Need help with electric conversion of 40" PT boat

[kgfly]

Hi Folks.

My 5 year old son discovered my 30+ year old R/C boats that I built and ran when I was between 10 and 15. We brought in the small electric scale fishing boat and spent time installing new electrics, rewiring the servos for a Spektrum Rx and got it all going this last weekend. Very nice

Now he wants me to get the ~30" all plywood nitro PT boat going. There's nowhere within 50kms that I can run a nitro boat and the motor is seized and ruined so since these days I am heavily into electric helis I am considering an electric conversion.

While I know a fair bit about electric power systems for helis and a teensy bit for planes, I know nothing about what makes sense for a boat of this size and performance. I am not after wild performance but a nice cruising speed and good endurance. I carved a balsa water-skier that I used to tow behind the boat that was always a real crowd pleaser so low speed operation is important too.

Ideally I would like to find a suitable brushless motor to drive the existing single propshaft and propellor at an appropriate speed but I don't know what that would be. Ideally I would like a brushless ESC with reversing if such a thing exists. I am planning to run of either 4s or 6s A123 packs but I have to see what fits. Since most RTR electric boats seem to be designed for 6-cell NiCd the high voltage could be a problem. May 2s2p would make more sense ?

I am sorry but I cannot remember what size the old glow motor was, I think 0.25 perhaps, nor the prop dimensions/pitch. I did look up some current marine motors of about 0.20-0.30 size and I see that they run around 20,000 to 30,000 rpm and 1000W to 2000W peak power :shock: I guess that's for racing as I am sure my old motor was never in the sort of power range.

So my questions are:

a) What are typical usable RPM likely to be for 1m hull like on a PT boat ?

b) What kind of power levels makes sense (I was imagining 50 to 150W but really have no idea, my light 20" fishing boat runs at about 20W-30W) ?

c) What ESCs do folks use in boats, are there special marine ones with reversing ?

d) What brushless motors would you recommend for direct drive on either 12V or 18V (I guess this really relates to the answer to (a) )?

e) Are there flexible couplings suitable for this size setup is direct coupling a must have (I foresee problems with motor alignment to the existing fixed shaft) ?

f) Is water cooling of the motor and or ESC usually needed for moderate performance (not looking for wild speed here) ?

Thank you for helping a dad looking to make his 5 year old smile some more.

[waboats]

Not Sure but it depends on the weight..

My PT Brave Class (Pekasa) runs on a Direct Drive 785 on 12V Nimh..

Good Scale Performance but still a little heavy. Undergoing some surgery to improve on scale speed.

[kgfly]

With some digging around I found this kit which is roughly the same size:
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXE371&P=0

They recommend a Dumas 6V, 540-brushed motor and a chunky looking Dumas speed controller:
http://www2.towerhobbies.com/cgi-bin/WTI0001P?&I=LXE551&P=V
http://www2.towerhobbies.com/cgi-bin/WTI0001P?&I=LXE527&P=V

Seems that might perhaps give scale performance but it looks like a rheostat-based controller which is very inefficient. Besides, scale speed isn't quite enough !

I found this Traxxas Nautica controller which looks promising but is for brushed motors and there is no max current or power specification: http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXJM29&P=0

I found a whole page of reversible brushed controllers, mostly intended for cars/trucks but the "AI Quantum Runner" is waterproof and reasonably priced so could be a goer: http://www2.towerhobbies.com/cgi-bin/wti0093p?&C=MJD

-------
All the brushless boat controllers are very expensive as are the motors it seems, so perhaps a low-voltage brushed setup will be a reasonable choice. That might push me towards a 2s2p 4600mAh A123 setup (7.2V peak, 6.6V nominal, about 6.0V under load). If this is a valid assessment, is a "540" class 18 to 20 turn brushed motor a good choice and if not, what is ?

[kgfly]

Not Sure but it depends on the weight..

My PT Brave Class (Pekasa) runs on a Direct Drive 785 on 12V Nimh..

