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Author Topic: Prop size?  (Read 1477 times)

STEVO

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Prop size?
« on: June 16, 2010, 05:04:54 PM »

Hi all,any suggestions on best prop size for running on a 3/16th flexi,which is for a 50inch hull powered by a 5s lipo.

 ;)Thanks.
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martno1fan

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Re: Prop size?
« Reply #1 on: June 17, 2010, 08:09:33 AM »

Not sure whats available for 3/16" shafts but you will need a large prop arround 70 mm .If the power is compairable with the zenoahs you should have used a 1/4" flex but im not sure what power your putting out as im not into leccy power.
Mart
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ids987

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Re: Prop size?
« Reply #2 on: June 18, 2010, 02:41:13 PM »

Hi Stevo,

I'll start off by saying that I also know very little about leccy boats, but the prop selection will depend largely on the motor used - along with the Amp Hour and "C" ratings of the LiPo. The more powerful LiPos usually have a P (parallel) element in the rating as well as the S (series) rating. Cells in series gives Volts, cells or strings of cells in parallel give more current (Amps).
If it is a brushless motor, the main factors will be the KV (RPM/Volt) rating, and the power rating of the motor. The KV rating will tell you the maximum RPM the motor can turn for a given voltage - although the actual RPM is governed by the speed controller. For a given power, the higher the KV rating, the smaller the prop you will be able to turn (but at a higher RPM). The wattage of the motor will largely govern what size of prop you can turn at a given RPM. The bigger the prop you want to turn at a given RPM, the more power (Watts) you need. Usually, the power train (motor, battery and ESC) would be chosen to suit the hull, and how you want it to run.
For efficient running - in terms of prop slip etc, there will also be a minimum prop size (primarily in terms of diameter), which will drive the hull efficiently. For a deep vee of that size, as a rule of thumb, I would say that you really want to be able to drive a prop of at least 55mm diameter. Somewhere around 47mm is probably about the absolute minimum, but probably won't really push the boat very happily, bite very fast or accellerate very well.
With dog / surface drive props at least, the minimum pitch to diameter ratio is normally 1.4:1 (eg Octura X4 series).
I don't want to be negative, but if you want to go fast - or anything like fast, with a deep vee of that size, you need A LOT of power. Similar sized hulls on the MHZ website are suggested to be paired with the BIG Plettenberg motors - running on 30 odd Volts, and driving 60 odd mm props. Here's one example: http://www.mhzusa.com/Ocean-Spray-m-1757.htm

Of course, if your deep vee is more scale orientated than fast, or if you already have a motor, ESC etc which suit the hull, a lot of the above is irrelevant.

Not sure whether this is any help whatsover or not........

Ian
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STEVO

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Re: Prop size?
« Reply #3 on: June 18, 2010, 11:05:51 PM »

Yes,that very helpful Ian,the motor is 3300 kv (5hp) running on a 180amp water cooled esc powered by a 5000mah 5sp lipo turning a 50mm prop.

Stevo.
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nick_75au

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Re: Prop size?
« Reply #4 on: June 19, 2010, 12:48:43 AM »

Your being a little over ambitious with that setup

3300 Kv X 18.5 volts is over 60,000 Rpm

If the motor can draw its max wattage its going to pull 194 amps

you are going to need 40C batteries

you will get about 1.5 minutes runtime.

Nick

I would look at the RPM that the gas boats run then think about changing the setup, the 26 cc Zenoahs in full race trim have a maximum RPM of 16-18000 RPM.
so either gear that motor 3.5:1 or obtain a motor  closer to 1000Kv
An even better setup that is competitive with the gas boats is this, which is lower Kv and higher voltage
http://www.littlescreamers.com/id57.html
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John Mk2

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Re: Prop size?
« Reply #5 on: June 19, 2010, 06:25:29 AM »

If its the boat i think it is ok2 Its geared (Toothed Belt) @ 2 to 1 if i recall correctly ?
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John Mk2

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Re: Prop size?
« Reply #6 on: June 19, 2010, 06:35:36 AM »

 :embarrassed: It is the boat  :embarrassed:(in the sales section)Only just looked there. Well it is a bit early {:-{   
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nick_75au

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Re: Prop size?
« Reply #7 on: June 19, 2010, 08:57:32 AM »

Well the reduction helps, as direct drive it would not have worked.
STEVO have you run the boat? I'm curious to find out its performance, forgive my previous post, Obviously the whole picture wasn't there

Nick
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