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Old Aug 24, 2006, 03:03 PM
wja
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Motor/ESC suggestions for a 60Ē PT Boat?

I recently picked up a 60Ē PT Boat and Iím now trying figure out what electric Motors/ESC(s) might be a suitable for this boat. Iím thinking of using 3 direct drive motors with port and starboard engines controlled by an independent speed controller and the middle motor being turned on by a micro switch when the other engines are on full power. Does that sound like a reasonable setup? Would 3 AstroFlight Super Ferrite 40 Marine or Graupner Speed 700 BB TurboElectric motors be sufficient? If so what Speed Controllers would be suitable?
Iím not really familiar with RC Boating as my experience is mostly with RC Airplanes so any advice would be greatly appreciated!
Thanks in advance,
Wal
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Old Aug 24, 2006, 03:50 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
Atascadero, California, United States
Joined Oct 2004
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If the center motor only comes on at full power then the prop acts as a giant "Drag" at all other power levels. This will draw more juice from your batteries than necessary and possibly mess up the water flow around the other props.

What I've read some do in this situation is either go 3 ESC's or 1.

With a good computerized radio, you can mix the left right motors in with the rudder and the center motor is 100% throttle responsive. This has been done in aircraft 'tri-motor' setups as well. For best turning, this is your best setup. The left and right motors at straight ahead will echo the center motor. As rudder is fed in, the direction of the turn motor will fall off and the other will increase above (if at less than 100% throttle) the center motor, up to 100% throttle.

For least expense, go with one ESC rated at the capacity of all 3 motors added together and give yourself about 15% headroom on the ESC above that. So, if all 3 motors draw 15 amps short peak each, that's 45 Amps and a 15% buffer would put you at about a 50AMP ESC. If they are rated at 15amps continuous each, you might want to bump that up to a 60-80Amp ESC.

Just how I'd approach it, there are probably a bunch of other ways.
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Old Aug 24, 2006, 04:35 PM
wja
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Shaun,
Thanks for your input! Even though itís the pricier alternative, Iím really liking your 3 ESC approach suggestion especially since I heard that the rudder response on PT Boats tends to be rather sluggish! By the way, are there any Marine specific radios (F-14 and Nautical Commander come to mind) that provide mixing capabilities to accomplish such a set up or am I better of sticking with a good Futaba Computer radio, which is something Iím familiar with?
Thanks!
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Old Aug 24, 2006, 05:03 PM
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As described above with one speed control and the motors wired in parallel the current could be 45 amps. This setup is self defeating since with the higher current draw from the batteries your run time is reduced to one third. If the motors and batteries (3) are wired in serial to a motor controller rated for higher voltage then your run time is the tripled. With a boat of this size three battery packs would not matter.

I doubt you would need to reduce throttle on an inboard engine when turning at medium to high speeds and this would only be useful for lower speeds. You would have the effect of slowing down in a turn. Rudders at higher speeds are quite effective and useful since you only use small inputs anyway.

Three speed controls would be useful if you intend to do considerable manouvering at low speed for docking and such.
The nautical commander does not have any mixing capability, except for the dual throttle sticks which give you a natural feel for differential throttle.
I do not have a F14 but with the mixing capability it could give you the ability to pair two thottles together and mix them at low speed, turning off the mix at higher speeds.

All that said I find it hard to drive my PT boat slow, it needs to go fast.

Thanks
Steve
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Old Aug 25, 2006, 09:12 AM
wja
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Thanks for your input Steve; you pretty much confirmed for me that 3 speed controllers are probably the best way to go. Now to decide on the motors and ESCs. By the way, is water-cooling something that I should consider if I go with either of the Motors mentioned or for the Speed Controllers?
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Old Aug 25, 2006, 10:07 AM
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Yes you will need water cooling for both motors and ESC, both will get hot in the sealed hull.

Steve
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Old Aug 25, 2006, 10:51 AM
3 Blades to the Wind
Shaun Hendricks's Avatar
Atascadero, California, United States
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Steve...

I'd like to point out that if you are putting in 3 battery packs to up the voltage to push serial, it's just as heavy as using those 3 battery packs to run the motors in parallel for just as long. There is no magic way to 'gain' power (runtime) by changing wiring connection choice. Watts are watts. V*A=Watts. So, if you are targeting 15 amps, then you need 36 volts to run those same motors @ 12v serial. It's easier to increase AMP delivery than volt delivery. Volts requires more cells. Amps just require proper cells. 12v @ 45Amps is still 540 watts.

Higher voltage ESC's are usually more expensive than lower voltage ones that can handle higher Amps.

To put it mathematically, if you are using 3x3000mah batteries at 12v each. That's 3000mah of stored energy in serial. If you wire the motors for 36 volts (serial), you only need 15amp capable batteries (5C) which is underutilizing most batteries, even LiPo's, and you need an ESC capable of handling 36v (most aren't) this is still drawing 15amps (15000mah) per hour so your runtime is ten minutes. Whereas, if you wire in parallel, your voltage remains 12v, your motor amp draw jumps to 45 and your stored voltage becomes 9000mah. This means you need to have at least a 15C rated battery pack (even wimpy current drain LiPo's do this nowadays, NimH and NiCd scoff at this low drain and lead acid just yawns). It is 45000mah which is still ten minutes of runtime on 9000mah of stored energy.

