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[Colin Bishop]

I put this up initially on the Model Boats forum but thought it might be useful to invite comments on here as well.

The traditional fast launch model is becoming increasingly popular as are other models where the efficiency of brushless motors gives a performance hitherto unobtainable to mainstream modellers without the use of i/c power.

However my builds of the SLEC Fairey Huntsman and Swordsman kits have thrown up a driveline issue which I can’t see any obvious solution to without spending a lot of money! I imagine the same will apply to other smaller models of the same type such as the re issued Aerokits and similar ranges.

The preferred power plant these days is a combination of a small brushless motor combined with a 2S or 3S Lipo pack.

Both my models are really noisy when revved up on the bench although the Huntsman in the water is a bit quieter. (The Swordsman has yet to get its feet wet). However much I check and adjust the driveline using a sleeve between shaft and motor it seems impossible to achieve a vibration free power delivery. I am using heavy duty couplings with rubber inserts as the HUCO type wouldn’t last more than a few minutes with the power these brushless motors put out. Even temporarily substituting a piece of silicone rubber doesn’t do much to bring down the noise levels and the thinnish piece I have available twists too much to be a practical alternative on an M4 shaft.

Looking at it a bit more closely, I’m not sure you can expect more than an average result vibration wise. The coupling efficiency depends on the accuracy with which the rubber section is bonded to the ends and any error will be amplified by the high RPM of the motor. The plastic motor end mount allows for some flexing of the output shaft too, even when well screwed down. These little motors appear to be designed to have an aircraft prop directly attached to the shaft which of course makes couplings irrelevant. So I think we have to suffer the consequences when we try to link them up to a near foot long shaft with a big brass whirly thing on the end which probably introduces its own small intolerances. Precision engineering it ain’t!

On my Swordsman the motor starts up a bit unevenly, smooths out at around one third throttle then there is a high vibration band which settles down to medium vibration at full speed (by which time the noise level has shot up with the RPM). So the unevenness is not consistent through the rev range.

Dave Milbourn tells me that one of the people he sails with has removed the motor shaft and bolted the motor directly onto an elongated propshaft to give a rigid drive although the motor needs to be supported in the boat to absorb the thrust and stop it rotating of course. The motor I have does seem to have its shaft held in place with a bearing at the front and rear and a grubscrew securing it to the rotating case but I haven’t dared to dismantle it. It does have a 4mm shaft so should fit on a standard m4 model boat shaft plain end.

I would be interested to hear people’s thoughts on this.

Colin

[Leaky Bottom]

This is how I mounted the motor on my remora, the same as Dave Milbourn apparently. You do have to get the brass flange turned unless you have a lathe as I do not know of any commercially available. It's all self aligning if the flange in turned accurately

[Colin Bishop]

That is interesting. Graupner do rigid couplings which might have the same effect.

Colin

[kinmel]

On most out runners replacement motor shafts are available and they are also reversible. We have had to do that on some motors for Jet drives.
On that basis, there is no reason why an appropriate diameter propshaft cannot be extended through the motor, the removed motor shaft provides a perfect profile for the extension
 

[Colin Bishop]

So, is it simply a case of undoing the grubsrew and releasing the shaft then?

Colin

[kinmel]

So, is it simply a case of undoing the grubsrew and releasing the shaft then?

Colin

In principle, yes, but sometimes it can be an interference fit. If it doesn't simply slide out, use a vice and some timber packing to ease it out. It does not need great force.
Another option is straight brass couplers, see Taranis' coupling for the OceanWorks jet drive on his Shannon build as an example.
Either way you will have to line up the motor mount exactly.

[Leaky Bottom]

 There could also be a small circlip around the shaft which needs to be removed as well depending on which way out you remove the shaft

[Subculture]

Sometimes mount motors on polypropylene foam blocks (used a lot for packing TV's etc.) which is a chewy rubbery type of foam. This isolates the motor nicely from the hull, and the boats are very quiet.

[Taranis]

Hi Colin
My lobster boat build dates back to 1986 and was designed for brushed motor with universal coupling
I had the opportunity building from new to change the design with a longer shaft and a relocation of the motor to enable straight through connection and used a solid coupling for alignment and fixing the motor in place. I then changed to the flex coupling which isn’t bad but as you discovered even the slightest imperfection will cause micro wobble
The video here is with the flex coupling and not bad at all but I later changed back to the solid coupler and it is even better
https://www.modelboatmayhem.co.uk/forum/index.php/topic,62915.msg666378.html#msg666378


I think I built the mounting platform under the motor while it was all connected together or tacked it in place then reinforced it

[Colin Bishop]

Taranis,

Yes, I'm rather inclining to that view at the moment although it is interesting to see what others say.

Normally I build sedate scale models where these issues don't really arise. A brushless motor on 11 volts is a different proposition!

Colin

[canabus]

Hi Colin and all

I use both a solid coupling (locally made)or metal uni joints from Banggood.
The Banggood ones are cheap, but I have use them on both 28 and 35mm brushless motors.
No problem at all.

Canabus

[roycv]

Hi all, I have only run one launch with manic speed and it was noisey. 
If approached from the other direction if you want a sound effect then the speaker has to be very firmly attached to the hull to get as much sound resonance as possible.  It seems the opposite effect us needed to remove any oscilations from the motor / coupling resonating in the hull.

 BJ earlier made similar points, I remember that there used to be suitable vibration free  mounting for i/c motors, I do not know if something similar is still available.
regards
 Roy

[Subculture]

Don't wish to state the obvious, but also bear in mind that other parts of the system beyond the motor should also be balanced as well as possible. If using a metal prop, taking time to balance these with a magnetic prop balancer is worthwhile.

