep it simple.

The crescent shape was a nod to my still firmly held belief that a crescent shaped rudder is the best choice for canoe rudders (never stalls in a sweeper). It may be that there's a decent foil path down mother natures way but at the moment the NACA 6310 appears a better choice to follow.
I got one of the cheap ESC's along with the Xmtr/reciver and servo; it acted a little wonky when first hooked up and went back to switch on/ switch off. I appreciate the offer but at 83 I'm dropping more and more side pursuits as my mind slows down ;-).

This is interesting stuff. It starts with the premise that the most efficient propulsion results from moving the most mass the least amount. Big propellors do work better than small in this regard. A fairly wide high aspect ratio fin moving with an adequate amount of stroke impacts a lot more mass.
So many variables; span, stroke, area, plan shape, foil shape, frequency, angle of attack, centering spring tension and the interrelation of each provides an old man plenty to piddle with.

 

[Skip Johnson]

Interesting!

Would have expected that the circular shaped foils wouldn't perform as well

I agree, but Harry Bryant's Thistle, which is what started me down this path used a flexible fin to good effect.

 

[tropostudio]

Yes, many variables.

IIRC, Bryan used a flexible fin made from a sheet of thin polycarbonate. Foil x-section was irrelevant, but the sheet would be fairly big and flexible. I don't think the fins on the Hobie Mirage drive rely on flexibility, do they? It's about the oscillation of rigid fins. I may be wrong on that.

There have been MIT tank models of 'fin driven' robotic fish that used rigid sections connected by membranes to approximate fish. And membrane or elastomeric coatings to emulate shark skin for drag reduction. And then they find that the part of the biological organism they couldn't or didn't recreate contributed to the part being tested having poorer-than-anticipated performance! Biology is complicated compared to engineering.

Best of luck, and here's to forward progress!

 

[jehardiman]

For the 1991 International Submarine Races (ISR) Scripps showed up with an non-flexible oscillating fin design that used a mechanically linked trailing edge flap to alter the camber of the foil as it moved in each direction. They were the fastest non-propeller design and later set the Guinness Record in that category. You can probably find the paper in the IEEE OCEANS 1991 proceedings on the 2nd ISR. https://ieeexplore.ieee.org/document/606535

In the early 1990's there were several ONR and SEA GRANT study projects into using biometric oscillating fin propulsion, most notably (IIRC) done by the Ocean Engineering department of University of New Hampshire. The big issue with fish like body or fin propulsion is the manipulation of the vortices that are used to 'push' off of. For a flexible fin this means that the vortices at the contact surface must be accelerated along the streamline over the surface. Biotics, with continuous embedded sensors and muscle structure find this easy....electro-mechanical systems not so much.

 

[fallguy]

The first thing I noticed is there’s some up-and-down motion. It may not seem like much at this scale, but it cannot go into larger craft.

It is certainly interesting that man has not used the fish as the ideal when a typical wahoo hits 50mph.

 

[Skip Johnson]

Yes, many variables.

IIRC, Bryan used a flexible fin made from a sheet of thin polycarbonate. Foil x-section was irrelevant, but the sheet would be fairly big and flexible. I don't think the fins on the Hobie Mirage drive rely on flexibility, do they? It's about the oscillation of rigid fins. I may be wrong on that.

There have been MIT tank models of 'fin driven' robotic fish that used rigid sections connected by membranes to approximate fish. And membrane or elastomeric coatings to emulate shark skin for drag reduction. And then they find that the part of the biological organism they couldn't or didn't recreate contributed to the part being tested having poorer-than-anticipated performance! Biology is complicated compared to engineering.

Best of luck, and here's to forward progress!

The early hobie drives were somewhat flexible, I tried one of the very first models out at a canoe gathering and was not impressed. Some years later I was paddling with a group and asked a guy if I could try his Hobie; a much improved experience but I still preferred paddling.

 

[Skip Johnson]

For the 1991 International Submarine Races (ISR) Scripps showed up with an non-flexible oscillating fin design that used a mechanically linked trailing edge flap to alter the camber of the foil as it moved in each direction. They were the fastest non-propeller design and later set the Guinness Record in that category. You can probably find the paper in the IEEE OCEANS 1991 proceedings on the 2nd ISR. https://ieeexplore.ieee.org/document/606535

In the early 1990's there were several ONR and SEA GRANT study projects into using biometric oscillating fin propulsion, most notably (IIRC) done by the Ocean Engineering department of University of New Hampshire. The big issue with fish like body or fin propulsion is the manipulation of the vortices that are used to 'push' off of. For a flexible fin this means that the vortices at the contact surface must be accelerated along the streamline over the surface. Biotics, with continuous embedded sensors and muscle structure find this easy....electro-mechanical systems not so much.

Many thanks for the information, the only thing I had to go on was the old IHPVA stuff. I feel better now about not following down the "natural trail" it's far beyond my capabilities.

 

[SolGato]

This video popped up in my stream the other day so I thought I’d post it here after seeing your thread.

I believe he has been here on the forum in the past.

The Hobie Mirage drives were fascinating when they came on the scene and I was always impressed how fast you could get a kayak going with them, although I never cared for the back and forth motion compared to a round pedal motion.

 

[montero]

People use various types of fins; perhaps a suitable design for a propulsion fin is feasible. However, it almost certainly cannot be a rigid fin.

 

[Victor R]

Ultimate goal is to repower the craft with paddle system currently used by MR340 racers using same motor and compare results.

Hi Skip!
It would be interesting to know the parameters that typically characterize the operation of a fin-based propulsion system—such as the ratio of the fin's chord length to the vertical stroke amplitude, as well as the ratio of the fin's average speed over a cycle to the boat's speed.
Another question: what other parameters do you plan to use to compare the fin propulsion system with oars, aside from matching the electric drive power? Perhaps equating the surface area of the fin to that of the oar blades?