|Dave Cooper 6||04/08/2019 23:02:11|
|290 forum posts|
A few years ago my son and I built a small (16" RAF launch just for fun /experience of our own design. It had a 6v brushed motor and heavy dry cell batteries and a '40' prop. Performance was disappointing - ie it didn't plane !
Got it down from the shelf yesterday and decided to do a refurb' /upgrade...I've fitted a secondary 'stepped' fore hull (like a hydroplane), and now wish to go brushless as I've been very impressed with my brushless electric glider's performance..
A few questions :- what motor /controller combination to fit ? (even though it's a fast electric - hopefully - I'd like reverse as well).
What prop to fit. The Graupner rep was in the model shop when I bought the 40 and suggested a 2-blade racing prop. They didn't have one, so, I got a 30 3-blader instead. BTW do these numbers refer to Pitch ?
As a young lad I had an RAF fire tender fitted with a "Mighty Midget" motor. This was also slow !!! However, the motor pulley drove a twin prop system via a rubber belt and 2 further pulleys on each prop shaft. Quite neat and I wonder if this would be less trouble than a twin motor set-up ? Maybe belt slip is a potential problem with a fast electric.
Sorry, I seem to have gone on a bit here...
|harry smith 1||05/08/2019 06:21:58|
|1079 forum posts|
What size, type, voltage and mah is the glider's battery.
You maybe to use that !!!
On motor is a 28mm brushless T2836-1200kv, 340watts on 4S,shaft 4mm.
Unloaded rpm on 3S 13320 and on 4S 177760.
ESC with reverse Hobbyking car HK-60A (2-4S) weight 91grams.
Program card required(HK Prog-Card).
The larger Amp ESC will handle both 3 and 4S, so only upgrade you battery.
Hobbyking have some small lipo batteries in 3and 4S.
eg Turnigy 3S 1500mah 25C(73x33x27mm) and Zippy Compact 4S 1300mah 40C,
This will increase you power and loss a lot of weight.
1 Cutoff Voltage for 3S=10.2 and 4S=13.6
|ashley needham||05/08/2019 10:19:59|
7218 forum posts
Hi Dave and welcome to the forum.
Using the setup as advised by Harry would no doubt provide the best solution to your question, as Harry has a lot of experience. The boat would also be very, very fast...
Do you have other models, and thus batteries to use or will you be starting from scratch?
Dare I suggest it, but for this modestly sized vessel, as a bit of fun, a speed 400 brushed can motor on a 6-cell (7.2V) Nimh and a 35 or 30mm prop would likely propel the boat at a reasonable speed. If you are a Lipo user currently then by all means use a Lipo pack, 2s.
The can motor may well fit easily in place of the old. A 20A Mtronics Viper ESC would be suitable at about £28 ish, and provide reverse, as well as being reasonably small and light.
GENERALISATION: TO BE TREATED AS A GUIDE!.... Common 2 blade plastic props come in course "X" pitch or fine "S" pitch. The S pitch ones would be suitable for the motor, at 35mm (as 30mm ones seem to be not available). 3 blade "scale" props are not suitable for high speed work and give poor reversing thrust. HOWEVER it is worth trying these things out on the boat, plastic props being cheap.
A 28mm brushless of 1200 (ish) Kv would be another choice as Harry suggests. Possibly something of 80-100 Watts would give sufficient performance on 2s or 3s Lipo batts. This is not contradicting harry, just offering an alternative.
|Malcolm Frary||05/08/2019 10:33:04|
|1010 forum posts|
The prop numbers probably give the diameter in mm. Back then, electric motors were pretty dire, dry cell batteries were better at being ballast than supplying power. Almost any clockwork boat could outrun an electric one. The 40 was probably the best answer to matching up a poor motor with a battery of very limited capability, but back then they were the only games in town.
This changed with the introduction of Mabuchi and Kako motors - not very good by todays standards, but a vast improvement over everything that went before. Todays motors are generally better yet.
For simplicity in picking parts, I would go brushed - a 280 motor, a NiMh (or LiPo) of voltage to suit what the label on the motor says and probably a Marine Viper 10 to control it. On a 16" boat that started as an RAF launch, a very much smaller prop will give the speed wanted. Big props are good for discharging batteries rapidly, causing the motor to convert stored power into heat. Over powering any hull rarely ends well.
