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At speeds below the onset of compressibility effects there's no practical difference between air and water; both are incompressible fluids.

This means that the shapes that work well in air at low speeds will also work just as well in water.

The only proviso is that although the form drag in air and water is proportional to density, so around 815 times greater in water than in air for a given body at a given velocity, the kinematic viscosity difference between air and water is the other way around (by a factor of around 11.2) which means that viscous drag in air is actually greater, at the same velocity, than it is in water.

In practical terms this means that a large pressure recovery fairing aft of a prop will have a less deleterious effect in water than it would in air (assuming the same velocity).

 

Doesn't THAT make most peoples heads spin!

 

Jeremy
The pic on your recent post #1543 showing the prop fairing. What spins the prop? It's a very tight turn, is it a flexible rod or what?

Ian

 

Ian, that drive leg uses a double universal joint to turn through 90 degrees. I've found that these things are very low loss, but they do need to be derated a lot when working at such sharp angles. Being a double universal the whole joint ends up being constant velocity, although the joint centre section does go through a fair bit of acceleration/deceleration with each rotation.

This is the second drive I've made like this, and the first one to operate at 90 deg (my original ran at 100 degrees). The joint housing is made by laying epoxy glass over a mandrel cut from tube, then splitting it into two halves, inserting bonded-in bearing housings and glassing up outside the whole thing. I use a lip seal on the output shaft and then fill the leg up with very light synthetic oil, both as a lubricant and to help keep the water out.

The leg shown is for this years Cordless Canoe Challenge, so will be powered by a cordless drill. The same principle would work for an HPB drive I'm sure, provided you could turn the pedal crank drive through 90 degrees and increase the gearing before driving the vertical shaft down the leg. One advantage this system has is that it's easy to steer by rotating the leg around the shaft, so no added drag from a rudder. It can also be arranged to tip up easily if it hits an obstacle. Here are a couple of other pictures of the drill powered leg.

 

Jeremy
I tried the universal joints on one of the mods on my first boat. I had two fitted between the gearbox and prop and managed to break both at different times. In hindsight they should have been a lot bigger than they were, a good experiment at the time. Might try it some time in the future for a drive leg, would give you a much thinner fin than with bike chains.

Ian

 

Yes, they need to be pretty big to take the torque. That one in the photo is marginal at 90 deg (45 deg per joint) and it's a 25 mm diameter one, with a 12 mm shaft. It's rated for continuous use at 30 N-m torque, 300 rpm when run at shallow angles, but has to be de-rated down to just 7.5 N-m when each joint is running at 45 deg (so around 240 W at 300 rpm). Ideally I should be running the next size up, which is 32 mm diameter, as that would give me around double the capacity.

I'm hoping that the smaller joint will hold up for the race, as it'll only be run for around 1/2 hour at most. Also, I'll be running it faster from the drill, maybe up to 650 to 700 rpm if I'm lucky, and javaprop is indicating that the torque should stay under about 12 N-m over the expected operating range. I'm hoping these steel joints are specced on the basis of wear rate, rather than ultimate strength!

For an HPB the 25mm steel joint should be OK I reckon, as I doubt it'll see sustained power of over a couple of hundred watts for long. Being in an oil bath, rather than relying on grease or splash lube may help, too.

 

Nice bit of kit Jeremy

 

Hi, Scheny:

Thanks for your detailed response!

My setup resembled the traditional elongated football shape surrounding the prop all mounted on a long straight flex drive shaft. There was an open section at the prop area between the 2 cones which most likely is the cause of some losses....

Porta

 

Hi, Ian:

Curious what was the failure mode on the U- joints? Pins, metal ears, bushing surface, retaining heads, etc? These look similar to U joint adapters used in socket ratchet systems and should be able to take a lot of torque?

Porta

 

Jeremy,

I'm a bit in awe that this is where you ended up and that it works as well as it does.

What do you figure your percentage power losses are?

I'd be interested in two: one in the boat at the top of the strut and one at the bottom of the strut in the bulb.

I've been thinking about you in my servo research for foil control. There's some pretty good looking stuff in the R/C world. And water tight too!

Cheers!

 

Thanks for the kind words about the drive, folks.

The torque capability varies a lot with angle and rpm, according to the manufacturers data, which makes me think that they are taking wear into account for a good service life, rather than rating them on the load the pins will take.

For example, the 25mm diameter double UJ in the picture above is rated at 30 N-m (about 22 lbs-ft) when run at shallow angles (up to 10 deg per joint) and an rpm of 300, but only 7.5 N-m when run at 45 deg per joint and the same rpm.

The pins are around 6mm in diameter in the 25mm joint, and are hardened steel, so should be OK for an ultimate torque that is massively more than the rated operating torque.

I haven't checked the power loss on the 90 deg drive, as I have a surfeit of power from the drill I'm using, but on the 100 degree drive on the electric boat the measured losses from the shaft at the top of the leg to the propeller shaft (so bearing losses, UJ loss and seal loss) were just over 4 watts at 300 rpm. The 90 degree drive doesn't feel any different to the 100 degree one when turning it by hand, so my guess is the losses will be similar.

I can't see any reason why a low power sail drive type leg couldn't be made using another right angle drive like this. There is a little bit of vibration generated by the uneven rotation of the double UJ centre section, but that's probably less than you'd get from gears.

The major limitation with using UJs like this is the maximum allowable rpm (they are limited to under 1000 rpm as a rule) and the limited torque capability. The advantages are low power losses and low noise, when compared to bevel gears. There may also be a small cost advantage, as good bevel gears tend to be pretty expensive.

 

Hi Porta
The setup was one of a few I used on my first boat. I was going through a steep learning curve ( still am ). The U-joints connected a shaft coming off the gearbox with the prop for a total angle of 25 deg. size was 12mm dia. Way too light for what they were used for, the pins sheared off in both cases.

 

Has any one made a paddle-wheel vessel ? I was always led to believe that they were efficient at transferring energy into movement and perhaps it could be possible to design a hull utilizing the wheel quarters as hull extensions. I would assume the drive mechanism would have to be geared for paddle-wheels.

 

If you take the time to look back in this thread, or use the search function, you'll find it's been discussed many times, the last time just a week or two ago.

To be efficient paddle wheels need to be large diameter. For a pedal powered boat at least 5ft diameter or so to get even close to prop efficiency. This makes them a bit impractical, as a wheel that big adds weight and windage.

 

If the spec is similar to the make that I'm using then a 12mm UJ should be OK for around 4 or 5 N-m when running straight, or perhaps half that at an any appreciable angle. My guess is that you could pretty easily reach over 10 N-m at around 200 to 300 rpm or so at the prop, so no real surprise that it failed. The failure may also have been more likely because it was just a single UJ, rather than a pair. The shaft would have been subject to a fair bit of angular acceleration/deceleration with each rotation, adding to the peak loads on the pins a fair bit.

The 25 mm UJs seem to be around six times stronger than the 12 mm ones, and I reckon that a 25 mm one would only *just* be OK for a typical 12" to 15" diameter HPB prop.