Large scale sailboat model testing

Hi there,
I have designed a 44' sloop, I actually had stations cnc cut and made a 1/4 scale model. I'm aware of the laws of mechanical similitude but wound never-the-less like to make the rig and keel/rudder appendages and cg exactly scaled to the full sized boat. I plan to make duplicate glass 11' models and possibly test different keels and sail combinations.
My questions are: how would I scale the wind speed to correct for the scaled stability of the model? Also, does one need to ad areas of sand paper to the hull so it is not in laminar flow?
john

 

Hello,
Take a look at this discussion:
http://www.boatdesign.net/forums/sailboats/scaling-model-velocity-gradient-34519.html
To determine wether the flow will be laminar or not, you need to know the speed of the model. From that data you will calculate Reynolds number and see if you are in the turbulent regime (approximately above Re=500000). If you are not, then it si advisable to sand it or to apply some other type of turbulence generators (like sand paper strips). Perform separate calculations for the hull and for the keel/rudder, because of the big difference in reference legths for Re calculations between the two.
Cheers!

 

Thanks daiquiri,
As you can tell this is not what I think about all the time. That is the way I will approach this. It is logical. later if I need it I'll probably ask about "grit" or the courseness of the paper but that is great for now.
I am posting a picture of the model in my "galley" on the forum.
John

 

You need to trip the flow at the scaled separation point with a sand strip or studs or lines. Do this for all appendages as well as the hull.

 

Dear H20land,
I think you need to be realistic with what u can achieve by testing free sailing models. It looks like you will put a lot of effort and time into to it so I took a couple of minutes to give u my thoughts.
Forget trying to compare the perfomance between yachts. Impossible to measure free sailing. The best u can do is "race" them and see which wins the most.
From free sailing tests I would gauge the balance and the sailing qualities of the boat. For that worrying about turbulence stimulation etc is debatable. If you have the knowledge I would instead design sections that behave well and similar to the full scale ones at the low Reynolds number of the model and forget the turbulence stimulation.

If you really want to compare drag, then a old setup used in the past of using a scale(lever) type arrangement in the horitzontal plane to tow the models side bz side infront of a boat. Then depending on which model advances u can do relative drag comparisons. but that is wrong anyway since the wetted area is different between models.
There is a very old paper on it which I do not have at the moment

Good Luck