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Increasing model top speed

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Paul7519/02/2019 08:40:39
90 forum posts
5 photos

Hello Everyone.

I'm currently renovating a second-hand 1/24th scale Model Slipway model Drumbeat of Devon. I think I'm right in saying that this boat has a semi-displacement hull similar to larger RNLI lifeboats. This has two 38mm 4 bladed brass props, two geared Torpedo 500 motors and runs from a 12v 5000MAh NiMH battery.

I've just sailed the model in a lake the other day to carry out speed tests but found that the maximum speed of about 1.2m/s is approximately half the true scale maximum speed. I've checked the electrics with a voltmeter and the motors are receiving nearly 12 volts under load so I'm thinking that it's simply the maximum speed the battery/motor/prop set up can achieve. There isn't space under the hull for a larger prop.

From searching online I have seen a suggestion of using Graupner #66317 Speed 700BB Turbo motors for similar type boats on a 12v supply.

I'm not sure if these motors are still available but can anyone recommend these or another suitable replacement please?

Many thanks,

Paul.

ashley needham19/02/2019 08:59:56
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5521 forum posts
171 photos

Paul. Any idea of the gearing as fitted to the motors? Someone, for instance may have fitted 50:1 gearing which is quite low, and a change to 2.5:1 would speed things up?

A 2.5:1 mfa unit would manage these props easily on 12v, and a replacement unit would fit straight in.

if not marked then a bit of prop twiddling and counting will be needed.

Ashley

Edited By ashley needham on 19/02/2019 09:00:25

Paul T19/02/2019 09:55:42
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6722 forum posts
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Paul

Try changing the motors for a pair of Johnson 700 (683) motors these are nice and powerful at 100w with plenty of grunt from 5 poles, these are the motors that I used in my Slipway Trent which is very similar to the Drumbeaht and you shouldn't need to change the shafts of props.

At 10v they produce around 22,000rmp without any gearing, they operate anywhere between 5 to 12 volts and have a very useful in built fan.

They are fairly heavy at 250g but I found that two motors and a 12v sla battery provided sufficient ballast to get the model down to the correct level.

Look on ebay (or wherever) as new motors can be bought for around a fiver each.

Paul

Dave Milbourn19/02/2019 10:07:50
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3542 forum posts
218 photos

Model Slipway always used to recommend 2.5:1 gearboxes for use on 6v and 6:1 on 12v, but that always seemed to me to be a bit on the conservative side [small 'c']. Granted that my Envoy sailed very nicely using the 6:1 ratio gearbox with 12v, but that was turning a 70mm 4-blade prop. If your motors are 2.5:1 - and there's a label on top of the mounting plate which tells you which it is - then first try bigger props e.g. 45 or even 50mm. They'll still only be turning at 5800RPM with no load, whereas the sugested 6:1 ratio would produce a mere 2400, which is very slow indeed.

Gliders of Newark have the 700BB #6317 motors in stock **LINK** but you'll see that the speed on 12v is over 11000RPM.

Dave M

Edited By Dave Milbourn on 19/02/2019 10:08:21

Paul T19/02/2019 10:24:58
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6722 forum posts
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Dave

I found the recommended motor/gearbox to be very conservative as the Trent couldn't go fast enough to get onto the plane, out of desperation I tried a pair of old Johnsons that were lying around and was very pleased with the outcome as they produced a perfect scale speed and exactly the correct bow wave without having to change the props that were supplied with the kit( which would have been a problem when it came to show time)

Dave Milbourn19/02/2019 13:50:23
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3542 forum posts
218 photos

Sorry, pardon - I missed the bit in the OP about not having room for bigger props. I still reckon 22,000RPM is way too fast for those 38mm props. Drumbeat is at best a semi-displacement hull and not a fully planing type like the Trent. The 700BB 12v motors would probably be OK but 12v might be a bit too much. Still, you always have the throttle stick to slow it down!

DM

Edited By Dave Milbourn on 19/02/2019 13:52:32

Malcolm Frary19/02/2019 14:31:58
738 forum posts

Unless they were vastly undersized, bigger props do not make for a faster boat. Smaller props revving faster do speed it up, but only if the motor can rev faster. I suspect that there is too much gearing on the motors resulting in motors that rev as fast as they can while not turning the props quickly enough.

About 3mph should be about right for this model - the real thing would do about 12 and a bit Knots.

Provided that the NiMH battery can supply the current (nearly 12 volts? - is measured at the motor or is it the battery under load?) it should be just fine. If the battery voltage is dropping under load, and a 12 volt NiMH (10 cell) should show at least 13.5 charged, it is not handling the load demanded. Measured at the motor terminals shows the volts lost in the wiring and ESC.

I would avoid SLA in any boat requiring performance, they are great in tugs and cargo boats, but simply do not have the ability to supply continuous heavy current without gaining a short life and they have poor energy density - they don't store many Watt-Hours per Kilogram and for the same weight give a much shorter run time than other types of battery. When they were almost the only game in town, they were tolerable, but a problem to work round. With newer chemistry available, just not worth the effort. If one 5AH NiMH is too light, fit another for the extra run time.

Paul7519/02/2019 15:51:27
90 forum posts
5 photos

Thanks for your replies.

For info, the motors already installed are geared to 2.5:1. I understand that Drumbeat of Devon has a top speed of 18 knots but unfortunately my model only manages a scale top speed of 12 knots, which would be the cruising speed of the real thing.

I've had a look at the specs of some of the motors recommended.

The Johnson 683 gives 20384 rpm @ 9.6V and the Graupner 700BB Turbo gives 19200 rpm @ 12V.

Would these motors propel the model at scalded cat speed and/or would reduction gearing be needed? I understand the already installed MFA Torpedo 500 geared at 2.5:1 gives 6300 rpm at 12V.

Also, what does the the term "maximum efficiency drain" mean? Is see that the figure for the Johnson motor is 14.98A whereas the Graupner is 12.4A.

I measured the current drain of the model with the existing geared motors at 12V (with the props in the water) and the current used was a mere 4.5A. I'm a bit concerned that the alternative motors might be a bit current hungry and might exceed the P94 ESC installed.

Any advice would be much appreciated.

Many thanks,

Paul.

Paul T19/02/2019 16:48:51
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6722 forum posts
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Paul

You don't have to use all of the power / speed that the bigger motors provide but sometimes it is nice to have the extra oomph at your disposal.

I wouldn't worry about the P94 as it is rated at 20amps and the chap that makes them always under rates his designs, I have never been able to blow any Action ESCs (and believe me when I say that I have tried)

The twin Johnsons in my Trent drew 6amps at full load.

Paul

Edited By Paul T on 19/02/2019 16:56:21

Dave Milbourn19/02/2019 16:58:30
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3542 forum posts
218 photos

Paul

The problem is almost certainly because the geared motors aren't spinning the props fast enough to give the speed you're looking for. The #6317 Graupner 70BB 12v is quoted at 11,600 RPM at its nominal voltage of 12v. Check this link again **LINK**

There were several different types of Speed 700BB at one time - all with markedly different specifications. This seems to be the only type still on the market after the demise of Graupner, although you might actually be supplied with a differently-labelled motor, albeit with the same spec. I agree whole-heartedly with Malcolm about SLA batteries. I use them only as weights in the workshop these days.

The current drain at maximum efficiency is exactly what it says, and the difference between the two figures quoted hardly seems significant in the context of a P94, which will happily handle 20A for each motor all day. No need to search the web for verification of that - I've been making P94's since 2008.

DM

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