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Author Topic: what do i need for this boat  (Read 2856 times)

phillnjack2

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what do i need for this boat
« on: August 27, 2012, 09:28:31 PM »

not sure where to start so ill put down the size of the boat first.
length is 38 inches  width 11 inches at widest point.
the boat is a sort of fast rescue / patrol boat, with the fron deck slightly raised.

The boat is of the planning hull type but the strakes are small as this boat was a prototype of a
full size boat that i dont think ever actually got built,so realy is built to scale.

Now what i want is for the boat to go up onto the plane and atleast 15mph,more if possible.
This is because i want to use it as a rescue boat for my ice boats on the river and at sea.
Due to me using my ic boats in winter as well as summer i need a fast rescue boat due to high water in winter
causing river to flow fast about 10 - 12mph and need the boat to be able to go upstream as well as downstream.

I have tried a mfa barracuda but that engine is very heavy and needs big batterys to get it to rev enough .
so to get on plane with this is a no no.

Ive also tried a decaperm and that was far too slow, got about 4mph from that with more voltage than the motor is rated for.
if i was going to put an ic glow engine i would think i would need about a 30 - 35 size motor to get on plane and a 40 to be sure.

Any advice on type of size electric motor, what esc i would need and what battery's would be very good.

I am not worried about the motor being an outrunner as i will have lot of space and plenty of cold air running through the
boat.
Access to the hull is very big for this size boat as the whole top comes off.
I also dont mind if i have a single or twin installation as again i have plenty of room.
i will post a pic of it soon


phill





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nick_75au

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Re: what do i need for this boat
« Reply #1 on: August 28, 2012, 08:30:33 AM »

I would use a 1200 Kv on 3 cell li-po (12 volts) 36 mm diameter if the prop is around 40-45 mm, if you are using a bigger prop a 42 mm ou-trunner with the same Kv would probably be better, you could increase or decrease the voltage if you select a motor to give approximately the same RPM.

Nick
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essex2visuvesi

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Re: what do i need for this boat
« Reply #2 on: August 28, 2012, 09:13:26 AM »

be careful selecting the ESC tho as lower voltages require more amps to get the same performance
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phillnjack2

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Re: what do i need for this boat
« Reply #3 on: August 28, 2012, 03:16:40 PM »

what sort of run times would i get with this and what type of esc would be needed i.e amps etc

can i rapid charge the lipo battery or is this a big no no  ?

i am very new to brushless and have no idea about esc or chargers etc

i would like about 15 - 20 mins of running time then possible recharge on the spot.
hope this makes sense


can the brushless also run slow i.e for towing a boat back etc



phill




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nick_75au

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Re: what do i need for this boat
« Reply #4 on: August 29, 2012, 08:17:07 AM »

I use a similar set-up in a ski boat and get about 15-20 minutes on a 5000 mAH battery, it draws 40 amps static on a 40 mm brass prop.

90% of Li-po are charge at 1C which means set the charger to the same amp's as the capacity of the battery, it takes about 1 hour to charge at this rate, some newer Li-po's have ratings up to 5c charge so can be charged in as little as 15 minutes. This, for the larger capacities means a heavy duty charger and beefy power supply (big battery etc). This higher charge rate MUST be stated on the battery, charging a 1C battery at a higher rate would be fatal for the battery and possibly catch fire but I would do some reading about Li-pos and their potential hazards before jumping to conclusions ok2

this link has much useful information
http://www.rchelicopterfun.com/rc-lipo-batteries.html

The motors can run as slowly as a brushed motor, this does depend a little on the ESC, some tend to have a less low speed capability than others.

