Scale model trial

[fede]

Dear friends, I'm thinking about making a 1/10 scale model of a rib to test it in water towed from a boat.

Rib is 25 ft long scale model is 30 inches
Is that too small to get useful results?

How should I load the model to make a similar weight to
dimensions ratio? (full scale is 1500 kg)

What's the point I should tow the model from? CG?

Thanks for help.

[tom28571]

Some NA's who have done similar towing tests suggest that a model should be at least 4' long. That is what I used in towing tests for a boat I designed. For your model I would suggest a scale of 1/6. The model would then weigh 6.94kg. Some tow from the stem but I think that can mask some poor behavior so I tow from the CG. I attach a string bridle to each side of the model at a horizontal projection of the CG. May not be ideal but for amateur towing, it may be about as good as you can do.

good luck.

[fede]

All I wanted to know!
Thanks Tom

[tom28571]

One more thing. The bridle is in the shape of a V with towing line at the apex. This can make it a bit hard to get the boat started in the right direction and can cause another problem if the model founders. I attached another line between the bridle apex and the model's stem. this line is normally loose and has no effect other than getting started unless the model decides to do something crazy, like swap ends. This happened once before I came up with the loose bow line.

In loading, I set the model in a bathtub and added lead to put the model on its lines. To do this properly, you need to make the model lighter than the scale weight. I also put in a graduated plank on centerline so weights could be clamped and adjusted for varying attitudes and degrees of overload. I cast lead at 5% and 10% of design weight to make loading easier.

I can send photos if you email me.

[cyclops]

I build large scale RC boats . It should be a piece of cake to propel it with a motor and shaft to give a true condition of the prop pushing the boat. Feet per minute to the 1/6 scale would be very easy. 1/6 scale is now a RC racing Hydroplane class. You could change rudder shapes as needed for stability. And overpowering the model would reveal the probable upper limit. Turns at any speed are also done. ----------Sometimes technology is a blessing. Good luck.

[fede]

Thanks Tom and Cyclops, self powered test is something I was thinking about too...

[fcfc]

I am interested by scale model too, but what kind of instrumentation can you use on a scale model for speed, rpm , trim and (shaft) thrust measuring ?

Regards.

[fede]

I'm more interested in evaluating behaviour more than
performance (planing time, dynamic stability and handling).
I guess you could also use a dynamometer to measure resistence while towing the model.

[Thunderhead19]

For vessels less than 40' your results might not scale up very well.

[oceanmaster66]

Take a look at Professional Boatbuilder magazine issue #55, Richard Akers wrote an article titled "Model Testing".

If they are anything like mine - if they can't bust, sink, lose or burn it, in the first 20 minutes, likely as not it will last a lifetime and give great service (at full scale).

I was thinkin of renting em out as destruction testers for toy companys...

Kids....don't get me started today!

Cheers!

[heruzen]

i really need information about catamaran hull for water transportation such as LOA (15m-17m),B,H,T. for the information, thank you.