Afternoon All
Having realised there's more to scaling than I'd initially appreciated, I've taken on board the comments with respect to weight and done some more digging. As a sanity check I'd appreciate comments on the following approximations for scale speed and power.
I'm running same calculations for my 1/10 build of the Aquarama as a check that the right numbers come out, which generally they seem to.
Firstly Speed. I've looked at two approaches, what the scale speed should be, and then what it would take in terms of propeller pitch and shaft RPM to get to that kind of speed.
I saw this scale speed formula in March 2021 Model Boats, so for 1/10 its:

And my current 1/10 build is set up like this:

So that's a scale speed of 23.4km/h and a real world that's at best 20.86, but a bit less with some slip. And that feels about right, I'll get some hard numbers from the GPS on next outing...
For the 1/5 same calculations look like this:


So that's a scale speed of 33 km/h, and a combination of shaft speed and pitch that gets close to 33 km/h too, depending on real world slip...
I worked on a ratio of 1.2 for pitch to diameter on the prop which I think is quit conservative in terms of motor loading.
And then on a second related point, what power would one anticipate is needed to get to the scale speed, given scale size and weight, and to turn that size of prop at that shaft speed...
Seems the power calculation is quite straightforward and involves cubes again, so firstly for the 1/10th build. Its the full size power in HP (700), divided by the scale cubed (10x10x10 or 1000), so that's 0.7hp which is about 520 watts. My build actually has 2 x 340w (so 680w in total) motors, which on the face of it would appear to be slightly overpowered. I think they reach max rpm with ease, so basically aren't fully loaded and could take courser pitch props if I felt the need, which I don't.
Same calculation with the 1/5th goes like this. Full size power (700), divided by the scale cubed (5x5x5 or 125), and that's 5.6hp... Which is just over 4,100 watts. So that implies 2 x 2,000w ish motors are required to reach scale speed of 33 km/h.
And my feeling right now is A) that's a lot of power, and B) I think a 1/5th scale can be built at a lower weight than the 'scale' 32kg allowance. And that weight saving can be trade off against the predicted power requirement. So I'm inclined to look at 2 x 1,600 w motors and accept a real world speed a shade lower than true scale. But still quite perky I think.
Does this all sound realistic..?
Regards to all.
David.