# Lightest weight option to provide short bursts of power (250W x 4 seconds)

I'd like to mount mount a small pump to a surfboard to provide a small push to help catch waves. Normally to catch a wave you have to paddle furiously for about 4-5 seconds. So that's about ~ 1000 joules of total energy. I'd need about ~25-50 such pushes.

One of the most important things here is the total weight -- to prevent the board from becoming dangerous, the board is ~7lb and addons should be ~4lb max.

The engine will be probably 12-16amp. What would be the lightest-weight option for providing these bursts of energy? 16 AA-sized batteries would weigh about 1lb; Would a supercapacitor be lighter?

• Rule of Thumb: Anything that makes light weight power is explodey. – Henry Crun Aug 16 '18 at 3:10
• If you can recharge on shore every run or two, the amount of charge you could store in a supercapacitor bank is substantial. Could use flexible solar panels and or inductive charging if you know how to cut apart and reseal a surfboard. Make sure you think about where you put your prop if you use one. – K H Aug 16 '18 at 3:27
• youtube.com/watch?v=0V4qQvFzHlk – mkeith Aug 16 '18 at 3:33
• 3.4Ah 18650 lithium is ~44kJ. But they can deliver about 50W, and you want 200W, so 4 batteries meets your stated energy requirement. An external rotor model aircraft motor easily meets that power requirement, and the windings can be easily waterproofed – Henry Crun Aug 16 '18 at 4:06
• Supercapacitors are actually really not bad for short storage and significant boosts of energy, provided recharging between runs isn't a problem. They would go quite well with a surfboard sized solar array too =) more cycles and lower charging cost than lithium ion. Ultracap/Lithium hybrid batteries are quite nice too. Run a drywall screwgun for about 15 minutes and charge in 1 minute and they're lighter weight. That's probably the way I'd go to get the best of both worlds. 250W is a pretty significant motor, so for an electric option you may need to consider a larger weight. – K H Aug 16 '18 at 4:46

Compressed air. Weighs nothing. Can be put into fibreglass/carbon tubes inside the board. Could be recharged from a dive tank (with pressure reducer), or a small 12V compressor and solar panel.

You can probably make a single pulse jet where there is a tube inside the board with a jet nozzle at the back (and flap at the front) that fills with water.

When you gate the compressed air into it, it shoots the (single) water charge out the back bottle rocket style. A 2m x .1m sq tube holds 20kg of water, and can give a big impulse to an 80kg rider+board.

More likely you would use a small tube as a pulse jet and just put a series of bursts of air into it. Once each charge is ejected and the pressure drops, the flap at the front opens, and it refills with water ready for the next burst.

You are clearly able to work out the energy/mass/pressure requirements of such a system yourself

Another alternative is butane cartridges a la Pasload nail guns. But you do have to keep the igniter dry.

From Wikipedias 530kJ/5L@200bar I estimate about 3kJ/l of air at 100psi, in isothermal exansion. That might be possible if the air is forced through the water to regain its heat on expansion.

So 7 drink bottles for your energy needs. Plastic drink bottles are good for about 100psi, probably more wrapped with fibreglass tape (which stops the spherical bulging).

There are also carbon fibre 300bar tanks 0.5L/600g paintball tanks giving you over 50kJ and 3l/2kg scuba tanks for 350kJ

• .01 sq m cross section is required at 2m for 20L/Kg of water. – K H Aug 16 '18 at 3:17
• @KH yes 0.1m square = 0.01 sqm. But I am thinking more of 1L pulses really – Henry Crun Aug 16 '18 at 3:22
• Makes sense. Was just mentioning so you could edit "A 2m x.1m sq tube holds 20kg" to either "A 2m x.01m sq tube holds 20kg" or "A 2m x.1m sq tube holds 200kg" – K H Aug 16 '18 at 3:30
• I like the idea of this because you could have a lot of kick compared to an electric motor. – K H Aug 16 '18 at 3:32
• Lets not forget hi test hydrogen peroxide, which is monopropellant, no ignitor, and is perfect for sterilising shark bites if you surf in WA – Henry Crun Aug 16 '18 at 3:37

I propose the robotic mega surfboard solution. I really hope someone builds this, but it won't be me. I started looking into it and was amazed at how doable an electric version of your product will be, but for the full effect it won't be cheap.

