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Mating prop and motor. Help please

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Mating prop and motor. Help please

Old 04-30-2014, 12:52 AM
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TullyK
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Default Mating prop and motor. Help please

Hello,

A friend and I are attempting to build a propulsion system for a small wooden boat. This is not a model boat but a one you can sit in, we will however be using some RC components, as they're cheap and easy to work with.

This is the motor we're looking at, it has an 8mm shaft and we will be running it between 3000rpm and 6500rpm.

Finding a suitable prop has proved quite difficult, we're now considering a 75mm M5 Threaded 4 Blade Brass A-Type Propeller, which has the following specs: 75mm Diameter M5 Thread, Pitch 76.9mm.

My question is twofold:

Is there a significantly better/cheaper prop we could choose?

How would one go about fitting this prop to the 8mm motor shaft?

We are on a fairly tight budget so please no expensive suggestions, however much better they may be.

Thanks in advance for any help
Old 04-30-2014, 02:38 AM
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Without a link to the actual prop, no idea if something else would be better. Without knowing the expectations, same again.
The prop usually gets mounted on the outer end of a prop shaft, there are several ways of connecting the inner end to a motor.
Universal joint - just a question of finding one with the right size ends and a strong enough middle that can reliably handle the speed and power.
Rigid sleeve. Prop shaft size hole one end, motor shaft size hole the other.
Gears or belt drive. The motor can be coupled to the shaft using either gears or toothed pulleys/belts.
You say you are on a tight budget. Have you figured out or factored in how much controller you are going to need to control a motor this size? Have you worked out how to control the controller? Most RC ones need a timed pulse system to work.
Old 04-30-2014, 04:58 AM
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TullyK
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Hi, thank you for your reply.

I'm not sure why I didn't post a link to the propeller in my original post; Here it is. I'm looking at the 75mm one, possibly the 70mm.

We're fairly confident that the control gear will not be a problem. The plan is to get a cheap 100A controller and build a simple PWM circuit to feed it. The power source will be old car batteries and the mount will be fabricated out of aluminium sheet. It's just mating the prop to the motor that we're not sure about!

Thanks again

Last edited by TullyK; 04-30-2014 at 04:58 AM. Reason: Grammar
Old 04-30-2014, 02:20 PM
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You are unlikely to find better than Raboesch for props. It just becomes a question of whether the chosen prop when turned by your chosen motor will move a sufficient quantity of water far and fast enough to make the volume of your boat cover its own length in a short enough time. For the coupling arrangements you might find useful stuff in here - http://www.shgmodels.com/acatalog/ about halfway through the catalogue, pulleys, belts, gears. Some fairly knobbly items here - http://www.modelboatbits.com/ - as well.
A cheap 100A ESC might not be as cheap as it looks. Replacement with the right one is more expensive than buying the right one first. Boat ESCs tend to need a higher continuous rating because of the nature of the work and condition. The prop gives a continuous load, and there is usually precious little airflow to cool the ESC. Chinese amps as opposed to everybody else's seem to be smaller, more delicate things, as well.
Pulling 100A from an OLD car battery might result in early problems, both for the battery and anybody sat near it.
For a PWM source, think "servo tester". Fairly cheap, very likely to work, and, if a kit one, easy to fix the range to match the ESC. Lots of circuits via google.
Old 05-01-2014, 04:00 AM
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If you're making it big enough to ride, why not find a used motor from a trolling motor and use that or just buy a trolling motor of at least 30lb thrust. http://www.ebay.com/itm/Minn-Kota-En...#ht_1896wt_906
That little electric motor is made for r/c use and not a rideable boat. The 75mm prop which I have on a 49" boat weighing 18 lbs won't move a boat with a person on it. I made a motor for my inflatable boat using a weedeater motor and the prop was a 6" diameter trolling prop as not to over tax the 25cc gas engine.
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