Foils for amphibious waterkart

I have an amphibious warterkart build in progress in the electric propulsion category. This question is about a foiling that could be added on to significantly increase craft performance as an idea but well out of my experience

I have 1kw of electric plus human power available on the drive system. I have been thinking about allowing space for deployable foils under each wheel mount. The treads on the wheels are the water blades. Might it be possible to achieve a balance whereby the kart is lifted out of the water with only the wheel blades making contact?

 

I don't know whether treads on wheels are an efficient enough propulsion system that you can fly with maybe 1400 W, or if you can, whether the foils would actually have less drag. However, paddle wheels typically have quite a large stern wave behind them from shoveling water not only back but also up. That seems like a good opportunity for using foils to recover some of the energy that goes into creating that stern wave. There is a foil called the hull vane which recovers energy from the rather weaker stern waves of ships:

Put such a foil behind each wheel, adjust depth and angle until you find the combination that works best. I would start behind each wheel, a little higher than the deepest point of the wheel, and pitched down a little.

If you make the foils behind the steering wheels move with the wheels, you should also improve steering.

 

Let me guess. A rear paddle wheel that lifts water in the back would pull the stern into the water, so the bow would lift a bit, in that sense resembling a speed boat. But would the drag from the deeper stern be greater than the drag avoided by raising the bow?

Could you instead design wheels that would turn 90 degrees, and turn into back facing propellers rather than paddle wheels?

Perhaps with an adjustable pitch, so they wouldn't waste much energy moving air while on land?

It would have to be built rugged, for the transitions between mud and water. And the outer wheel would create drag in the water.


www.youtube.com/watch?v=XHloXe1wCv0 shows a Goodyear-proposed flying car tire, in which the tires become a tilt rotors that lifts the car into the air (I doubt you could do that with 1400 W, if it is lifting your weight too). (Also, in the wind, I think Goodyear's concept car would crash into other cars while taking off. ) But could you just tilt the front wheels of your cart, to lift the front of the boat, and use an ordinary propeller in the back?

I admit it intuitively seems unlikely that a wheel that also functions as a propeller could be particularly efficient at both. The outer part of the wheel would create drag.


There have been very light human powered hydrofoil boats. Maybe 1400 W is enough power? I.E., don't use the wheels in the water (or fold them into the structure for streamlining), just a propeller.

But if by "cart" you mean something that can carry much weight, that would be hard.


Why are you limited to 1400 W?

 

That is a brilliant idea. This vehicle should be light enough to use bicycle wheels. Some racing bikes have carbon wheels with three spokes. So it should be possible to build a carbon wheel with three or four spokes that form propeller blades. Insert that wheel into a tricycle with one front wheel and two rear. Drive the front wheel. When in water, turn the front fork by 90 degrees. Build the rim so that it is as streamlined when going sideways as is compatible with moving on land. In water, support the vehicle by catamaran hulls, placed in line with the rear wheel. Angle the hulls so that the front wheel is fully submerged when the vehicle is in the water.

That is all under the assumption that the rules require that you use one or more of the wheels for propulsion in water. If that is not so, you are better off building a separate propulsion method for water.

Do look up the IHPVA (International Human Powered Vehicle Association), if you haven't done so yet.

 

Keep in mind, there’s a big difference between 1.4kw of input power and what kind of power you are actually putting out at the prop.

ICE outboards are rated by horsepower at the prop.

Electric motors are compared to ICE motors in terms of propulsive thrust.

This means a smaller electric motor may be capable of pushing a vessel equally as well in part thanks to their torque, however most electric motors do not reach the same max RPM as their ICE equivalent, so you will not achieve the same top speed and will likely lack the ability to plane.

Most electric brushless motors at full speed are probably around 50% efficient.

Overall efficiency (which could be closer to 80%) is not as applicable if you are trying to get on plane or going for all out speed.

That means an electric motor that pulls 1.4kW might only put out 700W at the prop.

So keep that in mind when you are collecting data.

If you see reference material where someone is able to get on plane or foil with X amount of electric power, be sure to determine if the Watt spec is the input or output power.

In short, I think you’ll find you need a considerable amount of power to create enough speed to get on plane with foils.

Once on plane, you would be able to throttle back and take advantage of better efficiency of the motor running with less power to maintain foiling.

 

Power ratings on electric marine motors seem to be rated inconsistently. In the U.S., 1 hp is legally defined as 746 watts. Last I checked, which I admit was many years ago, when you look at the amperage and voltage draws, and hp output, that such motors are rated for, even major manufacturers sometimes claim as much as an order of magnitude more hp than the rated amps times the rated volts. While 1000% motor efficiency would be wonderful, it probably isn't possible, according to the 2nd law of thermodynamics, which most physicists and engineers would agree works quite well at macroscopic scales. Apparently, the U.S. government doesn't work very hard to regulate such things, which I find annoying. (Or has that changed?)

If they are rated by thrust while the motor is held still, that isn't valid, in terms of power imparted to a boat - effective power would be thrust times speed, and if speed is zero, then no power is imparted to the boat.

