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Dec 15, 2019, 08:37 AM
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Discussion

endurence hacks


hi all, sorry if this makes the racer guys cringe but..

i'm looking at low speeds with hi/long run times with a 1.5-2kg hull weight all in.

now i've seen the 'bait boats' that state 5 hours and the likes, and they're using geared 40mm props and do about 3mph top end for a couple of amps/hour with single lipo voltages.(the '3mph' range seems to be similar to scale boat speeds).

so to the question's, what sort of motor torque is needed? in grams per cm?

theres those little geared motors that go down to like 5 rpm, and i've found some in 1000-1500rpm with a stall current of about 200mah and 18g.cm in 3v, 300mah with 37g.cm torque @6v and 550mah stall with 550g.cm 12v.(i'm guesing a 15-20% loss due to exsagerasions lol).

i can 3D print larger basic looking props (but theyre flawed the more complex) or tubes etc but haven't got a clue renegading sizing things. The jet drives seem easy enough to print out, i've printed a few randoms and drawn a few, but.. again, i'm stumped regarding what motors do what.. the problem here is no one seems to want a slow boat that lasts ages.(in jest here but..i can burn 200 amps with just a peice of wire..lol..sy.). my sarkies aside, no doubt i'll be speeding things up at some point, yu know what it's like, buy one, mod it.. before yu know it yu cant help but popping a few amps, lower the plane hull..untill..lol, well..

how low can the rpms at the prop shaft go with larger 'flatter' props?

paddle steamers use about a 200rpm shaft speed, well.. thats all the info i've come up with on google.

(yes i've googled..boy have i gooogled, it definatly aint yu freind lol).
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Dec 16, 2019, 05:16 AM
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Displacement hulls are generally limited on top speed by their waterline length (google for "hull speed", it can be your friend)
Making a hull move its own length implies that the prop has to move the hull's submerged volume of water the waterline distance in the time that the hull takes to go that far. No system is 100% efficient, and you have to take into account the hull shape (how suddenly is it going to push water sideways, how smooth is the path to the prop?) as well as slippage on the prop.
For a scale boat, if I know the power of the original, converting the power to watts, then dividing by the cube of the scale gives a theoretical wattage for the model. Doubling this figure accounts for stated power on the real thing is power out of that engine - wattage of a model motor is power in. Similar with scale prop speeds - multiply the real thing speed by the square root of the scale.
A general rule of thumb tying motors and props, is that for direct drive, "the motor can shall be the same as, or larger than, the prop diameter, and the motor should not have fewer poles than the prop has blades". Gearing allows smaller motors to swing larger props, possibly with more blades, but always at the cost of speed.
The same rules apply to planing boats, but the consideration that is usually ignored is that when planing, ther is a lot less submerged volume, so a fast spinning prop moves water fast, resulting in the small submerged volume moving fast the opposite way.
Dec 16, 2019, 05:21 PM
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thank you mfr02.

regaurding the electric motors can you broaden on the power? are we talking work done power or the old amps times volts power?
Dec 17, 2019, 04:21 AM
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With model electric motors, invariably amps times volts. Many people can easily measure these, few people can measure output power. What you get out of the motor depends on a huge number of variables all of which affect the conversion efficiency in different ways.
Assume 50% and you won't be disappointed.
Depending on hull shape, after much calculation, the answer will probably be a 15 volt RS385 driving a 3 blade 30mm prop running on probably 3S if using LiPo or 7 or 8 cell of NiCad. This assumes a displacement hull.
Dec 19, 2019, 05:39 PM
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that rs385 looks very similar to the ones in the hull already.

had a play today with a simple twist prop in a short tube mount'd straight onto a prop, 3D printed, ehh.. wasnt up to much untill the 550 motor sucked 5 amps.

