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[DavidHughes]

Hiya - Needing some help/guidance and hoping someone here might be able to point me in the right direction...
 
Just starting work on a 2-and-a-bit ft long MTB.
 
I want a decent bit of performance out of it - reasonable speed (planing speed!) and good longevity (1 hour +), but at a reasonable cost (ie no Lipos and brushless)
 
Motors seems to be an absolute minefield.
 
Started on Cornwall Model Boats site by trying to find something with high rpm (ie 12,000rpm or more) and with reasonable current drain (only a few amps)...I know all the quoted figures are unloaded, and max efficiency, but I was using them as a rough guide.
 
I ended up looking at MTronix 500 and assuming battery wise I'd need a 6v/7.2v SLA with c6Ah to get the life span I want
 
Am I going in even remotely the right direction?
 
Thanks for any wisdom you are able to share

[derekwarner]

"2-and-a-bit ft long MTB"  ....David......

This covers approx. 300mm LOA to 800 LOA or more importantly a variance in displacement of 3kg to 8kg

You may get a better answer with more accurate building block nominations.... Derek

[inertia]

You'd be hard-pressed to get full planing performance for the length of time described from any sort of motor.
Brushless motor plus LiPo battery would be ideal.
The MTroniks details on CMB's website are contradictory i.e. it's supposed to do both 24000RPM and 6000RPM on 12v. The power rating however is inadequate for your model; 22W wouldn't even frighten the skin on a rice pudding!
If you want cheap then buy a MFA 540/1 with a 7.2v NiMH pack of the biggest capacity you can find - currently 5000mAH. Don't trust a supplier who advertises anything larger.
Avoid SLA batteries at all costs; they will not deliver the current your motor needs and will be too heavy for the model to plane properly.
Any good ESC from 15A - 20A will do the job. No specific type for preference but smart money would stay away from a 'Blue Meanie' (often described as "50A Forward plus 20A Reverse" in a blue anodised heat-sink case).
On my ruler '2 foot and a bit' is at least 650mm but maybe Derek's millimetres are a bit more flexible (or Chinese imports...) 
DM

[DavidHughes]

"2-and-a-bit ft long MTB"  ....David......

This covers approx. 300mm LOA to 800 LOA or more importantly a variance in displacement of 3kg to 8kg

You may get a better answer with more accurate building block nominations.... Derek

its a 72'6" Vosper @1:32,  So it comes in at 690.56mm (depending on how enthusiastic I am with the sandpaper)
 

[DavidHughes]

You'd be hard-pressed to get full planing performance for the length of time described from any sort of motor.
Brushless motor plus LiPo battery would be ideal.
The MTroniks details on CMB's website are contradictory i.e. it's supposed to do both 24000RPM and 6000RPM on 12v. The power rating however is inadequate for your model; 22W wouldn't even frighten the skin on a rice pudding!
If you want cheap then buy a MFA 540/1 with a 7.2v NiMH pack of the biggest capacity you can find - currently 5000mAH. Don't trust a supplier who advertises anything larger.
Avoid SLA batteries at all costs; they will not deliver the current your motor needs and will be too heavy for the model to plane properly.
Any good ESC from 15A - 20A will do the job. No specific type for preference but smart money would stay away from a 'Blue Meanie' (often described as "50A Forward plus 20A Reverse" in a blue anodised heat-sink case).
On my ruler '2 foot and a bit' is at least 650mm but maybe Derek's millimetres are a bit more flexible (or Chinese imports...) 
DM

Hmmm - I was concerned about the weight of an SLA, but then if I only end up adding ballast anyway, wasn't sure if it mattered.  But then for the capacity I'm looking for, Ive now worked out it'd be c2Kgs!

So a standard 540 type 3-pole motor, with a high capacity NiMH pack...useful info thanks!

[derekwarner]

As DM says...... "On my ruler '2 foot and a bit' is at least 650mm but maybe Derek's millimetres are a bit more flexible"


...well yes & no Dave........I was simply giving this member a little latitude, longitude or shotitude in the UOM he was offering.....however he now appears to have come to earth with two decimal points in LOA.......

