An Unexpected Success

[Dr. Jet]

I'm sort of building Prototypes# 4 and #5 concurrently, although I think #5 will go on hold at its current state of completion and I'll concentrate on finishing #4. The Silkspan and water-based sanding sealer finishing technique work fantastic. If you get a ding in the foam, add a little Model Magic Filler (the ultra-light stuff), sand smooth when dry, cover with a small scrap of Silkspan and sealer. A little finish sanding when dry and the repair is 100% invisible. Cool stuff. I had intended #4 and #5 to both be gear-drive versions, but for the sake of testing and the expedition of the production, I'm going to run the little outrunner in #4. That should give a good handling comparison for the hard-chined Prototype #3 without having to make allowances for different power systems. #5 will now be the vehicle for comparing the direct-drive configuration with the geared.

After #4 and #5 are complete, I'll find out which hull shape and power system work the best. I'm thinking the asymmetric hull may be the ticket, and it's easier to build in that you don't have to spend a lot of time making both sides exactly the same. The Prototype #5 geared version will use the 5/32" stuffing tube with the tapered drive dog and Octura thrust washers as Native Paul suggests. The laser-cutting for the motor/gear mount will have accommodations for either a 9/32" ball-bearing shaft or a 5/32" brass tube bearing/stuffing tube.

After all the testing is done on the various prototypes, I plan to build one final hull that will serve as my "Race" build using the best of everything I've learned from all the prototypes. That way, I can do the same thing that happens to Ringo at 0:47 in this move to anyone that tries to race mini single-step hydros against me...........

https://www.youtube.com/watch?v=A6nvyhz_FiM&list=RDA6nvyhz_FiM&start_radio=1&t=4

[Dr. Jet]

About to Repeat a Mistake?

Well, not a mistake per se, but rather a compromise.

The first MPR (Mini Pool Racer) was nothing more than a scaled-down version of a VERY OLD design. Jay's original design had a turn fin located in the center of the hull. I copied this layout on the MPR (a.k.a. Prototype #1) and onto Prototypes #2, and #3. It was only a couple of days ago that I got to thinking and wondering why the fin was located at the center as it was. It would seem that the longer the moment arm between the turn fin and the outside ride surface is in relation to the height of CG, the less inclined it should be to want to flip over in a turn.

My guess is Jay put it there so the boat would turn either way. Power systems were marginal at best back then and counteracting torque and propwalk was less of a concern as it is with today's high-power systems.

I was just about to cut the hull bottom doubler piece when it dawned on me that I should move the turn fin as far to the inside of a right turn as possible. I have no plans to turn left with this hull. All future prototypes and the "Final Build" will incorporate this feature.

Stay tuned for photos another day.

[Dr. Jet]

Prototype #4 Progress Photos and Comments

I built the motor mount from the laser-cut parts. I installed a motor, coupler, and short stub shaft on it and used this assembly to locate the through-hull hole in the step for the stuffing tube. Then, I made a small scrap plywood support to hold the stuffing tube at the right distance from the bottom of the hull. There were some minor adjustments to the motor mount legs to get the right angle with the motor and the shaft.

Once everything was right, I tacked the motor mount in place with thin CA, then used polyurethane glue for the stuffing tube/hull joint. Finally, I held everything in place with blue tape as the stuffing tube joint cures. After that, I'll re-glue all the plywood parts.

Interesting things to note:

1. I re-located the servo. The height of the servo arm in its previous location was always an issue as was the bend I had to put in the linkage to access the mounting screws. With this lower-profile design, I needed to do something...... I moved the servo to a new location just forward of the step and modified the mounting. The ESC should now occupy the space formerly filled by the servo. I may have height clearance issues with the Rx too, so I may have to get creative.

2. I have cannibalized the turn fin from the MPR and installed it in a new, inboard location.

3. A GREAT way to hog out Styrofoam from the inside of the hatch is to use a a Dremel tool with a wire wheel. It absolutely vaporizes the foam. Interestingly enough, the Silkspan and sealer is much tougher and you can grind the foam away leaving nothing but the covering. This is exactly what I did and I can actually tape down the hatch without interfering with the motor. I may still have to build a blister for the motor as the skin is REALLY thin above the motor.

