Boldly Going Where No Man Has Gone Before. RC Nano-Hydro

[Dr. Jet]

Jim Clark was nice enough to gift me a 1/43 scale (7-1/2”) Casper shovelnose hydro hull. If you do a search for Casper hydroplanes, you will find some links where these hulls are “raced” on a track with special turning vanes to keep the “free flight” boats going in a circle. I haven’t been able to find much more information on them; specifically motor/prop/battery details, availability, and build details. From the videos I’ve seen, these boats are more displacement hulls plowing through the water than hydroplanes and they do not appear to ever get on step. Since I am the de-facto micro enthusiast here, I thought I’d try to make this little hull actually work as an RC race boat. To that end, I have two goals: Get it to run on step and get it to be fully controllable

I'll be extensively modifying it for RC. Ride pads are definitely a consideration as the vacuum-formed polystyrene hull no doubt has high drag rounded corners. Looking at the vacuum-formed bottom, it may get some re-shaping of the aft bottom, maybe get a lot of air trap. I’ll know more when it actually arrives. Also planned is a homemade rudder, a homemade micro strut, a 1/32 wire drive, and a homemade motor/shaft coupler. Maybe a straight shaft would save some weight and complexity, but I like the adjustability of a strut; we shall see. The BIG problem will be a prop. As of this writing, I see a couple of options: I could modify a 20mm Raboesch prop (see: https://www.ebay.com/itm/Brass-3-Bla...19.m1438.l2649), or I could modify a 20mm micro Eco prop (see: http://doctorprops.torgg.com/collect...mm-doctorprops).

The Raboesch prop has an M2 threaded hub which means I would have to bore a 1/32" hole through an M2 threaded brass rod (cut the head off a brass screw). I have a mini lathe so this is not impossible, but it does not leave much meat on the threaded rod. The 3mm hub of the mini Eco prop would make this machining process a whole lot easier to accomplish as I can simply ream one of the 1.5X3 prop adapters to 1/16" I.D. and I'm done. It may even be possible to reduce the prop diameter a bit to more closely resemble (and behave like) a surface prop.

I’ve ordered the following motor, servo, and battery for it:

Motor: https://hobbyking.com/en_us/turnigy-...or-4500kv.html
ESC: https://hobbyking.com/en_us/hobbykin...c-1a-ubec.html
Servo: https://hobbyking.com/en_us/hobbykin...cro-servo.html
Battery: https://hobbyking.com/en_us/turnigy-...lb3002s35.html

At this moment, I only have the photo provided by Jim Clark. The paper photo of the Bardahl hides a lot of the sponson and hull detail. I kinda wonder what those rectangular “bumps” are on the aft portion of the bottom and located on either side of the driveshaft. It also appears the prop was designed to run inside of a “tunnel”. I assume this was for geometric and safety reasons. I plan to have the prop behind the transom as is normal for most RC hydros and I'll be closing off this prop tunnel.

This project will have to take a back seat to my newest project (The 1/20 scale Atlas Van Lines) when the wood I've ordered for it arrives. Be sure to follow the build thread I’ve created for it too.

[fweasel]

Love these tiny builds. So challenging in such different ways.

[srislash]

I like it, the shovels have such a classic look. Mini, mini!

I had recently seen a vid where some guys in Thailand made their own props for some mini boats.

[NativePaul]

I was pretty excited by these when Jim first posted it and I became aware of them, but it looks like they are out of production as the sources Jim gave had none, and the next race dates on their website is in 2016. I did find a couple on cragslist and one had decent pictures of the bottom, but it looked to me like the bottom moulding is suitable only for the bin wit shallow sponsons, round edges and lots of bumps, so I would be paying a lot to buy and ship a deck and I changed my mind on it.

I found out that the stock power system is a 23g 2s aaa alkaline battery an an 18g 130 motor, which I think is far too heavy to plane with the limited planing area available to a 7" semi scale hydro. I wanted to reduce the weight as much as possible and was planing along the lines of a 9g 1s 300mah LiPo, a 6g 1104 4600kv outrunner, a .7g 7a 1s esc, a 4.5g Frsky FASST RX, and a 1.7g linear servo, for a total radio and power system weight not much over half that of the stock powertrain, and should yield about 15mph on a 20mm eco prop.

Another prop option may be the plastic Joysway Magic Vee and Magic Cat props I cant find size or pitch specs for them but they come in packs of 2 dirt cheap, and the Magic boats are only 9" so if you can find them locally you may be able to talk you LHS into letting you measure one and ordering some if they are suitable. There are alternative 7000kv motors similar in weight available if the magic props are small enough.

[Dr. Jet]

Driveline Ideas

Here's a quick and dirty sketch of what I'm thinking for the driveline. Since everything is going to be small and light, I'll let the wire drive carry the thrust to the motor bearings. I couldn't do this in bigger, more powerful hulls, but I think it should be just fine in this application. The sketch shows what I would do with the Raboesch prop, but I can simply make another driveline if I decide to go with the Eco prop. It's well under a dollar and twenty minutes to make another driveline.

