drive train help
 

<p class="MsoNormal" style="margin: 0in 0in 0pt"><font size="3" face="Times New Roman">Hope some can help me develop a drive train for a PT boat that I am building. I am new to RC so have a lot of questions.</font></p><p class="MsoNormal" style="margin: 0in 0in 0pt"><font size="3" face="Times New Roman">The PT boat is at a scale of <st1:time minute="20" hour="13">1:20</st1:time> so therefore will be about 48&rdquo; long and guessing at a weight of 20lbs with batteries motors etc.
I am hoping to get it up on a plane and a run time of 30 mins. Is this run time achievable and realistic?</font></p><p class="MsoNormal" style="margin: 0in 0in 0pt"><font size="3" face="Times New Roman">I am considering Gaupner speed 600 or Astro marine 425 motors running on 12 volts each; with all 3 running forward and only the ctr <span style="mso-spacerun: yes"></span>capably of reverse. Will these motors achieve my desires or will smaller motors work?</font></p><p class="MsoNormal" style="margin: 0in 0in 0pt"><font size="3" face="Times New Roman">What would you recommend for batteries Gel cell or NimH cells? If NimH how should the pack be built; 10 cells in series = 12 volts but how many in parallel to achieve run time?</font></p><p class="MsoNormal" style="margin: 0in 0in 0pt"><font size="3" face="Times New Roman">Finally can you recommend a ESC that can handle the above? Will I require a ESC for ea motor?</font></p><p class="MsoNormal" style="margin: 0in 0in 0pt"><font size="3" face="Times New Roman">Thank you for your consideration and await your reply</font></p><p class="MsoNormal" style="margin: 0in 0in 0pt"><o:p><font size="3" face="Times New Roman"></font></o:p></p><p class="MsoNormal" style="margin: 0in 0in 0pt"><font size="3" face="Times New Roman">Robert G Wager</font></p>

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RE: drive train help
 

Ok, well yes you can run either of those motors. On that size of boat using three motors will certainly get you up and planing. With the Graupner 600, I suspect you can get it up on two motors.
If you want to run all three motors, with only the center running in reverse, I suspect that the easiest way to do this is to run two ESC controls. The two out board motors run on a forward only R/C type ESC with the braking feature disabled, and the center motor running on a forward/reverse ESC. You can look at ESC from Mtroniks, I have had good results with them. The Viper has a reverse that can be disabled, and the Marine series have proved to be fairly reliable.
As for batteries, a boat that large can carry SLAs, but they will be heavy. I would go with some NiMds and run them in two to three packs of 10 set in parallel, that should keep you running for a while. Plan on about 10-15 minutes full speed run time if you have some good matched cells. Keep an eye on the c-rating of your batteries, I would go for 15 C or above.

RE: drive train help
 

Thanks for the help; most apprreciated

RE: drive train help
 

You might also consider the Traxxas 14v Marine Titan motor. Good efficiency for the amount of thrust. As for the ESC, research all the various brands, Electronize, Traxxas, Novak, and Jomar. I have never tried the JETI controllers, but the Vipers seem to work quite well. As for Vantec, they have the most accurate amperage ratings. Not inflated to sell units. When they advertise "continuous" amps, they mean it. For some makers, "continuous" really means a short 2 to 3 minute contest between cars or trucks. Be sure to properly fuse your motors to protect your investment in the ESC. As for the three motor, two ESC arrangement, that is not a bad solution. It might be tough to find an ESC that could feed all three hungry motors. It also offers bit of redundancy for reliability. As for gel cells, only if you want ballast. Run times - 30 minutes is a bit of a stretch. Depends on how much throttle you want to use. Being a PT boat, you'll want to see it you on plane most of the time. I think Tachikaze has the right call.

RE: drive train help
 

If fitting three motors, it seems a waste to have the centre one just providing drag and slowing things down. 3 ESCs and a mixer seems more logical.
Saving weight on batteries is the best way to increase performance. LiPos could be a good answer for giving the higher capacity for increasing run time.
Having got motors with enough power to do the job, the next tick is to get the right props. Initial intuition points to big, heavily pitched props with lots of blades to shift a lot of water. Usually, this just flattens batteries quickly and cooks ESCs and motors. Think in terms of small, two (preferably for performance) or three (keep most of the performance and look good out of the water) blade props. The boat can never go forwards faster than the stream of water being pushed backwards. Plenty of other factors, but thats a good one to start with.

RE: drive train help
 

I have another suggestion,

Use the 3 motors of your choice, Astroflight 25’s Graupner 600’s whatever- I do like the Astroflight option though..., use one ESC to control the throttle of port & starboard motors, and use a simple “on- off” switch to power the center motor for forward only, like mfr 02 said, it is a waste to have the center motor for reverse only. The on-off switch can be powered by a servo which will be used for extra forward thrust only. The PT will really sing with all three motors opened up and will be manageable with the two motors on an ESC for cruising and scale maneuvers. You have plenty of cruising options there.

