Efficient electric boat

Rick,

I'm going to experiment with cell mounting, as I want a light weight solution that I can custom-fit to the shape of the decks. Luckily, the fore and aft decks of this small river boat won't get walked on, so I don't have to make the panels double up as walk ways. They still need to be able to tolerate occasional heavy loads though, as I'm sure that someone will put a foot on those decks at some time.

Having looked at several DIY solar cell designs on the web, I'm torn between laminating the cells between layers of EVA film, perhaps with a thin GRP sheet as backing, and bonding the cells, face down, to a sheet of thin polycarbonate sheet and then adding a layer of glass cloth over the rear.

Both require the use of vacuum, but really nothing more complex than the simple DIY vacuum bagging unit that I already have for moulding composites.

The EVA laminating film solution needs a means of evenly heating the film to the right temperature, to get the glue to melt, which really means seeking out someone with a suitably sized machine, as I'm not sure that it's a very practical DIY job (although some have done it OK).

I can't find anyone who has used the technique of encapsulating the cells in resin, face down on a sheet of transparent polycarbonate, although this technique has been used on sheets of glass. The advantage this method has for me is that it looks relatively straightforward to do as a DIY process. The special clear silicone resin needed, Sylgard 182 or 184, is expensive, but fairly easy to work with. The only challenges to overcome are finding a way to ensure that all the bubbles are drawn out from between the cell faces and the polycarbonate sheet. I'm fairly sure that a low vacuum bagging system should do the job, but I'm sure it'll need some experimentation to get it right.

I've deliberately bought more cells than I can fit on the deck area, so I have some spares to experiment with. If I can get 50 watts or so from the finished arrays on the decks then I'll be happy, as I think that will be enough to keep the battery pack topped up (the battery pack should run the boat for 8 to 10 hours on its own).

These cells are fragile, Rick. They are very thin sheets of silicon that are very brittle, so need careful handling. I'd definitely recommend getting some to experiment with, if the cost isn't prohibitive, just to gain the skills needed in handling them.

BTW, I like the analysis of solar vs diesel power you've done. I suppose the only thing I'd add is that solar powered boats (it's solar power than generates winds) have been around for centuries and many have circumnavigated the globe with no other form of power. The only difference with a solar electric boat is that we've removed the problems presented by a set of sails and replaced them with the problems of where to put a solar cell array. Just as sailing boats have evolved over hundreds of years to deal with the challenges of making sails work easily and efficiently, so a solar electric boat will have to evolve hull and deck forms that make this system work well. I sailed a 28ft yacht with no engine for years and never found it to be a big problem, I quite regularly went across the Channel to France (a trip of about 100 miles each way from where I then lived) and would only ever have wished for an engine when forced to berth in a marina, rather than just pick up a mooring or anchor in deep water. At least with a reserve battery pack this latter problem could be removed.

Jeremy

 

This is just me, but a set of sails is seems simpler and a less trouble prone way to harness solar. Motoring brings on many more issues than just where the solar cells go. Connectors, controllers, corrosion, mechanical servicing, prop dinging and tangling, etc. Plus, I wonder if outright speed of solar cell- battery can come close to matching sails for any distance, except in dead calm.

However, I am a tinkering freak and lover of gadgets so I would love to see what the state of the art produces with solar cells. I have an old Sovonics "flexible" solar panel that can be bent, shaped and even walked on without damage which I sometimes use with my micro eboat. It has survived dunkings, scrapes with rocks, and is handy for extended trips when away from the grid. Not efficient by today's standards but rugged, portable and not too heavy, so it suits the application. Here's a link that shows some of the possibilities http://www.siliconsolar.com/flexible-solar-panels.html

Godspeed in your project

Porta

 

Vic
I get the impression from this that you have not done a lot of ocean sailing.

Any cruising sailboat has sail issues, motor/prop issues and electrical issues plus there are inherent safety issues related to the high forces on sails, mast and keels (if a mono).

My constant annoyance was keeping cabling up to my nav lights. This was before LEDs. I ended up with all soldered connections encapsulated in silicone rubber. If a globe blew I needed a soldering iron to replace it.

The direct solar boat ends up a simpler proposition. Every technology needs development though.

