Balancing a cat - How sensitive is the CG in boats?

[jfrabat]

So, I am finishing the build of my 30" scratch build cat. Things seem to be balancing up close to what the plan says, but I am about 1/8~1/4 inch forward of the CG recommended by the plan (33% from the rear). I still need to do the servo mount (in the rear), so it may end up balancing fine, but just wanted to know how sensitive CG is in boats (1/4" is fine for planes), and is it better to be tail heavy or nose heavy? In airplanes, nose heavy tends to make planes more stable; logic tells me it will be the same for cats, as tail heavy would tend to flip, right?

[rearwheelin]

33% CG it just that designs recommended starting point. There are a few factors that cause handling differences, some builders put components in different areas that cause the best CG numbers to be different with the same hull between builders.

[Panther6834]

I'm just getting into FEs...but, I've been doing this long enough with land-based RCs to offer a solution. For off-road race vehicles, we use weights to help balance a vehicle...sometimes brass, sometimes lead, and sometimes other things. One of my favorite "other things" is something called 'Liquid Gravity'. I get mine from Amain (https://www.amainhobbies.com/deluxe-...lmbd38/p186992), but I'm sure others sell the same thing, or something similar.

One of the reasons I like this stuff so much is that you can pour what you need into any pocket, crevice, or whatnot, until you get the weight to less than 1/10 of a gram below the actual won't you need (the greater the amount of Liquid Gravity, the greater the difference between the "target weight" & actual weight). Then, to hold the Liquid Gravity permanently in-place, you add a little CA glue (when dry, the CA glue weighs less than a few tenths of a gram). If only a little weight is needed to balance things out (left/right, or front/back), Liquid Gravity is perfect. On the other hand, if greater weight is needed (like trying to weigh-down the front end of a 1/10 2WD buggy), that's when brass is needed. Not sure this'll hello you...but, hopefully it does.

~ More peace, love, laughter & kindness would make the world a MUCH better place [emoji1690]

[jfrabat]

Oh, it is my first FE, but I started in RC airplanes in the mid 80s, so beleive me, I can balance this thing. My question is more on how delicate the balance is with boats. Some airplanes are VERY forgiving with CG, others go ape *!***!***!***!** with the slightest move (I had an X-29 that flew for about 5 minutes with the CG wrong; most terrifying 5 minutes of flight ever! Ended qith a lawn dart!)

[NativePaul]

Not very sensitive, but you should definitely install your gear so that you can move it fore and aft a bit by moving the battery. While the designer may recommend 33% that wont necessarily be the best place for it with your setup, your prop may have more or less lift than his and you may be putting more or less power to it and altering the drive angle.

Having the CoG further back will increase the boats speed, reduce the amp draw on your electrics, increase the likelihood of blowovers (flipping vertically), as will a prop with more lift or angeling the prop skyward.

Having it further forward will make the boat run wet, increasing the amp draw on your electrics, making the boat slower, and making it more prone to hooking (spinning out), as will a prop with less lift or angeling the prop down.

[rearwheelin]

Not very sensitive, but you should definitely install your gear so that you can move it fore and aft a bit by moving the battery. While the designer may recommend 33% that wont necessarily be the best place for it with your setup, your prop may have more or less lift than his and you may be putting more or less power to it and altering the drive angle.

Having the CoG further back will increase the boats speed, reduce the amp draw on your electrics, increase the likelihood of blowovers (flipping vertically), as will a prop with more lift or angeling the prop skyward.

Having it further forward will make the boat run wet, increasing the amp draw on your electrics, making the boat slower, and making it more prone to hooking (spinning out), as will a prop with less lift or angeling the prop down.

Thanks for this explanation! I’m setting up my maiden run on my cat so this helps. I read something along the lines of what you said here 10 years ago so thank you for reconfirming it.

[jfrabat]

Defenetly a very helpful answer, I agree!

[jfrabat]

OK, guys, need your opinion... With all this quarantine lockdown, I cannot go to the lake, so I did the next best thing... Took my almost ready boat to the pool in the backyard (you know, those temporary ones that are about 12 feet round?). I know, I know... bad idea to run an FE in a pool... especially without grease in the flex shaft... But I could not hold myself. So, I had my kid recording. Seems to me, the boat is raising its nose when I accelerate. The are SUPER short runs, and I don't even get to half throttle, yet the boat seems to jump out of the water.

So, a bit more weight to the front, or too early to tell yet?

[jfrabat]

Here is the second run:

[Panther6834]

The short answer is, "You can't test a cat in a 12ft diameter inflatable pool, any more than you can test a 1/10 scale 2WD race buggy in an 18ft long hallway...or a 1/10 4WD stadium truck...or, God forbid, a 1/8 4WD buggy. Don't ask how I know these things...especially regarding the 1/8 4WD buggy. And, no, the wife will never find any evidence of such a thing having taken place.

~ More peace, love, laughter & kindness would make the world a MUCH better place [emoji1690]

[Panther6834]

Short answer is, "You never test a cat in a 12ft diameter inflatable pool", any more than you would test a 1/10 2WD in an 18ft long hallway...or a 1/10 4WD truggy...or, God forbid, a 1/8 4WD buggy. And, no, the wife will never find any evidence of any such thing ever having taken place. "I know nuuuthing."

~ More peace, love, laughter & kindness would make the world a MUCH better place [emoji1690]

[jfrabat]

In my defense, it is not "inflatable".... and the video is all special effects; I never even connected the battery. No, really! Ask my wife!

[NativePaul]

It's too early to tell.

Boats almost always raise their noses under acceleration, there are 2 main reasons for this:

A boat prop is generally at the bottom of the hull, so the CoG is almost always higher than the thrustline, causing a nose up under acceleration.

The front of a boat generally has a much higher angle of attack than the rear so provides much more lift when the speed is low enough for it to be in contact with the water, this causes a nose up at low speeds. (At high speed on flat water the hull only touches the water from the CoG backwards, so the bow angle doesn't matter (this also explains why you go faster with a more rearward CoG, you have a smaller wetted area))

When accelerating "out of the hole" (from stationary) you generally have both effects working to raise the nose.



Edit. I say generally because there are some unorthodox boat designs out there that don't have these effects, and if I gave absolutes someone would say "my S.W.A.T.H. hull doesn't have the second effect", or "my submarine doesn't have either effect", but all high speed planing boats do have both effects, including all planing FE model boats.

[Panther6834]

In my defense, it is not "inflatable".... and the video is all special effects; I never even connected the battery. No, really! Ask my wife!

You should be working in Hollywood...lol

Seriously, tho, why can't you go to the lake? I understand the "shelter in place order" (we have it here in CA, tho ours are among the most straight...especially here in the San Francisco Bay Area), but our parks aren't 'closed'. As long as the "social distancing guidelines" are followed, the parks are open, as are almost all outdoor places where RC is done (one exception is the airfield between San Jose & Morgan Hill, but that's closed for a different reason). Where I usually go in Campbell is open, as is Sprekels Lake in Golden Gate Park (San Francisco, which is one of only man-made lakes in the US build specifically for RC boating.)

~ More peace, love, laughter & kindness would make the world a MUCH better place [emoji1690]

[jfrabat]

In Panama, women can go out Monday, Wednesday and Friday for 1 hour according to the last digit of their national ID. Men can go out Tuesday and Thursday for one hour. Nobody can go out on weekends. If you get caught out when you should not be, you get arrested and taken to jail (not sure for how long, but not interested in finding out for running my boat!). But that is why cases here are going down, and mortality is low, so I am not complaining. With all due respect, I honestly do not think the US is a good example on how best to manage this pandemic...