I'm not sure that making the hull into a stepped type wil actually help - the Italians and Russians tried - if it had worked well, everybody would be using them.
|Dave Milbourn||05/08/2019 10:34:42|
4025 forum posts
I think that the motor and battery suggested by Harry would not only be too powerful for this small model but also too large to fit. My only experience with something similar was an all-balsa Swordsman I made some years ago. It was about 20" long and, like Ashley suggested, had a Speed 400 and 7.2v NiMH AA-size pack. The prop was a 25mm 2-blade bronze job from Propshop and it was pretty darned quick! A 15A ESC would be plenty to cope with that motor, which shouldn't be loaded to more than 10A.
|Charles Oates||05/08/2019 10:37:12|
623 forum posts
I was about to post suggesting a 400 motor but Ashley has beaten me to it. Would I be correct in assuming that the model has a relatively narrow beam, I.e. not built like a racing model. If so too much power will be bad idea. To get some idea about small fast boats look on YouTube at club 500 boats. These are a little bigger than yours, but using not much more power than we are suggesting go very quickly. They were also designed to be stable, yours might not be.
A modest brushless set up will go much faster, is that a good idea?
Hope that helps a bit.
|Charles Oates||05/08/2019 10:45:13|
623 forum posts
Sorry, I missed part of your question. Pulley drives are great for plodding models, forget about them for a quick one. Twin drive is possible in a quick 16 inch boat, but will need care, skill, experience and luck. Forget about that too.
|Dave Cooper 6||05/08/2019 14:10:41|
|290 forum posts|
Thanks for the very comprehensive replies everyone - obviously a wealth of experience here... some more data on the model and equipment currently available :-
Length = 16" , Beam = 4.5", Empty hull weight (with rudder and steering servo /linkage) = 236 gms.
Motor label says "380 /385S" and is 280mm diam. 6 volt (runs quite nicely both ways and is now suppressed).
Shaft diameter = 2mm (I think this was why I couldn't get a two-blade racing one originally ?).
Props = '40' and '30' both 3-blader's.
Controller = "Bob's Board" , Type PB2, rated at up to 2.5 amps - servo mounted.
Available LiPo's (from my aircraft stock) :-
2s 7.4 v 450 Mah 20c weight 31gms
2s 7.4 v 1500 Mah 25-35c weight 83gms*
2s 7.4 v 2200 Mah 20c weight 123gms
* Size for 1500 Mah is 70 x 35 x 18 mm
Yes, not sure about the stepped hull either. Maybe worth a try along with one of the above LiPo's and the existing motor as a first test ? The 'step' was installed a few years ago so would require 'surgery' to remove !
Going to a generic model show on Sunday so may be able to source a better shaft /prop etc. to experiment with...
|Malcolm Frary||05/08/2019 14:30:52|
|1010 forum posts|
My Lotse (about 22" long) performed quite indecently well on a Lightspeed 400 intended for, and run on, a 7 cell NiMH pack via a 24mm 2 blade prop. A 16" boat will weigh a bit under half a similarly shaped 22" boat. The Lotse, ready to go, weighed 1Kg, a 16" boat will be less in proportion, and will need less power to move it similarly, which is why I suggested a 280 and a 20mm prop. Maybe a 360, a largely forgotten motor capable of spinning quite fast but with quite modest current requirements.
A small prop spinning fast will create fast moving water which, since every action has an equal and opposite reaction, a fast boat. A big prop shifting the same volume of water will not move it as fast, but will try to spin the hull in the opposite direction.
Probably the most important way to getting performance from a small boat is to keep the weight down. Only needing, and therefore using, a smaller, lighter speed control is a good start, especally if this means a smaller battery.
|ashley needham||05/08/2019 16:02:03|
7218 forum posts
A 380 is similar to a 400, in that it is a revvy beast, and should be ok on a 2s Lipo. A 385 is a somewhat slower motor usually rated to 12v or so, but in both cases it depends on the manufacturer as to the actual spec. Luckily a swap to a “known” 400 or what have you should be simply a straight swap. A 480 is the same diameter but a few mm longer and is a bit more powerful. Those from JP products are rated at 9.6 v.
The shaft at 2mm is too weedy and needs changing to a 4mm one, as this is the most common size allowing a wide choice of props. Having dissed the 3 blade props for fast work, they are cheap and worth a go..35mm I would try.
ideally a wattmeter would be good to make sure the motor is not overloaded, but fitting a fuse and checking how hot the motor gets after a few mins thrashing about will do.
best way to fit a fuse is to use electrical female blade connectors, crimped on, and insert a car type blade fuse. An easy way to retro-fit.
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