it's something to play at i suppose. i noticed after fighting in blender that gear box's seam a little over priced..some rubbish plastic did the job very easily with a 20 minute print (0.8mm width x 0.4mm height layers), it's got me thinking, how long or how fast till the plastic cracks n blows lol.
Dec 19, 2019, 05:55 PM
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in forra penny in for a pound.. is there any reason to not use worm drive reduction gearing?
Dec 21, 2019, 07:11 PM
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Quote:
Originally Posted by carpmon43
in forra penny in for a pound.. is there any reason to not use worm drive reduction gearing?
Worm drive is fine for where you need both a big gear reducion and a 90 degree change in direction. The motor needs to spin the prop fast enough after the reduction to move the water fast enough to do the job. Wih a big ratio this implies a very fast motor. If looking for endurance, you need efficiency. Worm drives are not usually noted for this. The worm also needs to be very firmly fixed to its shaft. On a model railway, I have seen a worm pulled off its shaft by the action of the trains inertia pushing the loco when power was cut suddenly.
The 385s are probably right for the hull and props, the only way to get more endurance from what is basically a sound system is to improve efficiency to make better use of the limited energy in the battery. If running at constant top speed, this is probably a job for a brushless set-up of appropriate power, and is outside of what I can comment on.
Unconventional ideas occasionally work, but usually you find that conventional ways of doing things are like that because they do work.
Dec 24, 2019, 11:07 AM
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i tried the 500rpm geared motors with a 40mm 4 blade prop, i was suprised to find it would do small boats and foamey type toys, and draws piddly amount of mah's.. allas i think theyr at about 1/3 the speed of the '380' type motors and the baggy gear box that are in the hull as standard (couple amp draw).
is it just me or is there a small voise calling me, from the 1806 and 22' brushless motor box's lol...
hmm food for thought though.
Jan 09, 2020, 10:43 AM
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subject on hold due to smoking below decks lol,,,,,fffff

esc's dooont do 4 amps for long..dunno where the 10 amp constant come from though.
Jan 21, 2020, 09:41 AM
Big Boats Rule!
boater_dave's Avatar
A simple hull form like a kayak is quite efficient, and can support a lot of weight. For a model, try a 1/4 or 1/3 scale version. A small trolling motor mounted on a kayak will pull a person around the lake all day long.

Anything that adds drag through the water must be removed. Anything that adds friction and inefficiencies in the drive must be removed, or minimized. Gear drives may help, because of the slow shaft speed required for high efficiency props. But they also add weight and friction, so their use must be carefully considered.

Once you get that sorted out, test your proposed motor/prop and find the amp draw. Size the battery to give you the run time you need.

What is this project for?


Dave

PS: My 35 pound sailboat runs for 5 hours on a 2400 mah LiFE pack. Sailboats don't need propulsion power, but they do need wind.
Jan 22, 2020, 04:43 AM
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Quote:
Originally Posted by carpmon43
subject on hold due to smoking below decks lol,,,,,fffff

esc's dooont do 4 amps for long..dunno where the 10 amp constant come from though.
If it is an Oriental ESC, no real surprises. Their Amps tend to be smaller and more delicate than Western ones. But without knowing the make and model, thats just guessing.
Jan 25, 2020, 02:31 PM
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the esc's are the generic 'car' one's that are all over ebay and amazon, 240A forward 160A reverse, only mine wont reverse.. it did survive funily enough, even after smoking up the river for nearly 4 minutes lol!

so.. i did find a use for them this afternoon as a BEC for the servo tester, i popped two 2213/920kv motors and esc's in, the Air brand ones for large propped quads. (they were on 9 inch blades on a kilo quadcopter(battery included, a 5 amp 4 cell zippy), alas the asgards all in one FC literaly spat one of the fets off on low revs, all involved deligated responcability to the man the next door...i.e. gufored me off and i lost the dosh..

either way, the air 2213's are presently heatglued into a jabo bait boat with the 'came with' 42mm propellers and 2mm shaft'sin a straight tube, no bearing or bushings , bolted to a bit of rounded plyboard and plentifully glooped into place. (cant remove the motor bolt's after its stuck in basicaly.) Reason for the heat glue is so i could remove them easily with a dab of isoproponal if the exsperiment failed in a puff of smoke or worse, projectile bits lol.
rpms dry are seriouly hi on full thruttle lol 15v x 920.....13800 which i actualy clocked at the prop. motors and esc stone cold, obviously, the motors are rated to 18amp, esc's 20 amp.