[Dr. Jet]

The hatch is nearing completion. I got the motor blister a little crooked though. Oh well; it's a prototype, and paint can hide a number of mistakes. I still need to hog out clearance for the rudder linkage and the ESC/wiring. Rudder linkage is curing as I write this.

[Dr. Jet]

Well, it all fits in there. I need to solder a couple of connections, add some shrink tube, and maybe a Zip-Tie or two to finish up the guts.

Time to pull all the gear off, do some finish sanding, and shoot the final paint. Again, this is a "Quick-and-Dirty" foamie prototype, so don't expect a Concours d'Elegance contender in the finished product. It will be "Good Enough" and nothing more.

[Dr. Jet]

OK, Prototype 4 is all done. I'll take photos in the morning. The paint looks kinda hacked, but hey, I did it in really poor lighting...

While the Silkspan/Sanding Sealer skin makes a smooth, tough surface on the foam, it's REALLY thin and punctures and/or dents rather easily. The low-temp covering film is also subject to this kind of damage, but it is much tougher than the Silkspan. The Silkspan repairs easily, so I could see some real "Rat Rods" running at the end of a season of racing.

[Dr. Jet]

Here are the photos I promised. The internal layout is vastly superior in this version and improves the ability to create the uber-low profile. Prototype #5 hull is mostly carved now and covered with the Silkspan process. I have some guitars to work on today, so I won't be able to glue the hull bottom styrene sheet on until tomorrow (Sunday). I'm waiting for a little inrunner to arrive and I will attempt a geared drive in #5, just to see.

One of the initial issues I encountered with the MPR was the easy ability to vastly over-power the hull. I'm trying to setup the prototypes up with readily-available motors in the 75-Watt range. The initial MPR success used a now unavailable finned 2030 inrunner rated at 3300rpm/V, (currently residing in Prototype #2 - Homey). This is about a 75-Watt system, so I'm assuming the others will be similar in performance.

I have a bunch of 20mm inrunners from Hacker, Mega, and others, but I think they may be far too much power for these little guys, plus they are expensive.

My ultimate goal with the mini single-step hydro would be to see a "Spec" class formed that is uber-cheap, uber-fast, and more fun than a human being should be allowed to have. Something along the lines of Speed400 Pylon Racing back in the "Before Brushless" days. Rebel Racers Unite!! Forget NAMBA, APBA, and all those alphabet soup organizations that do more to cause anger between the members by making questionable rules and issuing autocratic dictates. You won't need a big "Sanctioned" body of water; a small pond or even a large fountain will do for Mini Single-Step Hydro racing. If you have a well-stocked parts bin, you should be able to put a competitive racer on the water for less than the cost of a propeller for your 12S monster.

I now have templates and tooling to cut foam pieces until the cows come home, a lifetime supply of motor/servo mounts, sources for driveline components, and a seemingly reliable source for motors.

The silence here is deafening. Am I the only one that sees the potential for fun with these?

[Dr. Jet]

I know there is at least ONE person out there with questions, here are some answers:

Weights (all RTR with 65 gram battery):

Prototype #2: 266 grams (9.38 oz.) *
Prototype #3: 276 grams (9.73 oz.)
Prototype #4: 273 grams (9.63 oz.)

* Homie used a different motor and lighter driveshaft.

With a 75-Watt power system, I may be in trouble..... 75 Watts into 9.38 ounces is about 130 Watts per pound. An RC airplane can hover then accelerate vertically at about 150 Watts per pound.

I plan go go to the lake on January 27 and will hopefully have someone there to take videos.........

[Dr. Jet]

I've been working on #5 for a while now and have great expectations for this one. I am concerned that the 75-Watt outrunner I selected to replace the finned 2030 motor may be too much power for this hull; testing later this month will either confirm or assuage these suspicions. The original MPR used this no longer available 2030 motor rated at 3300 rpm/V and ran great at about 50 Watts. I just discovered a substitute 2030 motor of similar Kv that may be suitable (See: https://www.ebay.com/itm/RCS-Model-K...QAAOSw6vBeGDw4).