I can already see where a good solar/wind/battery electric boat can set better averages over long distances than a sailing boat. Will certainly not match what a big tri can do in the tradewinds but up in the warmer parts where the weather is more hospitable most of the time the solar boat could be the faster choice.

Rick W

 

I tend to agree that sail systems are pretty close to be as good as they can get, so, at the moment, look a bit better than solar electric as a propulsion system. However, I think that we need to look at the state of maturity of the two technologies.

Sails have been perfected over centuries, and vast amounts of time and money have been invested in making sail systems relatively reliable, relatively easy to use and moderately efficient.

Solar electric propulsion is relatively new and is only now attracting some modest investment in development. Like all new technologies it will take time to become accepted, both from a technical and cost perspective and from an aesthetic one. This latter point is probably more important than we think, at least in terms of commercial success. For example, we're now used to cars being an aerodynamically efficient shape, but 25 years ago the first cars of this shape were almost universally reviled for their odd looks.

The reason I've opted for the compromise of a hull that looks fairly traditional is because I personally want something that isn't too radical, plus I'd like to show that it's possible to have a purely solar powered leisure boat for this limited range of uses.

Eventually I think we'll see commercial designs that are better optimised for using solar electric power, almost certainly multihulls, probably not dissimilar to Rick's faux trimaran design. It'll take time to convince people that such boat shapes are both attractive and practical, as I am convinced that most people don't like to stand out from the crowd by owning something that's radically different from a traditional boat.

One big advantage of solar electric over sail is that it scales well. Big sailing boats are harder to handle, mainly because of the heavier, bigger sails, mast(s), rigging etc. A bigger solar electric boat gains significantly more area for solar cells, with no associated handling penalty. The downside of this is that small solar electric boats, in the sort of size that enthusiastic amateurs like ourselves might want to build, are inherently power limited, due to the limited space for solar cells. I'd love to build an ocean-crossing solar electric boat, but the cost and complexity of building the boat makes this out of reach for me.

I think the key thing to get right is the balance between stored energy in the battery pack, the realistic generating capability of the solar array and power of the propulsion system. We're used to these compromises on auxiliary powered sailing boats, and often accept that performance under power may be limited. The expectation many buying a solar electric boat may have is that it's performance will be the same as a power boat, simply because it doesn't have sails and looks a bit like one. Once people accept that a solar electric boat has more in common with a sailing boat, and that they need to make similar compromises in terms of performance being weather dependent, then I can't see any good reason for them not becoming popular.

Jeremy

 

Hi Rick. Certainly not an expert at sailing of any kind. Just some speculation based on the idea that a solar boat would not lessen the already existing electrical/electronics on a sailboat. Electronics stuff can be quite tempermental as we have seen with the Toyota throttle recalls. So failsafe, deadman or redundant electronic systems should be the norm when working around saltwater is what I am thinking. On my projects, I make provisions for "limp" home mode that bypasses PWM should the need arise.

You mention LEDs which have turned out to be trading of issues in the USA. Just about every traffic light has some non-functioning in the array for my area. Either the Asians are cutting corners again, or weather/shock is decreasing performance. Since they save energy and don't generate the heat that bulbs do, and traffic lights in cold sections of our country often remain covered in snow with an invisible signal.

You know, I consider you in the league with Paul McCready, (one of my heros) but in the boat area. Some of the solar developments inspired by Paul turned out to be revolutionary, and may even substitute for satelites in niche applications. So don't think I am discounting your thoughts outright. That doesn't mean that I will always be a "yes" man, I don't think you would like that or learn as much. I appreciate patient explanations when you have time to give them.

It took me a long time for me to persuade anyone to try flex shafts if you remember, and some are still skeptical. You were the only one that did come around after a while and I am gratified to see it further developed and become useful in your own applications.

Regards,

Vic

 

Vic
I do not know if you ever saw this:
http://www.adventuresofgreg.com/HPB/2008_06_17_archive.html
about halfway down the page there is some comment on the strutless shaft.

Greg did not use a strut after that day which resulted in losing a prop at one point because it was mounted on a welded stainless shaft that broke. However the 0.1kph speed increase amounts to 2.4km over 24 hours. He better Carter Johnson's old record by 3km. (I might be repeating myself)

I have recently set up an unsupported shaft to power a 42ft cat with pedals. I learnt something new with this.

