Whats wrong with my calculations?

That's your main problem.

Model length should be at least 1.5m, preferably longer, to avoid surface tension effects.

I certainly wouldn't trust the ITTC line at the small Reynolds numbers of your model. Then again, I wouldn't use the ITTC line anyway - it is not a physics-based formula.

Good luck,
Leo.

 

i test models around 80cm size, in our own back garden test tank, don't have probelms...results never out by more than 5% from full blown tank test. As a quick rough and ready idea for R&P, very quick and cheap too....

 

Hello,
Do you have a link to some research on surface tension effects related to model testing? Or some other reference to recommend? I've tried with google but couldn't find anything useful.
Thanks!

 

Hi, instead of multiplying my boat's displacement and its maximum load by 20^3 to estimate the results of a full size boat, what would be a more realistic and convincing approach? (The fact that multiplying 330g and 480g respectively by 8000 would means the real boat will weight 2000+ kg and could carry a maximum load of 5000+kg, and powering the boat in this condition at 7 knots would only need about 55HP would not be convincing to anybody). Thanks.

 

Err... What exactly makes you think that volume or weight scaling by the third power of the scale factor is not realistic?

The only thing you could do in addition is to account for the density difference between fresh water (in which you tested your model) and salt water (if the actual boat is intended to float in). You then would have to divide your model's weight by fresh water density (about 1000 kg/m³, the exact value depends on temperature) to get its underwater volume, then scale this by multiplying with scale factor³ (you now have the actual boat's displacement volume) and finally multiply with the density of salt water (around 1025 kg/m³) to get the weight.

In formulae:

VM = WM / ρFreshwater

VS = VM * λ³

WS = VS * ρSaltwater

VM = Displ. Volume of Model
WM = Weight of Model
ρFreshwater = Fresh Water Density
VS = Displ. Volume of Full-Scale Ship
WS = Weight of Full-Scale Ship
ρSaltwater = Salt Water Density
λ = Scale Factor

 

What is wrong with my calculations

I do not look at this forum very often so I am late in contributing.

When I saw the test figures it struck me that they were very low. I think that you might be expecting more from Froude's work that you should. Froude was interested in merchant ships not vessels per se. He was in no doubt that test models should be 6 feet long or longer. Nothing has changed.

If you have understood the mathematics in some of the replies you should be able to understand Chapter 11 of my text book at www.ivorbittle.co.uk in which I give a digest of Froude's work.

Froude anteceded the Froude number.

Ivor Bittle