Good Scale Performance but still a little heavy. Undergoing some surgery to improve on scale speed.
Thanks for that. Is "Direct Drive 785" and motor brand/model or just a description ? Is that a brushless 785kv or a kind of brushed motor ? If it is brushless then on 12 x NiCd that would give at most 12000rpm, is that the right range, from the nitro motors I got the impression much higher rpm were typical ?

[pompebled]

Hi kgfly,

In this size boat, keeping it light is the key to performance.

I used to run a stepped MTB hull as a testplatform for motor and batteries, the hull is 30" long, like yours.

With a 700 series brushed motor on 12 cells sub-C, the boat was very fast, way too fast for my (at the time) nine year old son.
As the boat has subsurface drive, the steps drew air under the hull, once on the plane, the speed would build up untill the rudder stood in a pocket of turbulent water/air mixture behind the prop, rendering the rudder useless.
Only when the speed was reduced the boat became steerable again, very scary, running towards a brick wall, waiting for the hull to drop of the plain and regain steering control.

With the components mentioned above, the boat sat too deep, not a pretty sight.

A much better combo was a mild, 500 size, car motor, on a belt reduction 1:2,6 (as used in planes) running on 7 cells sub-C (8,4V) turning a 40mm X-prop (high pitch).

The boat was much lighter and ran swift and steered well, still capable of running faster than scale speed, but that's what an ESC is for...

The motor had brushtab cooling, as these motors tend to get rather hot in an closed hull.

The 7 cell, 40mm prop set-up may be a bit too much for your son, you can always run only 6 cells and try a smaller, less coarse prop, to accomodate his driving skills.

My son is 20 now and just beat me in both the 7 cell 600 motor class and the 700 motor 12 cell class in the Dutch Electrospeed Championchip, we ran yesterday...
Coming in second after my son, who ran boats, that are build and set-up by me, isn't that bad.

Regards, Jan.

[kgfly]

Thanks Jan, it must be great to have been able to share your hobby with you son over all that time. I hope my son and I can do the same.

So in short you recommend a 500-class brushed motor with integral cooling running on between 6V and 8V. Can you give me some suggestions for motors, esc's and reduction gears so that I can look them up to get a better idea of size, price and availability ? Or if you have a favourite online shop that stocks all the right bits that would be interesting too.

I see that the brushed ESCs tend to specify "at least 16 turns" or "at least 20 turns" which I guess relates to the motor specification, with more turns being perhaps higher torque but lower power and maybe lower rpm ? Is that right ?

At this stage I am not sure how much flexibility I will have with motor mounting in order to accommodate a gearbox so would prefer a direct drive solution with perhaps a Graupner universal coupling to allow for any alignment mismatch. If I stick to a 2s A123 setup the voltage range will be from 7.2 down to about 5.4 which should suit most brushed motors and ESC setups from what I have seen so far.

[mfr02]

Outside diameter of a 540 motor can is 36mm, which means that if your shaft has 18mm clearance, you will have no problem. I suspect that you have just the one shaft, and you might want to consider some surgery to fit two shafts instead. This will allow you to make any arrangements that are needed. The wooden hulls that I have seen from back then which were intended for I/C tended to be over-built because they had to stand up to the motor being started. You can reduce a lot of internal framing with electric power, as the hull does not have to survive the same handling problems.
The best way to ensure alignment is to use a piece of tube in place of the coupler to place the motor corectly, then built the mount to suit. Replacing the tube with the coupler makes for a nice quiet, efficient drive.
Power? About 100W seems about right. With direct drive, a tip I was given many years ago was to make sure the prop diameter did not exceed the motor diameter by much.

[kgfly]

Thank you mfr02 that is great info. Any idea on reasonable prop RPM range ? If I want to explore brushless I need some idea of the RPM range to match voltage and motor KV.

[Prins Willem]

Several of the guys in our club swear by Astroflight marine motors. Pricey, but very high quality.

