Scale Weight - RC Groups
Sep 18, 2009, 01:18 AM
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# Scale Weight

I don't know if there is already a way of determining what the weight a truly scale mofel should be but i found a way of doing it. I wanted to determin how much a model would weigh before making it. If you know what the weight of the 1:1 ship/boat/etc then all you have to do is divide by the scale three times. It seems to have worked. These are my findings, I'll use the Titanic as an example:

The Titanic weighed 46000 tons so i used this as a starting point.

A 1:150 scale rc model of a titanic is sold as a professional product(forget who makes it now). To determin how much it should weigh before hiting the waterline i take the weight of the real ship(92000000lb) and divide by the scale(150) three times. I get 27-1/4lb. The rc model is advertised as weighing 26.4lb. Not far off and if you look at a vid the model is sitting a little high(1/4 inch) below the waterline.

I have used this to determin these scale weights of the Titanic:
Scale - Weight
500 - 3/4lb
350 - 2lb
250 - 5-3/4lb
200 - 11-1/2lb
150 - 27lb
100 - 92lb
87 - 139lb
48 - 831lb

Remember this is all highly theoretical process and if you can proove it wrong then let me know. Or if you have an example of this process calculating the correct weight i would also like to know. - MARCRACER
 Sep 18, 2009, 03:51 AM Big Boats take Big Bucks! Well, I was ready to gently burst your buble. But now that I've done the calculations (so long as you are sailing in salt water) the differences are miniscual. Scale - Weight 500 - 0.736lb 350 - 2.146lb 250 - 5.888lb 200 - 11.500lb 150 - 27.259lb 100 - 92.000lb 87 - 139.711lb 48 - 831.887lb However, if you are going to sail in fresh water, the weight is proportionally less. To get the fresh water weight for proper displacement, see https://www.rcgroups.com/forums/show....php?t=1024644 Read Section 2.1 Displacement for the formulas. Calculations for the Titanic are: Scale - Weight 500 - 0.715 350 - 2.085 250 - 5.722 200 - 11.177 150 - 26.493 100 - 89.413 87 - 135.782 48 - 808.490
 Sep 18, 2009, 09:36 AM Big Boats Rule! The only difference is that ships are listed in long tons for displacement and a long ton (tonne) is 2,240 pounds. Real ship disp x 2240 / scale^3 = model weight in pounds Dave PS: to figure model speed, take the real speed and divide by the square root of the scale.
Sep 18, 2009, 06:52 PM
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Quote:
 Originally Posted by 1Ironhorse Well, I was ready to gently burst your buble. But now that I've done the calculations (so long as you are sailing in salt water) the differences are miniscual. Scale - Weight 500 - 0.736lb 350 - 2.146lb 250 - 5.888lb 200 - 11.500lb 150 - 27.259lb 100 - 92.000lb 87 - 139.711lb 48 - 831.887lb However, if you are going to sail in fresh water, the weight is proportionally less. To get the fresh water weight for proper displacement, see https://www.rcgroups.com/forums/show....php?t=1024644 Read Section 2.1 Displacement for the formulas. Calculations for the Titanic are: Scale - Weight 500 - 0.715 350 - 2.085 250 - 5.722 200 - 11.177 150 - 26.493 100 - 89.413 87 - 135.782 48 - 808.490
Thanks for the extra calculations, I completely forgot about the bouyancy of salt vs fresh water. I was rounding the numbers to the nearest 1/4 and i also started the calculations going off of 45000 tons. Made a switch to 46000 after duble checking online. I will have to remember the extra step from salt to fresh water before sailing though, i probly wont voyage in salt water.
Sep 18, 2009, 07:02 PM
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Quote:
 Originally Posted by boater_dave The only difference is that ships are listed in long tons for displacement and a long ton (tonne) is 2,240 pounds. Real ship disp x 2240 / scale^3 = model weight in pounds Dave PS: to figure model speed, take the real speed and divide by the square root of the scale.
I wonder if the titanic went off of long ton measurements. If they did then the 1:150 scale model would have to of weighed 30.53lb in saltwater,29.61lb in fresh water. Doesn't seem right to me. The titanic-titanic site says that the ship weighed 46000 tons but doesn't verify if it is long tons of reg tons.
If it did weigh 46,000 ling tons then it would have weighed 51,520 straight tons.
