12" scratch built cat

[Dr. Jet]

That is the question.

I've been running the Micro Scat Cat Twin without strakes and for the first few outings and this one will run without strakes as well. I've been thinking of how I could come up with some really sharp, smooth strakes with the proper angles. I think I have an idea. I ordered some 5mm phenolic sheet and I can cut off small angled strips to use for the strakes. My research on strakes shows they can be as much as 5 degrees below horizontal for some additional lift at the cost of a very small amount of drag.

My plan is to makes some strakes from the aforementioned phenolic sheet and temporarily attach them with rubber cement. If I notice a significant improvement, I'll paint them to match the hull and attach them with some epoxy.

What do you guys think about adding strakes on such a small hull as this? Will it make a big enough difference to warrant all the extra work?

[Jesse J]

I think strakes are not necessary I’m this small of a boat, but I may be wrong...

[Dr. Jet]

Getting close. I need to install the rudder linkage, add a water outlet, and mount the receiver. I'll be using a pair of Turnigy Nano-Tech 2S 1800Mah batteries in parallel. That may be a lot more battery (weight) than I need, but they were on sale CHEAP, and the little Aveox will draw 35~40 amps, even with a cut-down X427.

Which brings me up to another issue: The hardware I selected can not handle props much bigger than 27mm. If someone were to build this hull and wanted to use an air-cooled helicopter outrunner, a bigger prop would be in order. As such, I think I'll redesign the transom to fit the little strut that OSE sells (https://www.offshoreelectrics.com/pr...prod=ose-80046) and the extended rudder assembly (https://www.offshoreelectrics.com/pr...prod=ose-80047). These are the same struts and rudders I plan on using for the twin motor version, so all is good.

About hijacking Jesse J's original thread from a couple of years ago, he's cool with it and will probably be building Prototype III from the V2.0 drawings!

[Jesse J]

Ees all good man,
Glad to see this lil guy evolving so much!

[Dr. Jet]

Here's some more thinking. I'm not sure if I can get that little strut low enough as-is. For the single-motor version, I can use the same tab on the transom as used on Prototype I. But for the twin version, that tab arrangement may not work. So here's what I'm thinking:

#1. Fixed stingers. The Micro Scat Cat had fixed 9/32" (BIG! - See photos) stingers that worked really well, but were rather complicated with ball-bearings and a 1/32" wire drive. That may be something difficult to re-create with simple hand tools (one of my requirements for this project). So it just dawned on me...... Using telescoping brass tubing, it might be possible to put 1/8" Octura lead-teflon bushings in a 3/16" tube and have a 1/4" support tube enter the hull (see sketch), But as I thought some more about that idea, it occurred to me that the flex cable would be prone to flopping around without some support as well. So the 3/16" tube would need to extend to within 1/4" + of the motor coupler and be secured to the end of the 1/4" tube with short pieces of 7/32" tube. This plan would allow bending of the 3/32" tube to match the motor shaft angle and location. Complicated? Yes. Easily accomplished with ordinary hand tools? Yes. I would make some plywood sub formers that would secure the 1/4" outer housing tube in the sponson bottom, allowing it to be fairly short within the hull.

#2. I'm laser-cutting plywood with ultra-precision, why not just make a plywood bracket for the little strut? I'm using 1/16" plywood and I can double it so the brackets would be 1/8" thick. This would allow for some adjustment.

I'm thinking Idea #1 may look better and in the end be easier to build.

Time to fire up AutoCAD again......

I think I may insist that Jesse build Prototype III as a twin.....

[Dr. Jet]

Well, 4 hours (or so) of AutoCAD work; adding bits for built-in stinger struts, correcting some mistakes, changing the single-motor strut and rudder arrangement, adding some other cool bits, and it's progressing along nicely.

Stay tuned.

[Dr. Jet]

OK, it's time to make the rudder linkage. One of the challenges of these small boats is the rudder linkage. It has to be flexible enough to allow the radial motion of the rudder and servo arms, it has to be solid lengthwise, it has to be low friction (micro servos don't have all that much power), and it has to be watertight. How to accomplish all that? Often the micros don't have room for a rubber bellows to seal the linkage and again, it takes servo power to expand and compress a bellows.

