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Old Nov 09, 2012, 04:57 PM
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Bobbing motion at full speed.

Hi All,
What causes a V hull to have a bobbing motion at full speed? At about 3/4 speed it's very smooth but that little bit extra causes it to have a regular bouncing effect.
Is it a sign of bad balance? I see where COG should be about 30% along the hull (from the stern I assume???)
Is it simply a feature a faster speeds in a V hull - something like the Crackerbox? If so I'll just sit back and enjoy the added excitement.

Cheers,
David
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Old Nov 09, 2012, 05:01 PM
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Canada, ON, Whitby
Joined Aug 2008
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It should be able to be tuned out with some setup. The easiest to try is shifting the packs forward a bit to see if that stops it.
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Old Nov 09, 2012, 06:04 PM
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Hi,
Thanks for getting back. I'll give that a go but perhaps start by taping small lead weights on to the nose and progressively increasing the weight until the bobbing slows or stops. It shouldn't need too much and then I'll see about how much shift I can arrange with the battery pack.

Cheers,
David
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Old Nov 10, 2012, 08:08 AM
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Gordonsville,Virginia
Joined Nov 2009
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If shifting the CofG does not help you may ether have the strut to low or you may have a lifting prop , most V's don't like lifiting props . Only play with one adjustment at a time .
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Old Nov 10, 2012, 02:41 PM
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Hi,
Thanks for the additional tips. If the COG change doesn't do it I'll get back to you for upskilling on the finer points of strut geometry.

Cheers,
David
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Old Nov 11, 2012, 05:57 AM
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Stoke-on-Trent, UK
Joined Nov 2010
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Do you have trim tabs? If so they may require lowering by adjustment, you'll benefit from dropping the right side slightly more due to the torque twist from the prop.

Also as mentioned moving the COG will definately help, if you battery pack positions aren't adjustable then i would make sure they they were. I simply lay 2 lines of velcro out in the hull and two on the packs this allows me to lay them at variable points which i adjust every run depending on the weather conditions (rougher water move them forward, calm waters back) there is a limit to how far so i have marked with permanent marker the optimum COG for various conditions.

I would expect it to be one of the above rather than a proping issue and adding weights as you suggested is just adding weight (obviously) i would try every other avenue before considering adding weight.
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Old Nov 11, 2012, 02:28 PM
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Sneek, Netherlands.
Joined May 2004
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Hi David,

Which boat are you referring to?

Some boat designs have a rocker in the keelline and will start to porpoise above a certain speed, only trimtabs, with which you elongate the waterline will remedy that at the cost of topspeed.

Regards, Jan.
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Old Nov 12, 2012, 12:01 PM
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IMO, trim tabs are a last resort. You should be able to trim out most things without needing them.
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Old Nov 13, 2012, 12:19 AM
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Oh poo - I'm almost too embarrassed to answer this.

About 25 years ago I made a very fast but small boat off a UK plan (2kg FE??)but it had a nasty habit of rolling even on a smooth pond and in a straight line.
I finally had a guts full and bought this horrid NQD Silver Rocket - one of the few single motor cheapies. I gutted the electronics and fitted my own speed controller and a real servo for proportional steering. (I did a build log some months back) I just got around to hiking the battery from the stock 7.2v NiCd to 11.1v LiPo.

For a shitbox it's now a fun boat to use but I was curious as to the bobbing motion at full tick. If it's a smooth lake and it's running at constant speed then what provides the oscillatory movement?
Does it reach a speed where the bow begins to lift, the air drag increases, the prop angle dips and the speed eases back, only to repeat again? What's the physics behind it? What's the alignment of the steerable prop supposed to be?

This is why a bought a cheap boat to play with before trashing something more expensive but I've never used such a drive before.

To its credit the boat survived a nasty uphill rock climb when the Schottky diode across the (brush) motor fractured a lead and the 150A control FET died from repetitive avalanche breakdown. I thought things couldn't be worse than a maxed out speed control when I discovered the steering wouldn't respond either. It hit the rocks (large crushed rock) and came well up the bank but had only a tiny nick in the rudder. I would hate to do this with my heavy, doubled planked 1930 Chris Craft.

I enjoy and appreciate the input from the various gurus here and if these questions have been answered before I would appreciate any links to such.
Link to earlier NQD rebuild http://www.rcgroups.com/forums/showthread.php?t=1634303

Best regards to all,
David
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Old Nov 13, 2012, 06:36 AM
Boaters are nice people.
Sneek, Netherlands.
Joined May 2004
5,122 Posts
Hi David, if your hull looks the same as the one in this link:
http://www.google.nl/imgres?hl=nl&sa...9,r:5,s:0,i:84
Scroll down for HR pics.

Again, if the last third of the hull is straight, it should not porpoise (too bad).
Keep in mind the design was never intended to run as fast as modern motors can propell it, so there is a limit to what the design will take...

Regards, Jan.
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Old Nov 13, 2012, 11:32 PM
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Hi Jan,
That certainly looks like it but of course that boat may be the real deal whereas mine is the knock-off.
I'm not too worried about the bobbing movement as it's not severe and I still have to play with weight and prop angle. At the moment the prop is just slightly angled up relative to the bottom of the boat. The prop angle can be adjusted but has two tiny detents, possibly as preset assembly aids but these can easily be filed off to allow full adjustment.
It's a cheap boat for learning and it's already doing better than hoped for so I should just enjoy it until the wheels fall off.......

Cheers,
David
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Old Nov 15, 2012, 11:22 PM
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280 Posts
Your problem is typically caused by one of a few things:
-Too much positive prop angle
-Prop to low
-incorrect prop

Change your stinger/strut so that it is more towards the negative end of the spectrum (prop pointing down) and see if this fixes your problem. I would not try to fix the problem with trim tabs, as that will create a lot of additional drag.
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Old Nov 16, 2012, 04:20 PM
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Thanks for your comments. Changing prop angle should be easy and I can run trials at the pond to see which gives the best ride.
Can't wait to try that.
Much appreciated.

David
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