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Feb 07, 2012, 02:55 AM
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Question

Should I convert this to a surface drive or start on a new hull?


Hi, newbie here. Been lurking for the last month. Over the NY break I lashed out and bought the small Seaking 20mm 4800kV motor and 60A ESC to 'upgrade' an old catermaran I designed and built over 20 years ago (I was about 16!). I didn't copy any design at the time but I guess it looks more like a ferry. I only ever used it twice with an old brushed motor at the time.

Over the last 6 weeks I've made a stack of modifications including several to the bow profile. I've been amazed at how much difference small changes have made and reasonably impressed that it performs as well as it does - it's stable and turns well. Anyway, I've now got the speed bug - at a guess it's probably only managing 25kmph. So lot's of reading on this forum and it would appear that I'm running surface piercing prop 'fully submerged' and a little too large for the motor (Octura Y534). Currently only running 1500mAh Ni-cads.

Attached are some photos, my question to the experts here (before I go buy some Li-Po's) should I convert it to a surface drive or start building a new hull? Do you think this design would handle say 50kmph or become very unstable (roll over) in corners? All up the boat is about 45cm in length. I'll try and get some video of the boat in action.
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Feb 07, 2012, 07:09 AM
Boaters are nice people.
Hi Rhizome,

My first impression was that your boat looks like a baitboat, designed to cut through the water instead of a planing hull.

In order to reach 50 km/h, you'll need a completely different type of hull.
If you do a search, you'll find a number of designs / plans, on which you can base your fast electric.
The MaxCat is a good design, which existst in several versions:
https://www.rcgroups.com/forums/show....php?t=1279885

Changing the existing boat to reach 50 km/h would require major surgery...

Regards, Jan.
Feb 07, 2012, 07:46 AM
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Thanks for the comment Jan. I seem to remember some notion of also riding on a trapped air pocket when I designed it. I will start on a new design, I'll be keeping the size of boat similar, wider and much lower profile. Managed to get some video of it this evening:

Red Catamaran RC Boat (1 min 7 sec)
Feb 08, 2012, 04:06 AM
Boaters are nice people.
Hi Rhizome,

The boat doesn't run bad, with the power installed, but I wouldn't go faster, you'll most likely tip it over when turning at speed.

You are correct in assuming a catamaran rides on an aircushion, which litfts the hull up, once a certain speed is reached.

I'd advice to work from a know and proved design, to get the hydrodynamics right from the start...

Regards, Jan.
Feb 08, 2012, 09:27 AM
I feel the need for speed!
scorpion1's Avatar
Maybe start with something like this hull if you want speed........................needs a 3660 1800-2200kv motor on 4s minimum.

http://www.ebay.com/itm/29-Phantom-c...item20ad05e6e0
Apr 02, 2012, 05:03 AM
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Update to my RC Boat


Just a quick update to my old hull design. I lashed out on a LiPo battery and B6AC charger (3.8Ah Turnigy, 2S) - the power increase was rather surprising.

The torque roll at increased revs wanted to turn the boat over with either the plastic Octura prop or the metal (un-sharped) prop. After some searching on the forums I sharpened the metal one to a knife edge and this virtually fixed the problem!

Next I 'cut out' the old drive line and replaced the stuffing box, setting it further back and fitting precise brass bearings I machined up on a small lathe. Also changed the position of the rudder further back and too the right. All up the boat certainly ran faster but I am still unable to run at full throttle. Apart from the boat rolling on my 'round' hull profile, the tunnel seems to be too narrow and the prop suffers from cavitation at higher speed - it actually runs better in a long right hand turn. The last attached photo is a sneak preview of the new hull I'm building - I'll start a build log on this shortly.
Apr 02, 2012, 05:06 AM
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A video my GF took of one of the last runs for my old boat.

Red Catamaran RC Boat - with 3.8Ah LiPo (0 min 47 sec)
Apr 02, 2012, 03:43 PM
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Mikethk's Avatar
Well it depends of what you want.......

If you wanna make it faster, make it longer. . . 1 m is good. . .

Keep the prop submerged its fine. Keeps good grip in the water.....

Then make it 100% water resistant, and start to play in waves..... That would be a nice project and unique. And you already got the height needed for a wave rider. Make it happen.
Apr 02, 2012, 03:48 PM
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Mikethk's Avatar
Hmmmm. Put a prop in each side aswell. . . I would say 6 LIPO cells (22v2) and some 1200kv motors.

Its a little expensive but a funny project.....

Maybe add a little weight.... Or make it less high in the extra 0.5m you add to the end of the boat.... That would gief you a better grip to the waves, which could make it fly.
Apr 02, 2012, 06:45 PM
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Hi Mike, I agree that something that would be self-righting and sealed enough to power through waves would be pretty good fun - the small water jet boats seem to be good at this - a large boat would be entertaining.

For now, I want to keep the boat small - looking to see how fast the small 2040 motor will push something that's not a 'micro' boat. The new hull is much wider and lower and will have a couple of steps on the bottom. I like the engineering challenge of the design process, you learn a lot this way.

Regards, Richard


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