Not So Boldly Going Where Others Have Gone Before. RC Nano-Hydro

[NativePaul]

When I first saw the tiny 7" Casper hydro I was smitten, first I thought I would love to convert one to RC, and then after seeing some better photos I thought I would love to design and build something kind of similar myself to my tastes and specs, hoping to improve on it.

I have since been inspired and spured on by Dr. Jet's Casper hydro conversion that he is working on. Thanks Dr.

This is my atempt at doing something the same size but a little more racy, so it is based around specs I would give a Naviga Rigger, but massaged to fit a semi scale hydro body. Its basic dimensions are 7"x4", 1/3 sponson - 2/3 afterplane, 13mm wide sponsons, 10mm deep, with 6degrees AoA and 35degree anti-submarine wedges, 5mm of airtrap for 2/3 of the afterplane. it is the first boat I have drawn out full size on an A4 sheet of paper and had room to spare.

The gear I am using is :-
This 6.5g 4600kv outrunner
This 2g 5A ESC
This 9g 300mAh 1s 20c LiPo
This 1.9g servo
This 3.5g RX
And these Rivos XS props, I saw a Rivos XS today and thought it was a rebadged Magic Vee, but its prop is a 2 blader of about 20-22mm x 1.2 pitch with a 3mm shaft (by eye, I forgot the calipers) and a decent shape for surface drive, it should suit my needs nicely, I ordered a set so I will give measurements when I get them.

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[Dr. Jet]

Mmmmm...... I like it!! My goal on the Casper is to get it to run "on step" and be controllable. You plan looks like it will be a real goer.

What material or combination of materials are you planning to use?

[NativePaul]

The main build material will be depron, I have enough on hand to make 20 of these little buggers already, probably with carbon motor mount and bracketry, I thing I have enough 2x200gsm 0.2mm sheet to make them from.

[NativePaul]

I have been aiming for 15mph, but I just worked out the scale speed for a 1:43 scale modern unlimited which would be 30mph.

As Living on the coast I never get glass water and the smallest ripple is quite a heavy swell at 1:43 scale, I am still going to shoot for 15mph which is about as fast as I think I can keep it on the water.

I do have a plan if it looks stupidly slow, although unfortunately it is not FE.

While I was at the model shop checking out tiny props, I saw a cute little 450mm span EPP foam chuck glider called the Fox, for just a few quid, I bought it cut it in half and hollowed it out, making plenty of space for the gear above in it, I ordered some extra servos too.

[srislash]

Paul at 1:43 scale won't a Mr Turtle pool work? J/K... But a pool perhaps

[NativePaul]

The props arrived already, I only ordered them after work yesterday, Sussex Model Centre get a shout out for that as they are not an item that OSE stock.

Their Diameter is 17.3mm, the boss is 6mm, and the bore is 2mm smooth, there is no dog or thread, although looking at the boat it looked like a dog, but I think it is just compression between a flat collar where the dog goes and the prop nut. I don't have the facility to measure pitch, but they are VERY progressive, it looks like 0 at what little tongue they have and 1.6 at the trailing edge. If it still gets up onto the plane, performance should be increased greatly by further detounging, the good thing is they come in packs of 3 so there is some experimentation to be done without disaster.

[Fluid]

I have been aiming for 15mph, but I just worked out the scale speed for a 1:43 scale modern unlimited which would be 30mph.....

I am curious, are you saying that a 1:43 scale model would have to travel at 30 mph to be “scale” to an Unlimited Hydro? Actually, to match a full-scale speed of 180 mph a 1:43 model only needs about 4 mph. Just wondering.


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[Dr. Jet]

I am curious, are you saying that a 1:43 scale model would have to travel at 30 mph to be “scale” to an Unlimited Hydro? Actually, to match a full-scale speed of 180 mph a 1:43 model only needs about 4 mph. Just wondering.


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Speed versus scale is not linear. You can't just divide scale ratio into full-size speed to get scale speed. It's a function of volume (i.e. a cubic factor). I've searched the 'Net for an appropriate equation, but have yet to find one. On this size of model, 15~20 mph MIGHT be somewhere close to "scale". A lot depends on the eye of the viewer.

[Fluid]

Nothing depends “on the eye of the viewer”, it is physics. Unfortunately, I used the wrong spread sheet to calculate in my previous post, forgive me

The correct formula to calculate scale speed is the square root of the scale times the full-scale speed. In this case it is: square root of (1/43) x 185 = 28 mph

So Paul was, as usual, correct.



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[NativePaul]

I was basing it on a 200mph top speed as the unlimited site says they reach over 200 in the straights, but I have never seen an unlimited race, do they only hit 200 in unusually calm weather conditions, is 185 more realistic?

The scale factor scales the distances, but you have to scale the time as well to make it look realistic, which is why you use the square of the scale factor.

[NativePaul]

I recieved the final piece of the puzzle through the post this morning. Happily all the actual weight were lower than quoted apart form the servo, and by more than enough to make up for the servo.

My bank notes and coinage are different sizes to yours, so I have pictured the gear on a green and black store card of the usual size.

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[Dr. Jet]

I'm excited to see how this works out for you. I just went through my "Stash-O-Stuff" and discovered I have a ton of Depron in a variety of thicknesses, AND a Fox chuck glider........

Start cutting and gluing!!

[NativePaul]

I redrew the plans last night, I thought I recalled them being 7", not 7.5", so I stretched the width and lengthened the afterplane to make it 7.5x4.

I wish I had a variety of thicknesses, I have 2mm and 3mm, but I think 1mm would have been ideal for the sheeting, and 6mm+ would make carving the canopy easier.

Never mind, I will get building this evening.

It is surprising how differently we can attack the same problem, with my entire electrical/electronics package weighing less than your ESC, however while I am confident of reaching my original goal, I wish I had worked out the scale speed before I ordered anything. I was obsessed by the videos of them racing and them being effectively very inefficient displacement monohulls, that look like hydros. I wanted to avoid that at all costs so went for the lightest gear I thought would push it at a decent pace so it would be easier to get up on the plane. It is however slightly depressing knowing that even if it is a success, it only has the pitch speed to hit about half it's scale speed.

Not that if I was starting again I would go your route entirely, it would still worry me about getting up on plane only being a little lighter than stock, despite the extra power, but if I had of stepped up to a 10g outrunner, a 7g 6A ESC, and a 2s 300mAh battery, the package would still weigh less than your battery, yet would have the pitch speed to at least attempt to reach scale speed.

[NativePaul]

It took a lot longer to build than I thought it would, The Rocket odouerless cyano I bought takes a LOT longer to go off than whetever I used last time I built from depron, and it barely responded to the kicker I have, so a lot of taping, clamping, and waiting was needed.

I did get the hull built eventually though, it looks done, but I still have to cut the hatch and build a lip for it to sit on so there is maybe another .1 of a gram more to add to the hull.

It has the motor mount and motor fitted and built inside, but no hardware or electrics/electronics other than that fitted at this time. The weight as it stands is 13.2 or 13.3g my scales cant make up their mind. So probably about 8.5g for the bare hull.

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Opposite of normal, the pictures make it look terrible on my 42" monitor as blown up 4x bigger than it is in real life I can see all the flaws.

[Dr. Jet]

Looking good! Some thoughts: With a slightly different cowl, I see a Nano Atlas Blue Blaster... Are you going to make your plans public? The sponson length seems a bit short though; isn't the balance point going to wind up rather far behind the aft end of them making it harder to get the stern out of the water and riding on the prop? I suppose being all Depron, you could stuff some of the electronics in the sponsons and get some mass forward.