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Author Topic: First post/First RC Boat build - Advice needed.  (Read 7435 times)

pompebled

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Re: First post/First RC Boat build - Advice needed.
« Reply #25 on: August 27, 2013, 06:33:06 pm »

Hi Riley,

With a surface piercing drive, the stuffing tube sticks out of the transom between 10 and 13% of the hull length, in your case 6 - 7.8 cm.
The angle of the propshaft must be as shallow as possible, usually meaning a very long propshaft, or a flexshaft.
Too steep will lead to a catastrophic running attitude.
As an example the business end of one of my monohulls, a 63 cm boat with a 32 cm long propshaft.



Your shafts are short(ish) with only 25,5 cm between the motorflange and the rear bearing (I have two of these shafts).
The motor has to sit as low in the hull as possible, no Chinese iron angle, but a proper motor bulkhead, which allows the motor to almost touch the bottom.

When the boat sits on a flat surface, the rear bearing has to be about 2 mm above that flat surface:



Ignore the position of the rudderblade, that has to sit next to the prop on a monohull, see my first picture.

As your motor will have to slide back 6-7 cm, the propshaft angle will be too steep, and you'll need to drill new holes in the transom and bulkhead.

As you'll have two rudders (or none and steer with the props via a mixer function) I'd position them behind the props, this means a longer outholder for the rudders.

To be honest, I'm not comfortable with the flat bottomed hulls without any curving up at the bow, my guess is you'll be in for a very 'adventurous' running attitude, to put it mildly...

The two long bolts that come with these stuffing tubes are there because these shafts also live in the boats build by Dragon, which are powered by outrunners and cooled by an extra water cooled spacer (also available at HK), 9 mm thick, with a 10 mm hole in the middle for the solid coupler and the shaft.

As that the motorflange only allows for a Ø9,5 mm coupler, the hole is Ø10 mm, you'll understand these shafts didn't last long in my boats and were replaced by standard size motorflanges and propshafts, with standard size couplers.

Here the stock shaft with an aftermarket inrunner, notice the extreme close fit of the stock coupler:



And the new, longer propshaft with the stock outrunner on a home made watercooler:



I'm not sure if this is going to work, I already pointed out that the motors chosen aren't very suitable.
These revs are intended for a fast electric, not a fast moving camera platform with flat sponsons.

Running at partial throttle a lot, risks overheating both motor and ESC.

Regards, Jan.

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pompebled

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Re: First post/First RC Boat build - Advice needed.
« Reply #26 on: August 28, 2013, 08:49:08 am »

Hi Riley,

I forgot to mention to make sure the outlet is at the highest point to prevent trapping air in the jacket.

Regards, Jan.


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Riley_NZL

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Re: First post/First RC Boat build - Advice needed.
« Reply #27 on: September 04, 2013, 07:00:20 am »

Thanks Jan, due to the problems you've mentioned here I'm looking at a slower turning outrunner, would something like this make much difference? 
http://www.hobbyking.com/hobbyking/store/__5690__Turnigy_2217_16turn_1050kv_23A_Outrunner.html
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pompebled

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Re: First post/First RC Boat build - Advice needed.
« Reply #28 on: September 04, 2013, 04:48:46 pm »

Hi Riley,

On what grounds did you select this motor if I may ask?

With that motor on 3S you'll have about 10.000 rpm under load, for a surface piercing drive, that's way too low.
I assume you'll be going for a subsurface drive, with the prop under the hull.

In that case 10.000 rpm is the 'limit', depending on the size and pitch of the prop chosen.
As your hulls are relatively shallow, cavitation can be an issue with those high revs.
Lower is better in this case, compensate with bigger props which give you more authority as bigger props are more efficient.

The stuffing tubes you've bought are intended for a fast electric and are not suitable for a submerged drive, as there's just the one bearing at the end, which is a ballbearing if I'm not mistaken.
This type will fill with water, so unless the inside is (far) above the waterline, you'll have a leaking propshaft assembly.
Far above the waterline would mean a steep propshaft angle, which you do not want, it'll push the bow down and make the boat rather 'unseaworthy'...

Even with the inside above the waterline, water will travel up the shaft and be splattered into the boat by the coupler and the motor.

You'll need a second bearing close to the motorflange, so the stuffing tube can be filled with grease to stop the water ingress.
Mind you, the ballbearing at the rear end is by no means watertight, so water will always find it's way up the stuffing tube.

Regards, Jan.
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Riley_NZL

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Re: First post/First RC Boat build - Advice needed.
« Reply #29 on: September 04, 2013, 04:55:52 pm »

I picked that motor as it was the lowest Kv one I could find that draws under 30 amps. Are there prop shafts you can buy designed for submerged props, or is the 2nd bearing and grease method you described what most people do?


I could also buy some 2 cell batteries to drop the RPM futhur and build a Fast electric boat at a later time to use the motors and batteries I already have.
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pompebled

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Re: First post/First RC Boat build - Advice needed.
« Reply #30 on: September 04, 2013, 08:06:26 pm »

Hi Riley,

It's a very bad picture, but you'll need something like this, with a bushing on both ends and a propshaft with the thread that will allow you to run the props that are available.
http://www.rcnz.com/Shop/Service/Detail/Product/3029/Boat_Shafts___Rods/SAB___Bronze_Tube_250mm_5_32_M4_End.html
Personally I'd choose a M4 threaded end as the Graupner props are efficient and cheap, also Raboesch comes with M4 thread.USe a coupler with the fitting inserts to go on your motorshaft and propshaft:
http://www.rcnz.com/Shop/Service/Engine?PrCategoryLink=290
Props:
http://www.rcnz.com/Shop/Service/Detail/Product/332/Boat_Propellers/RCNZ___35mm_Plastic_Marine_Prop.html
Or the Dumas/Raboesch bronze props.

A motormount is easy to make out of aluminum angled material, or sheet
Check the pictures in my M.A.S. thread for inspiration.

Regards, Jan.
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