An Unexpected Success

[Dr. Jet]

I went to the pond today, and took the usual suspects; the yet to run prototypes and some of the tried and true with new modifications. I was about to back the truck out of the driveway and thought: Maybe I should take THAT one as well. After all, I put a new motor and propeller in it since it's earlier failures a year or two ago, why not?

What I'm talking about is a 10-year old scaled-down version of Jay Turner's "Pool Racer".

Why did I build one? It was cheap, simple to build, allegedly fast, and I had the parts on hand. It's nothing more than a couple of blocks of foam and a motor with a straight shaft. Pretty easy in my book.

That's where the problems started. Jay designed this back in the days of brushed car motors and NiCad batteries. Pretty beefy stuff. So a decade ago, I built a micro (now mini) version using the power systems we had developed for the mini hydros. This consisted of a 3000 Kv outrunner (26mm diameter), a 2S LiPo and an Octura X430. The problem was the power to weight ratio had changed considerably. I put it in the water, hit the throttle, and got an instant imitation of a Saturn V rocket launch. There was no horizontal movement, only vertical. Scared the cr@p out of me.

So over the years. I toyed with motors. Brushed motors, brushless motors, but I could never find a motor/prop combination to do it justice. Motor couplers were an issue too. Then a year or two ago, I found this motor: https://hobbyking.com/en_us/b20-30-3...or-3060kv.html. Cheap enough I thought, and about the same Kv as we were running on the other hydros, so I bought one. I had some problems mounting it, but nothing that couldn't be overcome. But what about a propeller? The small motor probably couldn't spin an X427 (the smallest decent propeller of the time). By then I had a better lathe, so I chucked up a driveshaft, stuck an aluminum X427 on it and turned it down to 24mm. While I was at it, I cut the tongues off as well. This gave me effectively an X624 prop.

So I stuck this thing in the water and it actually worked quite well. Jay's design has rather tall sides to accommodate the car motor it was designed around and I copied his proportions exactly. As a result, there's a lot of empty space in mine with proportionately large side and frontal areas. Going upwind it ran on just the step and the prop. Going downwind, it skipped and flew like a perfect throwing stone.

So now what? I think I'll get some more blue foam and styrene sheet and build a more streamlined and low-profile "Pancake" hull and move the drive train from this one to a new one. It was fast (sort of), it was cheap (REALLY) and it was (almost) out of control (I did flip it once ). A revised one will have a lower CG.

[Dr. Jet]

So I started doing some sketches of how I would revise Jay's design for a lower-profile motor and an 850~1200mah battery. Lo and behold, I came up with a low-profile design that uses 1" foam and the same motor/prop/shaft combination. The plan view is essentially the same except I rounded the nose enough to make it just under 350mm. The profile view resembles a Frisbee with a flat stern. I think I will make 3 templates from 1/4" MDF (the upper hull, the lower hull, and the hatch) and use my table-mounted router to cut the foam to shape. That way, I can make as many as I want. The rest is simple, some 1/32" styrene sheet, and I'll cut some motor mount parts from 1/16" ply in the "extra" space on a sheet from my rigger project.

This arrangement will allow me to get the motor lower in the hull, reduce the driveshaft angle (it will exit at the top of the step), and greatly reduce the profile. It's going to look like a little flying saucer zipping around the pond.

UPDATE: I have some 1" pink foam; I have some MDF to make templates; I have stuff to make a 1/32 wire drive; I have motor couplers; I have motor mounts. When I finish all the other projects, I'll start this one. The motor/coupler/receiver/ESC/propeller will all go into the new one. I may leave the old hull/driveshaft/motor mount/servo/rudder intact for future experimentation, but it will definitely be relegated to the "Retired" pile.

