Pedal Powered Boats

"Indestructible" comes mainly from being short and thick. I plan glass fibre reinforced nylon. It can be injection molded cheaply and has a high strength, but you can also use almost any material because of the thickness of almost 5mm.

The efficiency loss of rake is currently being evaluated by me, but it will take me a lot of time until finished.

I am designing a NACA 66-008 strut with a NACA 66-020 nose for the whole system. At first I wanted to incorporate nylon bevel gears (which could be injected together with the prop), but for sprints, that would mean 5 Nm torque.



I calculated, that for 5 Nm you need at least 7 cm diameter for a small gear, or 12 cm if you want to incorporate any reduction.

So at the moment, I favour using a 5M, 15mm HTD belt, which allows for steering of up to +/- 45°.

I have three different sketches using different combinations of bevel gears and belts, where the lower section with gears can steer 360°.

I could also design a sketch, where the driveleg can be steered for +/- 45° while being capable of being swung up by obstructions in the water at the same time.

You will receive this sketches in the next time for discussing which of them is best suited. The main advantage of all of them is, that they can modularly be used either as a driveleg, or a steerable Z-drive.

Andreas

 

Andreas,

A friend used bevel gears for a pedal boat drive and found that in practice the gear teeth needed to be large to take the very high peak torque load. I've attached a photo of the special gears he had made (he gave me a couple to play with).

The interesting thing about these gears is that they use a standard stainless steel spur gear coupled with a large "bevel" gear that has a clever tooth shape that not only engages with the spur driven gear but also has a larger tooth cross section than a conventional bevel gear, so allowing a higher peak torque load.

These gears have proved to be very reliable on a pedal boat drive and are acceptably quiet in use, too. I believe they came from a company in the Netherlands, but could probably check for you if you're interested. The larger plastic gear is about 85mm in diameter.

 

Jeremy:

The spur gear doesn't appear to have a taper to mate with the larger gear in the pic. Interesting, as the heavy duty spurs I have seen are cone shaped?

Porta

 

You're right, it's a very clever tooth form on the plastic gear that allows an off-the-shelf spur gear to be used. The clever bit is the way the tooth form is shaped so that the root of the plastic gear has full engagement, with the top of the tooth form being cut away to allow it to smoothly line up. When you look at the shape of the teeth on the plastic gear they are very odd looking, as the "valleys" are wider on the outer diameter than they are on the inner. Apparently they are a relatively recent development, arising from the ability to now CNC cut pretty much any 3D shape needed.

Here's a close up photo that perhaps better shows the tooth form.

 

Very interesting gear teeth Jeremy, thanks for that post.

I also like your rule!

As my Mechanical Engineering Technology metal lathe shop teacher taught me, "It's not a ruler.
There is only one ruler, and that is the Queen of England.
It's called a rule!"
This in his thick British accent of course.
I got 87% in his class, my best mark in school.

 

Jeremy,

I just read through most of your posts on this thread, again.

Can I ask you a few questions?

Are you planning to use the 25mm UJ's again soon (nudge, nudge, wink, wink, know what I mean)?

Would you think them appropriate for a 97 degree angle, 80 -120 rpm, human direct drive to a 500 mm two bladed prop. The prop centre would be deep, about 1.3 m below surface while on foil. Oil filled with lip seals.

What did you pay for them, do you recall?

I didn't see any pricing on their web-site.

Thanks,

 

Tom,

I'm not planning on making another drive leg for a while (unless fate intervenes!). The most acute angle per UJ is 45 deg, so 90 deg for a double UJ as I used, so 97 deg would be fine. The 25mm one would probably handle the torque OK, but as you've probably seen there is a big torque derating factor that has to be applied as the angle of each joint increases. My current high-ish power (around 700W maximum at about 600 rpm) leg is running a 25mm joint at an included angle of 100 degrees, rather than 90 deg that I used on the other leg, but there doesn't seem to be any appreciable difference between them, they both run pretty smoothly.

I bought the last one from RS Components, not the cheapest supplier, but I was buying other stuff from them at the same time so it was convenient. Here's a link to the UK page that might help: http://uk.rs-online.com/web/p/universal-joints/4923761/

Hope this helps.

 

Thanks Jeremy, that helps a lot.