I recently purchased a pair of Mtroniks Viper marine 20 ESCs. I haven't had a chance to try them out but I did a question on the forum and received positive feedback on these units: http://www.rcgroups.com/forums/showthread.php?t=737486.

The Astros push the Dumas Dauntless very well (we have a few in our club) so your model should not be a problem. On the Wisconsin Scale Boating Assoc. website links page are links to Astroflight and Mtroniks. www.wiscaleboat.org

[kgfly]

Thank you for that info. The Mtronik Vipers look interesting but seem hard to find outside the UK/Europe. I did find one vendor in the US (www.rcmart.com) that seems to stock them: http://www.rcmart.com/catalog/default.php?cPath=421_36_51&sort=1a&&page=2

This "Associated A.I. Quantum Runner Reverse" ESC is looking promising too: http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXKYX5&P=0

I am confused as the Dumas 6v and 12v motors are rated at only 5000rpm but I see other marine motors rated at 15000rpm. I get the impression the Dumas motors are intended for pushing around large scale models at scale speeds. Probably not quite what I am looking for.

I guess it all depends on the prop diameter and pitch, on the weekend I will have to dive into the garage and take some measurements on my old PT boat.

I am wondering if one of these ESCs together with a cheap 540 Mabuchi motor and a universal coupling might do the job ? I need to learn more about "no. of turns" and how to tell if any given motor will work with an ESC rated for "Rec. Motor Limit: over 18 turns"

[Prins Willem]

Yea, the Dumas Pittmans turn rather slow. I use them in all my tugs. They draw little amperage and are bulletproof. We have a member in our club running a Dumas PT 109 which I believe is larger than your boat. I'll e-mail him tonight and ask him how his is set up. May I suggest you look over the build threads on the Dumas Dauntless on the forum. Anything that will push that horse will launch your boat into orbit.

[tim slocum]

The number of turns determines power.A "standard 550" sized motor is usually around 27turns. High performance motors are usually around 20T or less. Traxas makes a 23T Titan motor that ideally is run on two 7.2v packs for 14.4v. It has an internal fan for cooling.To run this motor you will need a electronic speed control (esc) which can handle a 23T motor and at least 14.4volts. I'd check out www.towerhobbies.com they have a large number of motors,escs,props etc. Another idea would be to use the Aquacraft powertrain from their SuperV27. It's brushless,comes with motor and esc(w/reverse) and all the wiring. Its plug and play. It is powerfull. I think it retails for around $160US.Oh, Im not sure if someone covered this before,but , a 550sized motor has a case 550mm long, 540 is 540mm long and so on.

[kgfly]

Yea, the Dumas Pittmans turn rather slow. I use them in all my tugs. They draw little amperage and are bulletproof. We have a member in our club running a Dumas PT 109 which I believe is larger than your boat. I'll e-mail him tonight and ask him how his is set up. May I suggest you look over the build threads on the Dumas Dauntless on the forum. Anything that will push that horse will launch your boat into orbit.
OK, thanks again for the info, I will look into the Dauntless forum.

[mfr02]

I wouldn't get too hung up about actual claimed RPM. "High" or "Medium" or "Low" is enough, and an idea of probable power consumption helps. Both the electric motor and the prop act as slipping clutches, so finding a combination that is efficient is top of the list. It is often found with a particular motor that an increase in performance happens with a smaller and finer pitched prop, down to the point where it just can't shove enough water backwards fast enough.
With a scale model, even a planing type, control at all speeds is vital. I stand to be corrected, but control and power is still easier with brushed motors, especially at a sensible cost. You still need reverse, as that is your brakes. Nothing quite as dispiriting as a fast boat that is heading for an expensive concrete bank that doesn't want to slow down.

