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Author Topic: prop size?  (Read 3021 times)

steveb6

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prop size?
« on: July 04, 2013, 10:56:02 PM »

Hello to all,  I built a ep speed boat a few months ago and have not had much luck with prop selection,  need a little help.   The hull is an old 900mm long racing shallow v and it used to have a submerged drive and an ic motor.   I stripped out the old knackered stuff and put in a 4074 leopard motor, 180 amp esc and surface drive. On the first run  with a x443 prop,  it only managed about 20 mph, so I tried a 3 blade 437.  this made it run even slower.
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Mad_Mike

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Re: prop size?
« Reply #1 on: July 04, 2013, 11:48:36 PM »

what kv is the motor? Are you using lipos? how many cells?
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w3bby

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Re: prop size?
« Reply #2 on: July 05, 2013, 02:58:49 PM »

Some more pics of your stern set up would help and a better description. Is the motor revving up?

steveb6

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Re: prop size?
« Reply #3 on: July 05, 2013, 07:16:08 PM »

Motor is a 2000kv  , running 5s  40c lipo ,   the motor spins up fine, it seems that the prop isn't getting any purchase on the water. lots of spray but no go. 
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AlanP

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Re: prop size?
« Reply #4 on: July 05, 2013, 08:09:42 PM »

I think perhaps your prop is too high, try lowering it a bit at a time, think you might find this sorts out you problem

Alan
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steveb6

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Re: prop size?
« Reply #5 on: July 05, 2013, 09:06:40 PM »

Cheers Allan, will give it a go.      Steve.
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w3bby

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Re: prop size?
« Reply #6 on: July 06, 2013, 01:07:46 AM »

The extra pic didn't really help... A lower angle direct at the transom so we can see the position of the hardware and one from the side would be more useful..

Prop too high sounds reasonable, how high is your prop shaft centreline from the bottom of the V?

MPM

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Re: prop size?
« Reply #7 on: July 06, 2013, 09:52:37 AM »

You've done a really nice job there buddy. Top build!!  :}

steveb6

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Re: prop size?
« Reply #8 on: July 07, 2013, 10:52:28 PM »

Ran the boat today with the lowered prop, ran a lot better,  aprox 30 mph.  the exit of the propshaft  from the transom is about 8mm above the bottom of the hull, I am going to lower this so that the centre line of the prop is in line with the bottom of the hull as I have run out of adjustment to get it lower,    Running a 642 octura prop,  too small?
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black magic racing

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Re: prop size?
« Reply #9 on: July 07, 2013, 11:16:25 PM »

hi
you want to make sure that the bottom of the stinger is level with the bottom of the hull,use a straight edge :-))
you dont want it lower than the hull as this will force the front end down,
once you have done that you then need to have the prop at maybe 2 degree up from hull bottom to lift the boat to get onto plane,
stick with the prop you are using for now as you know it will run with that untill you have made the changes,you should once again notice a difference.then maybe try another prop,
hope this helps? oh by the way very nice looking boat
   kurt
   black magic racing :-))
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martno1fan

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Re: prop size?
« Reply #10 on: July 08, 2013, 10:35:30 PM »

I think your main problem is your motor is too high a kv for such a heavy boat.You would be much better going with less kv and 6s in my opinion.Your prop is spinning at around 31000 RPM which is too much for a heavy boat in my opinion.
If you  went with a bigger motor and less kv on 6-8 s you could then spin a bigger prop  and that would push your boat much better.
Mart
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steveb6

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Re: prop size?
« Reply #11 on: July 08, 2013, 10:56:23 PM »

I have lowered the stinger so that its flush with the bottom of the hull and will run it like that,  I will make angle  adjustments at the lake on sunday. Thanks for all the advice , put me in the right direction. stuck with this setup at the moment, wife says no more toys......
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David Shaw

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Re: prop size?
« Reply #12 on: July 09, 2013, 06:34:44 AM »

Try the Graupner range of K series props (affordable and no balancing). RPM no problem, then try different angle of the stinger.
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steveb6

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Re: prop size?
« Reply #13 on: July 09, 2013, 08:11:58 PM »

Thanks David will look into them.    Steve.
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bj

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Re: prop size?
« Reply #14 on: July 19, 2013, 08:11:58 PM »

Hello to all,  I built a ep speed boat a few months ago and have not had much luck with prop selection,  need a little help.   The hull is an old 900mm long racing shallow v and it used to have a submerged drive and an ic motor.   I stripped out the old knackered stuff and put in a 4074 leopard motor, 180 amp esc and surface drive. On the first run  with a x443 prop,  it only managed about 20 mph, so I tried a 3 blade 437.  this made it run even slower.


You are way under pitch - you need to be looking at X9** or in that region ...........
Try HOR Racing for props at a more sensible price.
My X937 cost $10USD and was precision machined so no need to balance.
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pompebled

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Re: prop size?
« Reply #15 on: July 20, 2013, 11:22:50 AM »

Hi Steve,

Something else to consider is the distance between the transom and the front of the hub.
Rule of thumb is between 10 and 13% of the hull length, in your case that would be 90 - 117 mm.

You've chosen to add two large trimtabs, which effectively make the hull longer and narrow the gap between the prop and the transom.

Once a boat pics up speed,  a 'dent' in the water forms behind the hull, if the prop is too close to the transom, at some point the prop will stand in that dent, there's less blade surface in the water, the motor can rev out and all you get is a huge roostertail without the matching speed.

If adjusting the stinger doesn't have the desired effect, you may try removing the trimtabs and run the boat to see if that makes a difference.

Apart from the fact most well dialed in hulls don't need trimtabs, it also slows you down...

Regards, Jan.
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steveb6

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Re: prop size?
« Reply #16 on: July 24, 2013, 11:43:01 PM »

Thanks Jan, Hull to hub distance is 100mm . the trim tabs are raised up above the bottom of the hull to see how the hull performs without them. I will adjust them as necessary.   Steve.
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