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Thread: 12" scratch built cat

  1. #91

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    Sorry Dr Jet, I had a senior moment, the wedge under the back will move the CoP back, but it is the opposite of reflex and the CoP will move futher forward with incrased AoA rather than backwards
    Paul "tug Killer" Upton-Taylor, Cat lover.
    FastElectricBoats.co.uk

  2. #92
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    Quote Originally Posted by NativePaul View Post
    Sorry Dr Jet, I had a senior moment, the wedge under the back will move the CoP back, but it is the opposite of reflex and the CoP will move futher forward with incrased AoA rather than backwards
    I know what you meant. It's all good and I'm going to employ a "trim tab".
    A nation of sheep breeds a government of wolves

  3. #93
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    Default More Minor Revisions

    I put the motor mount and servo mount together today to see how everything fits. The holes for the servo mounting screws were a little too big, so I reduced them to virtually nothing on the plans. Additionally, I neglected to account for the "ribs" on bulkheads E and F so I'll have to trim this motor mount to fit. The final drawings will have my typical locked tab and slot feature.
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  4. #94
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    Well, I bought some more glue and finished sheeting the bottom of the hull. Since I'm using balsa instead of ply for the sheeting, I put some carbon cloth on the innards to protect against flying batteries and gear.

    To seal the wood and apply the cloth, I used my customary LG-285 epoxy. It's pretty thin to start with, about like pancake syrup. I thin it out with denatured alcohol until it is like a thin paint or shellac and paint it on. The thinning allows you to paint a very thin coat which means less weight. Once everything is fully cured, I'll paint the carbon area one more time, let it soak in a bit and wipe off the excess with a paper towel.
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  5. #95
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    "Look good doin' it"
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  6. #96
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    Got the hull sheeted. Now, to get all the ride surfaces dead flat.
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  7. #97
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    Bill,

    I've been watching this thread for a while and wanted to say that I really like this! Once you get to electronics, send me a message. I may have some good suggestions that will save you a lot of time, energy and money. A perfect match for this size might be the little Etti outrunners that they have been running in Europe for 5-years or more.

    For durability, you might consider sliding in a piece of Lexan at the inside of the transom. You can always add ride pads/steps later if you want to run in rougher conditions. Have you picked-out hardware yet? I'm not sure if I saw that. If you decide to fabricate some inline hardware, it could really help with holding the bow down and more predictable behavior as the hull airs out.

    As a final thought, if you do add ride pads later, you might consider installing them so they are towed-in slightly in the front. That will do wonders for something this size.
    IMPBA: 7-Time FE World Record Holder "Don't think outside the box. Rather, refuse to admit that the box exists in the first place!"

    MGM Controllers - Giant Power Lipos - ML Boatworks - Wholt's Wire Drives & Struts - Nano-Oil

  8. #98
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    Quote Originally Posted by Meniscus View Post
    Bill,

    I've been watching this thread for a while and wanted to say that I really like this! Once you get to electronics, send me a message. I may have some good suggestions that will save you a lot of time, energy and money. A perfect match for this size might be the little Etti outrunners that they have been running in Europe for 5-years or more.

    For durability, you might consider sliding in a piece of Lexan at the inside of the transom. You can always add ride pads/steps later if you want to run in rougher conditions. Have you picked-out hardware yet? I'm not sure if I saw that. If you decide to fabricate some inline hardware, it could really help with holding the bow down and more predictable behavior as the hull airs out.

    As a final thought, if you do add ride pads later, you might consider installing them so they are towed-in slightly in the front. That will do wonders for something this size.
    I plan to use this strut/rudder assembly: https://www.offshoreelectrics.com/pr...prod=ose-80972 I have a number of outrunners in my motor drawer, but I'm going to start with an Aveox 1005/2Y (Kv=4900) that has a water jacket already installed. The outrunners I have are in a bit lower Kv range, so I suspect they may have quite a bit more torque than the Aveox inrunner and can spin a bigger prop. I'll probably start with a de-tongued X427 and go up from there. The twin version will have long-can outrunners (Kv=3800), and will start with 32mm x 1.6 pitch counter-rotating 3-blade props.

    I'm not sure about what you mean by the Lexan and where it would go. The transom on this guy is rock-solid and tied into two bulkheads with 1/16" ply doublers. I will make the first runs without strakes, but I fully plan on adding them to the finished version.
    Last edited by Dr. Jet; 03-09-2018 at 04:25 PM. Reason: Had wrong Kv for the Aveox
    A nation of sheep breeds a government of wolves

  9. #99
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    Haven't tried that hardware, so I can't really comment. Regarding Lexan, I find that by using it as a doubler, it keeps the nuts and washers from compressing the wood as much. I've used Lexan on 2s setups with soft transoms and it has survived crashes well into the 70s. That includes flips where Speedmaster rudders have bent.