I like to use a 1/32" music wire with sections of 1/16" stainless steel tubing attached to it (brass tubing will also work). I glue (green Loctite) short pieces of 1/16" tube to the ends where they attach to the servo and rudder arm connectors, and a length of 1/16" tube where it goes through the hull. I glue a 3/32" tube in the hull and generously grease the pushrod assembly where it goes through this tube.

Voila! Stiff, flexible, watertight, and low-friction.

Sorry for the one blurry photo, but you get the idea....

[Dr. Jet]

Batteries have arrived, radio and ESC are programmed, all cooling plumbing installed, batteries charged. Weather today is warm and sunny; forecast for Saturday..... 60% chance of thunderstorms.

[Dr. Jet]

Safe easy transport and storage in a handy-dandy gun case.

[Dr. Jet]

Just to bring this thread back to the top and share some info, I have the ability to make laser-cut framework short kits available. All that would be included is the 1/16" laser-cut plywood, some 1/8" square basswood strips, the yet unwritten instructions, my hardware recommendations, and of course, 100% customer support.

Right now, the drawings are set up as a choice of either single or twin, but duplicate parts may make things confusing. To make things easier to understand, and less fragile to cut, I have decided to offer the option of EITHER a single-motor kit OR a twin-motor kit. This, of course, may require some lead time to cut the appropriate kit. The laser guy charges me a minimum, so I want to give him as much work as possible to keep YOUR costs down.

For what it's worth, I've revised the strut/rudder hardware I used in the single-motor prototype to a strut and offset rudder to allow for a larger prop. I've revised the dual strut arrangement in the twin for fixed stingers with a central rudder arrangement.

[Dr. Jet]

UPDATE:

I finally ran the cat. It's great at high speeds, but at low speeds it ABSOLUTELY needs spray rails and probably a strake too. I'll make some and install them before the next outing.

[kfxguy]

UPDATE:

I finally ran the cat. It's great at high speeds, but at low speeds it ABSOLUTELY needs spray rails and probably a strake too. I'll make some and install them before the next outing.

Mine works good at speed or low speed. I do have a thin piece of carbon blocking the underside of the rudder mount. I’ve only run it about 30mph tho. I had to tape a gps to the top of it and it went like 29.9mph. I bet it would go a little faster without the extra gps weight. Visually it looks faster without it.

[Dr. Jet]

I took my video spy glasses there to record everything, but the card was blank when I got home. I made sure the surfaces were flat with sharp corners and at low speeds it shed thin sheets of water off of either hull near the bow. It was as if it had wings. Once on plane it was better, but a spray rail at a minimum is definitely in order.

[Mtrfpv]

I would like to purchase a twin motor kit! This is amazing! Beautiful work!

[Dr. Jet]

Send me a private message and we can discuss it.

[Dr. Jet]

Here's some photos of the partially-completed 16" Twin (Prototype 2).

[Dr. Jet]

The little Cat project hasn't died, it's just had a rearrangement in my priority list.

What's been delaying it is the need for strakes. What has something seemingly so easy been such a huge delay you ask?

Because I want strakes so sharp they will cut your fingers to shreds if you mishandle them, I have been pondering how I could make something that is the length needed (14" or so), and the width and depth as well. The piece would have to have a 100% consistent width and depth throughout its entire length, so that means some sort of machining process that can be repeated. That also means the strake has to be made of a hard material that is easily machinable, rigid, but somewhat flexible, with a smooth ride surface that is 100% flat and does not require any additional finishing.

I wouldn't want to use most woods because I don't think they are hard enough to have the necessary consistency for the machining process. Carbon fiber is too expensive where the strength really isn't needed. The solution? G-10 (a fiberglass laminate product).

Now, how to cut it? A table saw comes close, but because of the tiny dimensions (1/4" high and maybe 3/32" thick) I don't think the consistency would be there and thus the long time to come up with a solution. What to do?

