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Proper Prop?

Old 05-31-2005, 10:46 AM
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bassman
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Default Proper Prop?

Hi guys. I am very new to RC boating. I have a 38" RC PT boat with a .30 nitro engine. The prop I have on it seems to break loose in the water when the throttle is advanced to quickly or the boat is turned with any speed on it. I am an very experienced RC air guy. Do the same rules apply to boat props as air props? I would suspect that I need , larger diameter, a change in pitch, or more blades. Can you get me pointed in the right direction? It has a Octura 1440 prop on it now.



Thanks

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Old 05-31-2005, 04:26 PM
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pompebled
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Default RE: Proper Prop?

Hi Bassman,

What you describe is called cavitation, could have a number of reasons, a too small prop, if the prop is metal, is it sharpened and balanced?
Is the motor geared down or direct drive?
Geared down you could try a larger prop or a three-bladed one, if a two-bladed is stock.

A prop that's too big will choke the motor and won't allow it to get revved up.

How fast is the boat at present?
The shape of the boat will allow air to get under the boat when run significantly over scale-speed, this in turn will result in cavitation and loss of rudder control.

Just some thoughts...

Regards, Jan.
(drummerboy)
Old 06-02-2005, 12:18 PM
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bassman
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Default RE: Proper Prop?

Thanks for getting back to me. The motor is direct drive to the prop. The boat top speed is good, over scale yes but not to much. 3/4 throttle looks pretty scale. The boat rides a little nose high and I was thinking about adding some trim tabs on the transom to fix that. Because the top speed is pretty much where I want it can I assume that the prop pitch is correct? The prop is metal. I worked it with a fine pitch file to take the burrs off of all the edges, used a dremel tool to polish it to a dull shine on all surfaces, and used my top flight prop balancer to try to get it balanced. On a flight prop I would remove material from the front side of the baldes to balance, Is it the same for boat props?

Thanks for the help, I am really new to boats. I will try to get some pictures of the drive and prop/ rudder area to post.

Bassman

Old 06-02-2005, 01:00 PM
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pompebled
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Default RE: Proper Prop?

Hi Bassman,

Yes, sharpening and balancing the prop means taking material from the back of the blade (the side that faces to the bow)

If you're happy with the boat speed you should work on the ride attitude by moving the CoG forward, to get a more level ride.

I run electrics and have my batteries to shift around, in your case it means ballast in the nose I guess, trimtabs on that hull would not be very scale-like, but effective, keep them as small as possible (and painted grey).

I've been experimenting with a similar hull 29,5" long, testing different motor set-ups. The shape of the hull made it run best with a light 7-cell geared down 600 motor, heavier motorisation with 12 cells and a direct drive 700 motor on a 40mm prop, made it blistering fast, but also unsteerable as the entire hull would be on the plane and the rudder would stand in an airbubble, very scary when running towards a brick wall...

In my gallery there are two pics of the scaled up version, 43" long, not quite finished due to lack of time. The Plettenberg motor in the first pic will be able to propell it to very un-scale-like speeds.

Regards, Jan.
Old 06-02-2005, 01:45 PM
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bassman
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Default RE: Proper Prop?

I will work on ride attitude. Is there a basic formula for prop slection that takes into account , motor size hull lenght, weight, desired speed etc. or is it an experimentation thing? What would be the effect of using a prop with the same diameter and pitich but three blades instead of two?

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