[kgfly]

Great, thanks for the advice Tim and mfr02. I am fond of brushless due to their inherently greater efficiency but as mfr02 says, "but control and power is still easier with brushed motors, especially at a sensible cost". This project was never really included in the hobby budget but I want to do it for my son so a $40 ESC and $25 motor is sounding like a good option right now. I just have to try to find a suitable "540/550" motor that is compatible with the A.I. Quantum 25A or Viper Marine speedies. The Traxxas Titan 550 ($21) is interesting since it is fan-cooled and rated to 14V. Otherwise there is a bewildering array of 540-class 7.2V motors in a range of turn-counts and either single or double wound (what's the benefit of double wound?) ranging from about $18 on up. These all seem around the 100-150W range.

[boater_dave]

Prins Willem, both the big PT, a Dumas 48" w/glass hull, and the Daunltess use twin Astro 40 ferrite marine motors. They are running somewhere in the 12 to 18 cell range.

Dave

[mfr02]

(what's the benefit of double wound?)
Factors in motor power are ampere-turns. The more amps, the more power, the more turns, the more power. A lot comes down to how much wire you can fit into a given space, and if you take all the space up with thick wire, you get few turns, but a heck of a lot of amps, but the motor spins at a furious rate of knots. On the other hand, if you put as many turns as possible in the same space, the thinner wire gives a lower current, but many times more turns. Result is torque but less speed.
The double winding is two coils in parallel resulting in effectively twice as many turns. The current in each coil is added, so this is a way of increasing both the amps and the turns in the same space. Downside is that it's a more expensive way of producing a motor.
Another deciding factor is the strength of the magnet, a stronger magnet makes for a potentially more powerful motor, and thus you get exotic mixes for making magnets that concentrate more force into a smaller space. Cobalt, ceramic etc.

[kgfly]

I see, so all else being equal (which it usually isn't!), a double wound 20t motor will have more torque, more power but half the rpm of a single wound 20t ?

[mfr02]

So I have been told - perhaps an enquiry in a fast electric forum wil give chapter and verse. The numbers will head that way.
Personally, I just run to ordinary motors and try to stop short of actual abuse.

[kgfly]

Well I got the old girl out of the garage today and learned a few things:

Length: 40" (102cm)
Weight (motor removed): approx 6lbs (~3kg)
Prop: 42mm diameter, about 16mm along shaft from blade top to root
Old motor: HB 0.25

Looks like I can fit 2s4p 9200mAh A123 where the fuel tank used to be so that should be plenty for some reasonable run time, around 15 minutes at 25A/150W.

[kgfly]

Photos here:

[ptjim]

The Dauntless by Dumas looks like it would make a great Crashboat conversion.
I am still new to this and will be posting many questions once I start to build.
Great to be here!!!

[kgfly]

Update - leading contender at the moment is Graupner Speed700 BB Turbo (12V, 967kv) with an Mtronik Viper Marine25 (or maybe Marine40) ESC and a A123 4s2p 4600mAh (13.2V nominal, 12V under load) battery. That should give approximately 5000-10000rpm under load and around 10-15km/hr if the calculations hold up!

I am still trying to find a suitable coupler (3/16 threaded prop shaft to 5mm motor shaft) and then once the whole system is assembled, hope to try various props. The existing red plastic prop has "X40" embossed on the back of one of the blades. An experienced modeller has recommended I try a 50mm Raboesch C-type 3-blade or B-type 4-blade.

The search continues...any comments welcome :)

ps. I found this useful link with data about lots of motors: http://www.motocalc.com/data/motor.html

[tsenecal]

kgfly:


http://harbormodels.com/site08/main_pages/propulsion.htm


first entry "Kardan Style Motor Couplers"

allows you to select "end1" and "end2" connectors, both 5mm and 3/16" connectors are available. don't worry about the fact that 3/16" side is threaded....

have used them in the past, very high quality.

Tim

[kgfly]

Tim - Thanks for that, I'll see if I can find a similar product locally but at least I know I can get it at HarborModels.

Still looking for a suitable speed controller. I was leaning towards the MTronik Viper series but there is a potential problem. They are only rated for 12V. I want to run 4s A123 which is 14.4 hot off the charger, 13.2V nominal but only 12V under load. I am concerned the over-voltage, even though it is only at no-load, will burn up the ESC. On the other hand a 10-cell NiCd is about 13.5V fully charged. Any thoughts folks ?