Surfboards vary in size and this one will have a bit of extra weight and it may have limitations in use for pro surfers, but if you've got a lot of money or a sponsor or something and you know how to build surfboards, I think a minimum 6'8", 19.5" board is in order.

It will have a top side surface area of roughly

83% * 2.032m * 0.495m = 0.8474m$^2$

which will give you space for roughly 22 1.54W flexible 15.4V military grade solar panels. That will bring in 33.88W in bright sunlight at a cost of about $1000 (less in american), but only adding 312g plus wiring. Even when you're surfing the parts of your board with no shadow will generate power. I believe measures should be taken if possible to spread the heat from the cells, perhaps share it with the bottom side with an internal water loop because many of the mini jet motors available appear to have engine cooling anyway. A flexible aluminum mesh behind the panels and following the interior curve of the board and several thin aluminum plates on the bottom surface might be in order. You could get away without it, but a bit of reinforcement and also heatspreading for the other components could be in order anyway. Blackish things get pretty hot in sunlight and the better it sheds heat the more power you'll get from your panels. Motors... Water thrusters won't clobber people as bad as a propeller, so I think they're the way to go too, and looking at the options available, perhaps a pair of 40mm pumps placed on each side and angled slightly outward are in order, intakes facing down so they're not blasting down the middle where you are. If they don't come with the thruster pump heads, perhaps 2 250W RC motors, allow 1lb per motor and pump assembly with motor driver and add 1 lb total extra for heatsinking. If weights start getting too high it may be necessary to add voids and/or use materials like carbon fibre tube, sheet and cloth to reinforce the board and increase buoyancy. You'll want some power storage, maybe up front to help balance the weight in back? A decent size Lithium ion battery bank will cost probably$200 or even \$400, and the larger the battery bank the less the batteries will heat up at a given power draw. Perhaps augment it with a substantial sized capacitor bank to take in power from the solar and provide burst power for the motors. If the capacitor bank is below the voltage the motors can sustain continuously, have the batteries charge it, and if the voltage is close to the maximum the panels can put out, instead have the batteries take a charge from the capacitor bank. This way you'll always have a mimimum amount of thrust that won't cook your board until the battery bank runs out, and while you're in the sun the board will constantly be preparing for the next boost of thrust and charging the battery bank with remaining energy.

Wireless charging would work fine too so feel free to add that.

All told you can probably squeeze this in under 4 pounds with existing factory made parts and add just the right amount of buoyancy while you're at it. Perhaps instead of a wood spine a hollow aluminum heatpipe would work.

• Water thrusters (jets) are not as efficient as propellers at low speed. I am not sure if you are aware of it, but there is a product category now called the e-foil. This is a hydrofoil surfboard equipped with a small BLDC motor and propeller. No solar panels, though. – mkeith Aug 18 '18 at 17:28
• That is likely due to smaller prop size and constriction of the water flow path. While I do see the merits of a prop, efficiency being one, these water jets do have their merits too, especially safety. I'm seriously considering putting a few little ones of those on a foam waterboard and seeing if they're strong enough to pull my nephews around. I think if you went with a prop it would be wise to use a ring guard around it. I bet getting hit by the fins of a surfboard is pretty painful. 2 extra pounds all in one place and a spinning blade worry me. – K H Aug 18 '18 at 19:44
• I think another advantage to the jet design is many provide a water cooler for the motor and they're designed to run at the same speeds as lightweight RC motors, which are often silly high RPM it seems but are designed to shed heat. I think the efficiency might be an order of magnitude worse, but I still kind of want to put the Ol' 746 watts into a pair and see what comes out. – K H Aug 18 '18 at 19:50
• Foiling boards are potentially dangerous for sure. The motors only make them more dangerous. – mkeith Aug 18 '18 at 20:17
• To be fair, it is likely possible to adequately protect a larger propeller, perhaps something akin to (the waterlily charger) [treehugger.com/corporate-responsibility/… but one problem I see is that drag would be inevitable when the prop wasn't running. I personally like the idea of a sealed unit, with little change to the profile of the board, but obviously there are concerns when running fully sealed Lithium battery arrangements, but if they can make a waterproof phone a waterproof board should be OK. – K H Aug 18 '18 at 22:01