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

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

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

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

Hi Paul

Yes thats the one, just check the shaft diameter will match your propshaft connection.

Try mounting them in your existing geared motor mounts first to see what the performance is with the gearing, (always a good move to change just one thing at a time)

Paul

There is probably too much prop for the motor. While it is close to the "rule of thumb" of prop size not being greater than the motor can diameter, the ability of the motor to power the prop might have been compromised by either the pitch of the blades or the number of blades.

The maximum power of the motor quoted in the link is a little over 105 watts, at 12 volts that is about 9 amps. If it pulls more than that (i.e. by blowing 15A fuses) it indicates that the motor is being dragged into its stall condition causing current demand to rise rapidly above optimum – more power being converted into heat than motion. Fit smaller props, or props with fewer blades, observe it go faster for longer, probably with the motors running cooler. Some accelleration will be lost, but as with all engineering using finite resources, there is always a trade-off.

I recall when Drumbeat was a new model that there were letters in magazines discussing the problems of overheating the then current ESCs in Drumbeats. Some problems never really vanish, they just move around.

So the Johnson "555" has resurfaced as the Johnson "683"? I suspect that the cooling fan is really needed, but only effective if the motor is running really fast, i,e, with minimal load. A "free running" draw of over 2 amps is bad news – its using a couple of amps just to turn itself before any work gets done. A couple of very ordinary 545 motors (540s but with 5 rather than 3 poles, and saying "15 volts" on the label) would probably do a better job without blowing fuses.

When the model was designed, SLA was pretty much the only thing available, and its severe limitations were lived with. Nowadays, with other choices, they are best used in boats that do need ballast.

All this talk of 6 and 12 volts implies SLA ballast. Maybe split the difference with a 7 or 8 cell NiMH pack?. The reduced weight will probably mean a faster boat. Probably give a longer run time as well.

As said earlier, Drumbeat is not a planing hull, so it has limits on what can be done without infinite power, but a lot can be done by losing unneccessary weight.

"Simplicate and add lightness" – maxim of William Bushnell Stout (Convair developer), adopted by Colin Chapman.(Lotus cars)

Paul

No need to feel sheepish, old dear. There are almost as many different Johnson motors as there are different hatchbacks. Some wouldn't pull the skin off a rice pudding and others would pull your arm off – and you couldn't easily tell because they're all in more or less identical cans.

Even the much-missed and well-documented Graupner series could be very confusing. For example, there were five or six different 700-sized motors from the relatively slow but very powerful Speed 720 Torque to the viciously fast 9.6v Speed 700BB Turbo. Only the label would provide a clue without a careful look down the vent holes at the windings and armature.

I'll admit that this one-time critic of the rush to brushless motors is now a total convert. It's a lot easier to identify the likely performance of one just from the name, and there are hundreds of different sizes and specifications.

As for SLA batteries, Malcolm and I are "on all fours". They have their place, but it's not anywhere near a model of mine except maybe as a weight to hold parts down flat while the glue dries.

Dave M

Edited By Dave Milbourn on 08/05/2019 17:44:36