Thread Tools
Sep 17, 2021, 09:57 PM
Registered User
Thread OP
Question

Gas to electric conversion


To start off I have been in and out of RC for over 30 years now. 1/10, 1/8, 1/5 cars and trucks, crawlers, boats, planes, and even tried my my hands with a helicopter once (didnít end well), electric, gas, nitro. And things have changed a lot over the years.

I currently have one of my old builds that I want to convert to electric. It is a Davette Boats 34 inch Sprint hull that had a modified Zen 26cc powering a Graupner Jet unit 5 pump. The boat ran well (a little over powered) when I was running it, but havenít ran it for several years now. I would like to convert it to electric (cleaner, no gas/oil to mess with). I contacted one of the RC dealers and was suggested a Leopard 5698 1100kv on 2 4s 5000mah lipo in series for 8s and a 200 amp esc. They may have been mistaken on battery choice, but I donít think I need 33,000 rpm. My thoughts are to keep the rpm around the 16,000 range or less. And this is something my 73 year old father would like to run too.

Can I get by with a 40xx motor or should I go to a 56xx? The charger that I currently have can charge up to a 6s but would like to stay in the 4s lipo. Would a brushless motor 40xx or 56xx require higher voltage to run efficiently and not draw too many amps and build up heat? The last lipo batteries I bought was 11 years ago, so battery recommendations would be helpful too.

I guess the short of it is, what brushless motor, lipo battery, esc setup would run the hull and drive? I will be running it in streams and a small rivers in my area (PNW), my father would putt around in the slow water and back eddies. I do subscribe to the buy once cry once way of thinking.

Thank you in advance.
Sign up now
to remove ads between posts
Sep 18, 2021, 12:11 AM
Platinum Card Member
sundogz's Avatar
I'm running a TP Power 4070 (40x100mm) 1090kv on 6S in a Davette with a Gravitix 49mm jet drive and it runs respectfully. See post 856 for a video. I burn up a cheapo $50 speed control which ends the movie. I've even installed drivers and a roll cage since, just for sh*ts and grins.
Last edited by sundogz; Sep 18, 2021 at 12:16 AM. Reason: added pics
Sep 18, 2021, 08:38 AM
Registered User
unusual_rc's Avatar
It is still for sale in the New Zealand store, linked by Sundogz in the original post. The link is dead, but search the website for jet-drive and you will find it.

Of witch material are the impellers made of?
And are the 3,4 and 5 bladed impellers equally pitched?

I wonder why they recommend for a 49mm drive a minimum of 200W electric motor?
And 700W for the 5 bladed impeller. Sounds as way to less. For comparison a 28KMB drive can run on maximum 800W at 20.000rpm!
Sep 18, 2021, 10:24 AM
Platinum Card Member
sundogz's Avatar
I think he already has a suitable jet drive (Graupner jet unit 5). And the motor I used is still available here. Or for a similar price you could go with this SSS 56104 (1000kv) for a little more pep. Of course you'll need a more powerful speedo. The 300A Swordfish esc I used was more than I needed (got it for the data logging and never used that feature) and should be fine for the SSS.

But on your other questions the impeller is made of plastic (not 3D printed) and all impellers have the same pitch. I think they left a zero off the end of the motor sizing!
Last edited by sundogz; Sep 18, 2021 at 10:39 AM. Reason: answered other question!
Sep 18, 2021, 10:56 AM
Registered User
MiteyF's Avatar
Lill, if you'd like to sell that boat/hull and start fresh, please PM me. I've been looking for one of those (to run with the Zenoah).

Thanks.
Sep 18, 2021, 11:52 AM
Registered User
unusual_rc's Avatar
Quote:
Originally Posted by sundogz
I think he already has a suitable jet drive (Graupner jet unit 5). And the motor I used is still available here. Or for a similar price you could go with this SSS 56104 (1000kv) for a little more pep. Of course you'll need a more powerful speedo. The 300A Swordfish esc I used was more than I needed (got it for the data logging and never used that feature) and should be fine for the SSS.

But on your other questions the impeller is made of plastic (not 3D printed) and all impellers have the same pitch. I think they left a zero off the end of the motor sizing!
The jet unit 5 was high on my list when I was young and could not afford it

I guess that must be right, they left out a zero. They should have written down kilowatts

The reason I ask about the blades is that a non overlapping impeller seems to require less power. The 5 bladed version is almost overlapping (but not completely, otherwise it is not mouldable).

Eager to test that one-day in our drive.

But do not let me hijack this topic.
Sep 18, 2021, 03:16 PM
Registered User
Kayaker's Avatar
This 180 amp (8,000 watts at 12s) ESC made by Flier is good for this size jet drive with a 56mm motor:
https://www.ebay.com/itm/180A-Boat-F...AAAOSwFFVdoXTw

I like 4 batteries to get the weight right, down low and close to the center line with one up front.
Sep 18, 2021, 04:21 PM
Registered User
Thread OP
Sundogz, I was hoping you would jump in here. I have been reading your thread you started and donít know how I missed your maiden vid. Wish I knew that Dana was making cowls for the electrics too, or they mite not been available in 2010 when I got mine. It looks like the TP 4070 runs it fine, my 26 Zen was a bit faster. I will compare the numbers of the TP 4070 and some of the 56 series motors. I will consider stepping up to 6s lipos over 4s. Question, does the longer motor can give more torque?

MiteyF, thanks for your interest but I will keep her for now.

