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Author Topic: What prop do I need ??  (Read 7508 times)

Drkomen86

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What prop do I need ??
« on: October 06, 2010, 06:01:49 PM »

I'm in total confusion at the moment, I have just fitted a 3600kv brushless motor, a 60A ESC and a 3cell 5000mAh Lipo to my 42" Cat.
The only problem is I'm not quite sure what prop to put on it, I have a few here ranging from 30mm - 50mm and all they do is froth up the water behind it an not really move.

Please can some body help as it is quite upsetting when my yacht is faster than my speed boat  :}
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essex2visuvesi

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Re: What prop do I need ??
« Reply #1 on: October 06, 2010, 06:14:17 PM »

I'm in total confusion at the moment, I have just fitted a 3600kv brushless motor, a 60A ESC and a 3cell 5000mAh Lipo to my 42" Cat.
The only problem is I'm not quite sure what prop to put on it, I have a few here ranging from 30mm - 50mm and all they do is froth up the water behind it an not really move.

Please can some body help as it is quite upsetting when my yacht is faster than my speed boat  :}

3600rpm per volt and 11 volts, that prop is spinning some!  :o
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Drkomen86

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Re: What prop do I need ??
« Reply #2 on: October 06, 2010, 06:17:29 PM »

if thats how its worked out then yes but I'm not to sure  :embarrassed:
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Sub driver

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Re: What prop do I need ??
« Reply #3 on: October 06, 2010, 07:47:51 PM »

Is it surface piercing or submerged drive etc.???

if its frothing the water it is cavitating, ie not gripping the water, just dragging air in from the surface,  it could be a load of reasons depending on the question ie too light in the stern etc if its submerged drive etc etc, but you need to ascertain what type of drive it should have. 

sub.
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Drkomen86

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Re: What prop do I need ??
« Reply #4 on: October 06, 2010, 08:08:34 PM »

here is a picture, hope it helps
 

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w3bby

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Re: What prop do I need ??
« Reply #5 on: October 07, 2010, 07:25:25 AM »

Hard to see from the pic but a rough guess is that your prop is way too high. Take a couple more pics, one from directly behind the boat and one of the back of the boat from the side.

w3bby

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Re: What prop do I need ??
« Reply #6 on: October 07, 2010, 07:40:28 AM »

Found a pic on RCU that gives a better idea of placement.
You have removed all possibilities of trimming your cat to run at its best by not using an adjustable strut. Are you using a flex cable?

HS93 (RIP)

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Re: What prop do I need ??
« Reply #7 on: October 07, 2010, 07:45:57 AM »

what are the other figures of the motor, just the KV is not enough info as you can get a very small 3600kv or a very large 3600kv yes the voltage will give the revs but you know the size of motor to get a prop size.


Peter
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Sub driver

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Re: What prop do I need ??
« Reply #8 on: October 07, 2010, 08:58:49 AM »

drkomen 86  that to me looks like it should be a surface piercing set up and the prop from the pic looks way. to close to the transom. useually the are away from the transom and on flexi drives and run clever type props as per the other pic posted. the race boys should be able to help more post a pic of the stern side on.  regards sub.
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Drkomen86

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Re: What prop do I need ??
« Reply #9 on: October 07, 2010, 10:58:47 AM »

Rotational Speed    3600 RPM/V
Continuous Current    45A
Max. Current            52A
Input Voltage            8 - 18V
Max. Efficiency            90%
No Load Current     1.9A
Internal Resistance    0.0119Ω

Hope thats better  :-)

Its a solid shaft and was pre fitted to hull when bought so would be a massive job to change  {:-{





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martno1fan

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Re: What prop do I need ??
« Reply #10 on: October 07, 2010, 11:11:05 AM »

Props way too high and looks way too small to move the boat well ,look at the size we gas boaters use for 42" cats theyre arround 65-67 mm minimum as seen in Ians pic,you def need to ditch the solid shaft and switch to a  flex setup in my opinion,easy enough to do then use a strut on the transom which will allow you adjustment to get the right setting on the prop depth etc.Usually they start by sitting the boat on a table with rudder hanging over the edge and drop the strut so it sits flush with the sponsons and then the adjustments start from there making small adjustmenst after each run till you find the best setting for speed and handling.Cant help with the motor im afraid as i only run sparkies.
Mart
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w3bby

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Re: What prop do I need ??
« Reply #11 on: October 07, 2010, 11:43:26 AM »

Sorry but as Martin says if you want the boat to work you have no option but to rebuild the whole of the drive train and your best option would be a flex cable setup.
I can't help you with regard to whether the motor is capable of pushing the boat to a reasonable speed as electrics are not my thing. However a quick trip to and a look around http://www.xtremercboats.com/phpBB/index.php coupled with a few questions should get you some good answers.

