What you want won't work by modding an 24" RTR cat, you can make it go at the scale speed but you will never get the bottom loading down to what the real boat would be.
The BJ24 probably only gets up out of the hole and onto the plane at 10+mph, imagine ballasting your full size cat down so much that its planing speed was 40+mph, any data that you got from running as such would be very skewed and pretty meaningless compared to its normal weight.
Honestly I don't know how to scale the bottom loading. I suspect that the bigger lifting surfaces get a significant efficiency advantage from their higher reynolds numbers and the actual bottom loading has to be much lower on the model to behave in the same way as the full size.
Test models are normally much bigger than 24" which makes bottom loadings easier to achieve, reduces the differences in reynolds numbers with a smaller scale factor, and makes any given defects in the model more accurate relatively.
You should also make a scale model of your boat rather than use a RTR, as you don't know whether it is interactions with features and dimensions that yours doesn't have that makes modifications effect your model positively or negatively. Maybe consider having someone take a mold of your drag model to use, or have a model builder build one to your specs.
You might want to look into the SM Modelbau Unilog2 and Eagletree Elogger v4 for your GPS and Datalogging needs.
If you know a lot about Arduino programming or can employ someone who does, you can build or have built an Arduino based brushless ESC, and I suspect that you could take the data output from the loggers that is normally used for On Screen Display and input that into the Arduino to have GPS closed loop regulation of the speed. That would allow you to maintain 20mph and see what differences to the power consumed the modifications make, which is what it sounds like you are asking for.
I am not sure that is what would be best for you though, as your full size outboard has a limited power output, I think that simply limiting the power output of your model and see what differences to the speed attained the modifications make, might be a more useful as well as a much simpler experiment.
Paul Upton-Taylor, Greased Weasel Racing.
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