# Half Scale

Discussion in 'Boat Design' started by Jammer Six, Aug 3, 2014.

1. ### Jammer SixPrevious Member

Tell me what would happen if I took a standard, trusted sailboat design and built it to half scale. That is, with everything cut in half-- the weight of the keel, the height of the mast.

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It is sadly not as simple as that

If your boat is scaled down, a geosim (which is an exact scale in all direction, x,y and z) by 1/2 the displacement is reduced to just 1/8th of the original. So if the boat weighs/displaces say 1.0tonne or 1000kg, then it will weight just 125kg once scaled by 1/2.

3. ### Jammer SixPrevious Member

I'm considering a 1/2 or 1/3rd scale Dragon.

What are the calculations I'd have to look at?

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### TANSLSenior Member

Sorry to disagree but I think any element of composing the weight of the boat is directly dependent on the scale, you apply to it, or 1/8th of the scale.
If you want something done right, you have no choice but to estimate the weights, item by item, with the precision you want.
Some examples:
• The accommodation may depend on the number of passengers, rather than the scale.
• The weight of the wheel, can be the same.
• The weight of the hull may depend on many factors. One, for example, is the ratio V / SQR (L). If this relationship is not maintained, the weight of any element of the structure will change, but not according to the scale.
• The weight of the engines never depend on the scale,
• The weight of the rigging depends more on navigation conditions than on the scale, and it determines the weight of the fixed ballast keel.
• etc ...

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The new displacement is based upon the change in scale. So for 1/2 scale:

(scale)^3 = (1/2)^3 = 0.125

So the new displacement of your design/hull at 1/2 scale is old displacement x change in scale = 0.125

Thus if 1/3 -> (1/3)^3 = 0.037 or 1/27th of the original or just 27kg of a 1.0tonne hull

6. ### Jammer SixPrevious Member

Thank you.

What about the forces on the mast, and the weight of the keel?

Do they scale at the cube, or do they scale to 1/2?

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### TANSLSenior Member

I think you should think about what I said before. If it is not clear, ask the opinion of a third party.

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### SukiSoloSenior Member

Just look at scaled down J class yachts. If you scale the keel dimensions it just goes sideways.....

Hence stuff like the 2.4 meter class.

http://www.inter24metre.org/

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### TANSLSenior Member

Jammer Six, allow a simple comment, from a person who is not sailing boat designer: The force exerted by the wind on the sail is based, inter alia, on the sail area and the wind speed squared. The wind speed does not depend on the scale of the ship. Therefore I, who am not a designer of sailing boats, might think than riggings, sail's area, their weight is not 1/2 nor 1/4 nor 1/8 of the original ship.
There are many other examples where the weight of the boat does not depend at all on the scale. So if you want to avoid dismal failures, do not start to build your boat until you have things clear. And carefully analyze all the advice we give you.

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Tansl, I think that Ad Hoc meant to refer to the displacement, while on its' designed lines, of the scaled down boat, not the weight of the boat. The weight of a half Sized dragon, or a half sized anything cannot be anticipated by a simple formula, but of course the scale displacement can.

I'm guessing that the completed half Dragon will weigh more than half that of the full sized Dragon. Therefore scaling down will not yield a competent boat. She is likely to be way over her lines.

For the OP. The answer is: No, that will not work.

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Sadly this is getting beyond you even from the start. The displacement scales to the cube and sails to the square. Which is why if you deviate by more than around -10% of a sailing vessel when scaling, it is a total redesign because the original assumptions and calculation no longer remain valid.

I suggest you find a vessel that is exactly the size you want, or seek a local NA to do all the sums you want and then see if what you end up with is what you expect!

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13. ### Jammer SixPrevious Member

Thanks to all of you.

To give all the options a fair hearing, (before I reject them [joke]) do any of you have suggestions for what is, in essence, 10 foot Dragon, Dark Harbor or Skerry Cruiser?

The only thing I've found in that range is Chesapeake Light Craft's "Skerry", and that's not even close to what I want.

What I want is to a Dragon what a Millimeter is to an IOD, in wood.

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### TANSLSenior Member

Sorry to say, Jammer Six, but I do not understand what you're asking for. What you want is someone to design your boat?

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### daiquiriEngineering and Design

Neither do I.

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