Zippkits mini sprint

[dahodevil]

I scratched built one very similar to the mini sprint but I built 2 small sponsons to the rear of the hull. The recommended prop for this on 3s is a 427 but I can't get it on plane. It just ventilates. Tried with the prop 3/16" below the sponsons and still ventilating. I installed a 430 and it hops right up, even set flat with the sponsons. The 427 looks awfully small but that's what's recommended. Any help is appreciated.

[NativePaul]

What does it weigh? Is there any driveline drag? My stock, kit built Minisprint uses a 3000kv 72g 400 size motor on 3s 20c1500-35c1800mAh with an X427 which is heavily de-tongued for me to get the 5 minute +mill +in lap I need for the Mini Hydro class here, it weighs 680g all up with 1500mAh and 710g all up with 1800mAh and I have no issues with my hole shots (unless I ding a prop, at these sizes it doesn't take much tip damage to seriously effect them) I used to run it with a 4800kv 72g 400 size motor on 2s 20c 2100-2200mAh with the same 680g all up weight and a heavily de-tongued and back cut X427 which also had no issues with its hole shot despite having a very small blade area. My prop hub is roughly 1mm above the ski.

What motor are you running? X430 is probably fine unless the motor is really weak, Anything from the above modded X427 to an X435 works well in mine, If I didn't have rules to race by I would probably run high C 2000ish mAh and an X632 which really scoots.

[dahodevil]

I have the 3500kv motor from zippkits. Running the plastic blades for now till I find what works. 2100 mah 20c and a wire drive. I don't know what it weighs but it's mainly 1/16" ply and foam core sponsons with carbon fiber tubes. Motor is mounted way to the front between sponson tubes, battery in the middle. CG is at the back of the turn fin.

[dahodevil]

Just don't know if I run with the 430 if it will run to hot or draw too many amps.

[NativePaul]

Theoretically that should give you 5000+ more RPM than I have, although with a cheap Chinese outrunner vs a quality European inrunner you may not be seeing that much difference in practice.

I have a stock of worked Octura BeCu props and I threw the black plastic prop it came with straight in the bin unused as it felt rather flexible, I have however used Octura plastics in the past and the red x431p is decently stiff, and cut down to 28mm de-tongued, thinned and sharpened is a good match for a BeCu x427 in terms of speed but has a little more thrust and amp draw.

1/16 sounds very beefy for a rigger of this size, although the Zipp kit is about the same and well overbuilt too but I think made with Poplar liteply rather than birch, if I made another wood one it would be 1/32 birch to lighten it up. It could just be that yours is a bit of a pig, buy a set of digital kitchen scales to accurately weigh it all up. When I was scratch building I bought a .01g digital drug dealer scale if you weigh every part when building a little sanding, drilling, choosing glues carefully, etc that wouldn't show up on most scales, can add up to a surprising amount when done to every little piece over a build.

What size booms did you use? They have a LOT of drag when they are in the water at very low speed. Does it go fast enough to get them out of the water or is it dragging them through the water all the time? The stock ones were an imperial size just under 5mm but due difficulty of avalibility here I changed mine for 4mm, mine had no problem getting up before but the 4mm definitely have less drag and is still up to the task.

Back in the day of brushed motors and NiCad batteries some riggers wouldn't get up on the plane by themselves with optimum race props on and had to be launched, if you feel comfortable doing it safely you could try launching it underarm under full throttle (when the wind is blowing towards the bank or in shallow water (as you may not get it right first time)) and see if it hits the water at planing speed whether it slows down and drops of the plane, or accelerates away.

My CG is in the same place.

My motor runs lower than ambient with an X427 or X430 and just warm with an X632, but it is a quality inrunner and watercooled, yours can't run as cool as it has no watercooling, but it may still run cool enough with an X430, try it for 30 seconds, bring it back in and rip the tape off as fast as you can and take the temperature of the electrics, if the temperature is good top up the charge and let it cool down then add another 30 secs to the runtime and repeat till you are happy with the runtime and temps, or it comes in uncomfortably hot around 80C/180F is too hot for a motor and 60c/140F is too hot for a LiPo or ESC.

[dahodevil]

Thanks much NativePaul. I have 5/16 booms and the back one never gets out of the water. I will try the hand launch. I'm used of that (nitro). Waiting for my esc to come in, I'm testing with a 20 amp esc now just to get it on plane.
What motor and esc are you using and what speeds are you seeing?

[dahodevil]

Wow. 5mm is about .2". Mine are .3". Guess I overbuilt.

[NativePaul]

Blimey, that is 7.94mm I bet those booms have a lot to do with it not getting up, that is the same size booms that my JAE.21 came with, mine are almost exactly half that size at 4mm, the booms and brass for them must weigh a lot more too and that doesn't help either.

My motor is a Mega 16/15/3 and the 2s one was a Mega 16/15/2, ESC is a Castle ice 50 stripped of its heatsink, I upgraded from a Castle Pheonix 35 as I wanted datalogging and stripped down it is not much heavier, 35A was plenty.

I don't know what speeds I am getting, and nor do I care as it is definitely not a SAW boat. It does 15 second laps in almost any conditions which is what is important to me. It is slower down the straights than much of the moulded competition which weigh a lot less, a lot slower than some if the water is flat, but it handles better so lap times are as good as most and it copes with rough water much better than anything else.

[dahodevil]

You're right. 7.94. No brass, just epoxied in. We'll thanks for your help. I know my problems now. Do you water cool the esc?

[NativePaul]

I have run water cooled ESCs in the past, but the Castles phx35 and ice50 are not.

[dahodevil]

Ok, I can run it on the 430 prop but I can't get it on plane with the 427. The back boom wont get out the water. Well I managed to get it going once but that was it.
The problem is heat. After about 1 minute it will get to 170 degrees sometimes. I was told by zipkits to keep it under 140. What do you think about running the 430 with a water cooled leopard ?
If so, which leopard?

[NativePaul]

I guess that is Fahrenheit? If it is 140 is about 60C which seems a pretty conservative limit but if they recommend not going over that I would listen to them as they know their motor. 170F is 77C (I don't like to run motors over 80C so within limits for the motors I use) but reaching that after just a minute is way too much.

I think your money would probably be better spent lightening and reducing drag of the hull, but I am sure it is neigh on unbreakable with booms that size and if you want to keep it that way a watercooled inrunner would be a good solution to let you run the bigger prop which gets it up onto the plane a leopard 2845 3540KV would spin a 430 easily without getting too hot, it is a little bigger and heavier than needed, it could handle an x432 if your ESC and cells are up to it.

[dahodevil]

Thank you again NativePaul. Yes the booms are epoxied in and already constructed of 1/16 ply. It handles really good in straights and corners so Ill keep it going with a leopard water cooled. all edges are sharp to shed water and it scoots along nice. I have the thunderbird 54 ESC and it never got over about 115 Fahrenheit.

[NativePaul]

That should be enough for an X432, plenty for an X430.

[dahodevil]

Cool, Now I'm stuck trying to find a 4mm x .062 coupling. I might have to use a shim on the motor shaft. Do you think a 4.1mm ID x 5mm OD brass k & s tube will work to fit a 5mm coupling on the 4mm shaft? Will the tolerances be close enough on the tubing as to keep it all centered and balanced?