12" scratch built cat

[Dr. Jet]

To Strake or Not to Strake.......

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]

Linking Things Up

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]

Just another reason The Doctor recommends micros

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.