Cheers,

 

Tom, if you are racing, might consider a larger size or so on the joints. Also, if your design has the joints on the high torque side. There is quite a difference in shock load of gradual PWM electric drive vs. leg torque. Even some very strong, massive gearboxes that have been re tasked to HP have failed, mitre, etc....

JMHO

Porta

 

Yah, I hear you.

Thanks, I tend to agree.

Although, I'm going by Jeremy's testimonial experience.

There are of course several other factors, but thanks for your input and concern.

I'll let you know what I decide.

 

FWIW I ran a bit over 1kW through one of these drive legs, driven by very heavily over-loaded Makita drill (overloaded to the point where it was running at about half no load rpm). The prop was a stainless steel 13" diameter one, with a fairly coarse pitch, that was designed to run at much lower rpm (it's pretty close to being pedal boat sized in terms of diameter and pitch).

This got hit with some pretty high shock loads, as the drill has one of those shaft locks that automatically engage when you let go of the trigger. The prop inertia was such that the torsional shock load up the drive leg every time I let go of the trigger was pretty severe, yet in the limited time I ran it I had no problems.

The bearing pins in the 25mm OD UJ are hardened steel of around 5mm diameter, and I think I worked out that the peak acceptable torque load was well over 100 Nm (~74 ft lbs), even though the max working torque was only around 35 Nm (before derating). The torque derating on these isn't to do with the maximum torque the joint wil take without breaking, it's a limit imposed by the bearing stress and wear on the pin bearings, as an increased angle means a faster oscillation rate on the plain pin bearings, which combined with the higher bearing stress increases the joint wear rate.

For an application with a highly oscillating torque load I think that the figure to use when selecting a joint should be the average load, whilst taking account of the need to keep the peak load well below the failure point.

If weight isn't ultra-critical (I'm thinking of Tom's predilection for submarines here!), then adding a flywheel to the output shaft, between the pedal drive and the universal joints, would damp the torque variations out a great deal and give the joints an easier time.

 

Thanks, Jeremy.

You seem to be on the mark with your considerations as usual, and I defer my considerations to your experience as an engineer.

I'm curious about failure modes. Wonder what the weakest point of your drive train/prop is as regards shock load, if not the U joints? Do you think a folding prop dampens the shock load? Would the drive train/prop survive a full power collision with the bottom or a rock? I know some of these are not considerations in your particular application, just trying to get the benefit of your expertise...

Porta

 

In my case the weakest point would probably be the pins I used to secure the UJ to the shafts. These are 5mm diameter stainless steel, in double shear through 12mm diameter stainless shafts. If they broke it would be a pain, as they are sealed inside the drive leg! A very rough calculation shows that a stainless pin will start to yield at around 5kN (in double shear), so at a radius of 6mm (the shafts are 12mm diameter) the max allowable torque would be around 30 Nm. In practice I suspect the true shear torque would be closer to 45Nm, as some testing I did many years ago on some parachute lanyard shear links showed that they consistently sheared at around 50% more than the predicted load. The drill I used (an 18V Makita with a lithium battery pack) is nominally rated at around 50Nm maximum, so it looks as if I was living dangerously when using it to drive this leg!

PS: Thanks for the kind words, but I'm no engineer, just an ex-scientist playing around in my retirement..........

 

Interesting you should mention that Jeremy, I've been toying with the idea of a small flywheel for some time for just that reason.

It could be within the submerged pod at it's maximum diameter (~20cm), so of less weight with such a large diameter. Hard to justify the added weight even though it is in the best place, low.

The biggest catch is, I couldn't put it in front of the UJ's...

My thinking is increased technique. Less pounding on the pedals and higher RPM's especially to get on foil.

But, there may not be the issue here we're speculating on. Perhaps time will tell...

The other alternative is a Rick style flex drive.

That is a challenge for me to get into a cowlling or shroud.

 

It'd be less effective (because of the lower RPM), but could you add a flywheel to the pedal cranks?

There's been mention here before (by Rick, I think) of the effect of reduced flywheel effect on a pedal boat drive train, when compared with a bike. Whether it's worth worrying about I don't know, but I seem to recall those using flex drive shafts reporting that they tend to feel springy during the pedal torque peaks if they aren't stiff enough. Makes me wonder how much (or rather how little) extra mass you'd need on the cranks to reduce the peak to peak torque fluctuations. Not something that's easy to calculate, but it may be easy enough for someone with a flex shaft pedal drive to do some simple experiments.