Wja...

I wouldn't recommend a high voltage setup. It's generally unnecessary and causes other problems besides increased cost.

You can get 3 'Proboat' 30A 12v waterproof style ESC's from Ebay for about $20 each (50A ones are about $30). Or you can get one 100A one for about $45. Your cost will come in when it comes down to the radio you buy. As Steve points out, you'll probably want to kill the mix at a certain level of throttle if you go the 3 ESC route, but your own testing will determine that.

It is possible to start with a 1 ESC setup and an inexpensive radio to just get the 'feel' of the power you want the boat to crank out. Your manuevering will be sacrificed at lower speeds but you might be like Steve and you may just like running fast all the time.

My recommendation would be to start inexpensive, get a feel for the boat then start putting money into it.
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Old Aug 25, 2006, 12:00 PM
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At 45 amps everything needs to work harder and things become critical. Things like voltage drop, ESC resistance, wire length, connectors, workmanship, and battery selection. All batteries are rated for capacity at 1C as you increase this, usable capacity decreases. So battery capacity decreases with higher loads. If you really look at it increasing power by increasing voltage is easier to do and the benefits are greater.
You also have to watch how ESC are rated, a 100 amp speed control may only be rated at 100 amp for a peak. At 45 dollars I doubt it can handle 100 amps for anything less than a peak of several milliseconds.
Those 200 amp speed controls that you see for cars and such are only a theoritical value of the fets in parallel. The heavy wire coming in and out of the speed control cannot even handle 200 amps for any usable period of time.
If three ESC are used then the current levels and run times are reasonable.

Thanks
Steve
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Old Aug 25, 2006, 04:10 PM
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I just crunched some numbers into a formula to convert prototype HP to scale Watts, proto displacemnt to scale displacement and max proto speed to speed for scale wake.

For the 1943 series 80' Elco boats:
3 engines at 1350 hp each = 3 motors of 250-285 Watts. Calculated Amp draws are 6V = 47A; 7.2V = 39.5A; and 12V = 24A.
51 ton displacement comes out to 27.9 pounds.
43 knot full speed equals 10.8 mph for scale wake (the speed over ground will be higher than scale speed)

For the 1945 series boats:
3 engines at 1500 hp each = 3 motors 270 - 315 Watts. C Amp draws are: 6V = 52.5A; 7.2V = 43.5A; and 12V = 26A.
61 ton displacement becomes 33.4 pounds in the model.
41 knot speed becomes 10.3 mph for scale wake.
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Old Aug 25, 2006, 05:12 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
Atascadero, California, United States
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Traditionally, "peak" ESC's run about 80% of their value at continuous. So a 100A peak rated ESC could handle about 80amps all day long, more if it's watercooled. Wja's PT isnt' going to pull anywhere near 80amps. I could drop 3 Cobalt brushed motors in there and not pull 80 amps. You can splash 13ga wire with 300 amps for a short period before it blew, again, we aren't drawing those kind of amps here. By CG Bob's figures, he's looking at a 30A ESC being able to handle his PT.

Many batteries are indeed rated at 1C drain for capacity. The industry is slowly changing over (driven by LiPo's) to rating the battery's in a dual sense, at 1C and at max continuous discharge (usually 10C-15C). There are several manufacturers which are starting onto the 'truth in advertising' kick and doing real capacity ratings. All in all, I call it a wash because I don't usually go around at full throttle all the time, which means in the end, my runtime is fairly close to the estimates given. Sometimes more, sometimes less. Chemistry also comes into play. LiPo is usually further out than NiMh or NiCd. Lead acid is the least affected by high drain.

I like the 3 ESC plan as well, but that's a larger up front $$ commitment, when he can just buy 2 more later on and easily convert it when he has the computerized radio. So, buying a large ESC to start and going from there as he has the dough makes sense from a build and grow standpoint. He's out a max of $10-$15 or can buy 3 new matching ones and put the other in some other project. I'm just recommending from a growth viewpoint.

As for serial versus parallel, I always go for the latter as it has more survivability options for a multi engined vehicle. If you lose one motor in a parallel setup, you can still bring the boat back. Lose one in serial and all of them die. It's like lightbulbs.

