The Rise & Fall of A Young RTR

[David MeierParticipant @davidmeier28154]

The Rise.

 

When I was approaching early retirement I thought that I might take up Model boats as a hobby. To test the water I bought a cheap vacuum formed twin motored RTR speed boat through the Internet.

While it performed reasonably well I was disappointed with it primarily because there was only full speed or stop and it was steered by the means of one or other of the motors switching off.

As you know, it takes an increase in energy to change the direction of a body that is travelling at a steady rate and here was the boat dropping to half power to make a turn.

 

Never-the-less I was bitten by the bug and I bought two books, one called Radio control Model Boats by Phillip Connolly and Vic Smeed and the other was The Complete Marine Radio Control Manual by Hugh Bright. I also consumed every back issue of Model Boats magazine that I could lay my hands on in the local library.

 

I decided to exercise my new found knowledge and fit the boat with rudder steering and an electronic speed controller. Fortunately for me a colleague at work took an interest and persuaded me to re-power the boat with brushless motors.

We measured the current under load and the free running speed of the existing 360 type motors. My colleague then selected a pair of brushless motors that would give twice the power and would rev higher using a three cell 11.1 volt Lithium Polymer battery. The great thing is I would get a slightly longer run time as well.

 

I gutted the boat and using basic tools such as battery drill, hacksaw and file, I made a wooden mount for the two new AeroDrive B2040-2080 brushless motors and fitted the two HexTronik HXT36 brushless ESCs inside a plastic container to protect them from moisture.

[David MeierParticipant @davidmeier28154]

[David MeierParticipant @davidmeier28154]

The shaft to motor couplings were made from 1/8 stainless steel tube. One end was drilled 2.8mm dia to fit the prop shaft and the other end was drilled 2.3mm dia to fit the motor shaft. These were first used to align the motors in their housings, then when the motor bedding glue had set, the motors were withdrawn and flats filed on the motors and prop shafts. The tubes were slid over their respective shaft ends and deformed onto the shaft flats using a toolmakers clamp (or smooth face pliers). This process changed the tube to shaft fit from a snug fit to a small clearance fit therefore providing both a drive connection and a small amount of universal joint effect.

The tubes are working very well as a shaft coupler and their small diameter and mass mean that any out of balance will be minimal.

 

[David MeierParticipant @davidmeier28154]

I fashioned a rudder and mounting assembly from some odds and ends of sheet metal and brass stock that was lying around.

[David MeierParticipant @davidmeier28154]

The boat performed very pleasingly when given its maiden run on Henley Lake. It is hard to say how much faster it went than the standard boat but it was certainly noticeable and having the full power when turning made the boat much zippier.

 

[David MeierParticipant @davidmeier28154]

The Fall.

The following week, it was back to Henley Lake with the boat for another run. The lake was about 300mm lower than the previous week and the weed correspondingly closer to the surface. Things were going well, the boat was fair hiking along and people were commenting on how fast it went, when it did a violent leap sideways and then seemed to go a bit slower. It was also very unresponsive when turning to starboard. Now someone less of a novice or with more active brain cells might have concluded Gee I must have hit a bit of weed but I managed to convince my self that it was the off-shore breeze that was hindering the turning. With some persistent effort and steering an erratic course, I finally got the boat back to shore. Surprise, surprise there was weed wrapped around one of the propellers; there was also a burning smell about the boat and upon taking off the cover I discovered a haze of smoke inside. The motor driving the fouled prop had revolved in its mount and twisted the wires tightly together, the wires forced through the heat softened insulation causing a short circuit; end of motor and speed controller.

I have learnt a few things from this mishap. The obvious one of dont be so thick next time, also I will positively key my motors in future so they cant rotate and will buy the more expensive speed controllers that have current limiting built in.

 

Some mothers do ave em.

 

David. 

[ashley needhamParticipant @ashleyneedham69188]

David. No indeed. Valuable lessons learnt there. A fuse is needed to protect the motors. I always use a proprietary motor/shaft coupling.I have seen too many bods boats stranded due to all sorts of odd couplings failing (been there, done it..). The motor securing you have used, the bar-over-the-top and post/nuts, is one I use myself, However a post either side to get enough clamping force may be needed , I use some rubber from an inner tube under the bar to provide some grip and a measure of resilience. If you were to water cool then a “proper” screwed in type bracket would be needed. i have seen some natty heatsink type clamps for motors, not so sure these would be suitable for a boat though. Ashley

[David MeierParticipant @davidmeier28154]

Ashley, you no a like a  my shaft couplings eh. They were the positive bit of the exercise, they worked a treat! in fact I bought a couple of proprietary couplings and they were so large and clunky looking compared to the size of the shafts that I couldn’t bring myself to use them. Inner tube rubber under the motor and clamp is a good idea though and I agree it would have been better to have a separate clamping bar for each motor with a post screw each side. The way I had it the motors could have been clamped with different forces. One screw in the middle of my single bar might have worked as the bar would have deflected and acted like a leaf spring and still clamped if one motor settled a bit.

Lets face it the main problem was the nut loose on the TX.

 

David.

[ashley needhamParticipant @ashleyneedham69188]

David, always the weakest bit of any design, the nut behind the wheel. I have used a centre pair of screws before (in-line, rather than side-side) and this works well, as the clamping force has to be evenly spread betwixt the two motors, however I have made the clamping bar the same width as the length of the motor, and I have been using a bit of Dural plate, 3/16 of an inch thick,  that my brother…sourced…. from Hawkers Aerospace before the factory closed. I dont think you want to overdo the rubber, just a bit under the clamping bar should be enough, as I have found that using too much lets the motor wobble. Ashley

[PierreParticipant @pierre]

David

A very entertaining and informative post – I thoroughly enjoyed it and learned something as well!

I’m not as brave as you and am not going to share my various disasters!!

Cheers – Pierre

[David MeierParticipant @davidmeier28154]

Hi Pierre. I think I am safe, all seems quiet on the ArnieS front lately.

I recently met some people who have built 900mm long cracker box boats with petrol weed eater motors in them. They meet at a local lake and informally race them each Sunday. The boats are robust and cheap to make so it doesn’t matter if they get knocked about a bit. I bought a Chinese made Australian brand weed eater for $128.00 NZ last week and have completed about 80% of the hull, Hope to have it ready for next Sunday. Am really looking forward to joining in the fun. I think this one will last a bit longer.

 

Cheers.

David.

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