Also, many ICE motors, in both cars and boats, are rated by power output at some optimal rotation speed, and under a specific load, which may not apply under real world conditions. I've wondered if they are rated just as inconsistently.

 

I'm glad you think it brilliant - but

I assumed he was building the craft for fun, not competition. It is unlikely that a boat with propeller wheels would be as efficient as a conventionally propelled boat or hydrofoil of the same power. Why would you assume it is for a competition?

Also, I have no idea whether such a boat would plane (lift) much, or whether designing a planing cart at that power level would be more or less efficient than a displacement hull, or a true hydrofoil. E.g., the majority of small whitewater playboats and open water surf boats are designed to plane (lift) slightly, because it improves their surfing abilities, and makes them easier to control - but normally can't go as fast as paddled displacement hulls, let alone hydrofoils like Flyak.

 

Robert, let's see if I understand you correctly. The foil would be mounted behind the wheel and the leading edge a bit above the waterline at the wheel? Basically forming more of a trim tab type rake over the water and the water being pushed back and up by the wheel would encounter an angled flat face? Can this be added to the back edge of the fender over the wheel?

I can design that, but that system would not be a direct transition from sand to water and add to the parts count
The motor is 1kw E-Scooter axial gearless and brushless. I think Robert may have overestimated my human power contribution abilities. I think it would be safer to say around 80-100w of human power at cruise, so the number is more like a realistic total of 1kw. The controller is rated 3kw but 1kw seems like a good starting point balancing battery size and life and parts price

Its for fun so no rules. The wheels turning is the main attractions, and it will spend time running in very shallow water

Understood. The controller that I like is rated 3kw so still some options to upgrade the motor

For fun yes. I am ok to be immersed hips deep at rest on water or slow cruise while semi recumbent, and it would be cool to lift out underway

 

Hey guys, maybe a bit more info is needed. I had a build thread with photos on SUP conversion project in the electric section. The project has grown out of the SUP into something different but still retains the optional use of the SUP. The flotation is now just enough to float everything at hip deep on beach day and fit the SUP on river cruise and fishing

The structure is a 1.8 m longitudinal stringer based on 2 mm wall 100 mm x 75 mm aluminium rectangular tube. This mounts the seat towards one end, a crank towards the other, and a pair of 1.2 m long, 32 mm aluminium tube with 3 mm walls, live axles at both ends. The wheels are fixed to the ends of the live axles. The back axle turns like a dog trailer type or swivels. Steering is pulley based but I am considering electric too

The wheels are bike ones with a 26" pair, 20" pair, 16" pair and some 16" x 4.0 x 8 and 16" x 6.5 x 8 trolley types with a nylon hub that I have on hand to try out. The fins on the wheel are not done yet but will be made from TPU in an exoskeleton shell over the tyre

I am at the cutting the aluminium stage at the moment. Bearings are on order. The electronics for the front axle were 2x 250w BLDC hubs, battery and controller and ordered but not yet shipped and may get automatically cancelled by Ali if not shipped on time. The main back axle 1kw motor and electronics are yet to be ordered

So a very basic amphibious quad cycle kart wearing just enough flotation for the beach and a 3 m SUP for the rivers

 

If you want to get a good idea of how much power is required to get a single passenger vessel on plane with a foil, take a look at E foil boards.

They use very powerful high voltage high RPM motors typically running through a gear reduction in order to achieve planing.

If your motor controller has a max input of 3kW, that’s still only about half that in propulsive power, and most motors are not efficient at high RPM unless specifically designed, and then a gearbox is needed to multiply torque to allow it the ability to turn an aggressive prop.

For example, a popular foil board company’s board has the following specs:

5oooW input
3790W output
50v
100A
4500 RPM at the prop (but some of the foil board motors are hitting 20K RPM which is why they sound like George Jetson flying by).

So unless you are talking about a foil assist and not full foiling, you are going to need a lot more power and/or some specialized components.

Not trying to be discouraging, but it’s more challenging and complicated than you may think, and people are often met with disappointment because their expectations weren’t met due to misunderstanding the power and component requirements.

Early DIY E-foil builders learned the hard way burning up ESC’s, motors, etc.. by trying to turn too big a prop, overloading components, or just plain missing the mark by not achieving the speed required for lift.

 

"Plane" doesn't mean the same thing to everyone in the boating community.To some it means you ride marginally higher in the water. Strong whitewater paddlers sometimes plane just below the surface for very short spurts while moving up through rapids. To many speed boaters, it means only bottom part of an outboard motor is in the water. To others it means that only foils are in the water. Those require progressively more power.

I don't know if 400 W is enough to provide perceptible lift with you on board. Maybe not. Individual paddlers on even very light boards and boats only have enough power to lift much for a short spurt, by working very hard.

Many, maybe most SUPs slightly plane (ride slightly higher), as long as waves don't crash down on the front, because they have a relatively flat cross section that displaces water downwards when moving. That might be enough that they can turn most efficiently like a whitewater playboat or surf boat, leaning into turns (like a bicycle), rather than (most efficiently) out of turns, like a sea kayak. That won't make your board fast, but it makes it easier to balance and control.