However as we see this does help   .... Derek

[inertia]

David
Don't ballast the model just to get it down to the painted waterline. Use ballast ONLY to trim it fore and aft if necessary. Adding weight to a performance hull does only one thing - it wastes power just pushing dead weight around. SLA batteries are suitable only for low-current motors when used in displacement hulls.

Derek
"Shotitude"?? As in Ned Kelly?

DM

[John W E]

Hi David
Is the MTB you are building this one  http://www.modelboatmayhem.co.uk/forum/index.php/topic,31199.0.html  ?
if so the motor in the build model is a Graupner 500 speed running on 8.4 400mAh battery pack

aye
john

[DavidHughes]

Hi David
Is the MTB you are building this one  http://www.modelboatmayhem.co.uk/forum/index.php/topic,31199.0.html  ?
if so the motor in the build model is a Graupner 500 speed running on 8.4 400mAh battery pack

aye
john

Hi John

Yep - that's the one...

I've got the original plans, plus your excellent build log, and a set of scale GA drawings for MTB347 - I've redrawn the plans slightly to lengthen the hull by 20mm (to make it truly 1:32), moved the wheel house back slightly (extending the fore-deck), reduced the rake of the bow to match the GA drawings and am going to try adding more detail.
My aim is to end up with a hull that is proportionally accurate(ish) albeit carries flat-profiles, and a profile and look that is reasonably accurate and detailed...

That's the plan anyway - I'm sure lots of mistakes, frustrations, swearing lie ahead, but then isn't that half the fun?

I started searching for the Graupner 500 Speed motor (rather than deliberately going off-piste), but couldn't find it listed anywhere...hence why I started trying to work it out for myself...?

Battery-wise - Your suggestion is similar to above I guess? A NiMH pack (8.4v rather than 7.2v) with decent capacity for a good bit of running time??

Thanks all - I really do appreciate your help!

[sigfortunata]

David  I cant say I've got half as much experience as many on here however I've just built my first model  a 28" planing hull and I too have had similar problems with motors, and run longevity etc.


I started off with 2 500 motors turning 35mm 3 blade props powered by 2, 7.2 NiMH  (4200mah)  No ballast was required( weight about 2.5kg with batts and running gear)


Planed beautifully at a scale speed but run time was pathetic ,about 10mins.


Turned out my motors were rated 4.5 -15v so at 7.2v weren't even reaching full revs and also drawing quite a lot of amps to turn big props.


On advice I down sized to 2, 400 motors rated max 8.4 v , 20mm 3 blade props and same batteries.   I now get a lot my revs out of the motors and in the bath at least current draw is much less.


Hoping for a full size open water test this weekend 

[DavidHughes]

Let me know how that goes...
 
The more and more stuff I read across the various threads on this forum, the more and more complicated it gets!!!
 
I don't think it helps that I'm trying to find the perfect balance between a nice turn of speed and a good bit of endurance, whilst keeping costs as low as possible (too many other family things requiring money for me to go mad with expensive brushless motors and LiPos)
 
I've sort of got the impression that I'd be better going for a slightly smaller, lower voltage, lower current drain higher revving motor - I saw a 480 size motor somewhere that suggested it only pulled a handful of amps at 6 or 7.2v...
 
I guess so much of this is trial and error really - but then again, it'd be boring if it was all straightforward wouldn't it?

[John W E]

Hi, this so reminds me of many moons ago, when Model Boats Magazine published a free plan for a 'semi-scale MGB - boat' which is a very similar size to Glyn Guest's MTB where I slightly altered the MGB by making it twin motors using the 480 Rocket motors.  All that was available to me at the time were 2 small transmitter battery packs of 9.2 volts and about 800 mah each.  These were conveniently stuffed into the model along with a hard-wired Bob's board.   The model was then placed in the Lake and full throttle was applied and say about 10 yards up the Lake the model became airborne and turned over When the model was retrieved there was very little left of the wiring and the battery packs and a strong smell of 'Oh my words it's been on fire' in other words - don't forget - The model you are building is very light.

