An Unexpected Success

[NativePaul]

I tried a bunch of X428Ps, X430As X427s cut to different shapes and sizes and had good luck with many, my best was probably a thin X427 at near stock diameter, but with extreme tongue and back cutting to reduce the load. Even better than that though was an almost stock Graupner 2318.26. That ws all before CNC props were available, but the Graupners have been proven more efficient than metal props, even when the blade shapes are copies of the Graupner's.
I know the M2 thread is a bit of a PITA, but I would recommend you check one out.

[Dr. Jet]

I actually run a bunch of Graupner M4 props on several of my minis with great success (See photos). My main reasons for going with Octura-style 1/8" drive dog props on the MPR is availability and diversity. There was a time when the S-series Graupner props (1.6P) were unavailable. I bought a bunch of them at the time, but now with the advent of the CNC Chinese props the selection of M4 props is much better.

[NativePaul]

The 26mm is M2, not M4, pretty unusual.

[Dr. Jet]

The 26mm is M2, not M4, pretty unusual.

Hmmm...... Nothing like that in my parts bin. I did a web search and found these: https://www.graupner.com/Carbon-Hydr...ries/2318.26L/ and http://www.sgmodell.hu/hajocsavarok-...m-231826s.html

The latter seems to be an "S" Series prop (1.6P) with M2 threading. It would probably work, but it's definitely a PITA to source.

[NativePaul]

It is the first one. God knows what is with that second advert, it has the right part number but for the S on the end and Ss should be 2326 series not 2318 and it says karbon, but the photo is of a plain plastic.

Steve has 6 in stock, though erroneously in his 4mm plastic section https://www.offshoreelectrics.com/pr...od=grp-2318.26

[Dr. Jet]

It is the first one. God knows what is with that second advert, it has the right part number but for the S on the end and Ss should be 2326 series not 2318 and it says karbon, but the photo is of a plain plastic.

Steve has 6 in stock, though erroneously in his 4mm plastic section https://www.offshoreelectrics.com/pr...od=grp-2318.26

My head is exploding!!

This is why I decided to go with 1/8" drive dog props........

[Dr. Jet]

I Have Some Extras

I have a spare sheet of 1/16" ply in my inventory, so I've decided to go ahead and have a bunch of motor/servo mounts laser-cut so I can finish the second prototype MPR with the "new" motor choice. I have enough pre-cut foam (already glued together), motors, motor couplers (precision reamed to work with the supplied motor), and super-precise stainless steel propeller shafts to go along with the motor/servo mounts to make 3 "Short" kits. You would have to supply the 1/32" styrene sheet, the rudder, the 9 gram servo, the (10 Amp minimum) ESC, the 5/32" stuffing tube, the drive dog and prop, plus the turn fin in order to finish it. I'll include a CD with full-sized drawings and instructions (*.pdf files).

Anyone want one? $40 plus shipping.

[Dr. Jet]

Did Someone Say Extras??

It looks like I have LOTS of extras. In this case, it's servo and motor mounts. The laser guy has a minimum charge, and I had a 12"x24" sheet of plywood........

The mount fits the motor perfectly and the angles result in the perfect driveshaft location and angle. It's almost as if it were computer-designed with accuracy unattainable by human hands. Oh, wait a minute........

[Dr. Jet]

MPR #2 Taking Shape

So I'm sanding MPR #2 to shape and some ideas cross my mind... This thing looks like ......... Maybe I should have a vinyl wrap made of photo #1....

I've decided not to put non-trip chines on it at this moment. I can always come back later and add them. Jay's original plan had them optional, although I wonder if not having non-trips would cause the model to "trip" in a turn. Jay is one of the Mods here, maybe he could share some insight and his experiences.

[Dr. Jet]

A Little Support

I did something different for the prop shaft support. The shaft is actually quite solid without it due to the way it exits the hull, but this adds a little extra bit of support in the side-to-side direction. I tacked it into place with thin CA then came back later with low-expansion clear Gorilla Glue.

I'm just waiting for the rudder to show up to finish it. My plan is to use a styrene doubler on the transom, then drill and tap the styrene for the rudder. I used this method on the original JTPR.

[Dr. Jet]

Ready to Wrap

And it will definitely be a bit more conservative than "Homey".

