Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: funtimefrankie on August 04, 2006, 02:38:43 PM

I sent the Hunsman out today with a GPS on board and it came back with a Max speed of 8.1 mph.
How does this scale up to full size speed (1/12 scale , well 1/11 actually)?
Frank

A maths boffin is needed on this one ???

It's my opinion that speed doesn't always 'scale' to well. If it looks right, then it's close enough.
 'Doc
PS  Also a fairly obvious ploy to not learning how to do the numbers.

I have always thought that speed scales with the boat. Thus if your boat is 1/11 scale and travels at a real 8.1mph then the scale speed is 11x8.1 or 89.1 mph. :o Given that the power unit is far more powerful (in scale) than the prototype this is surely not surprising.
Has anybody a mathematical explanation as to why this should not be?
Doug

Try multiplying the actual speed by the square root of the scale. In your case, 8.1 x 3.32 gives 26.9mph, about right though some of these boats were a lot faster than that.
The real problem with this kind of exercise is that we are using scaled down boats in 1:1 water and wind. I guess this is why model planes always take off in such a short distance.
Jonty

Totally agree Jonty.
If it looks right with the correct wave formation it usually is right.
Yes can bore you to death with the formula, but I dont use it as the wind and waves are 1:1
Bob

Got to agree with Shipmate60 and Jonty: if it looks right, then it is right. Look at the bow wave and the wash and judge for yourself. I did find somewhere on the web a site which explained all the maths (scared me half to death!) and the guy at the end said...if it looks right, then it probably is. Sorry I can't be any more specific. I spent ages trying to get my MTB to have a perfect scale speed (now got so many sets of props I could open a shop!). I don't know if its is cale, but she looks great!
Mike

If your boat looks right on the water, and you then do the maths according to Jointy's recipe, you will find that it all works out very closely. If you look at a model you can work out how much power it will take to make it perform like the original by dividing the original power by the cube of the scale. We would probably want a percentage more to cope with nonscale waves and wind, but its nice to know where the goalposts are before committing to a project.

I wouldn't bet one using a scale down (cube or whatever) to find a reasonable motor for the model. Our electric motors are normally stated in Watt and normally the stated power is the input power to the motor (Voltage multiplied with the Ampere). The output on the shaft is quite different. You have to find the efficiency factor for the given motor (vary with the load) and you have to find the loss due to the shaft and not at least the propeller efficiency. Our scaled down reallity is quite different from a real ship. In a model (of a reasonable size) the propeller is turning quite a bit faster than the one on the real ship and quite a lot of the power is used for making airbubles. Again  a good guesswork combined with emperic tests and a squinting look at other models built by friends is the best way to find the appropriate motor and propeller combination.
Aage

The scale speed equals the square root of their scale,times the full size speed.
george

Right, now let's make this really complicated, and talk about scaling down colours. We can now, thanks to White Ensign Models, get the exact colours we need to make our warships match the original. But does this really work?
The point is that to get an impression of the real thing, rather than just an accurate small replica the colours do need to be muted or toned down. What do the judges in scale competions make of this? Or is this just going to open up the wounds discussed under the Mountbatten Pink thread?
Jonty

I ahve had this discussion before, as with the scale speed I look to get a shade I think looks right. When I was in the navy an opo of mine did paint a model in real top coat grey. even with a satin varnish it diod look a bit too bright. I am testing various shades on my current model.
I don't know what speed my model does, but it looks right and it loses the right amount of speed in the turn ands accelerates slowly.
If it looks right it probably is.