Cheers
Nick
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phillnjack2

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Re: what do i need for this boat
« Reply #5 on: August 29, 2012, 09:10:01 PM »


Ok im beginning to see its very different kettle of fish to what im used to.

what is the benefit of the lipo batts to nimh  ?

its a shame my sla are too heavy as ive got plenty of them  ha ha

just weighed one  5 amp = 4 pound in weight, bit too heavy.

must be a cheap way of getting the power surely

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nick_75au

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Re: what do i need for this boat
« Reply #6 on: August 30, 2012, 08:03:42 AM »

The big advantage is power to weight ratio, Li-pos pack a lot more power in a smaller and lighter space, and are almost as cheap as an SLA with far superior performance and probably cheaper than a good quality Ni-mh that generally have a capacity of 4200 Mah for a high discharge type which realistically are limited to about 35 amps.

5 amp hour 3 cell 2C Li-po is $28 Us Dollars , it can output 125 amps , an SLA looses a huge amount of capacity at anything over 1/2 C rate discharge, so your 5 AH SLA will give about 2 amp hours at 2C (10 amps) Li-pos will output nearly their whole nameplate capacity even at high C discharge, a 25C Lipo will probably still give 4 to 4.5 AH at 25C discharge.

The Lipo weighs 450 grams, the SLA weighs more than 4 times as much at 1900 grams and the Ni-MH around 700-1000 grams.

Nick
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phillnjack2

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Re: what do i need for this boat
« Reply #7 on: August 31, 2012, 11:39:20 PM »

ok i think im now getting it  (but probably not)

if using one motor does this seem an ok set up 
 (i am just trying to get to grips and using this as example)

the motor im thinking about is this
Input Voltage : 7.4V~11.1V (2~3S Li-po)......Kv : 1450rpm/V within 10%
Max. efficiency current : 15~25A............Current capacity : 35A/30s
Dimensions : 35mm X 36mm / 1.38in X 1.41in.......Shaft diameter : 4mm / 0.16in
Weight : 106g / 3.74oz..........Number of poles/magnets : 14
Power equivalent : .25 to .30 IC glow engine......Power: 400W+

the battery this
battery Spec.   
Minimum Capacity: 5800mAh .....Configuration: 3S1P / 11.1v / 3Cell
Constant Discharge: 25C......Peak Discharge (10sec): 35C
Pack Weight: 482g.....Pack Size: 149 x 49 x 30mm
Charge Plug: JST-XH ..........Discharge plug: 5.5mm Bullet-connector
   


i have no idea about what esc would be needed though for this set up, and what is the 25c stuff ?

sorry to sound ignorant about all this  but i grew up with old money and feet and inches, when  a li poly was a
little baby parrot
to me a battery is something with just positive and negative, now its all getting a bit i tech.




phill
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nick_75au

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Re: what do i need for this boat
« Reply #8 on: September 01, 2012, 02:42:46 AM »

I think the Kv may be a little high on the motor but it really depends on the prop, an increase in RPM makes a huge difference on the amp draw.

The battery is fine, it will give you a good run time.  "C" is the capacity of the battery and the C rating is the discharge capability of the battery, I'm going to point you back to the link I posted previously because it really explains these things better than I can ok2 Suffice to say though the battery has more than high enough ratings for what you need

I would get a minimum 40 amp ESC but would seriously recommend a 60 amp or more to give a little "headroom"

Cheers
Nick
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essex2visuvesi

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Re: what do i need for this boat
« Reply #9 on: September 01, 2012, 08:33:10 AM »

I think the Kv may be a little high on the motor but it really depends on the prop, an increase in RPM makes a huge difference on the amp draw.

The battery is fine, it will give you a good run time.  "C" is the capacity of the battery and the C rating is the discharge capability of the battery, I'm going to point you back to the link I posted previously because it really explains these things better than I can ok2 Suffice to say though the battery has more than high enough ratings for what you need

I would get a minimum 40 amp ESC but would seriously recommend a 60 amp or more to give a little "headroom"

Cheers
Nick

And if you are buying from "Value" controllers from China and Hong Kong its wise to go to 80-100A as Chinese amps are somewhat smaller
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