I found this Electronize FR30 controller (6v-24v, 30A), anyone have any experience with it? http://www.electronize.co.uk/fr30_series.htm

[pkboo]

So I have been told - perhaps an enquiry in a fast electric forum wil give chapter and verse. The numbers will head that way.
Personally, I just run to ordinary motors and try to stop short of actual abuse.
My kinda man Mfr02!

[kgfly]

Someone found this ESC kit for me, 12v-24V, 500-1000W, that ought to do it!
http://secure.oatleyelectronics.com//product_info.php?products_id=206

[kgfly]

I have taken the plunge and purchased a motor, cooling coil, mount, 4mm-t0-5mm coupler and ESC all for under $100 which was the original budget.

Motor: Johnson HC785LP-001 http://johnson.manowar.e-pro.de/index.php?IdProduct=476
ESC: 50A unit from here http://stores.ebay.com.au/RevolutionShop_RC-Boat-Accessories_W0QQcolZ4QQdirZ1QQfsubZ3080451QQftidZ2 QQtZkm

The motor is higher KV than ideal. The ESC is rated for 10-cell NiXx so I am hoping it will cope OK with up to 14V at it idle and 13-11V under load. It is a more convenient solution than the Oatley being a sealed,waterproof package that incorporates a BEC. If the ESC runs hot I have the option of adding some water cooling or using an external switched-mode BEC.

If it doesn't work out I guess I'll have to consider upgrading but it should be interesting finding out. I did some trial fitting of the motor in the boat last night and it only just fits! It will take some time to optimise the alignment but I think I have it close. The existing water lines from the nitro installation can be adjusted to fit which is nice.

[tim slocum]

www.offshoreelectrics.com is your one stop location for all your go fast needs. I am not affiliated with them in any way,but if you want to go fast, they know how and carry the stuff to get you there. Theres also a forum. I found an esc there called an Jetti Navy 600,which can handle...get this.... 8 to 30 cells! and costs about $68.00. I would'nt tempt fate by using an esc close to its max capablity, you really dont want to get the mysterious blue smoke in the middle of the lake..do you? I've had to learn the hard way,dont make the same mistake.

[kgfly]

Tim, thanks for your advice. I am not after especially high speed in this model but do want reverse, which the Jeti Navy 600 does not support. You are right about the risk of running the 10-cell ESC at the high end of its voltage range. On option I have is to only charge my A123 packs to about 90% capacity. This will keep the voltage well within bounds for only a minute or two reduction in run time. Since A123 cells are perfectly happy with partial charge cycles this is a viable approach. I hope to keep the magic smoke inside all the parts ;)

[Ghost 2501]

try the boat on 10-cells though, the additional weight of the ni-xx batteries may give you better performance because the prop is under more water

[pompebled]

Motor: Johnson HC785LP-001 http://johnson.manowar.e-pro.de/index.php?IdProduct=476
ESC: 50A unit from here http://stores.ebay.com.au/RevolutionShop_RC-Boat-Accessories_W0QQcolZ4QQdirZ1QQfsubZ3080451QQftidZ2 QQtZkm

The motor is higher KV than ideal. The ESC is rated for 10-cell NiXx so I am hoping it will cope OK with up to 14V at it idle and 13-11V under load.

Running that ESC on a higher Voltage than specified, isn't a very good idea; unless you are lucky and get one with Fets, that exceed the manufaturers specs, the ESC will most likely die on 14V.
Don't use the BEC if you decide to try it anyway, it's the BEC circuitry that will start to buckle first, heating up the ESC to the point of destruction.

As I'm on the other side of the globe and not aware what's available locally, I can only recommend what I know, such as this one:
http://modellbau-regler.de/xtcommerce/product_info.php?info=p25_AS12-30RW-BEC.html&XTCsid=8g9cpa3gftde8nj0vaqbjouh86

The next model is even more powerfull, but the price rises rather steeply with more capacity...
I know these guys used matched Fets in their ESC's, and I haven't heard of any failures, besides the ones that were 'abused to death' in the test phase.

