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Author Topic: Propulsion advice  (Read 5848 times)

Shipmate60

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Re: Propulsion advice
« Reply #25 on: May 21, 2007, 10:54:42 PM »

If the motor is giving that duration (but on what size battery) and is only warm, you have 2 options.
To try a larger prop but make sure you keep monitoring temp, or fit a higher performance motor.
I assume you dont want to go to 2 shafts?

Bob
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DMSmith

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Re: Propulsion advice
« Reply #26 on: May 21, 2007, 11:20:51 PM »

Bob & Malcolm
Thanks.
I'm using 2x 6V 4A jelly cells in parallel, and yes I now think I'll stick with one motor if I can.
You mention the Decaperm o/put via g/box is c 2750rpm. That makes sense looking at the prop free running speed. However I'm even more confused when I look in Howes catalogue and see motors like Graupner 3308.700 turbo 4.8-14.4v 15,000 rpm. This appears to be a direct drive motor. Should I use a gearbox? What voltage should I run it at?
Other motors are also listed e.g MFA torpedo 800 12V 5167rpm & MFA torpedo 850 12V 9778rpm
Having purchased a 15A speed controller I guess next step would be 2 more batteries for series parallel set up 12V 8A to keep current down. Now which motor to go for?  ???
As before, any ideas welcome
Cheers
Dave
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malcolmfrary

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Re: Propulsion advice
« Reply #27 on: May 21, 2007, 11:35:10 PM »

The best way to increase performance with a particular motor is to reduce weight.  It might well be that using just one 6V 4A battery will increase perfomance and still give an acceptable run time.  Only problem then is how to mount the battery and still leave the boat balanced.
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Shipmate60

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Re: Propulsion advice
« Reply #28 on: May 23, 2007, 07:26:42 PM »

DMSmith,
if you go for a motor with much higher performance on direst drive you will have to reduse the prop size.
The faster motors will struggle to turn a large prop and pull horrible amounts of current.
These motors need to be able to rev, so you must remove the load by reducing the prop size.
It might be worth asking some of the fast electric boys how they would go about it.
I have 2 x Graupner speed 600's on an epicyclic gearbox.
These motors go like the clappers, but pull over 6 amps in air withput a shaft.
One of these would suit, but you would need a boot full of batteries!!!
I haven't found a use for these yet!!

Bob
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DMSmith

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Re: Propulsion advice
« Reply #29 on: May 23, 2007, 11:32:44 PM »

Thanks chaps for the advice again. I'm pretty much getting the picture. Its very much a question of boat mass (displacement) v's thrust. Small prop big rpm, large prop lower rpm and finally current required dictating battery size bringing me back to weight again. I'm struggling to remember my o'level physics from the 70's but its coming back slowly.
The Decaperm with its gearing is able to turn the 55mm prop OK but is pretty much on its limit. It does appear to give what looks like a fair scale speed. Although reading up as best I can about this motor makes me realise it is better suited to a model tug boat or similar.
I'm a bit worried about the thought of 5,000 or 10,000 rpm through the current (old) shaft and the ability of  my children to handle what could become a real missile. So I'll leave as is for this summer and we can all get some fun out of it until motor or shaft gives up.
On the subject of scale speed what guidelines do you follow? My guess is that a real Perkasa is c.100' so 4' = 1:25 scale so 50mph top speed would translate to 2 mph. Obviously that would not look at all right and the boat could not plane at that speed. My guess is she can reach 5mph at present although not timed. Is that a fair enough speed or should I aim higher?

Thanks for your time and trouble

Dave

ps I'm trying to talk my friend, who gave me the boat, into letting me have a go at getting his 30" Fairey Huntsman going using what I have learned from you.
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Shipmate60

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Re: Propulsion advice
« Reply #30 on: May 24, 2007, 12:01:08 AM »

Dave,
As a rule of thumb, if she looks right she IS right.
I can give you the formula, but why bother. As I have said in previous threads I ALWAYS overpower my models as the wind and waves aren't scale.
As to fast patrol boats MY preference is for 2 shafts, not for power, but if it is planing with over 1/2 the length out of the water, 2 props "digging" in the water makes the hull that little bit more stable.
If you are happy with the deccaperm stay with it, but these motors reach 25 plus on eBay which is just about enough to get 2 x Motors and shafts and fast props.
But you might have to upgrade speed controller.

Bob
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boatmadman

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Re: Propulsion advice
« Reply #31 on: May 24, 2007, 12:18:58 AM »

I agree with the looks right / is right approach.

Its v hard to scale speed, cos water and waves (ripples) dont scale.

Ian
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malcolmfrary

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Re: Propulsion advice
« Reply #32 on: May 24, 2007, 01:09:23 PM »

Speed does scale, thats why test tanks work.  To get the scale speed, divide the speed of the original by the square root of the scale - in this case, sq rt 25 = 5, so 50/5 = 10 mph.  The waves produced will look right, the only thing that doesn't scale is the creamyness of the wash, as the bubbles produced are full size.  This is a function of surface tension, and we are stuck with it.
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boatmadman

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Re: Propulsion advice
« Reply #33 on: May 24, 2007, 02:08:40 PM »

Thats me told!  ;)
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DMSmith

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Re: Propulsion advice
« Reply #34 on: May 24, 2007, 10:04:18 PM »

Thanks chaps, I'll be taking my stopwatch to the Marina next time, this is facsinating stuff. If she doesn't make 8mph at least, then piggy gets the hammer and it'l be new engine time. However the beasts below look a little pricey.


http://www.pandanmodelboats.co.uk/acatalog/Our_Online_Catalogue_Graupner___Robbe_and_Geared_Motors___Drive_Systems_250.html#a581

Cheers again

Dave
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funtimefrankie

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Re: Propulsion advice
« Reply #35 on: May 25, 2007, 08:52:58 AM »

If you have access to a hand held GPS...  you could send the boat out with it on board. Mine gives max MPH.
Adds a bit of weight though.
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