I have ordered a 50-Watt inrunner motor rated at 11,000 rpm/V to use initially in the gear drive that will go into #5 (See: https://www.ebay.com/itm/X-TEAM-Inru...D/133075674548). I can make several different gear ratios, but I will start with three that provide an effective Kv of 3765, 3575, or 3405 rpm/V. The design will also allow me to use this motor in the future: https://www.ebay.com/itm/Surpass-Hob...r/133027921021 with a different gear ratio to achieve the same effective Kv.

On all the prior prototypes, I have offset the driveline by 0.2" as is common practice in hydroplanes to keep it running straight with a minimum of trim drag. The asymmetric hull hull design will put the prop shaft in a location that will make it look like it is offset too much, but in reality it will only be offset the usual 0.2" from the center of the ride surface. What isn't really easy to visualize is the center of the ride surface is offset from the centerline of the hull. This "additional" offset will allow me to fit the offset gear drive and servo in the same location as #4. I may have to remove the case from the receiver to get it to fit in the remaining space. That's cool though, it will save a gram or two.

The sanding sealer is drying, photos to come.

[Dr. Jet]

It's Not the Ride Pad That's Offset....

..... Although it looks that way, but view it instead as if I hacked away the non-trips on the starboard side. The water doesn't see what's above the ride pad, and since I won't be turning right, I don't need those non-trips.

[NativePaul]

I read all of your posts with interest, as I am into Minis too and there isn't a lot of info out there on them. I have picked up some tricks from your posts, and I also occasionally have some input that I hope will be of help to you (or others reading in the future).

With that said, sorry but this particular hull doesn't do anything for me. Obviously I haven't seen it run, but from the pictures it looks like the crackerbox (which I dislike) of stepped monos, which you have turned into a trampoline of a hydro. I look forward to seeing the running video, and hope you prove me wrong.

I run all my boats at approximately 500W/Kg, which is a result of the Naviga classes I race to having about that, and I like the speed/runtime compromise that the rules push you towards, so go the same way in my fun boats too. I never bothered to work it out before but maybe not by coincidence 500W/Kg is about the same as the 200W/Lb I used to go for in my fun planes.

I got out of flying and went back to boats just before the outrunner revolution started, back then you needed 200W/lb for good vertical and even then the punching out of a hover was marginal, I guess with outrunners lower KVs and bigger props it takes less power to punch out of a hover, or is that specific to low pitch 3D specialised setups (not something I got into).

[Dr. Jet]

Hey NP,

Actually, your input has been of great help in several instances that I recall. I never liked crackerboxes either. The 1/10 scales were hard to set up to run consistently; plus, the motor/battery technology of the time was only slightly superior to several small rodents in treadmill.

I see the hydro - crackerbox connection you mention; cheap motors and cheap drives in a cheap hull..... I believe Jay designed the original Pool Racer about the same time that the 1/10 crackerboxes were popular as they used the same identical driveline. He probably had a crackerbox with a shattered hull and wondered what he could do with the running gear; he most likely carved some foam around that crackerbox gear and created the original.

If I'm WAY off base in my assumptions, he will no doubt step in and spank me....

The MPR finally ran good with the last little finned 2030 motor I found, but being based on a different set of design requirements, high winds (above 10 knots) and center of gravity issues limited its potential. I am in totally uncharted waters here, but I decided to substantially re-define the design requirements while keeping the overall concept. If it works, it should be a hoot to drive (more fun than a cracker). If it doesn't work, I only have about $20 of materials in the hulls. The drivelines can go into another Dr. Jet hair-brained scheme.

A trampoline of a hydro? I prefer to think of it as a pancake. I have the ability to make really good waterslide decals. I was thinking of doing "The FINAL ONE" covered completely with the pattern in photo #1 and the smiling face of the Patron Saint of Pancakes in photo #2.

