12" scratch built cat

[Dr. Jet]

While I'm working on this and several other concurrent projects, I thought I'd share some thinking and future plans. Obviously, the laser-cutting is aimed at the "kit" market. What is the market for a kit-built, all-wood Mini Fast Electric catamaran? I can count interested parties on one hand and have fingers left over. Still, I like the idea of saying "I designed that"...... Yeah, the basic idea was provided by someone else; but the fine-tuning and laser-precision was my doing.

I chose a power system for Prototype #1 that had a fairly high Kv (and thereby a fairly low Kt) for the first iteration. The Aveox motor I selected likes a small, cut-down X427 prop. A bigger outrunner of somewhere around 3800~4000 rpm/V with a bigger prop might be a better choice for this hull. Testing will provide the answers.

I've framed-up a prototype #2 for twin power, but it is currently at the bottom of my stack of things to complete. Prototype #2 used the original laser-cutting, but I made some modifications for the ride surfaces and the gunwales at the first two bulkheads. I think this will make for a better finished product.

[Jesse J]

So I have been building and tweaking this design Shockerman was so generous to give me for almost 10 years.
The plans I sent you, free of charge, are my v4 and the biggest revision so far.
I’d say this is a little more than “the basic idea”...

[Dr. Jet]

I’d say this is a little more than “the basic idea”...

Please don't take offense at that remark. The plans you sent were made for print and paste onto wood and cut with a band saw style scratch building. Your plans were MORE than sufficient for a scratch builder using that technique. What I'm talking about is translating a raster file into a vector file for laser-cutting and all the associated accuracy required for precise-fitting parts. For example, I take different cutting kerfs into account when using different materials. For 1/16" ply I allow an extra 0.002" on all the parts. The lines on any usable scratch-building drawing are going to be more than 0.002"

How I did this was to import your drawings into AutoCAD as a photo (raster) file. Then I traced each part to get a basic outline and then removed the photo file. That's where the REAL work begins and why I referred to your drawings as the "Basic Idea". I had to compare all the parts and make adjustments so everything would be square, parallel, and/or symmetric. Then I added tabs and slots for alignment purposes, motor mounts, servo mounts, hatch framing, hatch lips, a chine strip, options for single or twin power and so on. The drawings you provided were a good starting point, but they were just that: A starting point. Only in building prototype #1 did I notice a bust in the angles of the first two bulkheads that made a less than desirable break in the sheeting. While prototype #1 is completely usable as-is, I strive for perfection. I've gone back and corrected this, but I'm going to frame up a second prototype (from the original laser-cutting) to confirm my calculations. I must add that I am doing this in 2-dimensional space, not 3-dimensional. Had I done this in 3-D the glitch in the first two bulkheads would have been more apparent.

[Dr. Jet]

The bulkhead I didn't like and the fix. I attached the chine strip and let it go where it naturally wanted to go at the first two bulkheads. I'll build up the bulkheads with scrap material before sanding and sheeting.

All future laser-cutting will be correct after the first three prototypes are built.

[Dr. Jet]

Fixed, sanded, and ready to sheet. The photo is a bit bleached-out, but you get the idea.

[Dr. Jet]

Prototype #2 Update

Prototype #2 has the bottom sheeting installed, and the curves on the bow end are MUCH better; see photos (sorry for the one bleached-out photo). As I'm working on Prototype #1 and fiddling with the foam cockpit (which is necessary as the motor is taller than the top of the hatch deck), I got to thinking.... I really don't need a cockpit on #2 since the motors are buried in the sponsons. I've also changed my choice of motors and props for a number of reasons, primarily because I wanted motors with built-in fan cooling. This is the motor I'll be using: https://www.ebay.com/itm/4000KV-Brus...19.m1438.l2649. Gotta love a $12 brushless motor! It's a short can motor so I'll be a bit more conservative on props. I'll probably start with 32mmx1.4P counter-rotating props and keep a close eye on temperatures for the first few runs. I will reduce the prop diameter if things get a bit too warm.

[Dr. Jet]

Filling and Sanding on Prototype #1

Some progress photos on Prototype #1. I still have a few spots to fill in, but it's beginning to look like something now....

[fweasel]

Looking good!

[Dr. Jet]

Back to Works After a Long Haitus

Well, I've finally found time and space to work on these again. Prototype I needs one more decal before I can clear-coat it. I have a guy that makes water-slide decals with metallic ink for guitar headstocks, so I'll have him make some for this with the same metallic ink. When everything is dry and cured, I'll add the final linkages and electronics and it will be done. Prototype II is progressing.

[kfxguy]

Have you ran one yet?

[Dr. Jet]

Have you ran one yet?

Not yet. This is the first prototype. As I said in an earlier post, I'm going to use an old Aveox 1005/2Y (Kv=4900) on 2S first. The Aveox likes a small prop like a de-tongued X427. I may go to an outrunner with more torque and spin a bigger prop later on.

Here is Prototype II:

[Dr. Jet]

I've detected a few minor glitches with the plans for the twin-motor version; specifically, the location of the struts. As initially cut, they were too high (the strut arm is too short). Not a problem though, I can lower them on the prototype as I've done on the plans. One side benefit of this is I can get rid of the "Squared-Off" transom and go with the better-looking rounded corners like on prototype I. I've added mounting holes for this rudder: https://www.offshoreelectrics.com/pr...prod=ose-80047 This rudder is mounted from the inside so I've added the necessary structure.

[Dr. Jet]

Well, the graphics are done, it's time for some clear coat and final assembly. I have the drive system worked out and the servo tray mounted, all I need to do is add the rudder linkage and a cooling water outlet. Easy enough. It might possibly be ready by next weekend.

Then I suppose I need to get hot on Prototype II

[cjs]

Looks great!

[Dr. Jet]

Looks great!

A lot of thanks goes to Lone Wolf who came up with the original Shocker design , and to Jesse J who tweaked it to the configuration I have now. I essentially traced their design into AutoCAD and brought the precision up to six decimal places.