[Dr. Jet]

First glimpse of the Low-Profile Mini Pool Racer:

[Dr. Jet]

So, I've been massaging the drawings and parts list on this one. Based on the stuff I already have on hand, it looks like I can build one for under $20 in new parts. If I were to buy all new parts to build one from the ground up, it would be well under $100 and probably closer to $75. That's for a motor, ESC, driveline, rudder, servo, hull material and such. Not including Rx or battery, but with a 1Ah LiPo, the battery can be had for under $10.

Now that's what I call CHEAP! It's maybe 20~25mph, but for it's diminutive size it's smoking! You could probably run it in a large fountain or a moderate-sized a swimming pool. It would probably get lost in an Olympic-sized pool.

I decided to put a fin on the hatch so there would be something to grip in order to remove it....

[Dr. Jet]

Jay's original plans had the option of non-trip chines on the forward and aft sections of the hull (see photo). I've done some preliminary parts layout on the 2'x2' pink foam that Home Depot sells. Using my CNC cut MDF templates, it looks like I can make 5 hulls out of two sheets of foam. That works out to about $1.30 in materials per hull . Granted, it doesn't take the cost of the tooling into account, but that cost gets amortized over the number of hulls I make. There was room on the 1/16" ply on my Mini Rigger project to add three motor mounts for this project, and the styrene sheets I ordered came in a pack of two, so I think I'll initially build two of them for experimentation.

I'll build one with non-trip chines and one without. I'll do a 1/32 wire drive (straight shaft) in one, but it requires making my own tapered drive dog. Easy enough using the machine tools in my shop, but very time-consuming. I may try this in the other: https://www.ebay.com/itm/RC-Boat-Mod...fpPcCP6bG2v8Cw. $9.09 for a ball-bearing shaft, stuffing tube and motor coupler is a great deal. It may be necessary to reduce the length a little bit, but that's easy enough with simple hand tools. I doubt the propeller will be usable though. The best (cheap) propeller choice might be an Octura X431 de-tongued and reduced in diameter to between 24 and 26mm. These props can be found on the OSE site for $1.65. Cheap enough to buy them by the dozen.

I entitled this post “Too Much Fun” as that’s what these little things are. Now that I’ve found a reasonable power system, these are just the ticket for a large fountain, swimming pool or small pond. You could conceivably make a challenging race course in an area that is 30’ x 40’, although longer would be better. Handling on the admittedly top-heavy prototype was a bit sketchy in the turns and required careful throttle management. I think the changes I have incorporated in the revised design will help that a lot.

I think I may build a bunch of these since they’re so cheap and easy to build. Then, I’ll take several to the pond with me and let some of the kids (and adults) I always seem to attract drive them…

[Dr. Jet]

I had a few free moments today so I decided to build the 1/32" wire drive for the first MPR. The second MPR will get the off-the-shelf 2mm ball-bearing shaft I mentioned in the previous post. I'm using my preferred 1/32" wire drive inside a straight 3/32" brass stuffing tube. It is supported at both ends by some 1/16" stainless steel tubing. I made a streamlined aluminum drive dog and a 1/8" brass stubshaft to hold the prop. The 1/16" SS tube goes all the way through to the end of the stubshaft. The entire assembly runs as smooth and nearly as friction-free as a ball-bearing unit, probably due to the minimal bearing area. I made sure I left enough (3/4") at the propeller end to resist radial loads. The 1/16" SS tube at the motor end will get trimmed to the motor shaft length once everything is glued together. The drawings are far more accurate than my fingers can duplicate....

Sorry for the blurry "Fully Assembled" shot, but you get the idea.

[Dr. Jet]

I finally have templates to cut MPR hulls. Photos to follow. Tomorrow, I will create the tooling to cut the lower forward hull section to the correct 2° angle. Then, I'll cut enough foam parts to make 5 hulls.

This is going to be fun.

[Dr. Jet]

So here are the router templates for the MPR. Note they have spikes to hold the foam in place as it's routed out. There's a lower hull, the larger upper hull and the hatch. The hatch has threaded holes in it (hardened with CA) so I can attach a block of wood to it while the foam is still in place. Then I'll clamp it in the mill and mill a 1/32" slot in if for the fin/handle.