To help you on your way here is a build thread for a catamaran that should help answer questions about the installation of the necessary hardware http://www.modelgasboats.com/Magazine_Content/Boats/AC_R-42_Cat/. I'm afraid it is a gas build but the principles are the same.

John Mk2

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Re: What prop do I need ??
« Reply #12 on: October 07, 2010, 11:46:58 AM »

Martins spot on :-)) Also point of balance (c of g) i found 50% (ish) works for me  ok2 Again you must be able to alter the height to get it right !

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John Mk2

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Re: What prop do I need ??
« Reply #13 on: October 07, 2010, 11:48:24 AM »



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Drkomen86

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Re: What prop do I need ??
« Reply #14 on: October 07, 2010, 12:01:10 PM »

thanks for all the help, looks like I have alot of work ahead of me then  {:-{
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w3bby

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Re: What prop do I need ??
« Reply #15 on: October 07, 2010, 12:02:12 PM »

CG at 50% :o :o :o Wow, that is high...... More normal around the 30-35% mark.... Are you measuring from the absolute back of the overhang or the back of the running surface?

John Mk2

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Re: What prop do I need ??
« Reply #16 on: October 07, 2010, 12:52:49 PM »

Works for me  ok2 To far back & she porpoises & trys to fly {:-{
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ghost123uk

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Re: What prop do I need ??
« Reply #17 on: October 07, 2010, 01:18:05 PM »

thanks for all the help, looks like I have allot of work ahead of me then  {:-{

I am no expert on these type of boats (as I run deep V hulls with glow motors and fully submerged props) - however I don't think adding a suitable prop tube & flexi shaft, plus the adjustable strut shouldn't be too much of a job  :-)  - A bit of dremmelling and some fibreglass = a pleasant way of spending a wet weekend  :-))

I take it you know where to get a shaft & strut from ?
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martno1fan

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Re: What prop do I need ??
« Reply #18 on: October 07, 2010, 01:20:29 PM »

CG at 50% :o :o :o Wow, that is high...... More normal around the 30-35% mark.... Are you measuring from the absolute back of the overhang or the back of the running surface?

I agree with Ian  :o :o
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John Mk2

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Re: What prop do I need ??
« Reply #19 on: October 07, 2010, 01:26:45 PM »

Well Iam Happy with it ok2 And thats the main thing %)
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martno1fan

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Re: What prop do I need ??
« Reply #20 on: October 07, 2010, 01:43:06 PM »

You would be even more happy if the cog was at 30-35 % its where most fast boats balance at ,if you go further forward they run too wet so your losing a lot of speed.If its flighty then its down to adjusting the strut angle to trim the nose down a bit.But if your happy with it its your boat  ;) ;)
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John Mk2

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Re: What prop do I need ??
« Reply #21 on: October 07, 2010, 01:52:01 PM »

She does run flat  ok2 Turns Very well & goes like stink on lipo 4s  :-) On 6s  :o :o %% %%
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John Mk2

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Re: What prop do I need ??
« Reply #22 on: October 07, 2010, 02:18:57 PM »

Almost as quick as this one  :-) More than fast enough for an old f*rt like me :}

http://www.youtube.com/watch?v=KMqcX8Oy_Mg&feature=related
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Drkomen86

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Re: What prop do I need ??
« Reply #23 on: October 07, 2010, 04:01:08 PM »

I'm buying it all from ebay as a kit for 38 unless anybody knows where I can get it cheaper  :-))

I realy need cheap as this project was ment to cost nothing and with no job at the moment keeping my hobby alive is very hard  <:(
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martno1fan

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Re: What prop do I need ??
« Reply #24 on: October 07, 2010, 04:56:54 PM »

How about a link to this kit then we can have a look ?,i know a good place in China for hardware if you dont mind waiting a week or so.
Mart
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