Hi,
I’m a newbie to this site and regard myself as relatively uninformed with regard to RC boating.
After a 20-year RC boating lay-off and enjoying a holiday with the missus on the lovely Isle of Skye I decided to build a Robbe Katje with my lad, now 30, to sail on the loch beside the cottage we stay when we visit.
We had built several scale boats from plans back in 1993, eventually going for a Krick Avanti but a year later I’d sold just about everything to raise funds for my real love, rallying. Two seasons later and I was neither boating or rallying - I was in a dreadful rock band whilst training to be a paramedic.
I’d love to say that I gave my career up for major fame and/or becoming a motorsport legend in my MK 2 Escort 1300 but the plain truth is I’m still just a paramedic. This might be useful to others in need of a hand whilst escaping the clutches of the grim reaper but to me, I remain unknown (infamous!) and without a career in music or motorsport.
How times have changed in the RC modeling world in 20 years. None of those horrid resistor/mechanical speed controllers, brushless motors, 2.4ghz etc.
Two years ago I decided to build another boat. The Robbe Katje was ordered. I scoured the interweb and found all sorts of permutations/options for the Katje. Some built the kit as a scale speedboat, as Robbe had intended using a small 380 motor, some had installed brushless motors however, I settled for a Robbe Speed 600/Mtronics 25Amp ESC/7.2v NIMH.
The chap in Mick Charles models told me the boat would be over-powered which only encouraged me. Despite various adaptations he was quite right. The Katje wouldn’t stay in the water. It would either cavitate or launch itself into the air.
Unlike the fiberglass Krick Avanti my lad and I had enjoyed in the 90’s, the Katje had a light, molded plastic unstable deep vee hull more suited to a lower power. I tried trim tabs, different props, ballast you name it.
I tried to attach a Kodak Zi8 mini HD cam to the deck to capture the lure of a freezing Scottish loch however the footage was shaky/unstable due to the Katje’s light weight ‘bobbing’ tendency.
Planning ahead for another trip to Skye, my lads’ friend found an MFA Spearfish at a car boot sale for £15. At 900mm it provides the perfect platform for the GoPro. I installed an MFA 850/Mtronics 25Amp ESC and a 12v battery pack, built a 2:1 rotating gearbox in order to pan with the GoPro using spit, sawdust and an old 4-turn sail winch servo and bought a cheap 2.4ghz 6-channel radio set.
I learned a little more about motor/prop relationships here on this site. Really simple stuff that explains why the Spearfish kept dying in the middle of Loch Fada after every prolonged, flat-out session. The MFA 850 is the higher revving of the 800 series but this didn’t stop me putting a high-pitched prop more suited to chuggers! The 50mm prop would see the Spearfish clattering along at top speed for about 10 minutes before over-heating the ESC and shutting down. Reverting to a 40mm, finer pitched prop allows a sufficient turn of speed and nearly an hour in the water.
Having captured some truly stunning footage of the loch using this boat and a GoPro, the house we stay in three to four times a year and the magnificent Old Man of Storr there was still something missing.
We revisited this February but this time with an MFA Piranha mk2. I’d read an article on your website regarding the building of said electro and executed my own with my lad finishing it off with a professional spray in Ninja green.
This was a total departure from the Katje. The boat feels substantial, stable and fast achieving instant planing and returning fantastic on-board footage. Once again, I struggled to optimize the prop/motor configuration.
I had wanted to retain a 12 volt set-up as I’d already invested in 10-cell NIMH’s for the Spearfish however, there were no retailers stocking the Graupner Speed 600BB Turbo 12v. I gave it some thought and settled for the 8.4v Graupner SP BB thinking it would handle the excess voltage. I ran water cooling to the motor in anticipation of the extra heat I assumed over-volting a motor would cause.
The motor gave me about three hours of boating whilst in Skye before letting rip on a sunny, windless day towards the end of the week. I removed the motor and replaced it with the former Katje Robbe Speed 600 7.2v. I’d already over-loaded this motor with a 10-cell pack a year before but I assumed I might get another hour of it. I got 20 minutes before it let go.
I will add that the prop/motor alignment is pretty damned good for the banana-fingered clot that I am. I have also learned to start with a small prop, increasing size until the correct balance has been achieved between speed/performance/heat/motor life (really? Is that why you keep burning motors out eh Dave?).
I opened both motor cans after removing the capacitors I’d soldered on for reuse at a later date. Both cans were in a filthy mess. The brushes were still inside but in powder form lining the can. The com was torn to shreds and both rear bearings had melted and departed their plastic housing. The front ball race of the 8.4v SP BB appears to be in top condition, which is impressive but not very useful.
I have managed to get a Graupner Speed 600 12v BB Turbo and I shall install it next week prior to our trip to Skye in April. I shall be fitting caps, I have run the thing in for 2hrs off a 1.5v D cell (minus any of that water nonsense) cleaned and re-lubed.
My questions for the forum are:
Exactly what dictates the nominal voltage of an electric motor? Windings? Brush-size? Pole division (The 12v BB Turbo is a 5-pole)?
Yes, both previous motors got very hot despite cooling but was this because the armature was spinning at approximately 25,000rpm instead of the 17,000rpm intended?
Am I likely to see total meltdown with the 12v BB Turbo at 12v with a sensible prop or will its nominal 12v ‘build’ allow for less internal heat development?
Or is it likely that I’m a greedy man who wants his boat to go faster than it was intended and all the cooling in the world won’t protect a well-built motor from the unreasonable expectations of an idiot?
Is the answer brushless? Not hell-for-leather fast and still retaining a submerged prop. Must I have the latest lipo’s at £60 a shot?
How cheaply could I have brushless boat that goes no faster than it already goes? I have a Sigma mk2 pro peak charger already leaving the motor/esc/flexishaft/lipos. There’s got to be an esc/motor combination for my needs but I’m finding most information on the subject based upon model flight or rocket-like hydros hitting 40-90mph.
I did read on this forum that a jet-drive should sit nicely in the flattened rear half of the Piranha hull. This would allow for filming river runs without losing prop/rudder. I see two Graupner jet drive units at ?affordable prices. One with a 20mm impellor and the more expensive unit with a 40mm impellor. For a 500-550mm long fiberglass mono which one would suit best? How robust are the impellor blades? Do people report regular impellor failure from sticks/stones?
I intend to film as many lochs in Skye over the coming years using a combination of both Spearfish and Piranha. I haven’t used any of the boats down here in South London oddly enough. I recall people getting miffed years ago when my lad and I would turn up at local ponds with slow boats. I’d love to know what they’d do if anyone tried running a crazy, brushless powerboat locally? I suspect the police would be called.
Here’s a 4-minute film I made from our last trip in Feb. All the off-boat footage is filmed from the Spearfish. A friend informs me that the last crazy ten seconds of the video constitutes a “non-linear narrative transition”. I didn’t go to film school like he did, I just wanted to include the Spearfish rescuing the Piranha with a big stick taped to the stern along with the missus plucking it from the water!
http://youtu.be/IkhYU2mSmfE There is so much I have yet to understand!
Thanks,
Dave