    With a model this small, I doubt the mass will ever put much pressure on the transom, but did want to suggest for an option in the future, if nothing else.

    Here's a link to the Etti outrunner. It is a little different than the ones of years past, but it is built the same. http://www.etti.com.hk/product_info....pqgim6s4m1vjg6
    IMPBA: 7-Time FE World Record Holder "Don't think outside the box. Rather, refuse to admit that the box exists in the first place!"

    MGM Controllers - Giant Power Lipos - ML Boatworks - Wholt's Wire Drives & Struts - Nano-Oil

  10. #100
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    Quote Originally Posted by Meniscus View Post
    Haven't tried that hardware, so I can't really comment. Regarding Lexan, I find that by using it as a doubler, it keeps the nuts and washers from compressing the wood as much. I've used Lexan on 2s setups with soft transoms and it has survived crashes well into the 70s. That includes flips where Speedmaster rudders have bent.

    With a model this small, I doubt the mass will ever put much pressure on the transom, but did want to suggest for an option in the future, if nothing else.

    Here's a link to the Etti outrunner. It is a little different than the ones of years past, but it is built the same. http://www.etti.com.hk/product_info....pqgim6s4m1vjg6
    Structurally, the strut/rudder is mounted to a 1/16" ply box created by the transom, the next bulkhead, and some ply blocking that goes between them (see photo). Additionally, the transom is doubled to 1/8" where the strut/rudder attaches.
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  11. #101
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    Default Hardware and Electronics

    I think I showed these before, but not mounted
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  12. #102
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    Quote Originally Posted by Meniscus View Post
    ...........

    Here's a link to the Etti outrunner. It is a little different than the ones of years past, but it is built the same. http://www.etti.com.hk/product_info....pqgim6s4m1vjg6
    That's really similar to a whole class of readily available Asian-made "450-Sized" helicopter outrunner motors; many of which have built-in fan cooling. I like them for these little hulls as the air-cooling really works; and in reality, these motors aren't working THAT hard in these applications. Our original "Spec" motors for the micro hulls was a 3000Kv, short can, 26mm diameter motor that we ran on a 6-cell NiMh battery. I'm currently using motors that are 28mm in diameter and the same size or longer can than the Etti. My plan for the 16-Inch Cat V2.0 will be to use a pair of these motors: https://www.ebay.com/itm/Detrum-BM28...72.m2749.l2649 I chose this particular motor for its extra-long can (more torque = spin a bigger prop) and usable Kv range.

    I mis-stated the Kv of the Aveox motor that will initially power this hull. It is 4900 rpm/V , not the 3900 I initially stated. I originally had it in a 17" MHZ cat hull (See photo). As I recall, I used a de-tongued and cut-down X427 with that motor/battery combo. That's where I'll start with this hull. If I think I need to go bigger in prop size, I may go the outrunner route in this hull.
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  13. #103
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    Yeah, what I liked about the Etti outrunners is that they didn't mind being over-propped a little. When I tested others maybe 5 years ago, they didn't survive. That's why I sought the feedback from the guys in Belgium who have done more testing than I could ever imagine. They are the ones who suggested the Etti outrunners. I have a couple that still keep kicking. Also haven't had one fail, so I never sourced any others. I'm sure things have come a long way since then.
    IMPBA: 7-Time FE World Record Holder "Don't think outside the box. Rather, refuse to admit that the box exists in the first place!"

    MGM Controllers - Giant Power Lipos - ML Boatworks - Wholt's Wire Drives & Struts - Nano-Oil

  14. #104
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    I'm running a real similar motor in my 1/20 scale Atlas Prototype V1.0 With any luck I'll see it go tomorrow. With even more luck, I'll have a video.

    Again. a similar motor in my BBY Micro Turbine Hydro.

    Lastly, similar motors in my BBY twin Micro Scat Cat.
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  15. #105
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    Rain day. No boating, no videos.
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    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.
    A nation of sheep breeds a government of wolves

  17. #107
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    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”...
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  18. #108
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    Quote Originally Posted by Jesse J View Post
    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.
    A nation of sheep breeds a government of wolves

  19. #109
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    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.
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  20. #110
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    Fixed, sanded, and ready to sheet. The photo is a bit bleached-out, but you get the idea.
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  21. #111
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    Default 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.
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  22. #112
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    Default 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....
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  23. #113

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    Looking good!
    Vac-U-Tug Jr (13mph)

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