# Calculating 1:12 scale model weight from 4400 lbs. real boat.

Discussion in 'Boat Design' started by SBarlow, Apr 26, 2020.

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### SBarlowJunior Member

I am simply having a dickens of a time trying to find out how to calculate the correct weight for my Billings U27 r/c model boat. The original was 4400 lbs. Scale is 1:12. 366 lbs is a bit too heavy for a 30" boat and 1.1 lbs is too light. My dad would have known it instantly but I was too stupid to listen to him when I had the chance. Now I'm stuck. Help please and thanks. I don't understand why I can not easily work it out...CoG would be nice too. Thx.

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### bajansailorMarine Surveyor

Welcome to the Forum SB.

I am trying to find your boat on the Billings website, but no joy - Billing Boats Denmark http://www.billingboats.com/
Have you got a link to it?

At a scale of 1:12, with the model being 30" long, then the full size boat is 30' long.
And I am thinking that a weight of 4,400 lbs / 2,000 kg is probably a bit optimistic for a boat of this length, assuming that she has fairly 'average' Length / Beam and Beam / Draft ratios?

Have you started to build it yet?

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### Mr EfficiencySenior Member

To sit on the same waterline as the "real" thing, at scale 1:12, you would divide by (12x12x12), that is, 1728.

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### mudsailorJunior Member

4400/(12*12*12) = 2.54 lbs

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### bajansailorMarine Surveyor

2.54 lbs certainly does sound like a better weight than either 1.1 or 366 lbs!
Does this seem about right SB?

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### SBarlowJunior Member

Hello and thanks. It is the slo-mo-shun lV model boat. The historical information I found says the boat was 4400 lb at 30 feet as you said. The Allison V-12 engine alone probably accounts for a ton or more. I have a couple of other replies I'm about to look at and it looks as though someone may have come up with the right number for me thank you very much for the help.

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### SBarlowJunior Member

2.54 seems ideal. Just weighed everything on my wife's kitchen scale while she's out and came in a fraction under 5 lbs. Don't think I'm going to be able to lose 50% of the weight. Thanks for the help everybody I never would have figured it out and couldn't find it anywhere online until I found you all. Well done! I'll post a picture to when I'm doing the final assembly soon. Cheerio.

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### bajansailorMarine Surveyor

Excellent!
Do you have any links re if this supposed weight of 4,400 lbs was true?
8,000 lbs for the full size version is starting to sound more realistic, especially if she had a V-12 propulsion engine in her.
And this would be more in line with your model weight.

I was thinking that maybe she was a WW II MTB model or similar, re the V-12 engine, but I just found her on Billings, and I see that she is a hydroplane.
BB520 Slo-Mo-Shun IV | The Experienced | BB520 Slo-Mo-Shun IV from Billing Boats http://www.billingboats.com/44/16/boats/models-suitable-for-radio-control/P-bb520-slo-mo-shun-iv.html

And here is a link to the comprehensive building manual.
http://www.billingboats.com/components/com_redshop/assets/document/extrafields/1316770481BB520 SLOMOSHUN_Instruction.pdf

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If you build to a larger scale you can come nearer to scale weight. Mr Efficiency has given you the correct method. Use the cube of whatever proportion of size that you are building to. If you make the model at tenth scale then 10 cubed is 1000. Divide that into the full size displacement and you get ....4400/1000 = 4.4 pounds. One eighth scale ..................4400/512 = 8.59 pounds.......... etc.

Modelers very often choose a small scale that makes it most difficult to build to scale weight. In general it is more practical to build to a larger scale. I am having difficulty with a sailing dinghy model of a 4 meter boat. I have chosen one quarter scale and so my divisor is 64 ( 4 cubed). The target all up weight of the dinghy including skipper is to be 320 pounds. My scale weight is then 5.0 pounds. I am struggling to meet that model weight because of the need for a fin and bulb that has to weigh about forty percent of the scale displacement.

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### SBarlowJunior Member

The 4400 lb. weight that I'm working with is in the specs in the building directions and also in some of the other information I found online. There's a ton of information and quite a few good videos as well of this very famous boat. I plan to complete it as accurately as I can and then I'll try and shave off whatever weight possible and live with the rest. Possibly use a gravity reducer but the hobby shops don't seem to sell them. Maybe SpaceX or the military? Thanks for all the help.

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### SBarlowJunior Member

Thank you that's a good suggestion except I'm almost finished building the boat from a kit. Now that I have the target weight I'm just going to complete the kit and then do what I can to shave weight once it's complete. If she's still too heavy maybe I'll tie a helium balloon to her and see what that does!

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