Scratch building a slingshot for cmb 27

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Roxyflash

Well-Known Member
Joined
Aug 20, 2003
Messages
1,424
My friend Adam ordered a slingshot kit I’m going to build it for a zenoah.I’m copying the frames to custom build one for a cmb 27 evo we have.Started on it last Saturday started at on rear sponsons to get a feel for the kit.Monday started on the tub used 3/16 ply topped with 1/6 to frames on side of tub.Used 1/8 5 ply for cross members.Ditched the servo mounts used 1/8 inch ply for servo mounts on inside of radio box full length 1 1/8 tall with 3/8 square dowels for servo mounts.I glued the crossmembers first on the sides then I glued the top on so I could frame the inside of baitbox and radio box with 1/8 ply so I could trim router the top openingsAfter that I added the lip for the lids then servo mounts.Once that dried I glued the bottom on.Tomorrow will add the boom mounts and turn the inserts on the lathe.I also fabricated the motor mounts took a while to figure out the spacing but used 3/8 x 1/2 for the lugs then 1/8 x 1 t6 flat stock.Had to order 10mm x 20 mm with m6 stud and screws For isolators which came out to 4 1/2 inch so the tub is normal size.
 

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Spent a hour in half this mourning on the tub sanded the edge flush added a nose block out out poplar dowel round had some hard rock maple so I cut it for boom mounts and drilled the tub.Bought some 5/8 tubing off Amazon for boom tubes which the 1/2 inch tubes from Menards slides right in cut them to length front one is 1/16 shorter then rear tube..
 

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After I posted this mourning I reliazed I forgot the triangle gussets in the motor area.Cutting the gussets on the table saw from 3/8 square dowel wasn’t my favorite thing to do the blade is almost touching the fench but got a 3 foot piece cut.I also checked the fit of boom tubes in sponson former slid right on.
 

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Next thing I want to do is mount the engine and fit the stuffing tube.Being the motor is heavier then a zenoah do you want the motor as far forward to keep the weight on front sponson?Or do you place it in the same position as a zenoah?
 
My friend Adam ordered a slingshot kit I’m going to build it for a zenoah.I’m copying the frames to custom build one for a cmb 27 evo we have.Started on it last Saturday started at on rear sponsons to get a feel for the kit.Monday started on the tub used 3/16 ply topped with 1/6 to frames on side of tub.Used 1/8 5 ply for cross members.Ditched the servo mounts used 1/8 inch ply for servo mounts on inside of radio box full length 1 1/8 tall with 3/8 square dowels for servo mounts.I glued the crossmembers first on the sides then I glued the top on so I could frame the inside of baitbox and radio box with 1/8 ply so I could trim router the top openingsAfter that I added the lip for the lids then servo mounts.Once that dried I glued the bottom on.Tomorrow will add the boom mounts and turn the inserts on the lathe.I also fabricated the motor mounts took a while to figure out the spacing but used 3/8 x 1/2 for the lugs then 1/8 x 1 t6 flat stock.Had to order 10mm x 20 mm with m6 stud and screws For isolators which came out to 4 1/2 inch so the tub is normal size.
That jig is really cool!
 
Next thing I want to do is mount the engine and fit the stuffing tube.Being the motor is heavier then a zenoah do you want the motor as far forward to keep the weight on front sponson?Or do you place it in the same position as a zenoah?
I feel, and this is my opinion, that the boat already has some weight up front, especially with a full gas bag. If anything, I might be tempted to move the motor back a little, but then you run into clearance issues with the collet and the stuffing tube. With my zenoah mounted in the place the kit calls for, it’s a little tight. I guess you could shorten that stiffener box that the stuffing tube runs into if you wanted to try it. If not, I would just leave it where it is. While it’s heavier, it is right around the cg already so it shouldn’t make that much difference.
 
The jig was a dumpster dive.I was working on a 860 jlg that wouldn’t start it was parked next to a roll off.I was throwing away some filter boxes and seen the aluminum in there so I pulled 6 straight ones out.I was going to built a setup jig for the boats.I never got around to buying the pieces to tie them together.I had some long 1/4 inch screws tried one of them in the top slot fit good.Took some scrap wood drilled two holes and made a cross brace.I just clamped it the the build board.I used them in the framing of the tub to squeeze it together to keep it straight.While it was clamped to the sides they sit below the top of the side frames that’s when I came up with the idea for the cross brace.I was able to glue the top ply on while it was holding it straight.When I clamped the the bottom on i spread them out 1/8 to clear the bottom sheeting I used a piece of 1/4 x 1/2 that was clamped to one side to keep it straight once clamped I removed it.Tub sits flat on building board and is straight.The stuffing tube I will see how the motor fits and mock up the brace to see what kind of room it has we are going to try to use a woods oiler if possible.
 
Worked on mounting the engine and stuffing tube.Basiclly the motor is mounted where the zenoah would be.The gusset box I shorten to accommodate the oiler.That’s why the stuffing tube extends past the box.I have some shorter screws coming for the rubber mounts I used some nuts to make tighten for now.I got a little sloppy on the slot when filing it out.Adam bought some 410 fairing from west system I really like this stuff mixes nice fills gaps and thickens real nice plus has a wood color.
 

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Worked on mounting the engine and stuffing tube.Basiclly the motor is mounted where the zenoah would be.The gusset box I shorten to accommodate the oiler.That’s why the stuffing tube extends past the box.I have some shorter screws coming for the rubber mounts I used some nuts to make tighten for now.I got a little sloppy on the slot when filing it out.Adam bought some 410 fairing from west system I really like this stuff mixes nice fills gaps and thickens real nice plus has a wood color.
I like this setup. Shortening the gusset area gives you more flexibility and is what I will do if I build another. Even if you are not using a wire drive, there’s plenty of room to retrofit one if you wanted to as far as being able to get a clamp on the outer tube. Everything looks great.
 
Thanks Rich I’m going to start on the front sponsons I order a Sullivan s390 tank I hate gas bags tried them 15 years ago hated them tried them again this year on my seaducer and threw it in the thrash.We ordered a zipkitts rigger cowl going to cut it in half and use the front half for a half cowl.
 
Here’s a picture of the oiler need to drill the hole for oil to drip tru not sure what size hole to drill
 

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Been making the front sponsons I have the tips to glue on and form I’m in the process of glueing the ride pads on now.Hopefully tomorrow will glue the rear shoes on.
 

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Got the ride pads on front sponsons and nose tips on mounted the boom tubes and front sponsons.Received my Sullivan tank from Sullivan fitted it with tank hold downs fits like a glove 16 oz tank.Next is glueing the rear shoes on.
 

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Glued the rear shoes on went with the tub level to building board used a 1/8 piece of aluminum 1 inch wide for rear sponsons to sit on front sponsons sitting on the board.After that dried it fitted the throttle servo linkage rod used a quick disconnect ball joint for throttle arm going with the woods pushrod seals to match the woods oiler.Too bad Steve quit making parts.
 

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Made the x brace i had to have it removable to access the fuel tank I remove the eyelets for rubber bands as it’s not needed with the brace in place.I will need to get some flat head screws I used some blind nuts the side dowels are 3/8 square.
 

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