An Unexpected Success

[Dr. Jet]

So How Do You Mount a Rudder On a Foam Transom?

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]

Some Progress

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]

The Worst Covering Job.... EVER!

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]

Amazing 12-Step Recovery Program Reduces or Completely Eliminates Wrinkles!

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]

Wouldn't You Know it.......

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]

MPRs Coming Out Of My Ears

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]

It's Homey!

Homie don' play dat!

[Dr. Jet]

All Joking Aside.......

..... 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]

Did I Get Shafted?!

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 Driving Force

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.