Eureka! I have an idea.... I'm going to mill a slot in a piece of aluminum such that a square strip of G-10 I cut on the table saw will slide in the slot. Along one side face of the slot. I'll install a linear wave spring to keep the G-10 in constant pressure on the other side of the slot. I'll make a bolt-on top with another wave spring to fully capture the G-10 strip I cut on the saw, yet allow it to slide in and out the slot I created. Then I can clamp this aluminum block at the proper deadrise angle to the bit in my milling machine. I can then locate the mill bit such that pushing the G-10 through the slot engages it it the cutter and Voila`!! I have the pre-finished G-10 surface as the ride surface with a REALLY sharp edge that will require little to no fairing in to the hull bottom and have a razor-sharp chine. Best of all, this is REPEATABLE.


Stay Tuned

[kfxguy]

The little Cat project hasn't died, it's just had a rearrangement in my priority list.

What's been delaying it is the need for strakes. What has something seemingly so easy been such a huge delay you ask?

Because I want strakes so sharp they will cut your fingers to shreds if you mishandle them, I have been pondering how I could make something that is the length needed (14" or so), and the width and depth as well. The piece would have to have a 100% consistent width and depth throughout its entire length, so that means some sort of machining process that can be repeated. That also means the strake has to be made of a hard material that is easily machinable, rigid, but somewhat flexible, with a smooth ride surface that is 100% flat and does not require any additional finishing.

I wouldn't want to use most woods because I don't think they are hard enough to have the necessary consistency for the machining process. Carbon fiber is too expensive where the strength really isn't needed. The solution? G-10 (a fiberglass laminate product).

Now, how to cut it? A table saw comes close, but because of the tiny dimensions (1/4" high and maybe 3/32" thick) I don't think the consistency would be there and thus the long time to come up with a solution. What to do?

Eureka! I have an idea.... I'm going to mill a slot in a piece of aluminum such that a square strip of G-10 I cut on the table saw will slide in the slot. Along one side face of the slot. I'll install a linear wave spring to keep the G-10 in constant pressure on the other side of the slot. I'll make a bolt-on top with another wave spring to fully capture the G-10 strip I cut on the saw, yet allow it to slide in and out the slot I created. Then I can clamp this aluminum block at the proper deadrise angle to the bit in my milling machine. I can then locate the mill bit such that pushing the G-10 through the slot engages it it the cutter and Voila`!! I have the pre-finished G-10 surface as the ride surface with a REALLY sharp edge that will require little to no fairing in to the hull bottom and have a razor-sharp chine. Best of all, this is REPEATABLE.


Stay Tuned

I think you are way over thinking it. I used two square strips of carbon fiber.

[Dr. Jet]

I think you are way over thinking it. I used two square strips of carbon fiber.

Maybe, but then I can make dead-accurate strakes of almost any size and deadrise angle for as long as I want.....

[kfxguy]

Maybe, but then I can make dead-accurate strakes of almost any size and deadrise angle for as long as I want.....

I guess what I should have said was it’s not gonna make any difference. Split
ting hairs is my thing for sure but that little detail won’t make a difference. Just trying to save you some trouble. But if you are like me, I look for excuses to use my mill. Lol

[Dr. Jet]

I guess what I should have said was it?s not gonna make any difference.

Only in the appearance.... Maybe in ease of installation.... I want filling and sanding to be almost nonexistent.

........ But if you are like me, I look for excuses to use my mill. Lol

Exactly. I like making chips.... It's a cool mill project that will result in some tooling I can use more than once in the future to make something that looks uber-cool. Yeah, it will take time, but so what? It's time well-spent.

[kfxguy]

Only in the appearance.... Maybe in ease of installation.... I want filling and sanding to be almost nonexistent.




Exactly. I like making chips.... It's a cool mill project that will result in some tooling I can use more than once in the future to make something that looks uber-cool. Yeah, it will take time, but so what? It's time well-spent.

Gotcha. No worries then. :)

[Dr. Jet]

.......... But if you are like me, I look for excuses to use my mill. Lol

And it also gives me an excuse to get something for my milling machine parts bin that is currently missing and just the ticket for precisely establishing the deadrise angle of the strake you are making. A tilting table. This table will more or less permanently bolt to the tooling that holds and moves the G-10 through the spinning mill (bit). Bingo! Perfect strakes. Careful gluing with thin CA might make a joint clean enough to allow the paint to fill it.

The temporary strakes that are currently on the hull are crude by comparison and appear to have been made by some caveman.

