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Author Topic: UTE  (Read 2460 times)

GG

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UTE
« on: March 31, 2012, 02:41:31 PM »

Having just completed an aircraft carrier, I needed something a little less demanding for the next model.  Luckily last years holiday photographs came to the rescue.  We had spent part of the holiday visiting friends who lived next to the St Lawrence Seaway.  Their town's small dock contained several boats which where quite naturally photographed as possible building projects.  Looking at these photos again and the idea of a simple model based on the sports/utility/workboat types could be the modest challenge I desired.
It took a fair amount of drawing, rubbing out and redrawing plus some experimental sheet bending before a likely design was produced.  It did seem awfully "boxy" but hopefully the subtle bends and curves might fool the water into flowing smoothly around it.  The basic structure was built from balsa and liteply to give a light but surprisingly strong hull about 24 inches (60 cm) long and 8 inches (20 cm) beam.  Power was to be a mild 540 type of motor connected to a Graupner combined gearbox/motor mount driving a 45 mm dia two bladed plastic propeller through 2.5:1 gears.
The completed model with RC and a six cell Nimh battery pack only weighed in at 3.5 pounds (1.6 kg) and the model appeared to sit "on" rather than "in" the water.  The driveline only drew a current of 3.3 Amps at full power, a modest value that ought to ensure long sailing sessions.  The top speed proved to be around 5 feet/sec (1.5 m/s) at which the bows are clear of the water but not quite fully planing.
The handling, which I was initially worried about, proved to be good with no bad habits at any speed.  The simple gently curved hull bottom working surprisingly well.  Moving astern was something else as once the model started to turn then there was nothing you could do to stop it.  This was not unexpected as the model had a large flat transom and substantial skeg, both features which tend to make models have poor astern steering in my experience.
So, easy to build, relaxing to sail, a perfect way to while away the time whilst thinking about what to build next
Glynn Guest



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Tug-Kenny

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Re: UTE
« Reply #1 on: April 01, 2012, 11:26:44 AM »


A very tidy looking boat Glynn.   :-))    I was curious about the gears and current consumption.

Does this mean the load on the motor, although travelling a faster speed is actually drawing reduced amps when geared down?  I'm all for saving battery power at the lake and have never yet had gears in any of my boats.

Just a thought

regards

ken
 
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GG

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Re: UTE
« Reply #2 on: April 01, 2012, 12:45:59 PM »

Kenny,
        Gears and for that matter Pulleys, are only needed when there is a serious mismatch between the speed that you want to run the motor at and the speed that the propeller needs to turn for the desired models performance.
In this case the 540 motor needed to run at over several thousand RPM and the model needed a modest performance in keeping with the type of boat it was supposed to represent.  Direct drive would have required the use of a small propeller, very small diameter and fine pitch, which is often not an efficient combination in such a model.  The gears allowed the use of a larger but slower turning propeller which, certainly in this case, is more efficient but the motor can run close to it's maximum efficiency which is probably about 75-80% of the No-Load speed.
Finding the "sweet spot" with gears/pulleys and propellers might take a little experimentation.  In this respect the Graupner gearbox/motor mount is very handy as three different gear ratios can be easily tested.
Of course, models which a generous amount of energy available don't have to worry about efficiency, although combustion from overloaded motors might be a problem?
Glynn Guest
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Krishna

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Re: UTE
« Reply #3 on: April 01, 2012, 04:17:54 PM »

stunningly beautiful craft GG..  :-))
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Mad_Mike

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Re: UTE
« Reply #4 on: April 01, 2012, 10:04:56 PM »

A very tidy looking boat Glynn.   :-))    I was curious about the gears and current consumption.

Does this mean the load on the motor, although travelling a faster speed is actually drawing reduced amps when geared down?  I'm all for saving battery power at the lake and have never yet had gears in any of my boats.

Just a thought

regards

ken
 

when i made my trawler i put a gearbox in to reduce the speed. It has a 540 motor turning a 40mm 3 blade prop threw a 2.5-1 reduction gearbox. the batteries are 2000mah and it runs for the best part of an hour. In fact ive never run it flat yet because i got bored with it before it went flat.
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Tug-Kenny

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Re: UTE
« Reply #5 on: April 02, 2012, 11:37:40 AM »


Thank you for input and clarifying the reasons. I might try gears with a larger Prop next time.

cheers

ken

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