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Old Sep 29, 2005, 10:07 PM
apprentice shipwright
John Pilot's Avatar
Little Rock, AR
Joined Sep 2005
112 Posts
Motors, Shaft and Prop info needed

Although I used to build model airplanes back when I was about 13 or so, I am brand new to boat modeling. I have been meaning to do this forever, just never got to it.

I am scratch building a 1:12 scale model of a pilot boat and I am now planking the hull. I have enough woodworking experience and I have learned so much from reading your forum that so far I have not run into any major snags.

Where I do need some help is in the motor / driveshaft / prop department.

The boat is about 47 inches long, has twin props/rudders.

I figured that if I get two counter-rotating props, one left-handed, and one right-handed I could pretty much eliminate any torque working on the boat and have it going straight without having to compensate with the rudders. (I assume that is the way the real thing works)

I decided to use a two-motor set-up, using conventional brush-motors and an ESC.

Here are my questions:

1. Is my reasoning regarding the torque elimination more or less correct?

2. Are there any major drawbacks to using the twin-counter-rotating prop setup?

3. Should I put the motors in parallel or series, assuming I am using one ESC to drive both? Other than making sure I get the right voltage motors matching my batteries I can't think of a good reason why one should be better than the other. Anyone have experience with this?

4. I know this is probably hard to answer given the information I provided, but Iíll give it a go:

a. For this size boat, what kind of motor would you suggest (type and power)?
b. What kind of props would I use (size, number of blades, etc)? How do I make sure I get one left-handed prop and a matching right-handed?
c. Where can I get the recommended motor and props?
d. Where do I get shafts? I assume I'll also need one shaft with right-handed thread and one with left-handed thread.

5. What kind of servo do I use for the rudders? Given this size boat, do I need any "Hi Torque" servos or will a regular one do?

6. I live near Little Rock, AR. Does anyone know fairly local sources or should I get everything mail ordered?


Thanks,

John Pilot
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Old Sep 29, 2005, 10:54 PM
Useful Idiot
Asturias, Spain
Joined Mar 2001
3,545 Posts
1. Yes
2. No
3. Series (balances motor draw)
4a Mabuchi 550/Pitman
4b about 1 1/4"
4c http://www.loyalhannadockyard.com/
4d above
5 standard
6 no idea
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Old Sep 29, 2005, 11:02 PM
KC8WPF
CG Bob's Avatar
Euclid, Ohio, United States
Joined Sep 2004
3,748 Posts
Here's a PT Boat thread that has some pictures of my PT Boat - see post #3. The pictures may help with your set up.

All of question #4 - look at MACK Products, under "Running Hardware & Power Packages" check the set for the Dumas Dauntless, which is about the same size as your boat.

#4D - The shafts have a right hand thread, the props usually have a matching right hand thread. SOme props require the use of a drive dog and two nuts. The drive dog is installed on the saft, then the prop, then two nuts. Dumas and Octura props have a notch on the forward side of the prop hub for the drive dog. Better scale props, like those made by Prop Shop, Rivabo. et al, are usually threaded and have a set or grub screw. Thread the set screw out, so it doesn't distort or damage the thread on the prop shaft. I usually remove the set screw while I locate the where to file a flat on the prop shaft. Screw the prop on to the prop shaft all the way, you now have two options. #1 (my preferred method) With the set screw complete;y removed, imsert the end of a fine point marker into the set screw hole. This should leave a mark on the prop shaft. #2 - Tighten the set screw all the way down, until it makes contact with the threads on the shaft. It will leave a mark or burr on the shaft. Remove the prop from the shaft and file a flat spot where the shaft is marked. When you re-install the prop on the shaft, the set screw will engage the flat spot and hold the prop on. A little bit of lock tite helps too.

#5. You can use a singkle standard servo to control both rudders. My favorite method is shown in the pictures of my PT Boat. Mount the servo so the servo horn center is as close to the boats center line as possible. I prefer to use a two arm servo horn. With the servo centered, mount the servo horn so it is perpendicular to the boat centerline. Two equal length control rods connect the servo horn to the rudder arms. The servo works in a push-pull fashion with this set up. The rudder arms must be equal length as well.

#6. HobbyTown USA
9101 W. Markham St., Suite 18
Little Rock, AR 72205
Phone: (501) 223-5155
Fax: (501) 223-5179
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Last edited by CG Bob; Sep 29, 2005 at 11:25 PM.
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Old Oct 06, 2005, 09:52 PM
apprentice shipwright
John Pilot's Avatar
Little Rock, AR
Joined Sep 2005
112 Posts
Thanks & need a little more info

Thanks Bob and Martin for the very useful information.
Bob, I have a few more questions:
1. What kind of motors did you use on your PT boat?
2. How many volts do you run them on?
3. You mentioned Hobby Town as a possible source. If you have purchased from them, what's been your experience with regards to price, selection and availability?
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Old Oct 06, 2005, 10:52 PM
KC8WPF
CG Bob's Avatar
Euclid, Ohio, United States
Joined Sep 2004
3,748 Posts
John,

Here are the answeres.
1. MACK Products #3150's. I actually bought the MACK power kit for the Lindberg PT109.
2. 7.2V 3000 mAh NiMh gives me 30 minutes of running at full speed. 1 battery powers both motors thru a single esc.
3. My experience with HobbyTown has been good in CT, MI, FL, OH, & IN. Because each HobbyTown is an independent franchise, selection and pricing varies from store to store - even if they are in the same general metro area. The prices at the local HobbyTown are pretty good. They can special order items, and special orders are generally received in a week - depends on the suppliers availability. The HobbyTown closest to me, in Mentor, has a lot of trains, plastic models, r/c cars & r/c planes, and some boats and boat stuff. The next closest HobbyTown is about thirty miles away in Strongsville, on the west side of Cleveland. I haven't been to the Strongsville HobbyTown at all. How ever, my opinion of HobbyTown USA may be slightly biased, since I've been working at the Mentor store since I retired from the USCG in Sept 2004.
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Old Oct 08, 2005, 10:34 PM
apprentice shipwright
John Pilot's Avatar
Little Rock, AR
Joined Sep 2005
112 Posts
More questions yet

I think I found suitable motors for my project. They're MABUCHI RS-550PF's that do 9000 RPM at no load on 6V (two in series on a 12 V power supply using an ESC).

I'm thinking about using Raboesch 156-05 and 156-06 props. These are 3-blade 40 mm props with a 41 mm pitch.

I did some rough calculations and it looks like full out I'd be running the motors at about 5000 - 6000 RPM under load.

Do I need to gear the motors down or does this sound about right?

In addition, are there any formulas available to estimate the surface area needed for the rudders or do I just scale those down like the rest of the boat? (I'm worried that if I just scale the rudders down I won't get enough "turn" out of them)

Thanks
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Old Oct 08, 2005, 11:01 PM
KC8WPF
CG Bob's Avatar
Euclid, Ohio, United States
Joined Sep 2004
3,748 Posts
A general rule of thumb is that the prop diameter shouldn't be greater than the motor can diameter. The RS-550PF has a can diameter of 37.3 mm; your props are 40mm. You should be okay, as the prop and motor diameters are within 10% of each other. You may need to gear the motors, if the motors get too hot to touch after a full speed run.

Generally, scale size rudders work fine. Some people will make their rudders 10-15% over scale size to increase the rudder bite. Usually slightly oversize rudders are found on models of long narrow ships, generally with a single screw - like freighters, destroyers, & WWII fleet subs.
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