I'm new here

Curtis
The stern mounted foil is probably worth a test just to get a feel of things. It can all be worked out but there is nothing like actually trying something to learn what the issues might be.

I found making foils was easiest using pieces of aluminium. The attached sectional shape is something to aim for. It does not need to be precise.

Lets say the foil is 2ft wide and 6" long. I would make this out of 0.8mm sheet with a 12mm square tube as reinforcing. This needs to be hinged to the vertical stroking strut (needs to be narrow so it does not have a lot of drag). I placed straining wire through the leading edge to act as a torsional spring. It is a bit heavier than a clothes hanger. I can give more detail if you want.

The lever arm would need to be pivoted in a way to give a minimum of 300mm sweep. The longer the lever arm the better because you have less angular change at the foil. You might be able to see in my foil attached that I have an angle compensating linkage in parallelagram configuration for this purpose. This blade was swept through 600mm and was nice at 8kph (5mph) on a long boat.

You cannot reverse with this set up. There is no steering without a rudder,

If you set up with two of these, one on either side you could get steering. The best place to pivot would be up near the bow with the vertical strut just over the stern. Could be awkward to set up. If the pivot is on the stern then the foil will be further back and this will increase pitching moment.

Rick W

 

Yes, please. I was thinking of using wood, but the idea of rot bothers me.
Thanks,
Curtis

 

Curtis
I hope the attached image is self explanatory.

If you can get some heavy straining wire about 4mm diameter it will do both the torsional spring and the pivot. There are two nose pieces that are a bit less than half width. This allows the wire to be bent out from either side and clamped to the vertical strut.

You form the noses pieces over something like 1/4" steel rod to get a nice curve.

The tail piece is best bent very hard by clamping between two pieces of wood.

You only need 0.6mm thick aluminium for this. Our local hardware in Oz, Bunnings, stock this in small sheets.

The stiffener is 1/2" (12mm) square tube with 1/16" wall. You can pop rivet it together or screw it with self tappers.

You do not absolutely need the plastic bushes but they do stop the wire flogging about and it is quieter.

Hope it makes sense.

I would make one to test out so you get an idea of the forces and its effectiveness. We will look forward to your posts on building.

Rick W

 

Thanks, Rick. Finding the materials for it is no problem. It will be a little while before I'm quite ready to build this part, but I've printed the drawings and related text. Fabrication seems pretty straightforward.

Curtis

 

Getting hold of the spring wire is the hardest part. A coat hanger is just too tin.

I have a spring maker near me and I can get straight length of the best quality spring wire. 4mm wire has a yield around 1600MPa. This is super stuff. Despite the strength it has good ductility and can be bent in tight radius. They cold form it for springs to retain the temper.

Ideally the foil will flap through about 90 degrees when at rest and will be less once moving. It is best if the vertical strut stays vertical but if it is mounted on a pivoting lever arm then this is not possible. Any added angle on the lever arm reduces efficiency.

It is possible to get the vertical strut to stay vertical by setting it up as one leg of a parallelagram. This could be a refinement.

Attached shows something that could be made to clamp on the stern of the boat. Use one on each side. You need to pump in unison to avoid roll. If you went hard there might be a point where pitching is a problem but it will not take much power to get moving. Operating one unit and leaning into a turn should give acceptable steering.

Rick W

 

Looking at this again, I have to ask about the spring. What is the purpose of it? How does it perform work in the system? I know you mentioned torsion, and your sketch is clearly that of a torsion-spring arrangement, but I would have thought that the foil should be rigidly mounted, probably at mid-chord. Is the spring only needed when the foil is supported at one end? it would make more sense to me in that case, to prevent failure at that attachment point.

 

In operation the foil will have a low angle of attack. Maybe 2 to 3 degrees. So if you consider the situation when the boat is at rest the foil must range through almost 180 degrees to keep that angle of attack. As you increase speed the velocity vector changes such that the foil might only need to range about 90 degrees to achieve the best angle of attack.

Hence the torsional spring plays a vital role. It is similar to the reason foot flippers are made from rubber. Same reason the Hobie flappers are rubber. You can achieve a similar action with a rubber blade but it is not as efficient as the torsional spring on a rigid, high aspect foil.

The system works very nicely if the vertical strut stays vertical. It is works as a variable pitch prop as the force on the blade is almost constant irrespective of boat speed and irrespective of how fast you oscillate the blade.

Rick W

 

Cool! One more question

Is the torsion spring more efficient than rubber on low-aspect foils?

 

High aspect blades are better than low aspect blades for any given surface area.

Defined shaped, achievable with rigid foils, are normally better than a flexible shape achieved with rubber. The exception is where you create a variable camber foil that comprises a flexible membrane with a rigid frame leading edge. I have seen something shaped like a porpoise tail using a curved steel rod frame with a membrane glued around the rod. This can make a very efficient foil as it forms a thin, highly cambered foil in operation. It will work reasonably well at speed without a knuckle pivot; hence the torsional spring can be avoided. Without the pivot it stalls out at start but would be good enough to get you moving. The tilting blade has very high thrust at low speed.

It is possible to replace the steel torsional spring with rubber bushings that achieve the same result but they need to be quite low durometer rubber. I have found spring steel works best. If you were operating in salt water then you might need to think about more corrosion resistant materials. I paint any spring steel I use. Straining wire is galvanised so it is OK for a while - still best to paint. (Machinable aluminium rod with a good temper, something like 2011 T8, is a good substitute for spring steel and you could go to thicker rod as it has much lower spring constant. I expect 1/4" rod would be good.

