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Author Topic: robbe najade problem - boat almost flips!  (Read 8668 times)

Stunts

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robbe najade problem - boat almost flips!
« on: July 11, 2010, 08:00:54 AM »

Hi all, newbie on the forum, but I'm after some experienced help please. I am running a kit standard brushless equipped Robbe Najade. It runs a 35mm sport 2 blade prop on 2 x 8.4v nimh's in series (16.8v) as one of the options. I also run on 11.1v lipo's in series. The boat goes like stink, but there's an issue. When travelling at speed (near to full throttle) the boat will suddenly and inexplicably flip onto its side. It did this the other day and the coupler from the motor to the prop shaft came adrift. I had difficulty recovering the boat.

Can anyone tell me how to prevent the problem short of dropping the speed down on purpose? Contra-rotating props are not an option!

Is the boat suffering from torque roll? If so, why is it happening at near to full revs rather than lower down the range?

Thanks in advance,
Simon
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Mi Amigo

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Re: robbe najade problem - boat almost flips!
« Reply #1 on: July 11, 2010, 09:16:57 PM »

I can't say I am an expert but common sense would lead me to believe that when the hull gets up on the plane, (at nearly full throttle), there isn't very much of that thick viscous h20 preventing the hull from trying to prove Newtons third law is true.

The only suggestion I could make is to maybe experiment with a Contra-rotating cooling fan or flywheel inside the hull, something like a small brushless ducted fan, it probably wouldn't need to be nearly as powerful as the drive motor to help a lot.

Of course it would burn extra amps.
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scoop

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Re: robbe najade problem - boat almost flips!
« Reply #2 on: July 12, 2010, 01:36:42 AM »

Hi all, newbie on the forum, but I'm after some experienced help please. I am running a kit standard brushless equipped Robbe Najade. It runs a 35mm sport 2 blade prop on 2 x 8.4v nimh's in series (16.8v) as one of the options. I also run on 11.1v lipo's in series. The boat goes like stink, but there's an issue. When travelling at speed (near to full throttle) the boat will suddenly and inexplicably flip onto its side. It did this the other day and the coupler from the motor to the prop shaft came adrift. I had difficulty recovering the boat.

Can anyone tell me how to prevent the problem short of dropping the speed down on purpose? Contra-rotating props are not an option!

Is the boat suffering from torque roll? If so, why is it happening at near to full revs rather than lower down the range?

Thanks in advance,
Simon
Your last sentence say's it all, your boat is probably suffering from torque, some of the Club 500's had the same problem if the balance set up was wrong but not as bad as you are suggesting. I assume it's rolling onto it's starboard side, one way round this is to mount most of the gear (batteries etc) on the port side, another obvious way is to go a bit slower or throttle back as soon as it happens. You could also fit a less powerful motor(s). If it's speed you want buy a fast electric hull that is built and designed for speed and the more powerful motors. Most manufacturers of ARTF/boat kits supply the proper sized engine/battery configuration for their product, any upgrades may be too powerful for that particular hull design.

Regards
Scoop
ps If it's speed you are after you could always come racing with us at the British Model Powerboat racing Society (I.C engines only) see www.bmprs.co.uk for details
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Stunts

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Re: robbe najade problem - boat almost flips!
« Reply #3 on: July 12, 2010, 09:45:25 AM »

Hadn't thought of the fact of less H2O on the hull - d'oh! Ironically enough the boat is sporting the recommended power system - the Robbe brushless motor came with the boat and the speedo was the recommended one for the motor  {:-{ I'm thinking that the hull just may not be heavy enough. Even with 28 nimh cells in it....... (14 for power and 14 for ballast!)

