Efficient electric boat

Jeremy
Your first metal fabricated marine prop is a long way ahead of my first one. I still see good ideas from others that I incorporate into what I make.

I also have a good idea of what adds to performance and what doesn't so have never been to concerned about looks for the sake of looks.

Rick

 

Hi Harry,

There's less than a days work in those blades so far. I didn't get out in the workshop until about 10:00, and spent the first 40 minutes or so making the angle templates and jig. I cut the blank bits of stainless out before lunch and put the twist in. After lunch I spent maybe an hour getting the blade root stiffening pieces cut out, twisted and silver soldered in place. The first blade took about 1 1/2 hours to grind to shape, the second one less than half that time, as I knew what I was doing! I finished for the day and came in at about 17:00, so a total of maybe 6 hours or so spent on them. All told, I reckon I could probably make a pair of blades in a couple of hours now, maybe less.

The only reason for going back to basics and making props is because we can't buy an off-the-shelf prop that will give the required performance at the low power inputs we're working at. The boat I'm building only needs about 1/8 hp to run at cruise speed and I need the best efficiency I can reasonably get, because the only energy source available is sunlight.

 

Thanks for those kind words, Rick. If it wasn't for your work, and that of others on here that have added to it, I'd not have thought of making a prop like this. It's the combination of ideas from many people working together that's adding value, I think.

When it comes to looks, then I'm afraid I do get a bit sensitive, even though I know full well that an ugly part may well perform every bit as well as nicely finished one. Knowing that I'm probably going to post photos on here, and have them open to worldwide public scrutiny, is another incentive to try and get a decent finish!

Jeremy

 

G'day Jeremy

Thank you for sharing your most interesting project. I am in the very early stages of designing an e-powered 1 person inflatable pontoon based fishing craft.

Your work with outrunner brushless motors has certainly saved me some effort. I have a vast collection of RC e-power components already, I had intended to use an "off the shelf" trolling type motor but thanks to the ideas gleaned from your work, I think I'll have go at developing a powertrain. Ive taken a cursory look at the tailrotor belt drives used on some rc helicopters and I think that with some adaptation these configerations could be utilized.

Les

 

Glad the experiments so far have been useful. These little motors are pretty amazing. I recently bought a small milling machine, a Taig benchtop model. I bought it without the 115V motor (mainly as I live in 240V land) and instead fitted a small outrunner motor, with belt drive to the spindle. I now have a very capable variable speed milling machine that has more power than the fixed speed version.

The only thing I'd watch with the belt is the torque rating. I've found that I can get away with using 15mm wide 3M HTD belt, but I'm not sure that I could get away with the narrower 9mm belt on the same pitch. Using a bigger pitch increases the torque rating, but then the range of reduction ratios available in a reasonable size reduces. The bigger pitch belts are also stiffer, so absorb more power.

I'd better get back to the workshop, as I'm part way through machining the prop hub.

Jeremy

 

Hi, Les. Interesting about your 1 person inflatable pontoon fishing craft. I've cobbled together something like this from existing "kick" boats. Something like this: http://www.prophish.com/wskp-1-2.html So that might be a place to look for ideas. My fishing interests are in small rivers and protected waters, though I have used them on the "flats" at the coast on occasion. Lately, I have focused on going lightweight and compact as possible. The smallest is just under 6' and 35# fully loaded and can be carried in an airline case when I travel. Based on this model: http://www.4seasonsfly.com/11939/234630/Float-Tubes/Outcast-Discovery-Trekker.html Great for exploring remote areas where most other watercraft can't go. It fits in the back of my Prius hatchback with the pontoons inflated.

Hope this helps.

Porta

 

As Rick pointed out earlier, the pivot pin in the prop blade photo I posted yesterday could be mistaken for the shaft axis. Tonight I've managed to rough out a hub, milling a 1/4" slot in it for the blades, boring it for the shaft and making a start on shaping the outside into something that looks reasonably OK. There's still a bit of work to do on the lathe, as the taper on the end needs to be finer and I need to gain a bit more clearance for the thicker part of the blades as they fold back. Still, it's nearly there!



I may yet take a bit out of the root chord of the blades, as they look a little bit thick still. I'm inclined to just trim the trailing edge in about 5 or 6mm, as I think this may reduce a bit of swirl near the root. Hopefully this won't weaken the blade too much, from the feel of them I don't think I need to worry too much about strength.

Jeremy

 

Jeremy
Your prop looks good, quite similar to mine, the one thing that did worry me during the design is the flat area that is needed to swing into the hub. Will this cause loss of power. I tried to twist the blade as soon as it left the hub and this is what caused me problems with the blades locking up on the hub. The solution I found was a wider slot after the hinge zone.

