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Thread: An Unexpected Success

  1. #1
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    Default An Unexpected Success

    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.
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    Last edited by Dr. Jet; 03-31-2019 at 09:35 AM. Reason: Typos
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  2. #2
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    Default There He Goes Again.........

    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.
    Last edited by Dr. Jet; 04-02-2019 at 03:09 PM.
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    Default Preliminary Sketch

    First glimpse of the Low-Profile Mini Pool Racer:
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    Last edited by Dr. Jet; 04-03-2019 at 10:48 AM.
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    Default Some More Doodling

    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....
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    Default Too Much Fun

    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…
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    Last edited by Dr. Jet; 04-04-2019 at 10:07 PM.
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    Default 3/32" Stuffing Tube

    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.
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    Default

    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.
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    Default MDF Templates

    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.
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    Last edited by Dr. Jet; 04-09-2019 at 05:15 PM.
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    Default Angling the Step.

    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.
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  10. #10
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    Default A Stack of Hulls

    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.
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    Default I Got Some Ni-Chrome Wire Today....

    ..... 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.
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    Default I Did Some Sanding

    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.
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    Default Good News, Bad News

    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.
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    Default Cool-Looking or Dorky-Looking; You Decide!

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

    Whadda ya think?
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    Default Equipment Layout for CG

    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.
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    Default 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.
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    Default 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).
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  18. #18
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    Default 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.
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  19. #19
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    Default 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:
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    Last edited by Dr. Jet; Yesterday at 12:26 PM. Reason: Typos
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  20. #20
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    Default 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....
    A nation of sheep breeds a government of wolves

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