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Old Apr 14, 2011, 03:12 AM
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Deckie's Avatar
Australia, WA, Garden Island
Joined Nov 2010
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Scale propeller motor matching - Maersk B Class

G'day all.

I need to match the motor (spec sheet attached) to a suitable propeller from the Raboesch Catalogue below, preferably one of the 179, 185 or 168 models.

The scale of the model itself (to be an operable RC model of a Maersk Line B-Class vessel) will be based off the scale of the propeller chosen, and the main restriction is that I would prefer to use the specified motor already in my parts bin (A Johnson HC987). The model itself is likely to greater than 2.5 meters in length, with a 3 year build-time, on a part-time basis (which will probably blow out, knowing me).

Any ideas or advice for a new forum member on this predicament? or maybe different prop OR even motor suggestions?

Raboesch Catalogue:
http://www.raboesch.com/cat-2.pdf

Cheers
Mike J
Adelaide, South Australia.
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Old Apr 14, 2011, 03:56 AM
NeverAgainVolunteerYourse lf
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Australia, QLD, Regents Park
Joined Mar 2007
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For direct drive the motor is probably a bit fast at rated voltage for the large prop you would need for a 2.5 metre boat, for example a 100 mm Raboecsh prop has a maximum rpm of 3500 but for scale displacement under 2000 RPM would be more appropriate, the motor is rated at 8884 RPM no load at 18 volts, there is no reason for not reducing the voltage to reduce the motor RPM, at 6 volts expect 3000 RPM, loaded the motor speed would reduce.

Ideally you want the motor speed under load to be about 75 % of no load speed which will get you in the ball park of peak efficiency of the motor.

I think at 6 volts this motor will drive a 100 mm prop direct drive, 12 volts with 2-3:1 reduction would be perfect.

Cheers
Nick
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Old Apr 14, 2011, 04:31 AM
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Australia, WA, Garden Island
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Quote:
Originally Posted by nick_75au View Post
For direct drive the motor is probably a bit fast at rated voltage for the large prop you would need for a 2.5 metre boat, for example a 100 mm Raboecsh prop has a maximum rpm of 3500 but for scale displacement under 2000 RPM would be more appropriate, the motor is rated at 8884 RPM no load at 18 volts, there is no reason for not reducing the voltage to reduce the motor RPM, at 6 volts expect 3000 RPM, loaded the motor speed would reduce.

Ideally you want the motor speed under load to be about 75 % of no load speed which will get you in the ball park of peak efficiency of the motor.

I think at 6 volts this motor will drive a 100 mm prop direct drive, 12 volts with 2-3:1 reduction would be perfect.

Cheers
Nick
Alrighty, so based on that 6663 rpm at the motor for 2221 RPM at max efficiency, using 3:1 reduction. I think thats workable and would result in very low current draw as you allude to. The B Class uses a propeller of approx 9000mm - 9400mm in diameter from what i've been able to glean from photos and horsepower output relative to the E Class's Propeller diameter and output.

So I could opt for 1:115 scale for hull and prop, allowing me to use Raboesch's model 179 prop in 78mm diameter with a hull 2.55 meters in length. Excellent!


EDIT:

Im wondering if Raboesch can do custom work, a larger diameter model 179 off around 110mm would be even better, and allow a longer and larger hull. I'll email them and report back.
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Old Apr 14, 2011, 05:39 AM
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Australia, WA, Garden Island
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Another spanner in the works.

The motor is stickered as a Johnson, but this manufacturer

http://cf-motor.com/web/product_detail.php?sid=115

May be a contractor to them. The motor I have is a spitting image of those. I'll have to chuck it on the ammeter and see what it draws to find out which Model No. may match it.
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Old Apr 14, 2011, 05:59 AM
NeverAgainVolunteerYourse lf
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Australia, QLD, Regents Park
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Yep, that is a problem, I have 2 identical motors both 550 can size, one does about 2000 rpm on 24 volts the other one does about 30,000 rpm on 3 volts ( out of a cordless vacuum)