[Dr. Jet]

Here's the fixture for cutting the lower hull bottom to the right angle and dimensions. 2 phenolic resin templates for hot-wire cutting. The foam lower hull piece slips over the MDF template for accurate indexing and cutting every time. I did the original the hard way: Mark it and give it a ride on the belt sander.

[Dr. Jet]

Got some hulls cut this afternoon.......

I don't have any fresh Ni-chrome wire at the moment so it will be Friday before I can cut the lower hull angle.

[Dr. Jet]

..... and I cut the lower hull pieces. The cutting fixture I made worked perfectly. As long as I had my light table (a.k.a. Uber-flat surface) out for the Mini Rigger project, I thought I'd glue up some MPR hulls as well. I'm using expanding polyurethane glue as I think it's just the ticket for foam. Here's the first of two I plan to do.

[Dr. Jet]

And stopped to take a break. There's sill about 1/8" ~ 1/4" to remove, but I'll finish that up after the bottoms are glued on.

It's pretty obvious that the new profile will be affected far less by the wind and should also be far less prone to flipping.

[Dr. Jet]

The Good: The Chinese driveshaft showed up.

The Bad: The housing is stainless steel.

Well, it's not really THAT bad , but unless you have the right tools, it's problematic to shorten it to the correct length, plus it's heavier than necessary for this application. Fortunately, K&S makes an 8mm aluminum tube that could be used to make a replacement. An additional issue is the drive dog: The "tangs" on it are a bit bigger than the slots in an Octura prop. One could either just replace it with an Octura drive dog, file wider slots in the propeller, or machine (file) the tangs down to Octura specs.

When considering all the additional futzing around this commercially available unit involves, it might be just as easy to go with the 1/32" wire drive described above and skip the streamlined drive dog, or maybe just file the leading edge of the Octura drive dog to a more streamlined shape.

[Dr. Jet]

Here's some photos of the first MPR roughed-out.

Whadda ya think?

[Dr. Jet]

In order for this model to turn, the CG (Balance Point) needs to be fairly far behind the step. Here's my preliminary equipment layout to get that CG.

Wow, the battery JUST fits in the very aft of the hull.... It's almost as if someone designed the hatch to fit the battery. Oh, wait a minute! Someone DID...

The hatch will get notched to hold the battery in place without the usual Velcro. The battery will slip under the rudder pushrod wire and the hatch will be notched for that as well.

[Dr. Jet]

I gave this some thought and came up with this idea: A small block of wood, drilled and tapped for the rudder mounting screws gets glued into a square hole in the transom. The result? Rock-solid! The other prototype I'm building has a 1/32" styrene transom glued to the foam. On that one, I'll add an inset plywood doubler on the inside, then drill and tap the mounting holes. Thus far, I think this is the easiest method, but we shall see.

[Dr. Jet]

I installed the motor, driveline and and servo. I angled the servo a bit to provide access to the setscrew that locks the pushrod in place on the arm. Once the driveline was installed, I spun the prop and noticed the motor moving around on the 1/32" styrene sheet upon which it is mounted. I chucked the motor/coupler up in the lathe and discovered the shaft is bent. It had about 0.015" runout. I tweaked it as best as I could but there's still a few thousandths of wobble in it. I ordered a new motor....

I used the philosophy developed in the rigger for the rudder pushrod (sorry for the short depth of field in the photo).

[Dr. Jet]

On the first Pool Racer I built, I applied EconoKote low-temperature film. There were some wrinkles here and there, but not too bad. I also had some issues with the film not sticking along the edges. So I thought I would use some sanding sealer on this one to give a better edge for the film to stick to. The problem was, in so doing, there was no place for the air to escape as I was applying the film. Trying to iron it down, just heated the air under the film making the bubble bigger; or if I did get it to stick, it was just a bunch of wrinkles. Trying to heat up the iron to help the film shrink just melted the foam.

