New RC Cat Design & Build, 450mm long - RC Groups
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Apr 12, 2012, 03:50 AM
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Build Log

New RC Cat Design & Build, 450mm long

I mentioned on my previous thread I would start a build log for my new CAT design so here goes.

I specifically want to keep the boat small (but not 'micro'). Final length will be around 450mm, it is 228mm wide. It's my design using experience from my last boat and info from these forums and elsewhere.

The running gear will be the Seaking 2040, 4900kv motor & 60A ESC, 2S 3.8Ah Turnigy LiPo. Providing that setup has enough power to make the hull plane it should run as a surface drive. I'll be installing a solid drive shaft and hopefully the position of the prop will be close enough to make any small adjustments with CG (I could be wrong on that point!). For the rudder I've purchased a 95mm CNC with water pickup.

It's been a complex build from birch ply so far; here's the photos - you'll need to use your imagination for the front profile of the hull..
Last edited by Rhizome; Apr 12, 2012 at 05:41 AM. Reason: 228mm wide (not 280)
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Apr 12, 2012, 06:24 AM
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F1 madness's Avatar
nice idea but that vee could cause some issues with compression lift at the transom forcing the nose down or making the hull flighty im interested to see how you get
Good luck
Apr 16, 2012, 10:14 PM
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Hi F1 Madness, thanks for the comment. If it goes fast enough for air compression to be a problem I will indeed be pretty darn happy with the build!

Some more progress - starting to look like an actual boat now where you can see the shape of the front hull. The planning hull bottoms follow the curve up to a narrow point at the tip. Amazed at how much marine ply this has used so far. Almost a full sheet of 1/32" (aprox. 40cm x 100cm) birch ply.
Apr 30, 2012, 10:15 PM
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Almost there!

Well the hull is nearing completion - no hatch built yet.

Hopefully by the end of the week I'll have it primed sanded and an initial coat of paint ready for a trial run. I'll start with white - any suggestion for a colour scheme? I was thinking blue and gold..
May 06, 2012, 11:28 AM
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Maiden Voyage!

Finally finished the hull to a point where I can test my 'odd' design out - it does get up an plane on the little 2040 motor. It takes some careful throttle to do so but goes quite well. Seems to handle / turn well - this is (pretty avarage) video from the maiden voyage earlier today:

RC catamaran, 2040, 4800kv - 1st run (0 min 47 sec)

The drive shaft I think was slightly bent and all that plywood was amplifying the sound a lot! Seemed to be an issue as the second run later today the boat flipped only 10 seconds into the run. Unsure what happend but the upper mount of the drive shaft had snapped. Had to wade out and get the boat, lucky the lake is only knee deep and I had taped up the hatch pretty well! - only about 30ml of water inside.

Hopefully I have the drive shaft sorted now although it's really easy to slightly bend the 1/8" brass. I might need to find some 1/8" stainless rod.
Last edited by Rhizome; May 06, 2012 at 11:29 AM. Reason: spelling of title..
May 08, 2012, 08:04 PM
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Should have the drive shaft sorted now

Spent the last couple of evening sorting out the drive shaft balance problems. I found it was a combination of a couple of things:

1) the 1/8" brass rod I had used was not even consistently round (cheap imports?). In places it was slightly over diameter. It was also very difficult to get perfectly straight.

2) I found the brass bearings I machined up were also not perfectly centered despite using a pretty solid mini lathe. They were not a compression fit and were able to turn in the stuffing tube. This was also creating a problem, I have now epoxied these in place.

The hobby shops didn't seem to have any suitable stainless rod so I found the local steel merchant had 1/8" S/S rod - I now have 4m of it! (only $15.00). The diameter is spot on and was much easier to get perfectly straight (roll test on glass).

Boat Setup:
I managed a decent run on Monday evening (no video) and did several laps up on full plane. The boat rides very flat which could be an indicator that the air compression on the 'V' is having an effect. It sounds like a vacuum cleaner when it comes past close at speed? Is this just how a surface drive prop can sound?

The CG is also not back far enough, imho. I've shed some weight from my motor mount and added a little weight into the rear of the right hull which perfectly balances out the weight of the ESC in the left side - I was using battery placement to counter this; hopefully that will help.