[Dr. Jet]

One of the motivating factors behind much of what I do with these minis is that I want to make them available for others to build as well. Before I do that, I want to make sure they are thoroughly tested, work well, and as planned; I want everything as right as I can make it. Hence, the over-thinking and over-engineering of making strakes. It's going to take a long time to tool up to make them as I have so many other irons in the fire, but God willing, I will get it done. The best thing is I can make almost any strake with perfect surfaces and sharp corners.

My eventual goal is to provide laser-cut bones and other "Tedious to create" parts such as perfect strakes in a short kit. I'm still making improvements to the plans based on things I have learned from building the prototypes.

Stay tuned, and don't hesitate to ask questions.

[Dr. Jet]

So I pulled out my tablet of 1/4" grid paper, sharpened my pencil (true!) and started drawing up the tool I'm going to make in order to make strakes. I have all these grandiose plans of intricate machinery, but I have to KISS! I was going to make a power feed, but abandoned that idea (I can still go back if I want). I fire up the old hand-held manual eraser and start simplifying, but issues still arise. What is the maximum length of the workpiece I can accommodate? How will I load that length of work piece into the tool? As it turns out, this is a BIG deal! I could essentially make a bolt that opens a breech, but that adds to overall length and complexity. I need a permanently open breech so the piece will just drop in and push into the cutting tool, but how long can it be??

I diligently research Internet drawings of lead screws, lead screw pillow blocks, lead screw nuts, lead screw nut housings, and I discover all sorts of information. You purchase lead screw assemblies by the overall length, but you have to subtract all the other hardware to get the working length.

I want to be able to make a continuous strake that is at least 12 inches long, so I needed something more accurate than that sharp pencil and a scale. Time to fire up the old AutoCAD program and see exactly what will and will not fit... It didn't take long, but I got all the pieces and sub-assemblies drawn up; then, I began laying them out and found all sorts of ways I could simplify something, move something, or otherwise dial it in.

Finally, I have a plan. A minimal amount of machining required, mostly drilling and tapping holes. A main frame of aluminum with a few Delrin parts where things move. A couple of small parts to make that will keep the work piece from moving around while being cut and that's it.

[Dr. Jet]

Most of the parts I need to build my Strake-Maker tool have arrived. I'll go down to the shop and begin drilling and tapping holes to get the project started as soon as I post this. I think the Strake-Maker may work quite well. I will build one more prototype twin hull with perfect strakes if I like the performance of the single-motor prototype with the Caveman strakes.

Having the ability to make such a precise triangular cross-section piece opens up a whole bunch of possibilities. As should be apparent by now, I like to take an existing design and tweak it to my preferences, usually by shrinking it. I've had my eye on an unusual composite self-righting stepped mono hull that is already a "Mini" at 15" long. Since wood often comes in 12" lengths, I'm considering working up a similar design in wood and making it 300mm (just under 12").

I still have projects in the building queue (the rigger) that are in front of such an undertaking, and I would create another thread around it if I decided to proceed, but I wanted to share some of my current plans.

[Dr. Jet]

As long as I'm doing all this project analysis and queue management, I've been contemplating where this cat build needs to go. I'll be testing the prototype with the caveman strakes soon. Based on that I'll finalize the design. At first, I wanted to do two versions, a single and a twin, but in keeping with the KISS philosophy, I'm going to just do it as a twin. I would need to build one more prototype to confirm that everything was right, but that's a big chunk of building time away from other projects.

Hey JesseJ!!!

Do you want to be a guinea pig and build the prototype twin? I'm sure you'd do a classy job.

[Dr. Jet]

The Strake-Maker is done (well, ALMOST ) and I can cut 12" lengths. Tomorrow, I'll cut some 12 degree strakes and some 42 degree spray rails for the cat and see how it works.......

[Dr. Jet]

The 12 degree strake. Hardwood would work in my Strake-Maker tool just as well.

All in all this was a satisfying project. Was it totally necessary? Absolutely not. Will it allow future projects to benefit? Absolutely!

So when Jesse J builds the prototype twin, he will have super-sharp, super accurate stakes for his hull (and I'll revisit the hull on Prototype #1).

[Dr. Jet]

The last post really didn't show the precision of these, and this site rotated the photos due no doubt to their aspect ratio. Here's a cropped photo of the strake. It's about 1/4" wide and about 1/16" high.