Electric tethered hydroplane

Using electric power in a tethered hydroplane is not a new idea, but the advances in motor and battery technology recently, allied with developments in electronics and control systems, has made it a very realistic alternative to IC power. Martin Broad has utilised this technology, along with considerable skill and ingenuity to produce a very effective outfit, and we are grateful to him for sharing the details. (click here to skip to November update)

Martin Broad

Hull.
Dimensions roughly the same as for a novice hydro, 32" long, 2¼" wide approx 9½" across sponsons. Construction is a double skin 1mm ply with 3/16" gap between skins for bottoms and sides, this makes a very stiff structure and requires no formers inside. This allows more space and easy access for batteries etc, as well as making the hull narrower.

Drive
Direct drive, from motor with 3mm Stainless steel shaft. The skeg features two ball races and is angled at about 3½ degrees. The props mount on a 5mm shaft which the 3mm shaft is Loctited into.

Initial runs achieved 53.3mph over 5 laps, peak of 57mph at Victoria Park on a  48mm K series Graupner hydro prop. Hydro ran very cleanly and batteries were only just warm, got to find a bigger prop!

Following the "Workbench" article, Lynn has kindly asked me to supply further information on the electric hydro and follow its development on the website with updates, as it seems to have gained some interest!

Shortly after the pictures that were taken for the "workbench" article a significantly larger prop was tried. This was a proper hydro prop borrowed off a 'Novice' boat so it was in the region of 57mm diameter with around 7" pitch. The hydro is not fitted with a hook for bungee launching so a good "lob" was in order to get it away. Unfortunately the skilled under arm "lob" was rapidly followed by a nose dive, the hydro then re-surfaced with the motor still running and the prop proceeding to chew its way through the bridle. I managed to reach the hydro and switch it off but unfortunately the damage was done and that ended any further runs that day.

The post mortem discovered some damage to the nose, but the remains of the bridle had wrapped itself around the propshaft pulling it out of the skeg. I assumed that the coupling had slipped on the motor only to discover that the motor shaft had been pulled through the casing!

The next opportunity to run the hydro was the Victoria Park winter regatta on the 19^th October.

When I arrived at the pond conditions seemed far from ideal with a gusty wind causing a bit of a chop, I was a little concerned that further nose diving was a distinct possibility as it was far from the mill pond conditions I had first run the hydro on!

I fitted the smaller of the two JG props, to gain some confidence in the hydro, as I was unsure what power the new props would consume and I didn’t want to cook the system. The hydro was duly hooked up and the bungee attached, following a good launch it got away well, so it seemed…….. Before long, the rear end of the hydro started hopping, the oscillations increasing as the motor ramp up to full speed started. With a large splash the hydro nose dived and reappeared on its back with the motor running full tilt. To avoid another scenario as before and further damage, I waded as quickly as possible over to the boat, which as luck would have it, started to make its way back to me after rolling right way up. It quickly dawned on me that I was inside the circle and I was about to get wire wrapped around my legs !

Fortunately the hydro was not moving particularly quickly and I managed to stop it after its first lap around me! I switched it off and returned to the pits a little confused at what happened. The "experts" soon gathered inspecting the propeller, the conclusion was that the propeller had too much lift on it causing the oscillations and eventual nose dive. I was duly handed another propeller with the command "try this" ! I fitted the new propeller, which was basically an unworked 'Novice' prop as before, I also fitted a new set of batteries with a slightly higher discharge rate 30~40c (I will mention this later).

Back to the wire, I gave the bungee a couple of extra paces tension to increase the launch speed and switched on, I launched the hydro which, quick as a flash, nose dived again! As it was closer to the bank it was rapidly recovered and switched off, with no damage. Jim Free very kindly offered to try and launch the hydro again as it appeared that I caused the nose dive with a dodgy launch.

From these initial trials I can say that an effective electric hydro outfit is achievable. The current power system is certainly very capable of propelling a hydro up to 100mph, and with further development would probably exceed this. However, the current hull is not capable of this and will be redesigned over the winter. The following improvements will be incorporated into the new hull design :-

Sponsons: Reposition further forward with more up-sweep to reduce the chances of nose diving, only have one step on the sponsons to avoid "suck down" caused by negative pressure under the first step.

Hull: More upsweep under the nose, avoid airfoil type shapes to reduce hull lift, symmetrical section for the horizontal stabiliser with larger area. Reposition c of g to 10% of sponson/blade tip length. Use cylindrical rods to attach the sponsons to the hull.

Control System: Incorporate external kill switch and automatic power shut off if the hydro flips on to its back. I will also be adding a data logging system to the power system to log voltage, current and motor rpm. The data can be down loaded on to a p.c. and should aid propeller and power system development.

I have also decided to give it a name as I’m fed up calling it hydro or ‘it’. I’ve decided on the name ‘Fusion’ as it represents a ‘fusion’ of technologies as well as an electrical term!

If you were a little baffled by the earlier, brief description of the power system, I will try to explain how the control system works and how I derived the power system I have used. I won't delve into how brushless motors work and the ins and outs of lithium polymer batteries as there are plenty of sites on the internet which cater for this, just do a search on Google! Hopefully I will give you enough information so you can go and build your own and perhaps we can get an electric class running in the UK.