Amp stress is a thing of the past. With high parallel batteries now, you are seeing Amps out at 200-400 and nobody sweating the numbers. Me, those numbers scare me to death when you are talking some of these setups running at 72volts... That dumps and gets wet, you have an explosion as all the LiPo's water short and overheat, then explode. The high power guys scare me.... of course, they are over 100mph now right? I'll stick with low voltage, high drain when I need performance, it's safer than the higher voltage stuff. I belong to the 14.4v or less school when it comes to water.
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Old Aug 26, 2006, 12:43 AM
wja
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This is good stuff; thanks guys! So if I understand CG Bobs calculations correctly, for a 1945 series boat if I keep this boatís weight around 33 pounds, to achieve scale wake speed utilizing 12V power source I would require combined motor output of 315 Watts or more specifically motors capable 9 Amp current? I will probably go with the 3 ESC setup from the get go. I much rather set everything up optimally while everything is easily accessible. Besides, I can save some money on the radio for time being as I already have couple Futaba surface radios that I use with RC Tanks (I just didnít know whether dedicated marine radio like f-14 might offer some advantages). As far as battery selection is concerned, I must admit that Iím rather apprehensive over the prospect of using high voltage (series connected) setup as such I was contemplating using 3 quality 12 V lead acid or sealed batteries connected in parallel or having individual battery powering each ESC/motor. What setup would ideally offer longer run times and provide greater backup protection? Also, not having ever dealt with water cooling before, what is the best way to go, motors/ESCs already configured with water cooling or normal motors/ESC and a 3rd party water cooling option?
Thanks again,
Wal
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Old Aug 26, 2006, 09:26 AM
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McAlester
Joined Nov 2004
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wja,
Voltage. The only real requirement is that the other electronics will work on the voltage selected. Reducing voltage isn't all that hard to do, but it does mean there will be some expended in heat and circuit efficiency (might even knock a minute or two off the run time). Not all that much, but still there. A 12 volt battery is probably the high limit as far as availability and capacity. The other side of that is weight and how much room the thing takes up. The other side of that part is that you've got a fairly large area to 'fill up', and you are going to need ballast anyway. Best ballast for an electric boat is made from batteries! Temper the "best" thing to use with avalability, ease of use, and what you can afford. From there, the sky's the limit...
- 'Doc
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Old Aug 26, 2006, 11:17 AM
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Quote:
Originally Posted by wja
So if I understand CG Bobs calculations correctly, for a 1945 series boat if I keep this boatís weight around 33 pounds, to achieve scale wake speed utilizing 12V power source I would require combined motor output of 315 Watts or more specifically motors capable 9 Amp current? I will probably go with the 3 ESC setup from the get go. I much rather set everything up optimally while everything is easily accessible. Wal
NO. The combined motor output is 820-945 Watts at 78.5 Amps. You need three motors rated at 270 - 315 Watts each; three ESCs rated for at least 26.2 Amps at 12 Volts - 30 Amps would be safer. I did the calcs using a direct comparison from the Packard engines to recommended motor ratings. The Packard engines used in the late war PT boats produced 1500 HP each, three engines produced 4500 hp.
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Old Aug 26, 2006, 06:59 PM
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Wja, I would put the motors in series. I have a 50" cabin cruiser that I first powered with two MFA motors drawing 5A each at 10V. I connected the motors in parallel and the total current was about 10A. This setup was fine on one home-built ESC. I wanted more power so I put in 12 cells (more than 15V in practice on fresh charge), and the motors did not like it and one went up in smoke. As replacemt, I tried two 700BB 9.6V motors as these were relatively cheap. I first tried connecting them in parallel and have since lost count of how many blown fuses I created. These motors are really thirsty for current! I got hold of a 30A ammeter to measure how much juice these motors were drinking and at full power the pointer went off the scale. Finding fuses greater than 30A was getting a problem. The other problems were that the ESC was getting hot like mad, and the rather long connecting cables (battery was in two groups) were at the point of melting the insulation, it not the copper. My setup could not handle this current but for only a very short time. The only solutions that I saw were 2: either reduce the voltage, or put the motors in series. Putting the motors is series effectively halves the voltage seen by each motor, so this solution produces the same effect of using a battery with lower voltage. BUT the advantage is that the current is also halved. Now heating is proportional to current squared, so this settup produces only 1/4 the heat losses than motors in parallel.

The two 700BB in series now draw about 20A at full power, the ESC stays nice and cool and no more blown fuses. Remember that losses are proportional to current squared. It is not true that the situation is identical to putting the motors in parallel. With motors in series the power is halved, but the losses are down 1/4. This is why high voltage is used when high power is required, otherwise the losses (heat) would not be manegeable.
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Old Aug 27, 2006, 09:42 PM
wja
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Thanks again guys!
Antpsi, if I was going to use just one ESC I would probably go with a setup consisting of motors wired in series as my first option. However I was thinking of using 3 ESCs (one per motor) so as to gain more maneuverability at lower speeds, as such each motor will be powered independently.
CG Bob thanks for correcting my error in regards to interpreting your calculations! Iím a little (actually a lot) confused now though as the two motors that few people recommended I should consider, namely the AstroFlight Super Ferrite 40 and Graupner Speed 700 BB, have the max current range of 10 and 16 Amps respectively (that is if I read their specs correctly), which is no way near the 26 Amps that is required based by your calculations? Although I have read were the speed 700 BB motor will handle 12 cells and even Antpsi mentioned in his post that he got 30 Amps out of the 700 motors. So my question is whether the max Amps listed by motor manufacturers is understated and that most motors will handle higher peak Amps for short bursts of speed. I think I should have gone with a Tug where torque and gear reduction are the only concerns, something that Iím familiar with having few RC tanks! LOL
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