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Feb 27, 2019, 05:26 PM
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Quote:
Originally Posted by mfr02
Find out the power of your original, convert to Watts, divide by the cube of the scale, multiply by two, you will have some idea if you are in the right area.
I'm not sure I follow. Say it's a Fletcher class destroyer, approx 45 MWatts (full scale power) at 1/35 scale. 45,000,000 / 35^3 x 2 = 2099 Does that mean the 1/35 scale model needs 2100 watts worth of motors? I assume I'm doing this wrong... Thx, Cap
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Feb 28, 2019, 05:28 PM
Big Boats Rule!
boater_dave's Avatar
The formula was a general rule of thumb to get you in the ballpark for model drive power. Lot's of guessing involved, and remember that military ships had lots of HP for acceleration which your model may not need.

Original ship HP (per shaft) X 750 (to get to watts) / Scale^3 (to get to model size) X 2 to recover losses. In a Fletcher, 60,000 shp is for both shafts, so 30,000 hp each shaft.

30,000 X 750 / 35 / 35 / 35 X 2 = 1050 watts (each motor)

So yeah, a 1:35 scale model of a WWll Fletcher may need a pair of 1.4 hp electric motors. I think that might be on the high side, but not out of the question. Remember, at 1:35 scale it will be 10 foot, 8 inches long and weigh about 107 pounds. The props are 4.1 inches in diameter.


Dave
Mar 01, 2019, 08:18 AM
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Thanks Dave, In this case I think it's on the high side, why the X 2 to recover losses?
Mar 03, 2019, 05:25 AM
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Quote:
Originally Posted by capricorn
Thanks Dave, In this case I think it's on the high side, why the X 2 to recover losses?
Model props might not be as efficient as the real thing - apart from being blunter, they might not be the same shape. Model bearings might introduce more scale friction than they should.
Real ship power plants measure horsepower at the output shaft. Electric models measure power into the motor. Its anybodys guess as to what the exact motor efficiency figure is, then you start considering the other unscaled losses. X2 is a good ball-park figure to account for unknowns, also some extra power is usually good for getting out of "situations". Although the temptation is always there to shove the stick as far as it will go, you don't always have to.
Mar 12, 2019, 04:20 PM
Big Boats Rule!
boater_dave's Avatar
All good reasons. Also, for most scale applications a larger motor loafing along will sound better and last longer and draw less amps than a smaller motor struggling to deliver the power. I've also heard that scale warships are on the side of not needing very much power to glide along. They are much different than say a PT boat. Warships have lots of power because they 'may' need it. Ridiculous power to get that last 5 knots because speed is life. Freighters and such are designed for efficiency and economy and adding extra power for a trivial speed gain delivers a huge net loss value over time in more fuel consumed.


Dave
Mar 12, 2019, 05:55 PM
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Ridiculous power to get that last 5 knots because speed is life
This is absolutely on target. Example: Shokaku & Zuikaku, Japanese carriers, achieved approx 34.37-34.57 knots on 161,290 - 168,100 shp respectively, while at "cruise" speed, 26 knots, required 50,000 shp. The increase from 26 knots to 34.5+_ required an increased input of 110,000 shaft horse power.
Source: hans Lengerer, Warship International 2015.
Similar figures can be found for most warships.
Mar 12, 2019, 07:26 PM
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Just to throw a curve ball into the mix, a model requires far less power to achieve "scale speed" than to achieve "scale appearing speed". The latter is, of course, in the eye of the beholder, but most builders end up putting a far bigger power plant than is required to achieve even " scale appearing speed".
Mar 15, 2019, 04:59 AM
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If you don't have at least the scale power available, you can't get scale speed. Actual top speed of real warships is rarely witnessed, and in most cases is subject to the owners (i.e. whatever government) choice of what they want to admit to. Up to scare off the opposition, down to lull them into a potential surprise. Real warships going flat out tend to look unrealistic, and they are real.

This is sliding a bit away from boat wiring, which is why the thread was stickied in the first place.
Aug 05, 2019, 06:19 PM
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Now in a different vein.
I have several Robbe F-14 radios, all on 75 mhz, which I really like. I stick with them because I can use any of them for my submarines. That said, they are also all dual throttle radios which works excellent for my multi-engined fleet. What I really like, though, is the expansion modules which give endless abilities for switched and proportional functions.
However... these radios were designed (analog) and first produced back when a light called for a light bulb. Today, many of us use LED's ( Thanks Aimee). The problem I encountered and have since overcome is that because LED's need so little current to begin to glow, and that the Robbe expansion module do not stop all current flow thru them, is that even in the off postion of the switch, the LED still lights up dimly. I found this out after finishing wiring my icebreaker with 15 LED's, all of which remained lit, dimly. I do not know, but I suspect that the Graupner system is similar and suffers the same issue.
Thanks to a gentleman on Model Mayhem, I have the fix, which I present here. It is quite simple and is an easy retro fit as well.
The expansion modules use a standard servo lead that wires two functions. The Red is hot and the black and the white wires are the switched grounds. The load is wired between the red and the grounds. The FIX is to solder a resistor, I have used 1000 ohm and 2.2 thousand ohm and either does the job, between the hot lead(red and the switched grounds (black or white). It would appear to short the lead, which it probably does, but it is such a miniscule amount of current that it is of no concern. Meanwhile, it prevents the LED form lighting.
I have wired my ARS (10 LED's on 8 circuits) and it works beautifully. This winter the icebreaker gets a refit.
I hope this helps someone.


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