Many SUP can plane at or slightly below the surface while front surfing a wave (e.g., in a whitewater rapid), like a small whitewater playboat, or a short surf board, because gravity keeps it going, provided of course that the paddler has very good balance. My balance isn't nearly that good, but I've watched others do it. Amazing!

If you add a sail, it might have enough power in suitable winds to plane approximately on the surface, maybe even jump the waves, like a wind surfer.

Why do you need wheels on the board? Many people use carts, dollies (plus an elastic strap) or trailer to help carry their boats to the water. A cheap dolly is just a piece of wood, with attached wheels, and holes to thread the strap through - maybe you could adapt a freely available scrap pallet. Some lightweight inflatable boards can be carried deflated as backpacks. If theft is an issue, carry the dolly back to your car before going in the water, or lock it to a tree.

Any extra weight from wheels and motors on the board itself is going to make the board harder to move and maneuver in the water, less easy to plane, and mechanisms provide more things to break, or be damaged by water. If you really need a motor and propeller, you can still skip the wheels.

Sometimes simplicity is better.

Just my input. And I'm not an expert.

 

Because of the mention of an electric propulsion category, and because of the plan to use tyre tread for propulsion, which sounded like a restriction imposed by competition rules. But I was wrong.

I would initially put the foil as low as is compatible with driving on land, so below the waterline, then experiment with depth and angle of attack.

The aim is to redirect as much water as possible aft, instead of letting it go up and forward. If you can arrange a really tight clearance between tyre and trim tab, then using only the underside to redirect the water stream should be fine. But you need something to get a grip on the water, else you have no propulsion. That means a rough tread or paddle blades, and you can't really have a tight clearance to a surface that rough. I expect that after the lower side of the foil has redirected water that is beyond the wheel and being dragged along, the upper side of the foil can suck put water from the tread. Therefore I would try something directly analogous to the hull vane I linked to, not a trim tab. It would just be a vane for each paddle wheel.

That is because I mistakenly assumed a competition, and vaguely remember that a strong athlete may achieve 400 W for a few minutes. For someone commuting on a bicycle I remember around 75 W. You may be a bit stronger, or more motivated.

So there will be paddle blades attached to the wheels? I think that will work a lot better than relying on tread. Propulsion gets more efficient if you get the same momentum by smaller changes in the velocity of more mass. So you would want to shovel the largest mass of water you can grab, until you run into other constraints.

If you want to get out of the water without getting off and pushing, you will need front wheel drive. That is easiest to do if you steer the rear wheels, though then getting good road handling becomes very difficult. I found there was a thread on amphibious canoes that may give you some inspiration: Amphibious Paddle Wheeled Canoes. https://www.boatdesign.net/threads/amphibious-paddle-wheeled-canoes.59038/

If you would like to steer as well as drive the front wheels, cars have solutions, but I don't know how well they generalise to home building at a smaller scale. You could look up the Berkut recumbent trike. If you can't find good pictures of the relevant parts online, I can ask my brother whether he still has the one I gave him years ago.

 

All points noted. It would be nice to be able to out speed a mountain bike that is going along the sand

Yes the rear tyres will have blades of some sort. I found an aluminium 20" rim that will make attaching 5 to 10 blades very easily and the wheels will look right at home on this with their wave pattern spokes

 

Yesterday I went to get some stainless tubes for my axles. I asked for sizes available and picked 22 mm. I asked if it was exactly 22 mm and assured it was but the person serving me seemed more interested on his screen than what I was asking. Well, the 22 mm bearings do not fit and on digital calipers the tube measures 22.2 mm

I should have bought it from Ali, at least the Chinese will go a step over our local counter jockeys and post a million measurement pics in the ads for such items. I thought to buy locally to speed things along, but that turned out to be a joke

 

I have to say, all of you guys have made some very good points regarding putting the power down. Please accept that this thing is wearing 4 wheels for the roadster ambience thing. Surely four of those wave spoke wheels can carry enough blades to get a good bite in the water to get on the plane with some foil assistance? Is the foilboards high RPM system a compensation for the area and volume available in the tiny board vs the weight needed to carry? Imagine it with a lower rpm system. It would just sink with the weight of the size of prop needed!!! Just kidding, and maybe I am not thinking straight with my argument

Regardless, the space and strength is there in the chassis for a lot of kw if the need arises. I considered using ATV parts in the drive train, but that takes away from the fun nature of this project, and I am worried about the durability in the marine environment. For this reason, I am leaning towards less durable but easier hosed downed and replaced items like stainless sealed bearings and shafts. Maintenance should not be more than the hose and a can of corrosion-x

Foils are now a main feature of this craft. I am considering front and back aeroplane type wings with a small up sweep a few inches underwater to start off with. I would have these on servos or push-pull type control cables. The aim is to lift more as the speed increases and keep balance in fast turns

Hopefully tomorrow the owner of the steel shop will do the right thing and swap these tubes for the correct ones. A new SUP board will be delivered in a couple of days, hopefully. This one is more compact and allows more wheel and foil clearance. My current one is too tall. Let's keep the discussion going on the foils. I hope you guys have got the idea that it's going to be like a go-kart in appearance. I was initially thinking about deployable foils under each wheel but now leaning towards T type wings under the board