Model testing Cat hull designs

[otseg]

New member.

I am designing a 32' single engine 80 mph outboard Cat. I wish to model test hull design features with a radio control RC electric model before building full size.

Is there some kind of remote real time GPS logger that can be fitted to the model to report speed and power?

I suppose I could make two models and change one for comparison and run them side by side.

It would be valuable to test with partial power too.

Jim

[1coopgt]

Not to be a jerk ,but did you try a google search. There are all kinds of Gps loggers, You could also download an app to your smart phone also . I have a Dynamite GPS and Skyrc Performance analyzer.

[otseg]

Thank you for the suggestions.

The goal is the full size boat. Previous power and sailboat model testing we used a power boat with a boom that drags a model. I am thinking this time to buy a couple Pro Boat Blackjack 24 catamarans (only ones in stock presently) to do my mods to one hull. Using this formula (True scale speed is equal to the square root of the scale of the model multiplied by fullsize speed) 80 mph true speed is 20 mph model speed.

Is there a way to consistantly throttle the power that the electric motor produces so that before I make changes the model has a speed of 20 mph every time?

[1coopgt]

I have no Idea how you would do that sorry.

[NativePaul]

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.

[otseg]

NativePaul you have made some good points. Regarding weight. I hefted a Proboat 36" Geico Cat and could not believe how heavy it was. I remember building some large tank test models and having to keep the structure light. At the end of the day its going to be more about the aero. This is a 40' center console fishing cat we designed and built that achieved 95 mph. Not much boat touching the water.

[NativePaul]

The same applies to the wing loading of the tunnel as the bottom loading of the hulls. You will still have to build really light to replicate it and the Reynolds number scaling will still mean that you have to have lighter loading than scale to behave in a scale manner.

[otseg]

The same applies to the wing loading of the tunnel as the bottom loading of the hulls. You will still have to build really light to replicate it and the Reynolds number scaling will still mean that you have to have lighter loading than scale to behave in a scale manner.

Same true for airplane models?? I would think they are proportionally heavier too.

My friend is helping me with a wing foil between the hulls. His friend developed the winglets on airplanes. The wind tunnel winglet testing they would do a run in the wind tunnel, then he would use a wood rasp on the wooden pattern, vacuum up the debris, and give it another go.

I was a member of the AYRS in the '70's which I just looked and am pleased to see still active. https://www.ayrs.org/

I

[NativePaul]

Yes.