For smaller brushless installs (e.g. 28mm outrunner of around 100 watts) I find a simple coupling made from push fit plastic bosses with a sleeve of silicone tubing is perfectly adequate. This may not be enough for larger models, but moulded rubber couplings are available from SHG, designed for ic use, so should be more than strong enough for electric.

[grendel]

I had similar issues with a brushed motor, that turned out to be whip in the actual prop shaft where the flex in the shaft was causing it to contact the sides of the prop tube at high revs, this was on an 8" tube with a 12" shaft, maybe ball raced shafts would help, but I do believe my shaft was fractionally bent in installation, but there is a fair amount of whip on a 4mm shaft.
shaft alignment then motor alignment are critical to reduce noise.

[Colin Bishop]

Thanks for all the suggestions and comments guys. Lots of useful information and alternative solutions, one reason why I started the discussion.

Colin

[coch y bonddu]

Another problem that can arise and if often overlooked is twofold so to speak.


1 Length of shaft from the hull to the motor especially if it is NOT supported will cause vibrations


2 Obviously the same can be said of the propshaft length underneath the boat


The amount of revs transmitted through the  shaft can cause WHip inside the tube and will cause problems which is quite often overlooked....Had  a problem in a  recent purchase of a mersey where the shaft wa unsupported causing severe vibration....soon sorted with the finger on the shaft to stop in.....


Hope this is of help




Dave

[Colin Bishop]

The problem now seems to have been largely resolved with the arrival of an alternative rubber centre coupling which is much smoother. There is a slight vibration blip at around 25% revs but it smooths out above that and noise is reduced. Next step will be pond trials.

Look like engineering tolerances may come into it. What is OK for a relatively low RPM setup can struggle at the speeds some of these brushless motors achieve.

Colin

[Subculture]

Have you done things like ensured grub screws on the coupling are balanced 180 degrees apart, that can cause a lot of vibration.

[Colin Bishop]

The latest coupling has only one grub screw at each end. The previous one had two and adjusting them made a slight difference but not much. I think the real problem was a slightly misaligned central rubber section hence my comment about the engineering above.

Colin

[Subculture]

I just machine up some Delrin bosses, machine them a bit undersize so they're a press fit on the motor and prop shaft, and slip some silicone tubing over the bosses. No screws, very light, no vibration, silent and cheap. You do need a small lathe to make them though. Used them at power levels up to around 100 watts, bigger models/motors might need something a bit more stout.

[red181]

I've done a man read, so apologies if I missed something.


I wont suggest anything I've not done myself, or seen in operation, as "guessing" or "suggesting" really isn't a viable answer.



 I've been dabbling with brushless for many years now, with nihm and lipo as the powers source, which is irrelevant really! and in later years moved on to rc planes, which is now brushless due to weight loss and more power. I found that the higher kv brushless motors, over 1000kv, produce higher revs, but less torque, so lower kv (in my trials 700 to 900ish) had more torque. I build heavy, so need the torque to get going, as  I was sailing mainly on smaller lakes, the acceleration was more important than the top speed, which in my applications was becoming unstable on the water over 22mph. (using eagle tree data logging, so mph was accurate)

My most successful was a geared set up. We don't need to get into gearing, its been done to death recently, but this set up took a drive from the propshaft by a belt to the motor shaft, which could be anywhere, on top, at the side, and so on, providing the gear on the propshaft and the gear on the motor shaft line up. We don't have the weak UJ anymore. I found the propshaft does need to be supported  to stop a whipping effect, vibration, and subsequent bending. Prior to brushless I also used this on a 700bb grauper motor . I have also done the shaft removal with a brushless, in fact, it was a twin screw. Just buy a brushless motor that offers the option of replacement shafts, then you know the shaft will come out, generally on outrunner motors. This means its the propsfat that goes straight into the motor, this is the ultimate direct drive.


Im no expert, Ive got no special skills, but I like big heavy scale boats, so need some grunt to get them going. Also, brushless motors are so cheap now!



Pictures tell a story better! 


so here is my 4 foot aerokits fireboat refurb. I went twin screw, and knew it was gong to be very heavy. I had thought about removing the shaft from the motors for some time, and enjoy the building more than the sailing, so gave it a go. Im lucky, I have a small lathe, but that's just making it all very pretty, it could be done by hand. I made blocks that where drilled out to allow a water channel for cooling, it wasn't needed as an outrunner is a cool running operation. For those that have not seen inside an outrunner, here it is, two grub screws and the can pulls off. The grub screws can be tricky, they can be glued in with thread lock, and are usually cheap low grade so a good fitting allen key is needed, or drill them out as I had to. The motor by design will have a bearing which for plane operation will ne high revving, its generally at the end we screw to the mount, the better motors will have a bearing at the rear also. I used bearings in the mount also. A good solid bulkhead forms the strength, I reinforced either side

[red181]

here is my smaller 3 foot aerokits fireboat refurb, in its first reincarnation, using a 700bb turbo grauper motor. Again, ignore the gearing, if you used the same gears its a 1:1 ratio so direct drive. This again eliminates the cheap weak UJ which cannot handle the higher revs brushless will give


Its  bench mock up prior to installation. It looks all pretty as I have machines, but that's an easy arm aching job that can be done by hand, filing, cutting and so on. With brushed motor it needed to be water cooled, not necessary with out runner brushless, inrunner might need to be cooled

[red181]

and single screw, here the motor could be at the sdide, underneath, or on top of the propshaft, depending on the model. This is the fastest, and will reach 25mph depending on which ratio I use. But the propshaft is driven by gear and belt, so again no UJ etc.


On first test, the vibration was terrible, I almost scrapped the project, but  a simple shaft support at the rear of the propshaft eliminated the issue.


I would never have taken on this sort of work without the guidance and support of a pal, who at the time was a first class builder, and understood these building ideas