12" scratch built cat

[Dr. Jet]

Feeds, Speeds, and Materials

G10 is not the easiest material to machine. To do it right, you need diamond mills and constant-flow coolant; neither of which I have. So I'm experimenting with different materials. Wood seems to be the best option. It's cheap, readily available and easy to machine. I tried some pine first, but it just didn't yield the consistency needed. I tried some maple and while better, it still wasn't perfect. Walnut seemed to give the best results, but that was the extent of my hardwood inventory too.

I'll dig around and see what else I can find, maybe rock hard maple like they use for guitar necks would be the ticket. That stuff machines really well.

[Dr. Jet]

Went to the pond today, but spent too much time with other projects to test this one. Next time out, I'll run it first.

[Dr. Jet]

Went to the pond today and ran the cat. It wants to stick the bow in the water until it gets going, then it's insanely fast with that high-pitched scream that accompanies the REALLY FAST boats you see. It definitely needs some strakes to get the bow out of the water sooner and easier. There's just too much laminar flow trying to keep it stuck to the surface. I'll remove the taped-on cave-man spray rails and build a real set of strakes and spray rails for it. That should make it one frighteningly fast beast.

[Dr. Jet]

While I'm currently doing things other than making strakes for this cat, it is still part of my plans to do just that. In the interim, the twin design is 99.9% done; but then I got to thinking. (More trouble.... ).

When I built the Micro Scat Cat Twin, I used the TLAR Method (That Looks About Right) in establishing the length of the stingers. Is there a more scientific approach in determining exactly where you want the propeller to be in relationship to the hull/transom? The longer the better to a point? Where is that point? The next county?

Inquiring minds want to know........

Hmmmmm......... If the props WERE in the next county, it would be REAL slow to turn.......

[NativePaul]

I find that 10% of hull length works, but that is more or less TLAR. I have never played around with different lengths on the same hull so maybe 5% or 15% would be better for some, but at 10% your probably not going to break a hulls handling.

I don't think it is very sensitive or hugely important, when was using strut brackets I would just use whatever bracket length came with the strut and only once had cause to put them on spacers (a cat I made with way too much lift and needed training wheels/wheelie bars), and never cut one down, and never even bothered to measure them.

[Dr. Jet]

I find that 10% of hull length works, but that is more or less TLAR. I have never played around with different lengths on the same hull so maybe 5% or 15% would be better for some, but at 10% your probably not going to break a hulls handling.

I don't think it is very sensitive or hugely important, when was using strut brackets I would just use whatever bracket length came with the strut and only once had cause to put them on spacers (a cat I made with way too much lift and needed training wheels/wheelie bars), and never cut one down, and never even bothered to measure them.

[jfrabat]

Man, this looks interesting! My wife may kill me, but I could be interested in one of your laser cut plans of these... Twin setup. Let me know the pricing and and all that jazz.

The only thing I am worried about now is that you and Jesse are using 1/16 ply bulkheads with 1/32 or 1/64 sheeting (ply?). I used 1/16 ply sheeting for my scratchbuilt 30" cat, and only covered it in 2 oz fiberglass. I am now worried it may not be strong enough to survive an accident!

[properchopper]

Been following this.

"Is there a more scientific approach (than TLAR) in determining exactly where you want the propeller to be in relationship to the hull/transom? "


Thought I'd share my way of locating stuff : The Inter-Ocular Trauma Test : stare at it until the right location hits you between the eyes - then grab the drill (or whatever)

Must drive down (from Torrance) to play sometime when things lighten up

[Dr. Jet]

Been following this.

"Is there a more scientific approach (than TLAR) in determining exactly where you want the propeller to be in relationship to the hull/transom? "


Thought I'd share my way of locating stuff : The Inter-Ocular Trauma Test : stare at it until the right location hits you between the eyes - then grab the drill (or whatever)

Must drive down (from Torrance) to play sometime when things lighten up

Sounds like a plan!

[Dr. Jet]

Following Properchopper's advice, I used the Inter-Ocular Trauma Test to determine the location of the strakes and the spray rails. My goal is to break the sheet of laminar flow that forms on the very bow when starting from a dead stop. This laminar flow makes the bow want to stick to the surface instead of lifting out. Once given enough power for a long enough time it will overcome the sticking, but it's annoying. The strakes running all the way to the bow should break that laminar flow and allow it to get on plane much quicker. It still needs a bit more sanding and primer, but you get the idea,

[Dr. Jet]

I'll have to wait many days for the primer to completely harden before I can give it a final sanding and final primer coat. While I'm waiting, I'll talk about the power system. It is an old Aveox 1005 2Y from the late 1990s with a custom-made water jacket. It most recently powered an MHZ Micro Iceberg I had back in the NiCd days. The 17" boat was stupid fast, but would just roll over in the turn. I had the batteries down low in the sponsons, but the weight of the motor up high and the narrow beam was something I couldn't overcome. I sold the hull but kept the motor. If I could find an appropriate motor/battery combo that would both fit in the sponson (and balance the hull), it would be fun to build one as a twin. With the weight of the motors and batteries down low, it may corner a lot better.

This hull, while an inch shorter than the MHZ Micro Iceberg, is considerably wider and cornered well at speed, even without any strakes. I hope to get a vid when I run it the next time out.

[Dr. Jet]

Well, the last color coat is on the hull bottom and it's been allowed to dry overnight, then lightly wet-sanded with 2000 grit. It is now baking in the sunlight. It should probably do about a week of daily sunbaking before I shoot a clear coat on it. Then, after re-assembling the strut and shaft it will be ready to tear up the water.

[Dr. Jet]

OK, the first coat of clear is on the underside and it's baking in the sun. I'm going to shoot a bit more clear on the top as well, so when the bottom is dry enough to handle (24 hours), I'll flip it over and shoot a coat on the top. I'll just keep repeating the process with wet sanding and another day of sunbaking between the painting steps.

[Dr. Jet]

It's Done!!

Like most of my projects, this has taken quite a while to complete, but here it is. The last time out without the strakes it was really fast once on step running an X427 that had been de-tongued and reduced to 26mm diameter. This time out, I have another X427 that has a bit more blade area removed and it's been reduced to 25.4mm diameter. FE Calc predicts 35mph at 35 amps with that prop (on 2S), a couple of mph faster than with the bigger prop.

I used a ball thrust bearing which moved the prop back a little, so I had to shorten the bullet nut a bit to keep it from hitting the rudder. I enlarged the cooling water inlet on the bottom of the strut slightly so as to improve cooling water flow.

Hope to have vids once we get "permission" to go to the lake from all the smart people we elect and the bureaucrats they appoint.