The Evil Doctor's Next Project

[NativePaul]

My Antenna is up.

[Dr. Jet]

My Antenna is up.

I will have the cut wood this afternoon.

Water-jet cut turn fin blanks.

[Dr. Jet]

I'll be picking up the wood for this tomorrow morning. I need to box up the wood for the 1:20 roundnose kits, and I should be able to start framing up a prototype of this in the afternoon. If it all goes together OK and the hull isn't too flimsy (one of my concerns). I'll get in touch with NativePaul and make arrangements to send him Prototype #2 and see how he does with it.

I bought one of these motors to put in my version. https://hobbyking.com/en_us/sp-02435...rs-4800kv.html The Kv is right for 2S, the mass is right, and the price is..... well.......CHEAP! How can I lose? Remember, I'm not racing in some sanctioned event, just scaring the waterfowl and wowing the locals...

[Dr. Jet]

Well, I got the wood and threw one tub together quick and dirty with CA to see if it would be rigid enough. The result? Yes, it will work.. Now, I'm going to build one in the proper manner. There were a couple of dimensional busts which will require elongating one hole in the sides for the bulkhead tab and moving one notch on the hull bottom. The drawings have been revised accordingly. I'll photograph the next one as the construction will look a bit better and I can use them in a future instruction manual.

[Dr. Jet]

Laminating the sides. Photos to follow.

[Dr. Jet]

One Side Laminated

I paint the ply and balsa doubler with epoxy, then brush so much of it off that most people would think there's not enough there to get the pieces to stick together. I index the pieces with the bulkhead tabs to get them in exactly the right position on my flat glass light table that is protected with wax paper. Next, I use my uber-flat and uber-square Delrin blocks between the bulkheads and weight them down with cans full of lead shot. Lastly, I remove the bulkheads before the epoxy goes off as they will get installed later.

[Dr. Jet]

This is how much epoxy I apply to the side and doubler. Wet out the side to be glued, then scrape off any excess with a credit card until it's just a microscopic film.

[Dr. Jet]

Next....

Once the epoxy cooks off, I fit the sides together and sand them flush and square. This helps to keep the tub square.

[Dr. Jet]

Assembling the Tub and Installing Metal Bits.

Using a square, I install the bulkhead/formers into one side and tack into place with thin CA. Then I install the other side onto the bulkhead/formers. Next, the transom and nose-pieces are installed. Finally, the 1/64 ply bottom gets tacked into place.

Unfortunately, I made the holes for the rudder pushrod too big, so I shimmed them with small pieces of aluminum tube. The stuffing tube is glued into place, but the stinger won't get glued in until later.

[Dr. Jet]

Instaling Motor Driveshaft, Rudder and Servo

When I installed the motor, it was a bit flimsy feeling. I added a couple of braces to stiffen it up.

The driveshaft is my typical 1/32" drive. The coupler I decided to use was an insanely expensive 2mm x 2mm flex coupler I modified to fit 1/16" on one side.

I used the same servo mount as I used in other boats. It was a little big, but nothing the trusty old belt sander couldn't fix. I thought I would be able to reach the servo mounting cap screws with a ball-end Allen wrench, but it was just too great of an angle. Furthermore, when the top is glued on, I won't be able to reach the front screw with a standard Allen wrench, so I'll have to put some access holes in the side that will get covered with tape. That will make it easy-peasy to access the servo.

[Dr. Jet]

A Bit of Re-Engineering

I wasn't really thrilled with the rudder pushrod design. It had too many bends in it which created drag (friction) in the bearing tube, and the short length past the transom limited travel unless I used a more powerful servo to overcome the bending forces on the 1/32" wire. Remember, the wire must bend to compensate for the circular displacement of the servo and rudder arms. Linear servos wouldn't have such issues. Furthermore, all these bends in the wire made it difficult to remove and install.

The solution? Simple and elegant: I flipped the servo connector to the other side of the arm, putting the arc of the pushrod hole in the same plane as the bearing tube. I lengthened the 1/32" wire pushrod beyond the transom by going around the connector on the rudder. With a longer wire, the force required to bend it the same amount is reduced as it moves through the arc of the rudder arm.

[NativePaul]

That servo pushrod looks much better now. What weight is the servo? I usually use a 14-16g in minis and the one in the photo looks like it might be a bit smaller, is that rudder the 55mm Chinese one? i have tried one in the past and found it to not quite be enough and switched to the Chinese 62mm, version or a combo of the 55mm bracket modded for the 62mm blade holder, all boats are different though let me know how it turns with it, as the 55mm is cheaper and much more readily available.

Have you used one of those flex couplers with a wire drive before? I'd be worried that the wire cant support the back end of the coupler like a solid shaft does, so any tiny imbalance in it, or whip in the wire will throw the back of the coupler off center, then the mass spinning of center it will throw it further off center, in a positive feedback loop until it breaks.

[Dr. Jet]

....... What weight is the servo?

That's a 9 gram metal geared servo.

.......... is that rudder the 55mm Chinese one?

Yes.

I have tried one in the past and found it to not quite be enough and switched to the Chinese 62mm, version or a combo of the 55mm bracket modded for the 62mm blade holder, all boats are different though let me know how it turns with it, as the 55mm is cheaper and much more readily available.

If it's not enough, I will follow that advice.

Have you used one of those flex couplers with a wire drive before? I'd be worried that the wire cant support the back end of the coupler like a solid shaft does, so any tiny imbalance in it, or whip in the wire will throw the back of the coupler off center, then the mass spinning of center it will throw it further off center, in a positive feedback loop until it breaks.

No, I haven't used one of those 40,000 rpm-rated couplers yet. Remember, the coupler is not connecting to the wire, it is connecting to a 1/16" precision stainless steel tube that is fully supported in the 3/32" brass stuffing tube so it essentially behaves like a straight shaft; until it reaches the end of the stuffing tube where the 1/16" SS tube also ends and the 1/32" wire continues to the stinger. If any "whip" were to occur, I would expect it here. This is very similar to the arrangement I used in the mini hydros and the BBY Micro Scat Cat; all of which have been extremely smooth and flawless in operation. I'm a big fan of the 1/32" wire drives in these low-power (+200 Watt) applications.

[NativePaul]

That is clever, I hadn't twigged that it is a combination of a straight shaft and a wire drive.

I bought a 9g servo today so I'm ready.

[Dr. Jet]

Exactly! A straight shaft that transitions to a wire drive.

I don't want to send you anything until I've tested it out though. I'm currently gluing up MPR hulls right now; I'll start laminating the sponsons next week.