Linear dimensions are following directly the scale factor.
Surface dimensions are following the square of the scale factor
Volume dimensions go with the cube of the scale factor
simple example:
Real boat
LOA 10mt
SA 35m²
Dspl 2000kg
Model at 1/10 :
LOA 10mt : 10 = 1mt
SA 35m² : 10x10 (100) = 0.35m²
Dspl 2000kg : 10x10x10 (1000) = 2kg
the scale reduction, when important, reduces very much the model DSPL and render difficult to build a model with that scale unless the hull immersed form is modified .
ClaudioD
To continue, the stability scales by the third power. So if you scale down a boat to 1/10th, your stability drops by a factor of 10,000. You will need to do something to acomodate this. Things can include increasing the displacement to add more ballast, reducing sail area or making the keel deeper. This is why very small sailboats are very tricky and Footys look so funny.
All in All, scaling is not a good idea unless are static models.
Models shall be designed from scratch or according to Class Rules.
Another example : America Cup 2007
LOA 24mt
Displacement 24000kg
SA 350m²
The negative effect of the scale .
Assume a scale of 1/20 like the actual AC120
with a lenght of 1.20mt
model displacement : 24000kg : 20x20x20 (8000) = 3.0kg
Sail area : 350 : 20x20 = 0.87m²
Remark : Displacement too low to allow a construction, SA too large to obtain good lateral stability. To note that the bulb should be in the range of 60% minimum at model level. The real AC Bulb was 83%.
The actual AC120 models use Hull with 4.5kg dspl and max 0.80m² for very light wind conditions and 2.85kg ballast.
This is a great thread and I’ve learned much though some of it, for me is a little confusing (Froude etc). I do have a question though on my design using Hullform 9s my ‘area plot’, I think you use the term ‘curve of areas’ has a flat top on it. What does this mean? And what do I need to do to smooth it out?
Hi,
I have some difficulties to understand your question.
A Curve of Area (COA) is formed by the shadow’s immersed surfaces positioned at regular distance each other like in this attachement
I don’t see any “flat top”, please explain , for instance the shadow n°6 has an immersed surface of 56.07cm²…
for each calculated surface I use to fix a dot corresponding to the scale, left column. All dots are linked by a curve called Curve of Area or COA.
This curve provide the displacement, the Prismatic coefficient and the LCB position and not only since his shape suggest the characteristics of the model.
I should have worded my question better. The I.O.M. I have drawn does not have a nice rounded Curve of Area (COA) shape such as your attachment shows. Mine has a horizontal line across the peak of the curve and I’m wondering what I have to change on my design if anything? It does have a high prismatic co-efficient of 60 due to being quite narrow and full in the ends to achieve the displacement needed to meet the IOM rules.
Please show each shadows immersed calculated surface in cm² or mm² and the distance between all shadow from 0 to 10, unless you have different number of shadows.
A prismatic Coef. of 60 is acceptable.
Is your “curve” CAD made ? if yes, then being not familiar with your CAD, I cant help you, sorry !
if you have a “flat top” of the COA, two or more frames amidships have an identical area (below waterline). A cube, for example, will generate a fully flat line …