I started out with a 800 x 600 2-bladed prop spinning at 2X cadence on a 9mm shaft. The boat weighs 8t and I intended to make two pedal drives off the stern of each hull. However the initial testing was with one drive. The thrust required to get the boat moving was enough to cause the 9mm shaft to buckle and twitch violently rather than stabilising.

I ended up getting reasonably stable operation by reducing the spring steel length to 1500mm connected to a 1200mm rigid pipe section as well as fitting a 4-bladed prop. It twitched when accelerating the boat but stabilised once up to speed. It will work even better once there is a unit in each hull.

We did the testing on the Yarra River and a Police launch followed us for a little time chatting to us because they could not believe I could get a boat of that size up to 2kts by pedalling. With two fit pedallers they should get it to 3kts. (There is a yacht race here that permits human power and it becomes a huge advantage in low wind.)

Rick W

 

Rick, I did see Gregs setup with the flex shaft. Great accomplishment against an opportunist and olympic caliber athlete like Carter. Carter has competed in the Texas water safari here, but only under the unusual conditions of very high water which would be most likely to add to his collection of medals. Still, he is an extraordinary athlete, but I wonder how he would do in skinny water conditions with lots of portaging required.

Your experience with the large boat parallels what I have observed on my much smaller scale. The shaft does have length and springiness parameters which limit where it will stabilize as you discovered and stability is also difficult at low rpm. So a certain amount of "tuning" is necessary. Moving that size of boat as you did might qualify for a write up in Ripley's "Believe it or not" series.

Vic

 

As Denny pointed out, there's a wealth of theory here, and a growing wealth of practical. For efficient and low-hassle electric I'm studying the outboard mechanical configuration with different props and motors and drives. The coldtub test tank should provide controlled comparisons of parts, and what the cost is in watts. There's a centerline bulkhead so water flows around the perimeter, and I measure watts required to set the baffle plate on rollers to a given tension. Pusher or puller prop, that's the curiosity first. Volvo certainly likes pullers. Yes, that's Torqeedo's 8x8 kayak prop for starters.

 

I think your test set up will be valid if the force indicator is separated from local flow near the propeller.

The gross flow around the tank should have good correlation with the conditions on a boat hull. It is just the reverse to a boat with water being forced around the circuit losing energy to the fixed tub. However you will have more turbulent flow near the prop as there will be shear in the water here and it will be greater with the smaller diameter prop. So you need to keep the force measuring paddle away from this. The flow velocity ratio for inflow and outflow is an important function of efficiency. A small prop has higher ratio and therefore lower efficiency. You do not want your flow measurement to be seeing the local velocity near the prop. The velocity of circulation should also be a good indication of useful power being absorbed by the water. I would place the flow measuring on the opposite side of the central barrier to the prop.

There will be quite a lot of momentum in the water and it will take time to reach steady state so that needs to be catered for when making speed changes.

A nicely shaped blade will have cambered blades. The lifting face with the convex surface should face the oncoming stream. So when you do comparisons between pushing and pulling you need to reverse the prop if the blades are cambered, which they should be to get the best performance.

Normally a prop will work more smoothly if it is "pulling" as it works in undisturbed flow. In your case this may not be possible because the test rig will cause turbulent flow.

Look forward to what you measure.

Rick W

 

Good braincell stimulation there, Rick, thanks. I set the baffle close behind the prop conceiving that site as akin to headwind turbulence; i.e. the prop isn't biting cleanly, it churns in some of its own wash. I'll try the baffle also on the opposite side.

On push vs. pull, my stock apc and torqeedo props get their shafts inserted into the hub from the appropriate side, and the rightangle drive rotates 180. Indeed, like the MinKota 3X, I prefer to 180 the entire assembly for reverse rather than reversing the motor.

I recall there are exotic drives like kurt nozzles which drive props from the blade perimeter, so there's no turbulence from any central shaft or angle drive: superclean bite. Your shaft drives seem the next most efficient.