Regards, Jan.

[kgfly]

pmopebled - Thank you for your suggestions. I have not come across that brand of ESC before and they do not seem available outside of Europe. From the description they seem to have good features. For units with forward and reverse, only the 30A model has a BEC, the 60A model is opto-isolated so does not include a BEC.

10-cell NiXx straight off the charger is at 13 to 14V. If the ESC is rated for 10-cell NiXx as it claims to be, it must be able to tolerate 14V. Of course it may not so I will find out during initial backyard testing. If it survives initial testing it still might fail over time. I also plan to explore charging my packs to ~90% which will keep the peak no-load voltage down. Under loads of 15A to 20A they fall below 12V anyway. I have an external switched-mode BEC (up to 42V input) which I can use if needed and will also investigate making a water-cooling block for the ESC.

I do appreciate that using an ESC rated for 12-cells or more would be a much better idea and I may yet be forced to do so, but part of the hobby is about experimenting so although the ESC I have chosen is not ideal, perhaps what I have planned will work and perhaps not. If nothing else, we will learn something from the exercise.

Ghost - I don't have any NiXx packs and don't plan to buy any since I don't like their self-discharge behaviour, need for cycling, poor tolerance for partial discharge/charge regimes and the fact that they tend to die if left unused for months on end, which is likely to happen over time. The A123 cells have none of those characteristics and are what I have available. Ultimately I may have to add ballast to trim the boat for better performance. In that case I may use either additional parallel 4s packs or lead.

[pompebled]

The reason for not having a BEC on the over 12 cells ESC lies in the fact that, when not disabled, when using, say, 24V, the powerdissapation of the BEC (>20V!) will start to overheat the ESC to the point of malfunction, or shut down.

The 30A ESC should work fine in the set-up you're using.
As the ESC's are quite small, sending one to Australia in an envelope shouldn't be an issue.
If you like, I'll inquire about sending overseas.

You're right about the initial voltage spike, fresh from the charger, usually a short burst from the 700 motor will take care of that...

Nevertheless, disable the BEC on your ESC, by removing the red wire from the connector (lift the plastic lip) and use a separate receiverpack.
Also your idea of watercooling the ESC's housing is a good idea when riding on the edge...

Keep us posted.

Regards, Jan.

[kgfly]

pompebled thanks for the offer, if I burn up my 10-cell ESC I might take you up on the offer.

[pompebled]

pompebled thanks for the offer, if I burn up my 10-cell ESC I might take you up on the offer.

Take it easy on barbequeing your ESC; I've just been informed that a letter to Australia (with an ESC inside) would cost €12,00 (without insurance), a package would be €37,00 (insured), on top of that comes the €2 for using PayPal and ofcourse, the cost of the ESC itself...

Regards, Jan.

[kgfly]

Thanks for investigating, one of the complications of living Down Under is that we are a long way from everywhere and shipping anything is expensive.

[kgfly]

Update - Well the project was sidelined for a long time but this past weekend my son and I had great fun at the pond with the boat running beautifully on 4S LiFe. Performance was excellent and endurance much greater than expected. On a 35C day like today the motor gets very hot, even with the water cooling coil (I guess the flow is too low and the thermal coupling poor) and the ESC very hot too after 10+ minutes of continuous running.

Setup
Motor - JohnsonHC785LP-001
Prop - X40
ESC - 50A, nominal 10-cell NiCd/12V, Fwd/Rev
Battery - A123 LiFe 4s2p 4600mAh
Max power - 430W, 36A @ 12V

I might arrange a separate sBEC to reduce the heat load on the ESC as previously suggested but overall I am very pleased with the outcome and it has all certainly been worthwhile to have the pleasure of sitting on the bank of the pond with my boy on my lap spending time playing together :)

Thanks for all the help, suggestions and advice.