[Dr. Jet]

Yeah, I know weight is going to be an issue, but if it's too light, it may be uncontrollable. See this vid of a high-power Casper:

https://www.youtube.com/watch?v=5kG76oPygN0

Another thing I noticed in that vid was the balance point looked like it was way too far aft. Balancing this little beast will be critical to making it work. According to the data Native Paul was kind enough to supply, the "stock" power system was around 41 grams, not including the driveline. The motor/ESC/servo combo I have selected weigh in at 26.1 grams. I'm not going to use any connectors between the ESC and the motor; I'll solder the motor directly to the ESC and eliminate the ESC motor wires for maybe a gram or two savings. The Rx comes in at 5 grams, but I can save a couple of grams by removing the case. The battery I selected weighs another 26 grams, but it should give 4~5 minutes run time. This totals somewhere around 55 grams or about 2 ounces. The driveline in the sketch may be another 15~20 grams because the brass in the strut is a bit hefty. For the rudder, I'll probably use 1mm carbon fiber sheet and an aircraft aileron hinge.

NP is right that this may be a bit hefty, but putting sheet styrene ride pads on the sponsons with a bit of overhang will increase the planing surface area by quite a bit.

We shall see....

As I said: Going where no man has gone before......

UPDATE: I just checked my carbon fiber stockpile and I can probably make the strut from CF and save a lot of weight. I may be able to get an RC version going for an additional 15~20 grams or so.

[Dr. Jet]

Some more advance thinking

The hull kit has yet to arrive, but I'm doing more advance planning. Anybody can buy the latest 54" hull and bolt in a proven system. Since this uber-micro RC has never been done before (to my knowledge), it requires planning and forethought. I appreciate input from the folks here.

As stated above, the "stock" boats seem to have the balance point really far to the aft. Additionally, looking at the deck in the photo, it appears access hatches are limited in size and location. In order to have a minimum of flex in the driveline (less drag) and to get the balance point forward, it may be advantageous to have the motor as far forward as possible (See photo of my BBY Micro). This would allow access to the motor mounting screws and the coupler through the engine hatch. Sealing that hatch will be a challenge, but I'm sure I can come up with something easy to tape up. This arrangement will allow lots of room to move the battery around for a good balance.

[Dr. Jet]

The First Part

Well, I made a strut to see how well it would work. In its current form it's about 2-1/2 grams. Once I cut the tang to height and mill a slot in it, it should be right around two grams.

[Dr. Jet]

I made the strut bracket this afternoon. According to the USPS tracking info Jim sent, the hull should be here tomorrow. Once I have the hull in my hands, I can make some measurements and cut the tang to length.

The next thing to think about is the rudder, and especially the hinge. Due to the small size of everything, the hinge has to be precise, i.e. slop-free, and light weight. Brass tubing would provide more precision as opposed to CF tubes and rods, but they're heavy. I may make an upper and lower plate from CF with a reamed 1/16" hole for the rudder pivot, and attach a small piece of 3/32" brass tube to a CF rudder blade. See sketch.

[srislash]

Go with stainless for the pivot shaft.. the brass will wear quick from the CF

[Dr. Jet]

Go with stainless for the pivot shaft.. the brass will wear quick from the CF

I have some 1/16 stainless tube, so that will work. Alternatively, I could put brass bushings in the upper and lower plates. Or, I could just let it wear and replace the shaft every hundred hours....

[NativePaul]

How wide is it, I am guessing 4"-4.5"? I may try to knock up something the same size, if the magic props are suitable it should be on the water for less than half the cost of getting a deck shipped from USA.

[Dr. Jet]

How wide is it, I am guessing 4"-4.5"? I may try to knock up something the same size, if the magic props are suitable it should be on the water for less than half the cost of getting a deck shipped from USA.

I've been researching the whole Casper racing thing. It seems one of the allowable props was from the Kyosho Mini-Z Formula boat. See: https://www.ebay.com/p/Kyosho-Mini-z....c100005.m1851

How wide is it? I'll be able to give a more accurate answer to that this afternoon, but take a look at this:

[Dr. Jet]

Well, the hull didn't show up, but my rudder servo did.

[Dr. Jet]

More Advance Planning

After studying the photo of the deck, one thought I had was: How am I going to have access hatches with the "engine" in place and yet be able to seal them up?

I think I have the solution. I will remove the engine depression by scoring the inside of the depression with an X-acto blade. I'll also remove the rear depression as well. Then, I'll cut a thin styrene sheet so that it overhangs the engine opening in the deck and the aft cowl by about 1/4". I'll glue the engine depression to the bottom of this sheet which will give it the correct lateral curvature. Next, I'll glue the aft cowl to it, which should give it the correct longitudinal curve. Finally, I'll glue the engine to the depression and Bingo! A tape-down deck!