Graeme

The 900 used a nominal voltage of 12v for its quoted performance, which was 6500RPM; maximum efficiency @ 8A. That's not a powerful motor by comparison with most brushless motors (it's just 96W). I would suggest a brushless with a case of at least 42mm diameter (to give the torque) and maybe 500-600kv, which will give roughly the same speed as the 900 on the same voltage. Don't try comparing the quoted power figures!

Here's one I found on Component Shop's website, but note it's rated as needing at least 4S (14.8v). **LINK**

If you really want some serious grunt from 3S upwards then there's always this one **LINK**

Dave M

I have just picked my jaw up from the floor, where it fell having looked at DG`s link……..

Yes, these brushed motors don't actually put out much power, despite their reputation, and that's why everyone thinks brushless are so much better because they fit motors of quadruple the power to that initially fitted and say "cor these brushless motors are so much better" which on the face of it they would be.

My Project No3 has a dinky 28mm brushless motor, one of which (in this series of motor options) will produce 133W..on the face of it, a 900 upgrade easily, but of course it isn't is it? its just too small to do what a 900 does…but on paper..

Ashley

Edited By ashley needham on 27/05/2016 18:46:44

Graeme

The Caldercraft 900 is not the same as the Speed 900 in that it spins at only half the speed on the same voltage.

It's largely speculation and opinion anyway, but I wouldn't personally fit an inrunner using direct drive to the prop – especially if torque is the main consideration. Speed is not the same thing as power – see my comments elsewhere about Ferrari F1 engines and quarry trucks.

The nice thing about advice, however, is that you are at liberty to ignore it! Good luck.

Dave M

Graeme. Using a wattmeter on the setup is the only way to reliably see what extra power the motor is putting out.
​For tug use you need good torque. What size/type prop are you using?

A 35mm outrunner of low (sub-900Kv) Kv would probably do…the motors in this size get longer as the Kv goes down, and the torque goes up …however, the only way to gauge this as a comparison to other motors and Kv`s is to see how big an air-prop it will turn (on the specs)

Amperages of 15-25 are quoted as operating currents for instance on a FUSION 700Kv 35mm outrunner, on 3s, so in theory anything between about 150 and 250 watts without splitting hairs BUT obviously this depends on how much effort the motor spends in turning the prop.

A 50mm prop, for instance, is unlikely to make this motor notice that there is a prop connected to it, and the current may only be 4A, but a 100mm 6 bladed prop may make the motor draw 40A and melt.

Ashley (usual disclaimer)

I remember many years ago being sent to an obscure office of HMC&E to help do an audit. Unwisely I chose as my first task to have a chat with a senior member of the team.

This kindly looking old gent reminded my of santa so I felt comfortable in choosing him for the first interview……how wrong could I have been!

Behind closed doors this kindly old gent became a raging banshee, uttering such outlandish profanities as I had never heard before even during my service in the navy.

Weeks later I received a snotty memo from him so I audited his tax affairs and he has been paying double ever since.

Isnt karma fun.

Edited By Paul T on 28/05/2016 17:15:09

Paul

In your dreams, Dr Dude. As a former HMRC employee I get an Income Tax Personal Allowance of £50,000 as well as a gold-plated, inflation-proof pension which I find embarrassingly difficult to spend.

Here is the nightmare – in duplicate (as are all things Civil Service)…enjoy!

**LINK**

**LINK**

DM

Graeme

You may find this of interest. It certainly infers that the motor speed has to be carefully controlled if damage is to be avoided. That's critical as spares are increasingly hard to find for Graupner items. **LINK**

BTW if there are any Mail readers out there I'm sorry to disappoint you but the tax and pension figures I quoted above are manifestly untrue…..but I can dream, can't I?

DM

Graeme

On reading the thread I'm certain that the 8000RPM limit applies to the input speed and not the prop speed. You only have to read what the other owners are using. It seems that Graupner first mentioned supplying only 10v to their Speed 900BB, which equates to 5000RPM unloaded. If you really do believe that the Schottel unit – which is simply moulded plastic – will withstand over 3BHP then by all means do as you propose. I wouldn't want to be there when you do.

Dave M

Light the blue touch paper, and stand well clear!

Hi Graeme, how did the experiment go ?. I forgot to ask if your unit was well lubricated before the test.

Chas