 

push vs. pull

Seems to me that with a flex shaft arrangement, there is no difference between a pushing or pulling prop. The shaft cannot be turned so it is behind the prop because it will dive instead of providing propulsion. I have tested identical APC props made for pushing and for pulling by measuring current draw and speed while on my small e boat. With all variables remaining as constant as possible, the pusher seemed to be slightly more efficient than the puller.

Porta

 

I like the test tank arrangement. If you can measure the inflow velocity in front of the prop then this would be an added bonus, as it would allow a direct comparison with the output of something like Javaprop.

I'm indebted to Rick for his prop making ideas, as I spent some time today making a couple of stainless blades. I've been meaning to do this for some time, but I had shoulder surgery a few weeks ago and prior to that had virtually no use of one arm. Anyway, I used today as physiotherapy (not sure that my physio would agree, mind) and did some stainless steel surgery.



This is a photo of one almost finished blade, plus a blank already twisted to shape. I silver soldered root stiffening pieces in, as I wanted to use a single pivot with both blades working in a common slot in the blade hub. This is a 330mm diameter, 356mm pitch prop, optimised for 1.8m/S when driving my boat at around 400rpm.



This is a photo of the finished blades, minus the hub. I'm really impressed with this blade making technique that you've invented, Rick. Not only is it quick and easy, but the joy of seeing the sensuous curves of the finished blade emerge from a grotty bit of 316 stainless sheet makes it all the more worthwhile.

Tomorrow's job is to turn up the hub. I'm going to use a bit of 1.5" 6082-T6 alloy bar, as I happen to have a bit to hand, but may take a look at making a stainless hub in due course (not something I'm looking forward to, I intensely dislike turning stainless on my small Chinese lathe).

I've been assured by the boatbuilder that the hull should be ready by the end of May, so, with luck, I should be set for a Summer of experiments on the water.

Jeremy

 

Jeremy
Blades look nice. You will confuse some placing them on a shaft as you have. You will need to post a photo of them in the hub so the finished product can be appreciated.

Some of the things I have learnt with my own design and shaping - I aim to control the induced drag through angle of attack toward the tip rather than the blade plan shape. It means less trimming effort at the ends.

My own program hunts out the most efficient blade plan shape. I do not know if your method or Javafoil does this. For blades with high P/D it results in longer chord near the outside. For low P/D the chord near the tip is smaller than inside as you have. These days I tend to use such narrow blades that I stick with constant chord. I optimise the AoA across the blade but with the tip usually tapering to zero lift while adjusting a single chord value for the entire blade.

I also allow for the change in Re# across the blade and evaluate for cavitation.

Rick W

 

You're right. Rick, I should have pointed out that the pin the blades are on in that photo is at right angles to the shaft axis. The pin is only there to hold the blades in position for the photo, plus I used the pivot holes as an alignment aid when making the blades.

I've just let Javaprop dictate the blade chord, which I assume scales it to optimise L/D at each radial station. It looks this way, as far as I can see, my guess is that it reduces chord with span in a pretty much direct relationship to the increase in induced and viscous drag at that local velocity.

Thinking about it, the only real penalty for using constant chord blades would be the slightly increased viscous drag due to the greater wetted area, as at the very low loadings these low speed blades are running at viscous drag must dominate, I think. Overall, and based on your experience, it probably doesn't matter too much at all, provided the local AoA is adjusted to bring the local lift back to the right value with the broader local chord.

I did the best I could in terms of choosing values of Re and section at each station, within the limited range that Javaprop allows. The root sections are quite thick (due to the added stiffening pieces) but this seems to have had negligible effect on overall efficiency.

I made a jig up to test blade twist as I was bending them, with angle templates for the 25%, 50%, 75% and tip stations. I bent both the strips clamped together, when they were still just rectangles. The only bit I found challenging was getting the tight twist near the root; from the 25% point to the tip was pretty easy to get right. Twisting the short stiffening pieces to match the blades was an interesting challenge, as the short pieces didn't want to twist in the same way as the longer blades. With hindsight I should have left a bit more scrap material on the stiffening parts, as then it would have made the job easier.

Jeremy

 

The thought of making your own prop is a unique concept that is foreign to most of us living this century. Did you do that in one day? Harry