Batteries:

I think that NIMH or LIPO is the best way to go. You are going to need lots of “MAH” to give you runtime so (3) 5000 MAH packs would be the way to go for about 10-15 minutes depending on throttle habits. You can probably milk it with the one motor running on the on-off switch some of the time.

I have seen NIMH batteries reach as much as 10,000 MAH, perhaps 3 of them on 12Vcould help you get closer to the ½ hour mark, but running fast motors & an ESC that long will require water cooling to keep them from over heating.

Something to keep in mind is weight. PT’s have sort of like a pin tail type stern so it does not take much for the transom to sink below the waterline. For that boat, strategize battery placement and try to stay light, especially when powering with 3 thirsty motors.

ESC:

Use a marine ESC. Astroflight also makes an ESC if you go that route, or find any of the other marine type ESC’s. If you can get one water cooling ready, even better.

Here is the site for Hobby City, they have plenty of marine ESC’s there on the cheap. Pick one with reversing capabilities.

[link]http://www.hobbycity.com/hobbycity/store/uh_index.asp[/link]

If you want higher quality, go for the Astroflight. It has reversing capabilities.

[link]http://www.astroflight.com/index.php?main_page=index&cPath=17&zenid=docc10v0j ua7n668pom7i6bc9ggemdka[/link]

-Rich.

RE: drive train help
 

all 3 running forward and only the ctr capably of reverse

Argh, I misread you on that one. I was thinking of having it run in opposite rotation. Rich is right. Put some power behind it or leave it out. I have seen several 4' PT boats and you will need three modest sized motors or two very big, heavy, and amp-hungry motors. As for the ESC, don't skimp on price. Buy a good one and protect it. Fuses without a doubt. Cooling is good because heat is the main enemy of an ESC. I would like to hear more about LiPos. Sizes, prices, capacities, etc. Has anyone used them in a boat like this?

RE: drive train help
 

Thanks again fellows; this is exactly what i am looking "OPTIONS"

RE: drive train help
 

Hi Bikerbob,

I agree, it is good to have options & decide from there, knowledge is key!

RCShipyard,

I agree all around, it is better to spend the extra money on better ESC’s or electronics in general pays for itself long term. .

BikerBob/RCShipyard,

I think the PT boats were counter rotation port & starboard and the center prop turned left…or something like that…I forget. I know it is on the PT boat sites for reference, definitely check it out before ordering props. I would personally go 3 blade like the original PT’s to keep it authentic. (3) 3 blade brass props should do the trick. Just check the clearance between each prop before ordering.


I am going to run LIPO’s in my Dauntless which is about the same size & hull style as the PT. I am using LIPO because I am fitting the boat with a full interior and don’t have much space to dedicate to heavy or bulky batteries. LIPO is a good solution. My ballast will come from non battery sources. The motors will be (2) Astro 25's.

I like the idea of (3) 3S 5000 MAH LIPO’s in the 48” PT from a weight saving and performance perspective. If the boat was using 3 12V motors, and a battery pack for each motor for decent runtime, the LIPO will weigh much less than a NIMH giving you more power & speed and possibly better runtime. Recently I weighed a 3S 11.1 LIPO from my racing boat & it came in under 2 LBS. It was something like 1.85 LBS if I remember correctly. If it were a 12V NIMH pack which is comparable, (11.1 LIPO actually puts out 12.3V give or take) it (NIMH) weighs a little more than 3 1/2 LBS per pack. When you multiply that by 3 packs, the weight does add up which will weigh down the narrow stern.

For the weight of 2 NIMH packs, you can run 3 motors on LIPO giving you some extra speed and the PT realism of running 3 motors without losing performance or runtime. The extra weight that would have gone into battery weight can go toward onboard accessories.

This is just my opinion.

If considering LIPO, read up about it. LIPO is not as forgiving as NIMH and is a different beast altogether, both good and bad. The biggie with LIPO is the initial investment of the proper tools such as charger, balancer & LIPO specific cell meters which are all essential to the longevity of the LIPO. Once you have the tools and learn how to use them, it is really simple. Like NIMH, there are cheap cells and there are expensive cells, so it is up to you to decide on the budget. There are always discussions going on in the RCUniverse fast electric boat forums. There is a sticky on that page as well to learn more.

I hope this helps a bit further.[sm=shades_smile.gif]

-Rich.

RE: drive train help
 

Rick,

Thanks for the tips on LiPo. Since I need another charger, I have deceided to get a LiPo compatible unit. When NiMH first came out, the prices were twice what they are now. I expect the same for LiPo. The advantages are just too good to pass up. I have one model that absolutely needs NiCads because the heavier Nickel Metals weigh it down past the water line. That will be my first LiPo conversion.