Dennis

 

prop looks great

here is an idea Ive been kicking around lately

the prop looses efficiency at the tip and at the shaft

so why not a double shroud with one in the central say third of the prop area that the bladed attach's to and another in the more traditional location and attached to the tips

whole thing spins on a central shaft that is fatter than normal with a hydrodynamic shape

just a thought
B

 

Re ColdTub test tanks: the baffle on rollers yielded erratic results no matter what side of the divider it was on. I tried a few other schemes, and settled on a measure of the horizontal lb. thrust at the outboard downshaft. Even that was a fluctuating number, but the results were replicable through some twelve different trials.

And the Push vs. Pull winner is....

 

With the Torqeedo 8x8 prop, the PUSH mode consumed fewer amps than pull mode, as in all conventional outboard setups. Tests were for 100- 400 rpm range. Beyond that the ColdTub testtank allows cavitation and rpms rise dramatically, etc.

 

Hi Porta

Thanks for the info, the Prophish looks like a well executed concept, if a tad pricey, I may consider buying some of the components that they manufacture. My initial design is for a pontoon boat that can accomodate everything including "the kitchen sink". The main use will be as a Bass boat, I need to include a live-well, a drinks cooler (we often experience ambient temps of > 35 deg C), a rod rack for +- 4 rods and the electric powertrain and backup oars. I am having a custom set of pontoons made, I have not yet finalised the dimensions but I do know that they will be +- 2.1m long. The "deck" will be molded, I have some experience with aviation based composite structures. The craft needs to be sturdy & stable as some of the waterbodies we fish have healthy populations of Hippo and Nile crocs , these are not a problem as long as you don't go swimming with them.

As a later project I will look at building a "bare bones" craft, with compactness & portability as the main design criteria.

 

To get power delivered to the water you must have an indicator for the water flow velocity as well as force.

If you set up two markers on the return side of the tub you could use the time taken for a cork to pass between the markers. This will give flow velocity.

Right now there is a fair chance of concluding the lower pitch prop is better because it can generate more thrust than the higher pitched prop for the same power. That does not have any relation to what will happen on the boat.

For example if the big pitch prop produces the same thrust at 20% more power than the low pitch one but it is moving the water at twice the speed the bigger pitch prop is the superior prop. Your need to take into account the gearing of the prop in relation to the water ie the pitch.

The force measurement is not really necessary either in terms of determining the merit of the props. If you simply find the power it takes to keep the flow circulating at a given speed for each prop you can establish relative performance. A simple test with a cork to do 10 loops of the tub might be the best providing the cork does not get stuck in its journey.

Rick W

 

The push vs pull thing has been worked on a lot with aircraft. Which is "best" not as straightforward as it may at first seem.

With a pusher, the prop suffers from variable inflow conditions due to the slowing down of local flow (due to viscous drag) and turbulence associated with the supporting hub and leg. At high speeds (above the critical Re, i.e.non-laminar flow) this can make the prop operating conditions very challenging and result in considerable losses, or even structural problems due to the fluctuating blade loads.

With a tractor layout (a "puller") these problems go away, but are replaced with another set of losses instead. The local flow immediately behind the prop has a higher velocity than the flow into the prop. This means that the total drag losses associated with any hub or support leg behind the prop are significantly greater than for the pusher layout.

The problems associated with inflow turbulence with the pusher layout can be ameliorated to some extent by placing the prop some distance behind the support leg and making everything in front of the prop as streamlined as possible. If the prop can work in conditions where the inflow is as smooth and as near constant velocity across the plane of the prop as possible, and if there are no energy-sapping structures behind the prop, then you'll get just about the best low speed efficiency possible.

Overall, I'm not surprised that the pusher has come out best in this low speed test. What might be interesting would be to look at the effect of spacing the prop further away from the leg. I wouldn't mind betting that efficiency would be improved still further by doing this.

Jeremy

 

I decided to cut some of the root section of the blades away, at the trailing edge only. The blades are 1/4" thick at this point (the 316 sheet thickness was 1/8", 3.2mm), thanks to the stiffener I silver soldered on, so there was quite a bit of metal to play with. I'm happier with the shape I have now, and as a bonus they fold back much further, too.



I'm hoping that this will reduce the swirl losses around the root, as I've managed to get some semblance of a aerofoil section right up to the slot now. Before, the inboard root sections were virtually flat plates, and although the losses were most probably only tiny, they just looked wrong.

Jeremy