[Dr. Jet]

I've been away from the boat building for a while. That always happens every summer though: The flying gets good and the boats go to the back of the workbench. Now that the flying season is over, I've finished up Prototype #3 and am progressing on Prototypes #4 and #5.

I didn't like the way the Home Depot pink foam took to the iron-on covering and I tried a lot of different brands and techniques with equally uninspiring results. I decided to fall back on something I used to do on sheeted foam airplane wings: Water-based sanding sealer and Silkspan. Prototype #3 was the perfect platform for testing this technique.

I did a "Quick-and-dirty" paint job, just to see if it would work. Future versions will have a bit more attention to detail.

After completing prototype #3, I got to thinking about Prototypes #4 and #5. #3 balanced right at the forward end of the limits. It will take weight to move the CG back if necessary. I decided to see if I could remove more material forward of the CG in future models.

As a result, the hull has become so thin that a "bubble canopy" on the hatch is required to keep the outrunner motor inside of the hull. No problem, the Silkspan and sanding sealer technique work great with such features.

Motor choices have always been an issue with this project as I want to create a "spec class" where everybody has the same (or at least similar) power systems. This encourages innovation in hull and equipment design.

I have roughed-out Prototypes #4 and #5. Both of these prototypes will use a readily-available 1:24-scale car motor rated at 11,000Kv and a gear drive unit which will all fit below the revised hull profile and therefor will not require the aforementioned bubble in the hatch to clear the motor.

One of the big issues I initially encountered with Prototype 1 (the scaled-down Pool Racer) was cornering. It would roll over like a well-trained dog at the first provocation. All subsequent prototypes have 100% of their mass well below the CG of Prototype I. This has to help with cornering.

Prototypes #1 and #2 incorporated non-trip chines in the build. Prototype #3 has no such non-trip chines. Testing will determine whether or not these chines are necessary for good turning performance.

The Ultra-low profile Prototype #4 incorporates a full symmetrical hull bottom design with forward and aft non-trips. Prototype #5 incorporates an asymmetrical design optimized for right turns only.

Initially, I called this hull the "Micro Pool Racer" in deference to Jay Turser, who originally designed the "Big" and "Old" (very old) version. All I did at that time was to shrink it down to a different size. Now that I've made so many revisions to it, it doesn't look anything like the original, although the ride surfaces are still somewhat similar. I think it is OK to say that this new design is "inspired" by Jay's design rather than "based" on Jay's design. Stay tuned for a new name.

Stay tuned for more mini single-step hydro fun.

By the way, I'm going to try to make these work on a 10-Meter (between 2 buoys) course. It's going to be all about turning performance and not so much about top speed...

[Dr. Jet]

While I'm waiting for motors and gears to show up, I've roughed out the uber-low profile prototypes 4 and 5. Both of these will incorporate this motor: https://www.ebay.com/itm/XTeam-Inrun...sAAOSwrFlcxcib I plan to gear it initialy at around 3.3:1 which should provide the equivalent of a 3,300Kv motor. This geared arrangement eliminates the need for a shaft coupler and the motor mount becomes the gear drive assembly. I simply flip the motor to the aft side of the motor mount with the gears on the shaft and the motor on the forward face of the motor mount.

Thrust is now a concern with a gear drive, so I'll build #4 with the 1/8" shaft in a 5/32" brass tube and some Octura 1/8" thrust washers between the drive dog and the shaft. I'm not real certain on the current availability of these thrust washers (I have some very old ones on hand), so #5 will use a 9/32" stuffing tube with flanged ball bearings at both ends. These will take the very light thrust forces with ease.

[Dr. Jet]

The geared arrangement solves several problems: Finding an appropriate motor with the right Kv and enough torque to spin a commonly-available prop; it allows me to get the weight REALLY LOW in the hull; and it eliminates the need for a coupler.

The disadvantages are increased complexity and cost. Neither of these are bad enough to outweigh the advantages of gearing, so It will be propulsion method for the next two. Here's a sketch of the plan:

[NativePaul]

I run a flanged ball race on all my wire drives to take the thrust, it works well on everything from my mild minis to my wild weasels.

I put a bevel on my drive dogs to ensure that it is the inner not the outer portion of the race that makes contact.