so...bench tested with a clamp meter, at about 350mah drawing at the battery for 2 motors it was doing the 7400 rpms which were the maximum the brushed suppled motors managed on 7.4v...i havent ruled out a bad meter reading, it was reapeatable so..
(the brushed motors are 380/385 sized, 1:2 or slightly more reduction geared..)

so i filled up the bath to see if the hot cold thing would crack the heat glue loose, it' didnt...so i fired it up.

at 400 mah there was far more pull than the brushed motors which drew 2 amps per motor at piddley slow and 4-5 amps on full throttle each, at two amps toatal battery draw with brushless 2213's there was water going everywhere, throttle control not going above about 10-15% before the props were hunting for water and 'grabbing' (the props are in a weed caged tube btw, same princable as the jet thruster thingies).

so i wait till i can nip down the river and see what i get. i'm exspecting a glue release though...ffff. dunno how good heat glue is tbh.
(if any ones wandering, the jabo bait boats do go on a river, but...feck!! upstream takes like forever, they aint too quick...disapointingly slow upstream, not much quicker down stream lol, who knows, on a lake on a still day they may break a wake lolol.)

ooooo the suspence.. i have a torch .....hmmmmmmm...
Jan 26, 2020, 04:51 PM
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so, i tried the air 350 pack items out, even took the fishing line babey rein off. The quadcopter esc's worked no problems.

speed wise,ehh scale speed on a river against the wind.
2 motor no rudder steering vastly improved.
esc's got warm but arnt water cooled, the fact they were warm (30c ish) made me think more amps were being drawn than the clamp meter in the bath results.


it was perishingly cold with the wind, burning hands in under 5 minutes..after 15-20 minutes of running up and down at canal boat speeds i had to pack up with rain ensuing, the battery pack (a 5amp 4 cell zippy) hadnt quite used 0.2 volt per cell, that and the warm escs made me doubt my guestimasions and reading a bit, so i'll wait till it warms up a bit i think before i go for full pack time test.

i'm guesing1 1/2 hours...two at the outside at this point.

and of course i'll have to try a few prop combo's at some time, only half of the throttles being used before props wash.

i think my main conclusion is...........i dooont know lmao!

mfr02? am i on any right tracks with my vague plan so far? i had a look about at the 380 brushed motor you sugested and all the info i found pritty much upheld what you said, it was just bordem that found the brushless motors being heat glued in. I am still dubious as to how long the heat glue will last but hey ho.

any advise on making water gaskets for the hull/top and hatchways? i'm thinking some kind of silicone sealer and cornflour, theres a thin rim on this hull, like a trench that runs arounds the rim, it had a perished rubber 'bead' or 'string' like seal, alas theres bits of its border wall broken here and there.(it's a S/H hull..).
grease will get a bit messy..
Jan 29, 2020, 04:37 AM
Registered User
Sounds like it's coming together.
Manufacturers often use a noeprene "string" as a seal - your idea of silicone sealant sticking to one part and shaped by the other but prevented from sticking by the use of cornflour as a release agent sounds good. It will probably sound even better when I think of it myself.
Noticed the use of "cornflour" instead of "cornstarch". Are you in the UK?
Hot glue gun glue comes in various grades, don't know the actual temperature ratings other than "painfully hot" and really painfully hot", but it works fine at all "normal" motor temperatures. Removing/replacing a motor is simple - cut the "strap" that runs over the top, pop motor out, after the motor is replaced, a dab of hot glue reseals it.
Jan 29, 2020, 04:37 PM
Big Boats Rule!
boater_dave's Avatar
In the US, Amazon sells bulk O-Ring stock, usually 10 feet worth, for a couple of bucks. Might be a decent hack for your gasket. One drop of CA at the joint will keep the ends secure.


Dave


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