Have fun though.

Aye

John

[DavidHughes]

Crikey - hadn't even thought about that...
 
I was at Wicksteed pond earlier this year when some repeatedly flipped over a very lightweight powerboat... My little tug proved its use pushing it back to the side 3-4 times.
 
Someone cracked a comment about the prop on his boat being more like an airscrew given how much time it spent pointing at the sky...

[sigfortunata]

David, ran the RTTL today with the setup described;


2x 480 motors rated to 8.4volts (stats say 17000rpm at that voltage) each controlled from a separate tamiya ESC  each powered by a 7.2 V 4200mah Nimh battery driving 2 brass 25mm 3 blade props, One LH and  one RH turning in opposite directions

Run time was much improved, certainly 30 mins plus mixed pootling about testing turning circle with  planing runs and manouvering under speed.

Model came up nicely onto the plane at 80-90% power with the bow an inch  inch/half above the surface, it could maintain the plane if a wide turn but slowed and dropped off the plane under tighter turns (after discussion probably because the twin rudders are quite big (52mm).

Despite this there was no digging in on the turns.

Top speed was probably just above scale

Motors certainly got warm but not too hot to touch.

All in all I was impressed with the performance, next move is reduce the size of the rudders and if budget allows try running it with 2 7.4 LiPo batteries  ( brushless motors are just out of my price range at the moment although they would be the gold standard.

Steve

[DavidHughes]

Good news - I'll add your advice to my list and do a bit of online shopping tonight!
 
And the boat looks fantastic too...

[sigfortunata]

David


hope your build is going well and I've an update that may be of interest.  I splashed out just under £50 on 2  2S (8.4v) lipo's and a charger and some low voltage alarms to replace the heavy NiMH batts.


The difference was amazing, comes up on the plane at about 60-70% throttle and flies along at 100%, speed drop a little on tight turns but not on wider ones but the run time was well over 30 minutes on a mix of fast and slow..  The batteries started at 8.3v and the monitors said they had dropped to 7.6v at the end. There was still more go in them but I'd set the low volt alarms at 3.5v per cell but I might drop that a point or two.


Down side, hot motors so I'm at this moment fitting water cooling ( drop of in efficiency was noticable towards the end)


Considered opinion is If i ever fit brushless it will fly.


Steve

[DavidHughes]

That's interesting to know...
 
Just splashed out on a 540 type motor with a 7.2NiMh 5000mAh pack at a relatively decent price... a very old fashioned set up compared to some of the things Ive seen elsewhere on here!!!
 
Will give it a go, but my thought process was the same as yours - upgrade to LiPos if I want better battery performance.
 
Still cant make any sense of brushless motors - as far as I can see they are v expensive and high current draining...lots seem to b rated at 30A plus - which seems nuts!

[inertia]

Still cant make any sense of brushless motors - as far as I can see they are v expensive and high current draining...lots seem to b rated at 30A plus - which seems nuts!

Them's Chinese amps, m'duck.
I have a 2822/17 1050kv Turnigy  brushless motor which cost me less than a tenner brand-new and draws less than 1 Amp @ 12v unloaded. It propels a deep-vee powerboat model the same size as your MTB in fine style; uses a so-called 18A ESC (Fusion Hawk); doesn't get even remotely warm, and will run for well over 30 minutes without slowing down by any noticeable amount. That's with a 32mm x 3-blade scale prop.
If those silly ESC current ratings bore any resemblance to reality then the cables used on them would likely melt and the motors would run only for a very few minutes. Oh, and no-one in his right mind would use them.
Don't believe everything you read from armchair modellers; some of us here have practical experience. I wouldn't contemplate using a brushed motor in a planing powerboat now.
Suit yourself.
Dave M

[essex2visuvesi]

Brushless all the way here


There is even a class of brushless motor (3650) thats available in a variety of winds and it will fit anywhere a 540 motor will fit (right down to the screw locations), but will give you up to 3 or 4 times the power of a brushed 40