While I am all for DIY, building a PWM speed controller that has to decode the pulse from the RC is not exactly that simple or even cost saving considering you could just use one of these for 11 bucks:
or with fan and stronger wiring for less than $18:
HOWEVER a more unique approach to avoid cavitation(which I compare to wheel spin on the F1 grid) could be a simple two stage switching which you could adjust to always provide the perfect 'take off' while being still a DIY project.
The idea is one MOSFET switches on the motor in series with a 50W power resistor to get it spinning and start a good water flow without cavitation and maybe 20-100 milliseconds later the second MOSFET switches to full power using this simple adjustable timer module for $3:
The included relay makes things very simple to switch the MOSFET. The adjuster has at least 20 turns so you should find reasonable adjustment in the millisecond range. The clicking of the relay will help you there. Once you found the perfect 'take off' timing and with the right resistor you will always have it perfect for the race and out smart the others who have to rely on their RC skills using a PVM controller.
You will be the Hamilton of RC drag-boat racing
These are resistors I am talking about
:
I would suggest to get two 1Ω resistors which gives you a sensible range between 0.5-2Ω to play around with (parallel, series or just one). So your start up current will hardly exceed 22A (0.5Ω) or only 5.5A(2Ω option). I am guessing 1Ω should be just right. Never mind that the startup pulse is about 120W on the single 1Ω resistor. They can handle that for the 100 milliseconds or even a bit more.
All the links above are from Australian sellers so you should have it in 10 days or less.
Don't say you are not in a hurry. It is life and there is never enough time for it.
As for NOT finding things I am certainly in the same boat as you and Skepticist, have been all my life.
It comes with the cost of being creative.
I am more likely to find what I was looking for in a pile of junk than in thousands of boxes stored away in cupboards.
I compare my mess with the Universe and it all belongs to my theory of "Chaotic Order"
That said, I do have all my resistors and semiconductors sorted in trays like in a Jaycar store, it are those odd parts where you just don't know where to put them that end up in nomeat's "Universe" and are usually found at later event's or 1 hour after (re)ordered on Ebay.
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