Oh well, lesson learned. I have a bunch of foam hull pieces. I'll give this one a test-run, then replace the hull.

[Dr. Jet]

Just apply this special cream for a week.....

No really, I got to thinking that I couldn't show up in public with the covering job I did, even if it was just for testing. One of the contributing factors for my crappy covering job is I tried to do too much with a single sheet to eliminate seams. I decided to re-cover it it in a number of smaller pieces; but what about all the seams? Hey! If Eddy can do it to a guitar, I can do it to the MPR. Yeah, it looks sort of cartoon-ish, but it's totally fitting. The covering on the hatch wasn't too bad, so I left it as is.

Behold, the results of the recovery:

[Dr. Jet]

I finally found a motor/prop combo that works well in the MPR and now that motor is discontinued. At least I have another on order. The MPR was so easy to overpower, I wonder what readily available motor will work in it. I really need something around 75 watts maximum.

Now, I'll have to do more experimenting........ AGAIN....

[Dr. Jet]

I may have found another motor that will work. See: https://www.ebay.com/itm/Exceed-RC-H...frcectupt=true It's a bit higher in Kv, so there may be more prop experimentation. I may use this prop, de-tongued and reduced in diameter to 24mm. It will have less pitch (about 1.5) than the modded X427 (about 1.6) I'm currently running. https://www.ebay.com/itm/CNC-Aluminu...4AAOSwd0BVqlAX

The big issue with this choice is the motor shaft diameter. It's 2.5mm. I have a 2.5mm reamer and I can make/modify couplers to fit it, but what about someone else that may want to build one?

[Dr. Jet]

Another motor possibility may be this one: https://hobbyking.com/en_us/turnigy-...r-motor-1.html I'd have to modify the motor mount location, but everything else should work as previously planned.

[Dr. Jet]

I put in a low-ball offer for a bunch of those little outrunners listed on eBay but I got a counteroffer that I did not consider acceptable. I was thinking of getting some precision-ground 0.1247" stainless steel shafts, some 2mm x 3.17mm couplers, and a bunch of these motors. I would modify the couplers to fit the 2.5mm shafts and make them available to anyone that wanted to build their own MPR. I have the hull drawings available as *.pdf files, so anyone could cut their own hulls.

[Dr. Jet]

In the second MPR, I modified the servo mount to lower it (the arm conflicted with the profile outline) and I'm going to use a smaller motor mount to reduce the amount of "hogging out" the hatch will require.

The finned inrunner motor I used in the original is no longer available, so I'm going to try this little outrunner: https://www.ebay.com/itm/Exceed-RC-H...QAAOSw44BYVJxj

[Dr. Jet]

Lots of room for experimentation (see first photo). I may have the motor/coupler/driveshaft issues solved. I'm looking for a solution that anybody can do and it has to be CHEAP!

The "Clown" MPR uses a 1/32" wire straight drive. It's a bit of a PITA to build and requires specialized machine tools. I have a line on uber-precise 0.1247" stainless steel shafts which should be just the ticket and about $7.50 each if I buy a sufficient quantity. A 5/32" brass tube will make a perfect stuffing tube. I found aluminum couplers on eBay for under a couple of bucks each that can be reamed out to fit the 2.5mm shaft of the cheap outrunner I am going to test out. The reaming requires specialized machine tools but the setup time is minimal and I can make as many as necessary in 30 to 45 seconds each after the setup is done.

My goal is to design something that can be built for under $100 using all-new equipment. If you're like me, you already have a bunch of stuff on the shelf that could cut build costs in half.

Send me a note if you want full-sized *.pdf files of the foam and 1/32" styrene parts.

When I was originally building the first MPR, I was going to put a decal on it that made people think of pancakes, because the hull was rather flat like a pancake. I thought the Patron Saint of Pancakes would be fitting (see second photo). Now that it has a more clowin-ish appearance, I may put a different decal on it (see third photo).

[Dr. Jet]

Homie don' play dat!