It does roll to the starboard when it does the flip, the problem is that it does it so quickly that it's difficult to catch. I wanted an impressive motor yacht with a good turn of speed which is why I bought the Najade. Shame it doesn't come up to spec. I have tried dropping the voltage down and going slower but it has such a dramatic effect that it slows the boat far too much. I guess I'll just have to experiment with the cells used and try to find a happy medium of speed and stability  {:-{
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steamboatbob

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Re: robbe najade problem - boat almost flips!
« Reply #4 on: July 12, 2010, 12:28:44 PM »

I would actually sugest putting all the cells on the port side of the boat

If you had a video of it in action then that would be better for us to identify the problem

Cheers

Bob
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John Mk2

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Re: robbe najade problem - boat almost flips!
« Reply #5 on: July 12, 2010, 01:03:52 PM »

A mate of mine has one.  Running on 700bb (8.4V) 14cells 60amp e.s.c. Just right now :-)) He tryed more power & it was VERY unstable on turns  {:-{The hull does not cope high speed turns (To Much FreeBoard) So tends to fall off the plane.  
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Martin [Admin]

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Re: robbe najade problem - boat almost flips!
« Reply #7 on: July 12, 2010, 01:14:25 PM »

I sounds like the kind of problems I was having many years ago with my SHG Shadow.
In the end it was just too much power for the hull design...... {:-{


 http://www.modelboatmayhem.co.uk/My_models/02%20shadow.htm
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DickyD

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Re: robbe najade problem - boat almost flips!
« Reply #8 on: July 12, 2010, 01:15:16 PM »

Contact Ghost in the shell, he has one of these.
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Stunts

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Re: robbe najade problem - boat almost flips!
« Reply #9 on: July 12, 2010, 01:29:20 PM »

Thanks for the responses all. My boat goes approximately twice as fast as the one in the clip - I guess a triumph of power over design then  {:-{

I'll re-do the connector to run the 11.1v lipos I have in parallel, rather than the full on 16.8 - 22.2v beast!

Or maybe I'll just sell it and get a big, slow container or tug boat!  :}
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John Mk2

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Re: robbe najade problem - boat almost flips!
« Reply #10 on: July 12, 2010, 02:07:39 PM »

Why dont you just get a hull that can handle  the speed ? Lots of good hulls around :-))
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hmsantrim

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Re: robbe najade problem - boat almost flips!
« Reply #12 on: August 02, 2010, 10:03:43 PM »

 Hi Stunts.
                I am no expert here but I don`t see anyone suggesting TRIM TABS ...you could also move some weight forward to keep
               the bow down when the speed comes up, more contact with the H2o.

                                                          cheers Frank
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Stunts

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Re: robbe najade problem - boat almost flips!
« Reply #13 on: August 08, 2010, 09:18:31 AM »

Thanks all. I'm going to give it one more go, otherwise I might sell and buy myself a Taming big rig.
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Drkomen86

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Ghost in the shell

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Re: robbe najade problem - boat almost flips!
« Reply #16 on: August 09, 2010, 03:10:43 PM »

i run mine on 9.6v, seems to behave itself, though can sometimes act up on turns.  change the prop to one with a little less bite, say a raboesch 3 or 4 blade 40mm scale prop and try running her on 3s Lipo butt add some lead in her to bring her to the same weight as running with the 2x 8.4v packs in it.  running at speed she should run like this


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Stunts

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Re: robbe najade problem - boat almost flips!
« Reply #17 on: August 10, 2010, 12:53:42 PM »

Here it is on Wardown Park Lake prior to losing drive (again...... Darned props dropping off!) There's a lot of tree debris, so high speed runs were difficult   :-)

http://www.youtube.com/watch?v=RRUfwAFeJSE
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Dekan

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Re: robbe najade problem - boat almost flips!
« Reply #18 on: August 23, 2010, 10:14:40 PM »

I saw my first Najade last sunday, for its size it weighs very little with its stock setup.

I'm not surprised that it would almost take off with some serious power.

The hull IMO seem much too high to be stable with the kind of power being used... Even with the stock power setup  it seems to bounce up and down a lot. :}
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