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Did you mean the prop mounted in a pulling POSITION ie. with the flex shaft pointing toward the direction of travel or a PULLING prop mounted AWAY (trailing position) from the direction of travel as in Rick's boat? If you see post #596 on this list you can see that a pulling POSITION would jam into anything as you move forward, folding or not. I don't know for sure if it makes any difference if you use a pulling or pushing prop in a trailing position, so long as it is mounted properly (puller-face foward, pusher-face reverse). My measures with RC props seem to favor the puller, but not by much.

Porta

 

Sorry, last line, measurements favor the PUSHER, not the puller.

P

 

Yes Porta, a pulling prop would be off a flex shaft extending fwd in direction of travel. Indeed I saw Rick's video of the pusher prop off the stern of a yellow kayak. It's amazing how it is so well behaved. Same principles that say a bumblebee can't possibly fly are defied. And a pulling prop would dive. I reckon someone has seen this happen in practice?? And so for the race next year, Rick, you could just get rid of the debris-gathering strut?

 

If you go astern with an unsupported prop it dives. Same thing going forward with a downward sloping prop at the bow. That is one constraint - no astern.

The reason I had the side-mounted prop in the past was so that I could reach it to clean it. A side-mounted prop hits the hulls when turning and operating in waves if it is unsupported. It also hangs down when the boat is being carried.

The folding prop can so easily shed weed that it does not need to be accessible so I can now design with more freedom. Hence the attached concept with stern mounted prop. A shaft strut would be optional depending on the conditions.

Rick W

 

New Boat

Rick
I like your new boat design, very futuristic. Only downside I can see is dragging it up onto a beach, would the shaft in the hull be rigid and then flexible shaft in the water? How rigid will the outriggers be? If the outriggers were mounted forward of the gearbox would this result in less wash from the boat hitting the outriggers? It might be a tiny amount but in a race all the little extras add up.
I have put on the new flex shaft you gave me and have left off the skeg. Reason being its a new shaft and unlikely to break and also the skeg rattled around a lot in the guide. When I need to reverse I grab the shaft with my hand and raise it enough to get it straight, using my hand as a shaft housing.

Ian

 

Ian
The main reason to set the outriggers further aft is to prevent the flop roll when the boat is in a wave trough.

They will also be positioned close to where they will sit on the bow wave when I am at cruise speed. At present the bow waves go inside the outriggers.

I will probably have two part shaft with a more rigid section through the shaft tube and then a flexible section to get the curve. The boat will sit on the shaft when on the shore. I am not too worried about bending the spring steel. If I drop it there is a chance of bending the shaft but it will still work. I have straightened them by eye before. They take a lot of flexing before bending though.

Rick

 

Pulling props with flex propsa as you describe will dive, by my observation many years ago. The only way NOT happen is with a skeg or PERHAPS with a RIDGID shaft instead of flexible. But unsupported ridgid shafts introduce additional issues like tending to flail around instead operating in a smooth mode as you have seen with the very accomodating flex shaft. Ramming a forward mounted shaft into a log, bank or bottom would be pretty intense for the rider and equipment at racing speeds.

Wonder if Rick has worked out a way to save the prop, should the flex shaft break when a skeg is not used. Maybe imbed a small signal device, magnet or some kind of floating mechanism. Maybe using styrofoam spinners front and rear and a much lighter prop would work.


Porta

 

I have rarely used a prop without a strut.

Greg K lost the first milled prop he had made. I warned him because he was using a stainless steel shaft while waiting for spring steel to arrive.

It is easy enough to design the shafts for infinite life but I have pushed the limit to get the prop located beside me within reach.

Attached is the latest photo of my V7 boat. I have set it up to sell - need more space in the garage for V15. You can see how long the shaft is. It is 10mm 2011 T8 aluminium. It will give very long fatigue life with such a large radius curve making the cyclic stress very low.

Rick

 

The shaft specs of 2011 T8,10 mm: is that the optimum? I'm asking to use the flex shaft system to power at hull speed a 16' electric boat, using about 400 to 600 watts. I contemplate the unstruted loose shaft for max long-distance efficiency. I understand the bend, or radius of the arc of the shaft, should be as minimal as possible. I'll have about an 8' long shaft, with a two bladed prop 16" crafted per specs elsewhere in your postings. Shopping at present, and would like to know the optimum. I'd thought also of salvaged rod rigging, but infer that Al might be better than SS. Thanks in advance.

 

Do you expect to hit or tangle with anything in your travels with this motorboat? I don't think 10mm Al will survive peak human power (600 watts) at stall conditions with a stiff 16" prop. Even small Thai longshaft motors have sheathed, unflexible, heavy duty 20mm+ shafts and small, slippery props to avoid twist torques near stall and survive. Also the prop will push to get horizontal as it supplies thrust, which is another thing which will be a source of longitudinal stress. Rick has mastered the trick of balancing these stresses and allowing the prop to get in its natural position in operation while giving a good shaft life. My own approach is to use much more forgiving fiberglass shafts with my low power electric motors.

