An Unexpected Success

[Dr. Jet]

.......Adjustable I assume for different gears........

Exactly. I have different motors to try with different Kvs, AND I'll need to find a ratio that works with whatever prop I finally decide to use (probably this one with the tongues cut off: https://www.ebay.com/itm/CNC-Aluminu...t/162228388819).. I'm using gears from Tamiya 1/24 scale RC cars. They are a fine pitch designed for these motors, are very high quality, are super-durable, are available in dozens of tooth counts, and are somewhat reasonably priced.

[Dr. Jet]

What to do while I'm waiting for parts to arrive

The first MPR ran pretty good with the finned 2030 motor (3300Kv), although the bulk of the hull was a big impediment to going fast. The little outrunners (3500Kv) proved to have too much torque and seriously overpowered the hull. They will find a new home somewhere in another of my hare-brained schemes....

I think I will retire Prototype #2 (Homie), and transplant his little inrunner into Prototype #5. I'll also remove the straight CF turn fin and make a new, angled one from aluminum sheet.

In addition to the single-step hydro project, I'm also repairing/updating hulls that needed work before the next time out. While I'm waiting for parts to show up, I've made repairs to the 1:20 Atlas prototype (broke a driveshaft on a loose fishing line ) so it will be ready for additional prop testing the next time out. The 1:20 Short Circuit encountered some "Hangar Rash" and should be repaired within a few days (have to wait for paint to dry). I'll use the Atlas Prototype to find a prop that works well with that motor/battery combo and use the same thing on the Short Circuit for starters. I made some "Quick and dirty" strakes from PVC sheet and glued them to the bottom of the 16" Cat, so that will be ready to test the next time out as well. Stand by to see some posting in that thread too.

[Dr. Jet]

The Disadvantage of Inexpensive Boats.....

They (or the necessary parts to make one) often come on a slow boat from China.....

[Dr. Jet]

Well, the 11,000 Kv motors showed up today and I finished the geardrive. Man, this thing is tiny, but smooth as silk. I'm using little fine-point tweezers, the tiniest Wiha Phillips screwdrivers I have, and a high-power headband magnifier to assemble everything. I need a bit larger driven gear (on another slow boat from China) to get a ratio that makes about 3500 rpm/V. The gear ratio shown in the photos works out to about 3885 rpm/V. I have a larger pinion to go on the lower Kv motor I ordered.

This thing is considerably lighter and lower in profile than the outrunners I tested. I'm hoping the ultra-low CG will really help in cornering. I'm hoping the geardrive with less rotating mass will help in acceleration in that the motor may spool up to an efficient rpm a bit sooner. I'm thinking I could save a whole bunch of weight by going to a carbon fiber driveshaft. Its not like I'm putting 1500 Watts through this thing. I may need to add an intermediate ball bearing in the stuffing tube to keep the shaft from wobbling if I go that way, but that should be easy enough.

[Dr. Jet]

Prototype #5, the geardrive hull, is nearing completion. If you remember, I decided to try the gear option because I was unable to find 2030-sized motors in the right Kv. I think the geardrive will work great and I expect some interesting results, but it's probably just far too much ultra-fine, high-precision work to interest most people. With that in mind, I pulled the "Too Much" 3500Kv outrunner out of Prototype #4, and replaced it with the 3060Kv (I thought it was 3300 all this time ) inrunner from Prototypes #1 (the MPR) and #2 (Homey). Since I now know of a somewhat reliable source for similar motors and couplers; and in keeping with the KISS philosophy, I think this project needs to go in that direction and forget the gears.

I've changed the gear ratio from what it was in the prior photos to something that should give me 3100 rpm/V; or about the same as the inrunner going into #4. I used an acetyl nylon gear and it is incredibly smooth. It will be interesting to compare the two drive systems and how they perform. I'm thinking the geared version will come in significantly lighter and I may eventually opt for something smaller than the 1000mah batteries I'm using in all the other prototypes, since I do expect a lower current draw. The lighter weight (and maybe less torque-induced idiosyncrasies) may offset a lower total wattage. It might make it more flighty too. We shall see.

[Dr. Jet]

Another Thing I Noticed.....

When tearing the outrunner motor out of #4, I noticed the heat-shrink wrapper on the ESC had melted. For most of my minis, I use that cheap 30A ESC that can often be found for under $5 (See: https://www.ebay.com/itm/New-HW30A-B...r/392519751649). The problem is, it's rather large for this hull, so I used its little brother, the 10A ESC in all the single-steppers.

Normal running on the surface was fine for the ESC, and this makes sense as only half the prop is in the water, thereby reducing the overall load on the motor. The problem was when it was swimming around underwater like a fish; for a LONG time. It did this in full "Knife-edge" and with the partial loss of radio, it went to full throttle with an entirely submerged prop. I think that's when it was drawing enough current to melt the shrink-tube. The water in the lake was really clear and I could watch the entire aquabatic demonstration. Fortunately, after everything was done, there wasn't a single drop of water inside.

[Dr. Jet]

All I Need is Paint and Batteries....

.... And I HAVE the paint. Or at least the primer.

I decided to go with a smaller battery which will cut 36 grams off the total weigh as compared to the 1000mah cells in the other hulls. Deduct about the same amount of weight for the much smaller motor and this boat could conceivably be 25% lighter than the other prototypes.

[Dr. Jet]

Watching Paint Dry....

How exciting!

So I'm shooting the white primer on Prototype #5 and I'm wondering what scheme to do on it. I had thought of making a decal of a pancake or a wingtip shoe, but decal paper is expensive and a full sheet decal on a compound curved surface may not work very well. I'm kind of done on the Short Circuit theme for a while, so I may paint it gold and do another Auric Enterprises to match my BBY Mini Scat Cat Twin.

[Dr. Jet]

It's About Time to Clear Things Up....

..... And I'll shoot a couple of coats after the base coats and decals have had a few days to cure.

I may call this one "Goldbrick".

[Dr. Jet]

So I've just shot a single coat of clear over the decals and I think I'm going to call it good there. This is not a beauty contest. I have yet to rout the hatch for the battery clearance as the clear coat is still a bit soft and I want to give it a while to cure before man-handling it.

I decided to weigh #5 (the Goldbrick ) to see how it compares to Prototypes #3 and #4. Both of those came in at about 275 grams or 9.7 ounces. #5 weighed in at 199 grams or 7.8 ounces. That's a 27% reduction in overall weight with maybe a 50% reduction in overall power system wattage from before. This may work! I might take it out to test tomorrow, but the weather is predicted to be cloudy and windy with a high temperature of only about 60 degrees, so I may not go. I'll make the call tomorrow.

[fweasel]

I like the simple paint scheme. The gold is a nice looking color.

[Dr. Jet]

I like the simple paint scheme. The gold is a nice looking color.

You do get the gag about Auric Enterprises, right?

[NativePaul]

Looks more like a finger than a brick to me.

[Dr. Jet]

Looks more like a finger than a brick to me.

Not a gold brick? A gold fin.........

[fweasel]

You do get the gag about Auric Enterprises, right?

Goldfinger, indeed.