Nick

You could chuck the shaft in a drill with known RPM and measure the voltage, this will give you information to determine the RPM per volt (Kv)
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Old Apr 14, 2011, 06:02 AM
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I believe you are approaching this opposite from most modelers. Generally speaking a subject is chosen (which you have done) then a scale which is manageable in terms of construction, storage, and transport of the model is determined. A RC boat you can't get out of your house or to the pond is of little value, or if the Mrs. says one of you will reside in the garage. Prop performance doesn't necessarily scale down. You should expect that once to get the boat into the water you will end up experimenting with props and/or motors to achieve whatever performance goals you set. In a project of this size I suggest you opt for a standard scale where you can acquire fittings, figures, parts, and most of all for a container ship some cargo containers.
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Old Apr 14, 2011, 06:40 AM
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Quote:
Originally Posted by Prins Willem View Post
I believe you are approaching this opposite from most modelers. Generally speaking a subject is chosen (which you have done) then a scale which is manageable in terms of construction, storage, and transport of the model is determined. A RC boat you can't get out of your house or to the pond is of little value, or if the Mrs. says one of you will reside in the garage. Prop performance doesn't necessarily scale down. You should expect that once to get the boat into the water you will end up experimenting with props and/or motors to achieve whatever performance goals you set. In a project of this size I suggest you opt for a standard scale where you can acquire fittings, figures, parts, and most of all for a container ship some cargo containers.
You have a valid point. While it would be nice to work to 1:48 scale, which is in the ballpark of many RC tugs, this would give me a 6 meter vessel, which I don't have the means to transport. I can trailer up to 4 meters with existing equipment. So 2.5 to 3.5 meters is ideal.

Do you have a particular scale to suggest?
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Old Apr 14, 2011, 07:01 AM
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Another popular scale is 1:96 (close to HO's 1:87), either could be workable.
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Old Apr 14, 2011, 07:24 AM
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Originally Posted by patmat2350 View Post
Another popular scale is 1:96 (close to HO's 1:87), either could be workable.
I'll definitely look into that particular scale, especially re: availability of certain items. 1:96 gives me an approx 3 meter vessel. Im sure Adelaide tug drivers would still have fun with that....
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Old Apr 14, 2011, 08:35 AM
Retired for now
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I wanted to add that maybe you need to think about all those containers. In HO scale you can get tons of them in plastic but in another scale you'll have to make them all. Lots of extra work there. Sounds like agreat project. Hope you will do a build of it for us so we can see how you go about it. Pete
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Old Apr 14, 2011, 08:38 AM
Veni, Vidi, Feci
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A ship that size will also have bow and maybe stern thrusters, to avoid needing tugs. You can get those too!
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Old Apr 14, 2011, 09:12 AM
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Guelph, ON
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That is an ambitious project! I can;t wait to see the build log. (You are doing a build log, right? )
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Old Apr 14, 2011, 12:20 PM
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Hi Deckie

Do have plans for this ship to model it, and are they from the company if you do have them?

Duane
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Old Apr 14, 2011, 01:55 PM
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PDX, OR
Joined Dec 2002
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You can do what "HeavyCruser9" did to his USS Longbeach Cgn-9,
and split the boat into two halves... http://www.rcgroups.com/forums/showthread.php?t=1154170

1/72nd is a popular scale in Australia....

http://www.taskforce72.org/

.
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Old Apr 14, 2011, 04:46 PM
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1/8" scale (1/96) is popular for museum models and you often see bigger ships modeled in this scale. My SS Badger model was in part designed by the maker for model railroaders to use as a waterline model on their layouts. It is a compromise between 1/87 & 1/96.

http://www.rcgroups.com/forums/showt...ight=ss+badger

Some years ago I took a Hartman American Scout hull and did a container ship. For the containers I only used store bought ones in place that were exposed. For interior ones I made cubes out of thicker styrene, did the corrugated front with Evergreen plastic. Simulating the doors on the back was the easy part. On top I scribed the line between the containers and painted various colors. The top and ends were the only parts showing and I just matched the store bought ones as best I could. Didn't turn out too bad. I only have one photo of this model and it isn't very good.

The HO railroad section in the LHS can be a treasure trove. The cranes were two RR flatcar cranes I picked up on sale for a dollar a piece.
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