Thread: Discussion Bobbing motion at full speed.
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Old Nov 12, 2012, 11:19 PM
KiwiDavid is offline
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Joined Jun 2011
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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

Best regards to all,
KiwiDavid is offline Find More Posts by KiwiDavid
Last edited by KiwiDavid; Nov 12, 2012 at 11:27 PM. Reason: Link added.
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