The blade with torsional spring is quite easy to tune. If you find it too stiff then you can replace the wire with something heavier for example. Best to use self tapping screws for making the foil until you are certain the spring is right.

Overall I found the torsional spring the best solution. I made maybe 30 to 40 different variations from scale models up to many variants at pedal boat scale so there is a bit of background. I can do accurate force and power calculations but it is easiest if you just have a go. I used to make foils in an hour or two. Making nice mechanisms takes longer. I even got to using roller bearings for pivots and spherical ball joints for linkages to get rid of any floppiness. You do not need these to test something out though.

Rick W.

 

The reason I asked is because, for my purposes, the long foil approach is looking problematic; the necessary mechanism seems more complicated than I first thought, and the addition of a rudder adds more machinery. I think I'd be better just going with my original first idea. It's less complicated all around, and provided it works as I imagined, will give me better maneuverability, plus the ability to leave it permanently mounted and just swing it out of the way for beaching and transport. Also not as likely to get hung up or damaged if I hit a snag. (I've already driven over a submerged log; no damage, but it was a real surprise.)

Kjell Dahlberg made some of these at small model scale; I don't think it would be too hard to do. What's best for the membrane? I'm thinking of non-stretchable nylon cloth, like Cordura, sealed with RTV silicone, sandwiched between two sheets of stainless, probably pop-riveted together, as shown below. I don't know if a peduncle would help or not.
Please explain the "knuckle pivot."
Thanks again,
Curtis

 

Curtis
I envisaged two of the flappers each mounted off a small bracket that can be clamped over the transom. You could make them so that when the handle is in the lowest position the blade is higher than the keel. The lever handles would be placed about 2ft apart under the shoulders. You would pump the levers up and down about 8 to 12 inches.

Even the aluminium blade can be made to float by squeezing in some foam when it is assembled. The normal rest position would be with the blades up and the handles down. They will not add to draft for beaching.

With two flappers that are either side of the centreline you will be able to steer by being a bit more aggressive on one handle than the other. Leaning into the turn should also help.

The angle sweeping arm with the membrane is effectively the same thing but it has fixed depth so will not get shallower unless you pull it out. You also have to find a membrane and fixed it into place. I simply find working with aluminium so much easier than other materials. Light, easily worked, known properties and strong.

Rick W

 

If I understand you correctly, the first sketch below may be the answer to that problem. See also the sketch in post #6, it shows (with a note) that the hinge is on the outer bottom edge of the frame.
I don't know just how effective the fins would be in the intermediate position, but they might do something. I'll still have my oars aboard.
The fins and their drive shafts may need to be long enough to cause them to overlap when in the "up" position. If this becomes the case, I can simply make one of the assemblies to project an inch or so further off the back; the asymmetry would probably not be noticed on this boat.

The second sketch illustrates an entrapment scenario for the horizontal foil system, and is a very real possibility where I boat. Tough weeds are a lesser case of the same. The water is murky, so I can't see much of what's below the surface. The vertical fins would be far less apt to get entangled, and can be raised for clearing.

So, I'm pretty much giving up on the horizontal foil approach at this point. If you could help me design the fins, that'd be great.

I much appreciate your time and effort on this, and I'm still open to further ideas and comments. I'm still considering paddlewheels; there's a lot of appeal there for my uses.
Thanks,
Curtis

 

Curtis
I like the double waggle system. You could do some trials with a pair of large flippers attached to a tube. Still a bit of engineering involved but it does not need to be sophisticated for initial testing. This will be considerably less efficient than a rigid foil sweeping a good range but, as I pointed out, this is not such an issue with a hull that will only want to so 4 to 5kph easily.

The snag problem is not that bad with the flapper because it only has one narrow vertical strut that will flex quite easily to skip around obstructions. If you know they are there then you just sweep through a smaller range.

Rick

 

Double waggle, I like that. This thing needed a name.
Can you give me a rough guideline on the size and shape of the flippers? I'm guessing they need to be larger than swim fins, but beyond that, I'm -- well -- guessing.
Here are a couple ideas. The first is what I'd been thinking of and mentioned earlier, but the second is just a shorter, vertical version of your foil, including the torsion spring mounting. Does it even need the peduncle? Might be the best of both worlds.
The dimensions are just guesses; plug in better ones as you see fit.

Curtis

 

THe beauty of the swim fins is that the are available and you can get them quite cheap. They are rubber so will just bounce over things without getting bent.

The waggle system will not be very efficient. You would need a control linkage on the foil to get it efficient and that is more complexity. What you have proposed has a lot of moving parts with quite high forces. For you to get an understanding of this it is better if you build something and try it. Ultimately the forces are limited by what you can apply. If you really lay into you could easily break bits but you should get reasonable result with 5 to 10kgf on the handles which will multiply through the linkages depending on length.

You will find it quite awkward to row with the roll up feature because the handles will want to tilt up and down as you row back and forth. It is these sort of things that you have to contend with.

The parallelagram linkage I proposed gives nice force management. It will feel smooth. The applied force is straight up and down.

I could go on forever about the detail but it is better if you build what you think is best. Identify the issues and then redesign to overcome them. I have already done this and shown you what works best for me. You might find something different works best for you.

You could make a simpler waggle device to trial just by fitting the handle to the vertical shaft and setting it at the back of a boat like a rudder. Just waggle it back and forth. It will give you an idea of forces. If it is a flipper it will be more effective than the solid rudder used on sailing dinghies and they waggle or pump their rudder to propel in low winds. Two such devices on either side of the boat at existing oarlocks would be much simpler to engineer.

Rick W.