Any suggestions as to ideal CG for a cat?
May 09, 2012, 05:37 AM
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3rd Run - Full Throttle

Here's my thrird run with the bugs ironed out. The adjustments in weight balance seem to have done the trick. Slightly nose up, remains pretty flat in the turns.

RC Catamaran, 2040, 4800kv - 3rd Run (3 min 24 sec)

For my 2nd RC boat and the 2nd boat I've ever designed and built I'm pretty darn happy with the result!

May 09, 2012, 02:24 PM
Boaters are nice people.
Hi Richard,

Looking good, but sounding terrible!

As if you're wearing out the bushings at an alarming rate....

Something definitely is wrong with your drive train, even my noisiest boat doesn't make such a racket.

Regards, Jan.
May 09, 2012, 04:04 PM
Thermite + ice = Big boom.
boaterguy's Avatar
Have you balanced your prop? What type of coupler are you using? I had a boat with a dog drive and no matter what I did it vibrated out of control.
May 09, 2012, 07:19 PM
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Hi Jan & Boaterguy, thanks for the comments.

Yes I agree it's still way too loud. I have been trying to sort the drive train vibration. I'm using one of those plastic universal couplers which I've now covered with heatshrink to see if that helps. High quality collet style couplers for the small shaft in that motor seem to be rare to non-existent. Considering buying 28mm Turnigy motor in the high 3000kv range too.

I've only sharpened the prop but not balanced so yes I'll need to find a prop balancer as well. Trying to source some 30mm Octura props as well (or similar).

Any RC prop experts here in Australia?
May 10, 2012, 05:58 AM
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Fourth run on Bonython RC boat lake - flips at the end!

This afternoon's run was much better. The heatshrink I applied to the universal coupling made a marked improvement to the virbration issue. Video:

RC Catamaran, 2040, 4800kv - 4th Run - Flips at End! (1 min 7 sec)

Very happy with the performance thus far. Right at the end the boat flips (just out of shot). I'm pretty sure both flips have been the result of a torqure roll due to a failure of the upper bearing on the stuffing box.

Finally found a great local source for RC boat parts - Dale at Adelaide Model Boat Centre, he was very helpful. I now have an Octura x427 prop and a balancer too boot. Tested the 31mm prop I've been using and it's pretty good.
May 21, 2012, 04:33 AM
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Installing a flex shaft soon

Well I setup my old hull as a rescue boat using a small 1380kv aircraft 12 pole outrunner. Runs very smooth and is an ideal speed/toruque for a rescue boat on 7.2v Ni-Cad packs.

Turns out it's had lots of use too! With the upgrade to the Octura prop the Cat runs on a knife edge and flips very easily due to the angled prop pushing the nose down at the higher revs.

So I've now ordered from OSE an Octura .130 flex cable and shaft to suit a 1/8th prop, a strut to match and bracket. Still haven't found a Coupler for the 2.3mm shaft in the 2040 motor so I've ordered a 3.17mm Etti coupler and will need to shim it - not the ideal solution but I do have access to a lathe so hopefully I'll be able to get that balanced. Very keen to see how quickly the new parts get to Australia!
May 21, 2012, 09:16 AM
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wparsons's Avatar
Sounds MUCH better in the newest video!
May 31, 2012, 11:58 PM
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Questions before I start cutting the Flex Cable!

Thought I'd ask a few questions from the experts before I start cutting the flex cable set the stuffing tube in place with epoxy.

Q1: is it OK to leave some exposed teflon tube between the nose of the prop strut and the brass stuffing tube. The Octura teflon tube and nose housing are a pretty snug fit so I assume this will give me room to make slight angle / height adjustments if needed.

Q2: Will I need any additional support for the stuffing tube? ie: a short bracket from the hull to the rear of the brass tube..
Jun 01, 2012, 07:48 AM
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wparsons's Avatar
A short exposed bit of teflon is ok, but you don't want too much at all. By the look of it, you should be fine with how much is showing. If you did have too much the cable could whip at higher rpm's.

A good starting point for prop depth is the bottom of the strut level with the bottom of the sponsons, if it's deeper it can lift the rear out of the water too much and either drive the nose down or make it too loose. I can't really tell how deep it is, but it looks like its in the right ballpark.

The only thing I would really change is to run a locknut instead of the bullet for the prop nut. Those bullets are notorious for coming loose mid run and losing your prop on you.

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