Okay, if I may make some general guidance observations, there are a couple of maths formulas that can be used but the most useful one for us is Power(in watts) = Volts x amps. (P=VI)
So for practical purposes if you make up a jig with just one motor, prop shaft and propeller that you intent to use, just try it in the sink and measure the current in amps(I). You know the voltage, you then know the amps and you can calculate the wattage so total power which will give you a guide as to speed controllers.
Now if you put a 550 on 12 volts with a prop say 1" in diameter I would suspect the amperage will be quite high and the water will probably leave the basin! So if one motor draws say 7.5 amps four will draw 30 amps, so power consumption is excessive.
However, if you take a 550 and gear it down two to one the current consumption will generally be at least halved but the actual thrust not much reduced (I tried it years ago and was surprised).
Now its very easy to say gear it down but in practice this is not so easy because most gearboxes make a lot of noise. You used to be able to purchase toothed belt reduction drives, I don't know if they are still available but they work well.
It ultimately comes down to a cost/performance analysis:
1) Not all 450 or 550 motors are the same - you can get high and low current versions - you want the low current versions and they will cost "X". If you make a test rig you can find out if you need to gear them down or not before you purchase anything else.
2) How much will a toothed belt reduction unit cost? the answer is "Y" so X+Y is your cost per shaft. How much for the speed controller?(say Z)
3) How much will a brushless motor cost (say A) and how much will the associated speed controller cost (say B) so which solution is cheaper X + Y + Z or A + B?
4) Bear in mind the answer will have to be multiplied by four for four shafts and the cost can be surprising high.
5) More importantly which gives you the performance level you want. Bear in mind the lower the current consumption the longer you can sail the model.
6) In my general experience for a model of Dreadnought at 1/96 scale at scale speed you should not be drawing more than about 5 amps in all. Anything very much higher means the drive train combination is inefficient.
7) There are other ways to reduce amperage and that is to use a smaller propeller or 3 rather than 4 blades however to gain the practical experience also costs money!
As an added complication, say at full speed the model is going twice as fast than you want, then half speed will slow it down and generally reduce the current consumption. Electronic speed controllers (not sure about brushless) generally work by rapidly switching the power on and off. So at half speed the power is only switched on for half the time (so many times per second/frequency) so half speed = half the power consumption because the engine in only on for half the time - this is just a very general guide to the concept.
The benefit of this site is that people have already done a lot of the experimentation for you so we all gain from each others experience.
In my general view direct drive is the way to go
with the right motors as its fundamentally a simpler and therefore more reliable solution (always use double Huco couplings per shaft as this is much better than a single coupling and allows for more misalignment without power loss. Perfect alignment is hard to get!
Asa previously mentioned I uses two car blower motors (from behind the dash) as they will run on 12 volts or 6 volts with low revs and very high torque, I don't know what they now cost from a breakers yard but two will easily provide more than adequate power to drive four shafts using a single speed controller. Manoeuvrability will be fine as Dreadnought has twin rudders in line with the two inner shafts. You really don't need to make the installation too complicated.
Hope this may help.
Cheers
Geoff
Author
Topic: Deans marine HMS Dreadnought - First build! (Read 33799 times)