Unusual_rc, the Swashdrive was on my list back then, but I could only find the Graupner. I think the Graupner is a good unit, but I think the units from Down Under were a bit better.
Sep 18, 2021, 05:05 PM
Platinum Card Member
sundogz's Avatar
Quote:
Originally Posted by Lill73mule
Question, does the longer motor can give more torque?
Yes, more torque (watts) and pulls more amps.
Last edited by sundogz; Sep 19, 2021 at 09:32 AM. Reason: watts, amps
Sep 22, 2021, 01:05 PM
Registered User
Thread OP
Well I think I have settled on a combination that should work, I will run it passed you all to make sure my thinking is good. TP Power 5660 (56mmx92mm) 926kv, to save a bit of weight. Swordfish 300A esc. And a 6s2p battery setup with 4 3s Turnigy Graphene 5000mah. I should have room for the 6000 lipos but I will get the motor and running gear installed first and see how my COG is looking.
Sep 22, 2021, 01:51 PM
Platinum Card Member
sundogz's Avatar
Theoretically the TP5660 on 6S at 926kv (205572 rpm's) shouldn't go quite as fast as my 1090kv TP4070 at 24198 rpm's. I'm thinking that you're surmising that the smaller motor isn't powerful enough to reach peak output. It's smallish appearance in the hull may be deceiving, especially when compared to the space a gas motor occupies. Perfectly understandable....we'll see!
Sep 22, 2021, 02:34 PM
Registered User
unusual_rc's Avatar
1. What's the maximum power output of your Zenoah 26cc?
2. What is the maximum rpm of your zenoah under load (combined with your Graupner jet-5 pump)?
3. What was the advice of Graupner concerning maximum rpm for your pump?

Since you are going to power the same pump and adding more weight due to the batteries, your current power plant should be a good reference/starting point for an EV conversion.

Electric motors are in general more powerful than your zenoah if chosen correctly. However large RC jet-pumps do not like high rpm in general.

An electric motor does drop rpm under load, like any type of combustion engine. As a rule of thumb you can use a factor between .70 and .85
So 20.000rpm unloaded for example (voltage multiplied by kV rating) will be somewhere between 14000 and 17000rpm in reality.
Where exactly is very hard to say.

10 Ah of capacity and, let's say a current draw of 230 (6s = 24V x 230A = 5.5kW of power (that is what I found) Please check what this motor is rated for as maximum power output.

If you are using 10 Ah capacity batteries, you run out of power within 2 minutes and 40 seconds.
Last edited by unusual_rc; Sep 22, 2021 at 02:43 PM.
Sep 23, 2021, 01:49 PM
Registered User
Thread OP
Sundogz, I was thinking that a larger dia motor with a higher continuous watts rating wouldn’t load up as much and pull the rpm down as bad. And speaking of motors, do you know why Leopard motors have such a low watt rating compared to other motors roughly the same size?

Rc, The Zen 26 was bought 11 yes ago. I think it is a WHH/Model Gas Boats M5 rated somewhere around 4-4.5 HP and 15,000-16,000 rpm.

I never ran the boat with a tach or gps (for speed).

This is from Graupner spec sheet “ Suitable power plants are internal combustion engines of 15 to 30 cc capacity, and electric motors rated at a minimum of 700 Watts output power. “

The TP Power 5660 specs are watts cont 5500, watts max 10000

I didn’t think the power drain would be that bad! And I was hoping to keep the weight down going to electric. Dry weight now is just under 17lbs and I had to add 40 oz of lead to the transom to get the COG set (because the radio box was forward of the gas engine, I am fixing/moving that now above the pump). I guess I could keep it lighter for 2 minutes of fun=no fun.
Sep 23, 2021, 03:44 PM
Registered User
unusual_rc's Avatar
So 4 horses makes 3.0kW of power at 15krpm. Nice point to start with

I also read Graupner advised 1 horse (.75kW) for electric power. Much less than your Zenoah. A bit of a weird advice!?

I have a 3S 5Ah soft lipo here, which brings 350grams to the scale x 4 + weight of the motor + Esc. Does it stay under the weight of the Zenoah? If yes, that would be great.

To make 15krpm you have to multiply it with the inverse of .7, which gives 21.5krpm unloaded. So devided by the voltage of 6S (24V) gives 890kV.

Maybe put the krpm to 16.
More power means it can spin the impeller more easily.

At least this is an indication. I would keep the kV around this number, maybe a bit more. And more power than 3kW continuesly for sure.

For my application with the 45mmFJD and a 4092 inrunner at 1.5 to 2.0kW the rpm drop is .8
So there is a margin here, which is hard to predict unfortunately.
Sep 23, 2021, 09:09 PM
Platinum Card Member
sundogz's Avatar
Quote:
Originally Posted by Lill73mule
Sundogz, I was thinking that a larger dia motor with a higher continuous watts rating wouldnít load up as much and pull the rpm down as bad. And speaking of motors, do you know why Leopard motors have such a low watt rating compared to other motors roughly the same size?
That's what I suspected. As to 'why do Leopard motors have lower wattage ratings'? Is it less copper content (higher resistance) in smaller diameter windings combined with lower quality bearings/neodymium magnets and thicker & lower quality steel laminations in the rotor and stator cores which aren't as meticulously balanced? Or that they use 2 poles instead of 4 or 6 as in more expensive motors, or am I way off base here?


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Percentage weight penalty convering from I/C to electric oneday Electric Plane Talk 13 Jun 02, 2017 04:28 PM
Lazy cell. OPtion to conver 4S to 3S jferfre Batteries and Chargers 3 Apr 18, 2016 05:55 AM
Question Price Of Gas Vs Electric Power Systems MikeDobies 3D Flying 21 Nov 01, 2015 07:22 PM
Discussion gas or electric Ferb487 Fuel Heli Talk 2 Oct 26, 2015 01:30 PM
Discussion Winner Model 78in MX2- Electric only, no Gas CNJ3 3D Flying 0 Oct 13, 2015 11:27 PM