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U-47 battery problem. Need help!!

R/C Submarine modelers

Postby preveze » Tue Mar 09, 2004 6:20 am

U-47 is my first boat. I do not have much modelling experience and especially electrics. I use two 6V 4AH paralell wired lead acid batteries but runtimes are very short, approximately 20 minutes.When I measured the current I saw that motors draw 10AH current at the start and the current falls to steady 4.5 AH in 3 minutes out of the water. I fried a cheap esc because of high current. I think most of the friction is caused by heavy shaft grease which I bought from a Shell fuel station. It is a slicone based grease. What can I do to reduce the friction and increase the runtime. I tried to use the shaft seals which I bought from subconcepts but I failed to disassemble the rear hull from the wtc cover. One of my friends suggest using motor oil or vaseline for shaft sealment but I use my boat in swimming pools and I fear of pollution caused by oil leak.
I need advice of professionals please help
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Postby safrole » Tue Mar 09, 2004 10:15 am

I have a page showing how you can install your shaft seals. Maybe it would be difficult since your boat is already built, but here is the link.

http://www.modelbrass.com/U47Modifications.htm

Frank at Subconcepts will also sell you some lower draw motors which should help. Also check out my home page, as tomorrow or the next day I should have brass decks for sale on it. Quite fetching.

Another thing to note is that since you're drawing the high amperage at the beginning because your grease is cool and thick you might gain a small amount of run time by getting them warm with a hair dryer or something, instead of using battery power. I don't really know if that would be worth it; maybe you'd only gain five minutes of runtime but have to listen to ten minutes of your wife complaining about you using her hair dryer. :)
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Postby preveze » Tue Mar 16, 2004 6:32 am

Is it normal that two engines draw a total of 4.5ah out of the water or should I make some modifications on the wiring?
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Postby safrole » Tue Mar 16, 2004 3:32 pm

Someone more knowledgeable than I needs to address the amperage draw, but it does sound too high simply because when you're out of the water you are at the "zero load" state. But in this state you have 4.5 amps of pure drag, which sounds excessive. The heavy grease is undoubtedly your culprit, as you suspected.

I believe you can install your shaft seals without removing the aft hull from your WTC cap. You can get a little abusive when removing the the old hardware, as long as you leave a little shoulder on the cap to which you will epoxy your brass tubes. Did you check out that page I linked on my earlier post?

Maybe you could cut the tips off with a hot knife. Or I have a cheap hack saw frame that holds the blade at one end, so the other is unsupported and can be used for something like that. That or if you don't have a dremel, you could get one now. Some of the best money you'd ever spend.
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Postby JWLaRue » Tue Mar 16, 2004 3:58 pm

I would try disconnecting the two motors from the prop shafts and then see what kind of load you then get. If it's close to what you are currently seeing (pun intended), then I'd look at replacing the motors. On the other hand if the current draw is significantly less, you've got a lot of binding in the prop shafts.

-Jeff
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Postby Bob the Builder » Tue Mar 16, 2004 6:28 pm

I may be buttin in where I don't belong, but didn't the Robbe kit have big problems with this? I'm sure that I'd seen something like that posted on the internet over the last couple of years.

If I'm not mistaken, Ships N' Things sells a retrofit kit that significantly increases run time to over 2 hours (or something like that).

If you don't have the stock Robbe kit, disregard my post, but if you do, and you are experiencing these problems, I'd give Al a call.
Bob Martin,
RCSub homepage: http://www.rc-sub.com
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Postby preveze » Tue Mar 16, 2004 6:33 pm

I heard the gear reduction kit suitable for robbe U-47 but it is an expensive upgrade. Can bigger motors prove a solution. They have higher torq values. Can I longer the runtime by changing motor sizes?
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Postby JWLaRue » Tue Mar 16, 2004 9:17 pm

Ah, yes...Bob is correct! The stock drive system can be a problem.

If there is a binding/friction problem, then even bigger motors won't really help. Yes it may run longer, but it'll require more amp-hours.

-Jeff
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Postby Ramius-II » Wed Mar 17, 2004 1:25 am

Hi Preveze: :)
Just a couple of comments is hopes to shed some light. First, placing 2 batteries in parallel is not the best plan if they are "hard wired" together. If one battery is weaker than the other, it will suck the life out of the better battery. When a battery discharges, it's internal resistance becomes lower thus the battery becomes a load until the other battery reaches the same point of discharge. According to your first post, if you are getting 20 minutes then you total current drain is more like 24 amps per hour. Two 4 AHR batteries in parallel would give you 8 amps for 1 hour. Since you are only getting 1/3 of an hour, you would have to be 3 times the batteries current rating or 24 amps. If you wish to use two batteries in parallel, then it is best to couple the batteries with diodes. The - side of the diode to the + terminal of each battery. Then the + of each diode tied together to feed your circuits and motors. Such a device is already available at most RV store as it is used to charge the RV and vehicle battery from the same generator/ alternator.
Next and very important is to ensure the wires going to the motors are nice and heavy. Many times the use of small wire results in greater power consumption. As the wire tries to pass heavy currents like when a motor first turns on, the wire will act as a resistor and get hot.
Lastly, you might consider changing to 12 volt motors. Use one battery for your 6 volt needs or better yet a BE/VR. 12 volt motors will use 1/4 the current of the same 6 volt motor. This surprised me as I originally thought 12 volt motors would use 1/2 the amps of a 6 volt. There is a very detailed explanation in the past posts on this subject if you're interested.
First and formost, of course, is to ensure there isn't a major drag factor.

Hope this is of value.

Best, Ed :)
Just one more wire!
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WB6NSN
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Postby preveze » Thu Mar 18, 2004 7:16 am

Ramius I will try both solutions and evaluate their performance. Can you send me some simple drawings about diode connection and 12V motor use with 6V batteries.
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