[Dr. Jet]

..... The "inexpensive" little outrunner motor showed up today. I had to modify the motor mount drawings slightly to get it to work, but it was a piece of cake. I have couplers on order, but the cheap ones take a while to ship from China. So for now, there's not a whole lot more I can do on MPR #2 until parts arrive on that slow boat from China.

Looking at all my incomplete projects, I think the next one that should get my undivided attention is the shovelnose. I'll update that thread in a moment.

[Dr. Jet]

While some epoxy is cooking off on the mini shovelnose, I decided to pull the trigger on the most expensive part for this project: The propeller shaft. The JTPR I built "back when" used an off-the-shelf 1/8" shaft that had one end threaded to 5-40 to fit the commonly available prop nuts. I believe this was from an old DPI kit (a cracker box or maybe the American Dream Hydro). There were a number of models out there at that time that used that style of shaft. A decade or two later and the Chinese 3mm and 4mm shafts are all that you can find now in the way of straight shafts. DPI is long gone, no one builds little crackerboxes now, and 1/8" shafts are a distant memory of the past. Personally, I liked them for straight-shaft applications. Add a 5/32" brass tube and you were done.

In my ongoing quest for a simple shafting solution that was SMALL and relatively lightweight, I located some uber-precision 303 stainless steel shafting. Not drill rod mind you; it was dedicated shafting material that was 0.003" smaller than 1/8". Just the ticket. The cool thing about these shafts (besides the 0.1247" +0.000/-0.0001 diameter, 8 microinch finish, and 0.0002 in/in straightness ) are they're in an unhardened condition. That means I can cut 5-40 threads on them with the existing equipment I have in my shop!

With shipping, the shafts cost about $13 each. Granted, that's nothing if you're building a 60" twin 12S cat, but my goal with the MPR is CHEAP FUN. At $13, the shaft will be the most expensive item on the boat. I still think the average Joe could have one ready to go for $50 to $75, depending on the spare parts he has in his shop.

My plan is to run an Octura X431P (plastic) prop, reduced in diameter and de-tongued. Steve sells these here for $1.65. Cheap enough to buy a dozen.

At around 75 Watts, this hull is on the verge of being overpowered. The prop will have to be cut down to stay below 75 Watts (10 amps). You could use an aluminum X427 as a start (like I have on "Homey"), but those are more expensive.

[Dr. Jet]

That is to say: The dive shafts showed up today. Were they worth $13 each, or did I get the shaft?

In a word.........YES! Er,........ that is YES, they are worth it.

They are smooth, they are shiny, they are stainless steel so they will REMAIN smooth and shiny, they are straight as an arrow, they are a perfect slip-fit in a 5/32" tube, they machine well (cutting threads), they fit the prop and drive dog perfectly. Yeah, these might be the most expensive part on the whole boat, but overall, I'd say they're worth it. I expect a super-smooth running driveline.

Waiting for a 2.5mm reamer to show up so I can ream the coupler to fit the motor and finish up the driveline before I install it in the hull.

[Dr. Jet]

The first JTPR used a 3060Kv in-runner motor with an Octura X427 that was de-tongued and reduced in diameter to 24mm. That makes essentially an X624 prop (24mm diameter x 1.6 pitch). That motor/prop combo are now in MPR #1 "Homey". Since I am now using an out-runner motor with a slightly higher Kv (3500 rpm/V) in MPR #2, my first prop choice will be one of the Chinese CNC aluminum props (See: https://www.ebay.com/itm/CNC-Aluminu...4AAOSwd0BVqlAX). Stock, these are 26mm diameter with 1.4 pitch. I have de-tongued it and reduced it to 25mm. That brings the pitch ratio to about 1.45. I'm guessing slightly lower pitch will offset the slightly higher Kv, and the slightly larger diameter will be offset by the slightly higher Kt. If everything works as hoped, it should perform in the same manner as the original in-runner motor.

The mounting holes in the out-runner are different than the prior motor, so I'll have to make a different motor mount.