If you consider the output from a standard 6.5cc nitro engine is about 1.1-1.3 horse power and convert this into watts you’ll get 820-969 watts. So this gives you a basic power rating system for your electric hydro, how you achieve it is another thing!

If you visit your local hobby /internet shop you can buy brushless motors which are rated as power equivalents for I.C. engines. These are normally rated in Kv, so a normal .40 equivalent will be around 600-800 Kv, this means 600-800 RPM per volt off load. You also need to consider what voltage you intend to run the motor on, as this will also effect what Kv motor you choose. Going back to our initial power requirements of 969 watts we can achieve this by using different cell quantities and currents ratings, known as C. Lithium batteries are normally sold in quantities of 2 up to 6 cells in one pack, each cell has the nominal voltage of 3.7 volts so if we were to use one pack of 4 (14.8 volts) cells we would be looking at 969watts /14.8volts = 65amps. This means that we would need to pull 65 amps from one pack, this is possible if we use a pack with a significantly high enough capacity and C rating. Most average reasonably priced pack are rated at 20C constant discharge, so, 65/20 = 3.25 or 3250mah. The current pull will also dictate what size speed controller you’ll need in this case a 70 amp will do. When you are considering the capacity of your pack you also need to think of the run time, but as our hydro will be only running for a minute or so, this pack would easily cope with time required or, you can work it out, 3.25amps/65 = 0.05 x 60 (for minutes) = 3 minutes run time.

When I designed "Fusion" I intended to run it on an ‘A1’ class line to avoid lots of line drag and consuming too much power. This meant that the all up weight had to be no more than 2 ½ lbs. The restricted weight also reduced my choice of power plants as weight would now be critical. Using one large high capacity pack (3250mah) would not be possible as the weight of some 300 gram’s (10 ounces) would be a significant part of the hydro.

The other problem I foresaw was that the RPM from a typical 40 equivalent brushless motor wasn’t really up to the performance I wanted. Even on a 6 cell pack the RPM from a 800 Kv motor would only be 17,760 RPM (3.7 volts per cell x 6 cells x 800 Kv). And at the time I wasn’t expecting to be turning large diameter propellers so I needed more RPM. After looking around several websites my attention was drawn to some new "out runner" type motors, these are designed for high RPM, high torque conditions, typically for powering 3D helicopters, the motors have to be powerful to provide the torque for sudden direction changes and huge pitch curves on the blades.

The motors start at 1210 Kv 1300 watt rating, up to 1900Kv. The 1210 Kv motor looked promising, but I still had to find some lighter batteries.

I decided to look at the power requirements again using the simple formula watts = volts x amps. If I increased the voltage and used a 6 cell pack I could significantly reduce my current and therefore my battery capacity would reduce also, so I could use a smaller, lighter pack. So using a 6 cell pack , 3.7v x 6cells = 22.2volts x 1210Kv = 26,800 RPM great, and also 1300 watts / 22.2 volts = 58 amps even better! The motor is rated for 22 volts, 70 amps and is worth checking before you buy, but generally speaking it’s the current that kills the motor not the voltage. I decided to use two 3 cell packs in series (this is an advantage with lithium packs you can build them up in series/parallel with no problem). These are rated at 2200mah at 30~40c which easily copes with the required 58amps.

The other advantage with using a higher voltage is that, as there is less current draw on the batteries, motor, and speed controller will not get so hot, which is ideal as there is no water cooling. The other advantage is that the LiPos are likely to maintain their peak voltage longer as there is less current drain. Using 2 packs also eases installation when trying to achieve the correct c of g and also avoids one solid mass, + if a cell blows in a pack you only have to replace 1 of the 2 packs which makes running costs cheaper.

The control system presented a real problem as I needed to find a way to drive the speed controller without the use of R/C , although this does seem to be the safer way, it then seems pointless putting it on a wire!

You can buy servo testers which basically mimic the signal from an r/c receiver to drive a servo, they consist of a dial potentiometer and some IC’s. This seemed like an easy solution but it meant that you would have to wind the motor up to full whack before launching, plus, you had no way of stopping the hydro until the speed controller cut the motor out at the low voltage point (which is built in to stop you killing your batteries). This meant your batteries were flat and you wouldn’t be able to get a second run from them.

I was looking through eBay for a cheap servo tester when I found one from a British company, I went to their website to see what the original cost was before I put a bid on it (we all do this don’t we?), but browsing around the site I also found a programmable board which mimicked the signal from a receiver to drive a servo. The board contains a programmable chip which can record up to 4 minutes of servo movement, this seemed ideal and I ordered it. The board is easily programmed and so far has been totally reliable, it's driven from the speed controller voltage which would otherwise drive the r/c system installed in the plane/boat.

I hope these (long) notes have been useful and will spur you on to build an electric hydro, my email address is :- martin.broad@elekta.com if you wish to contact me. I will supply further updates when I start building, Fusion 2!