Ship model testing - precautions?

Dear Sirs,

If anybody has experience with ship model testing, kindly advise me:

(1) For a foam model of hull, the weight will be quite less as compared to the required model displacement. So some additional weight will have to be added with limitation that the model can not be made too hollow. In that case, does the difference in vertical center of gravity have much effect on the model test results?

(2) With one coat of varnish on foam and slight sand paper applied on top of it, how to consider the hull roughness for the results?

(3) The model testing machine itself acts weightless, but the pivot point will remain about 150 mm above the top surface of model. What effect will this have on the test results and trim?

(4) Angle and position at which the propulsive force acts will be different from the angle and position of the driving force applied by the testing machine. How do we consider this difference?

(5) Please provide any other suggestions regarding the precautions to be taken when testing and analyzing the test results.

 

1) Not so much for simple resistance testing. But it is always nice to be as close as possible for the VCG.

2) This is achieved either by studs or strips of sand paper

3) It effects the results a lot. You need to make sure the pull/trust line is in line with the actual propulsion line of the vessel.

4) See no.3

5) Make sure the water depth is deep enough not to be affected by speed of model. Also the walls are sufficiently far away not to effect the results too. Wall/Blockage Effect can be significant.

 

Thank you Sir for the reply.
Regarding (3) and (4) it is practically impossible to provide the pull line at actual thrust line in case of pod propulsion, SRP etc, because the thrust line is significantly underwater. How do we do it in that case?

Also, for foam models, it is risky to dig the model too deep as it may damage the hull surface.

The other points will be taken care of.

 

Your tow-line will introduce a bow down pitching moment = measured resistance*vertical distance (towpoint-pod)
You can compensate for this moment by moving the CG aft by a distance X
X = pitching moment / displacement
Your university, or the facility that operates the towing tank, will most likely have established operating procedures and a quality system that must be followed to get reliable results. You should consult these documentations.
Regards Uli

 

Not impossible, you need to look at a high-speed towing set-ups like those of Clement, Blount, and Savitsky.