Hope this helps.

Porta

pr

 

If you can get high strength stainless it could be better than the aluminium. The 2011 aluminium will corrode in saltwater. Painting it will slow that down.

My preference is spring steel but not for long duration in salt water. It corrodes even faster than the 2011 if not coated.

If you can get specs on the proposed SS I can work out the curve for infinite life. The best data is fatigue limit for it but if you have the yield stress that is reasonable pint to work from.

There is an electric boat thread that might be more suited to working through your proposed design:
http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-18.html

Certainly for trialling and set up purposes the aluminium is a suitable choice.

For the prop design you either need an accurate prediction of the speed at the proposed power or a good representation of the hull so the drag can be calculated with some precision.

Rick

 

Porta, I've scoped out Home Depot for 'glass shafts; they have 4' segments of about 1/4" for fishing wires. What's your source and how long typically are they? I think I've read most of the posts, but may have missed details. Thanks in advance.

 

I run at 50 watts maximum and that peaks at about 100 watts at stall with my small setup. Surprise that 5/16" tent poles spliced together have NEVER failed even under stall! Makes for an extremely lightweight power unit of about 3# which I can remove in a second from the mount and hold in my hand like a power drill if necessary.

Lowes has SOLID glass 3/8" chimmney sweep tubes that are stronger and would probably work at 600 watts, IF you baby them. Accelerate gradually and don't encounter STALL conditions by say tangling your prop to a dead stop position. I started with the Lowes rods and they were virtually indestructible, but too heavy and way over specks at my low power. I never worry about fatigue or a bent shaft which has to be straightened, everything springs back, they can even be packed in an arc position. Google them if you can't find at Lowes. If you do have a failure with the Lowes, then the carbon rods that I referenced for Rick should do the job, but they are much more expensive- see post #627. Much lighter and stronger than the solid fiberglass and with better torque resistance. That's what I would risk for a scale up version of my setup. You may have to go to the slightly higher diameters if the 0.415 OD don't hold.

Porta

 

With my boats I get about 10kts with 600W. The buckling force on the shaft is not that great and I have used as small as 1/4" spring steel unsupported.

If you are putting 600W in and doing 5kts then the force is double what I have tested. There is increased likelihood of buckling particularly if you use a lower modulus material like FG.

A small tension strut fitted with stainless bearings will cost 2 to 3W and overcomes the problem of the shaft dangling down when at rest. It also allows reverse.

The shear stress in the shaft from torque will depend on the diameter of the prop. Typically it is not very high relative to the shear strength of the materials used but it should be checked.

My main purpose of the curved shaft is not to avoid the strut but to get a shaft that transmits torque smoothly to a high aspect prop that is aligned with flow. If the prop is not aligned it puts high cyclic forces on the blades causing vibration and lowers the efficiency.

If you do restrain the shaft so it cannot fully wander to the flow you will find you get vibrations when you are turning. Once you observe this you begin to realise the advantage of having the prop lined up with the flow.

The design consideration for the shaft are buckling force (I have not found a limit but believe there will be one lower than the compressive strength of the material), shear strength from torque and fatigue life from the cyclic bending stress as the shaft rotates. I have explored the latter in a lot of detail and it takes a bit of design to ensure the configuration will last.

For fatigue the design with an electric motor is not constrained in the same way as a pedal boat because torsional compliance would be beneficial for shock absorbing. In a pedal drive torsional compliance is not desirable.

Most aluminum specialist suppliers in Australia have a machinable grade (2011 T8 or similar) 10mm round in 4m length for about AUD20. It is probably worth setting up with this. If you can get some high strength SS rigging rod it would also be worth a try.

One last thing. If you use a strut you can place a small fairing piece ahead of the bearings that is locked to the shaft. This means you do not lose the prop if the shaft breaks. If the fatigue stress is borderline for infinite life then you should make a second shaft and always have it on board.

I think you will find the curve shaft a really neat solution but take some time to set it up properly.

Rick

 

My fiberglass prop shafts are only 4', so it is probable there would be buckling issues if the same exact chimney sweep rod material is used for an unsupported 8' shaft at 600 watts. There are easy solutions involving trade offs to the buckling issues, such thicker diameters, shorter runs or supporting struts. Torsional failure has been a more difficult issue when working with existing shafts that have been adapted for MINIMAL dimensions and maximum life in my applications.

I suspect that fiberglass or carbon shafts specifically designed for drive shaft purposes might be the way to go in the long run. Corrosion, fatigue, weight, sudden break failure mode and deformation are essentially non issues compared to metals. Those have been my observations, anyway.

Could something be learned about buckling strength for fiber composites by studying the poles used in pole vaulting? I think FG still holds many of the world records there, sometimes with essentially infinite flexing life.

V.