Drumbeat of Devon batteries

[Clive HarrisParticipant @cliveharris32383]

HI all

I have just obtained a modelslipway Drumbeat of Devon,fitted with MFA 919D motors and electronize FR30HX esc.

What batteries do you recommend, also does anyone have any plans that they wish to sell ,?

Thanks Clive

[Clive HarrisParticipant @cliveharris32383]

[Dave MilbournParticipant @davemilbourn48782]

Clive

What ratio are the gearboxes on the motors? There's usually a little white label on the top of the mounting bracket, just under the motor. The usual ratios are 6:1 and 2.5:1. Also what diameter and number of blades are the props?

As far as I know the 'plans' are not to scale so might not be a lot of use anyway. Modelslipway might be able to help you.

Dave M

[Clive HarrisParticipant @cliveharris32383]

Hi Dave

Thanks for your prompt reply, the motors are 2.5 :1, with 4 blade 40mm dia props.

model slipway can't help with plans , so I'll make do with google images etc.

can you recommend any books etc that would explain how I could work out motor ,prop size's etc for any future models?

Thanks Clive

[Dave MilbournParticipant @davemilbourn48782]

Clive

This ratio is usually used with a 6v battery, although personally I'd fit a large-capacity 7.2v NiMH pack like this one (my dislike of SLA batteries is well-known!). **LINK**

In my experience the best way of selecting a prop is to look at similar models and start with the same size etc as they use. I don't know of any reference sources although I imagine there are some.

Dave M

[Malcolm FraryParticipant @malcolmfrary95515]

Looking at the motor info http://www.mfacomodrills.com/gearboxes/919d_series.html it says that the motor is intended for "4.5 to 15 volts", so 6 volts is down at the bottom of the power range. A bit more rarely does any harm, so the suggested 7.2 volts sounds good. Losing some weight will do no harm at all. Using the traditional rule of thumb about the props being smaller than the motor can, with the gearing this is well fulfilled.

Even in the full size world, there is a lot of empirical estimation (posh for guessing with experience) when fitting a new design of ship with its props, one of the reasons for sea trials is to see if the ship can actually do what the makers claim. Given that there is an almost infinite range of variation in the design of prop blades, and each variation has its own effect on performance, there is unlikely to ever be a definitive formula that will appear in a book suitable for modelers. For "standard" brushed motors, however, the working rules are that the prop diameter should not exceed the motor can diameter, and it should not have more blades than the motor has poles. Adding gearing obviously makes the motor look bigger and appear to have more poles.

[Dave MilbournParticipant @davemilbourn48782]

**LINK**

This table gives the output shaft speed for each of the ratios available and with supply voltages from 4.5v – 15v. The other thing to bear in mind is that these gearboxes are noisy, and the higher the speed then the noisier they get! I preferred the old MFA 'Olympus' 540 belt drive unit, but that was discontinued a long time ago. It also depended on setting up the belt and pulleys correctly or it would wear out the belts very quickly.

DM

[Malcolm FraryParticipant @malcolmfrary95515]

Interestingly, down the page on the link, timing belts and their toothed pulleys are shown……..

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