Help with Radio Controll Model

[BarendGrobler]

Hey all
I'm busy working on a design project that is going to require some model testing. As the towing tank we normally use can't go fast enough we are wanting to build a RC model. The scale speed required is 160Mph. I'm thinking of a model of about 1m that would give a model speed of about 45 Mph.
I don't know much about RC models so it would be great if someone could give me some advice; electric, nitro or gas engine; surface props required? etc.
thnx
Barend

[messabout]

Barend;
I think you need to re-calculate the model speed that will scale up to 160MPH. A quarter scale model would need only 10MPH to simulate 160.
The fastest model boats in the world are electric, over 100MPH. However the super fast models are freaks that will not likely be similar to the full scale boat you contemplate. They are 3 point hydrplanes called "pickle forks"

You can get a sizable model of 1.5 meters LOA going to the 45 MPH you mention. Boats that size are usually powered by gas engines. Mainly Zenoa or Fuji brands. Those engines are about 26cc displacement. They have clutches and built in recoil starters. Generally the gas engines are less quirky to use than nitro engines. Nitro engines are extremely powerful for their weight but they are messy and too tempermental to use in a research project.

In the process of scaling the model, weight scale is sometimes problematic. A boat like the one meter model is probably too small for the gas engine and possibly too small for the electric power because you will have trouble with weight. Batteries are heavy. The weight of the model is a function of the cube of the scale. A quarter scale model should weigh one sixty fourth the weight of the full scale boat. A scale of one tenth would mean that the model would weigh only one thousanths as much as the full scale boat. You can see that the larger model, the more easily you can duplicate scale weight.

As for props, there are many good ones available from firms such as Octura. Surface piercing props are usually found on the faster boats. For scale speeds you may not need the supercavitating (surface piercing) type. A fully submerged prop will go plenty fast enough for your purposes.

[BarendGrobler]

Thanx for your advice messabout, it certainly helps plenty, i initially figured electric would be the way to go just because it seemed simpler to me and perhaps a bit cheaper, but I suppose youre right that the weight of the batterias could become a problem.
As for the scale of the model, its not a quarter scale, the scaling factor is about 13 which then gives me the +-45Mph.
Thnx again
Barend

[messabout]

Barend;
Please post the math you used to arrive at scale speed. I'm puzzled. Maybe I'll learn something I did not know, but thought I did. Thanks.

[BMcF]

Speed scales as the square root of the scale factor.

so a 1/4-scale model needs to travel at half the speed of the full-scale it represents..an 1/8 scale model at 1/4 the speed full scale..and so on.

At 1:13 scale, 45 mph for the model is 162.25 mph at full scale speed

[BarendGrobler]

Thanx BMcF
Just to add to that; the scaling is done so that the Froude number (Fr=V^2/(g*L)) is the same for the model as it is for the full scale boat, which translates to what BMcF said.
Lekke dag!
Barend

[messabout]

Thanks guys. That is an elementary equation that was once familiar. Recent failure to think before typing may be a harbinger of my approaching senility.

Barend, take a look at www.fastelectric.com. There is a forum there that can and will answer questions and furnish advice about electric propulsion. Lots of things to know such as motor selection, speed controllers, and battery types. You may know that there are several types of battery NiCd, NiMh, LiPo, LiIon and so on. Each has different relevant features such as current density, appropriate discharge rates etc. The FE forum guys are very forthcoming with all that information.

[Quicksilver]

and go here

http://www.rcuniverse.com/forum/forumid_229/tt.htm
find find some hull designers

and here

http://www.offshoreelectrics.com