 Sep 18, 2009, 08:53 PM Damp and Dizzy member Hi BoaterDave. I disagree about scale speeds; the speed the model skipper should shoot for should be, to be realistic, the actual ship speed/scale, not /sqrt(scale). I have read the Froude analysis published by a British boater S.J. Booty (he has a string of engineering degrees behind his name). Booty is often cited for the model speed calc you presented above, which is why I mention him specifically. He misapplies Froude's work wrt model boat speeds. Froude investigated why shipbuilders could not use model results to predict actual boat speeds. The models always sailed faster than the real ship once it was built (that is, the equation real ship estimate=modelspeed * scale greatly overestimated the performance of the real ship). Froude calculated that if the ship builders did as you say (real speedestimate=modelspeed*sqrt(modelscale), then the equation would more closely predict the actual speed once the real ship was constructed. But, and here's the rub, you can't go the other way. If you want your model to mimic real ship speed, then you need to operate in the scale environment. For instance, 44ft/sec=30mph. If your real ship is 44' long, and it can travel at 30mph, then it is moving at 1hull length/sec. To mimic this in a 1:32 scale model, or in *any* scale, then the model must also move 1 modelhull length/sec. For a 1:32 model, 44/32=1.4fps. If you do the math, you find that the reverse Fronde calc would have your model skipper powering up his model to move faster than 1 hull length/sec (44/sqrt(32)= 7.8fps). For this specific example, the 1:32 model moving at a reverse calc Froude speed would be moving 5.5 hull lengths/sec. This would not look real. And explains why models often look zippy compared to the real ship they are trying to portray. Models live in the scale world. For distance and speed (and size of craft, eg LxWxD), simple linear adjustment is the appropriate calibration to use. For using a model to predict what a real ship of the same form will achieve, then use the Froude equations. To move a model at the speed that corresponds visually with the real ship, slow down :-). Booty's article: http://members.fortunecity.co.uk/sjb...s_on_scale.htm Last edited by Brooks; Sep 18, 2009 at 09:00 PM.
 Sep 19, 2009, 01:12 AM Nickel Belter Good article, but he obviously has the luxury of a more 'controlled' environment for sailing. It's worth repeating that you can't scale down the weather, so unless you want to build a huge model, you might have to add a bow thruster or independant paddles to maintain control.
 Sep 19, 2009, 08:22 AM Registered User Brooks, if you consider that for the model, time is scaled, everything falls into place. Take a 1:100 scale model for a 1 mile walk. If it looks right all the way, it will take about 1 hour. The model has sailed 100 scale miles. Using the square root rule, it has sailed at 10 mph. 100 miles at 10 mph gives a 10 hour cruise. The 10 hour model cruise has taken 1 real people hour. This works with all scales.
 Sep 19, 2009, 02:30 PM Registered User The odd thing is, if you use the square root theory, it looks right when compared with pictures of the real thing. This why the cinema crowd, when using models, run the model at the square root speed, filming at a high frame rate, then playback at a normal frame rate. Result, best realism for the model. Back to weight - Merchant ships have many tonnages - gross, deadweight, lading, registered and probably a few more. Occasionally displacement may be quoted. With military vessels, a displacement figure is usually quoted. This figure is usually manipulated by the government involved, upwards if there is a need to intimidate neighbours or bolster home appeal, downwards to lull aforementioned neighbours into a false sense of security or to fit in with a treaty. If a scale model is made, floated at waterline and weighed, you will probably have a more accurate figure than that published. If the drawings were, in fact, actually accurate.
 Sep 20, 2009, 07:42 PM Big Boats Rule! Wow, what a debate. All good, of course, because we do this to have fun. And yes, operate your model the way you enjoy, because that's all that really matters. I agree that there is way too much going on for a single simple formula to accurately predict what the full size ship is going to do. But, I also think that a 1:100 Titanic that is over 8' long taking almost half a minute to pass by would not be representing the prototype as she would appear at 23 knots. The difference is more apparent as the scales get smaller and smaller. Perhaps the best advice would be to operate the model at a speed in which the wave shape looks right. Choosing to operate your model in a scale like manner goes a long way as well. Dave
Sep 20, 2009, 08:55 PM
Big Boats Rule!
Here's more ammo.
The real ship is the US Coast Guard 110' Island Class cutter. Wikipedia says 29.5 knots max speed. Now, I don't know what speed the boat in the first photo is going, but I would estimate 20 knots.
The model is from Imex and is modeled in 1:48 scale, so it should be 27.5" long. The second photo is from our model clubs' fun run this past July.
According to the formulas, a pure scale 20 knots should be 0.4 knots (0.7 feet/sec) and the square root version 2.9 knots (4.9 feet/sec).
I didn't measure the speed of the model, I'm only looking at the bow wave. They both come up to the chine. The video shows the model driving around the lake at a slightly slower speed. Unfortunatly, there is no fixed reference to determine speed. But does is it appear to be going faster than 1 boat length in 3 seconds?

Dave

 Cutter in Kohler (0 min 32 sec)