[Dr. Jet]

I've been working up the drawings for the motor/servo/geardrive mount. My plan is to make them all into one unit. With what I've learned on the first four prototypes, I can make mounting the servo and building the rudder linkage far easier than the initial builds. I'm going to use a rubber band to hold the servo in place in a box-like space instead of bolting it into place with tiny little hex head screws that are nearly impossible to access.

As I was working out the details, I got to thinking that the availability of gears for the 8700 Kv Surpass motor may be better. Both of the little motors I'm considering have 1.5mm shafts and I found some 13T 0.4 Module pinion gears from a small RC helicopter that will work perfectly (http://www.megasmicros.com/product_p...b-13t-0p4m.htm). By using the lower Kv Surpass motor, I can use readily available Tamiya gears (https://www.ebay.com/itm/Tamiya-4226...72.m2749.l2649) instead of buying higher tooth count gears at SDP/SI (https://www.sdp-si.com/) for the higher Kv motor. The lower Kv means a smaller spur gear so everything can be lower in the hull. I also think the Surpass motor may be more readily available in the future. As such, I am finalizing the drawings around the Surpass and will have them ready to send out for laser-cutting when the motors arrive.

[Dr. Jet]

I had another thought that I may try out on the "FINAL ONE". There's a lot of "Hogging Out" going on inside of the hatch to make room for all the stuff. The Silkspan/sealer form a hard shell around the foam. If I were to carry the Silkspan along the sides of the hatch, leaving only the bottom open, I could pour acetone (or similar V.O.C. compounds) into it and dissolve all the foam away leaving only the Silkspan shell. We used to use this "Lost Foam" method when making airplane fuselages.

I suspect the Silkspan skin would be rather delicate, so I might want to beef it up with some 3/4 oz. fiberglass cloth and epoxy. Stay tuned to see how I decide to do this....

[Dr. Jet]

Thoughts on the geardrive

When Fast Electric was in its infancy there were extreme limitations on power systems. With only a few European Super-Motor exceptions, the only readily available motors were those intended for RC cars/buggies running on 6~7 cell NiCd batteries. In order to up the cell count (more power) in marine applications, a rather clever fellow by the name of Ed Hughey figured out a way to run two motors on 12 NiCd cells spinning a single prop.

How you ask? Simple: He realized that the electric motors of the time were far more efficient at higher rpms, so he decided to put two motors in a gearbox driving a single shaft and select an appropriate gear ratio to allow the motors to spool up while spinning a larger prop at a lower rpm. This allowed the motors to spin at higher rpms when running (better efficiency) and put twice the power into the propeller. He kicked a lot of FE butts back then with his geardrives.

But then the brushless motor revolution occurred, then the LiPo revolution occurred, then the low Kv outrunner revolution occurred, and the need for the complexity of a geardrive is just a distant memory.

I toyed with the concept using a couple of Speed 300 motors in a "mini" Hughey gearbox decades ago. I've attached a photo of that dual gearbox with a single motor in it (first photo). I still have it in my inventory, so in the future, it might get a couple of those Surpass motors and go into something fun (maybe my 16" cat...)

This mini Hughey gearbox ran remarkably smooth and quiet, but back in those days the selection of gears was limited, so I only had a few ratios to play with. As things went along, I built another geardrive to put into my diminutive H&M Mini Shovel to try it out (second and third photos). The results were excellent, and it too ran like silk. That gearbox ran with great success. I then built one for my MHZ Gecko, but it placed the motor higher in the hull, and spun it the opposite direction which limited the hull's self-righting feature. It currently resides in my "spare motor" parts bin waiting for a new home. With that gearbox/motor combo, the motor would contact the 9/32" tube that contained the bearings. The solution was simple, just cut away some of that tube (fourth photo).

Because I'm doing things no one else has done, and trying to engineer a way to use existing, off-the-shelf equipment, I find myself in the same position Ed Hughey was all those years ago: Finding a motor of the right size and Kv is limited. I may have to consider a way to pick a motor and make it's Kv bend to my will. So I'll be making the Kv of that Surpass motor whatever I want it to be by using a geardrive.


Depending on the gear ratio I finally select, I may have to cut away some of the brass bearing support tube on this one as well, we will have to wait and see.