How do you calculate ballast?

[Kmot]

der kapitan commented "time to do the ballast calculations" in another thread.

Okay, so how do you do that?

[patmat2350]

Simple! Ballast is what you need to put in the model to make it weigh the right weight.

Easy to calculate if you know what the real boat weighs (assuming the hull is correct in form). Take the real boat total weight, and divide three times by the scale factor (or divide by the scale factor cubed).

Got a 1:8 scale motor boat that weighs 2000 lbs in real life?

2000 /8/8/8 = 2000 / 8^3 = 3.9 lbs.

Weigh your model, and figger out how much more lead to add.

[patmat2350]

Or have a 75,000 ton battleship modeled in 1:96?

Ships are measured in long tons = 2240 lbs.

75000 tons x 2240 lb per ton = 168,000,000 lbs.

168000000 / 96^3 = 190 lbs.

[Umi_Ryuzuki]

How about the volume of your monster springer, calculated to the waterline.
Then the weight of that volume of water...

:confused:

[Kmot]

Okay, so is there a real life Springer I can get a weight from?

I think your method Umi, will be doable. Thanks!

[CG Bob]

On merchant vessels like freighters, fishing boats, and passenger ships you need to use the deadweight tonnage or displacement tonnage. DO NOT USE THE GROSS OR NET REGISTERED TONS (often expressed as measurement tons or cubic meters) - these are a measure of cargo volume, not weight. A gallon of water contains 128 fluid ounces (volume ounces) and weighs 133.4 avoirdupois (weight) ounces (8.34 x 16 ounces).

[Hoghappy]

der kapitan commented "time to do the ballast calculations" in another thread.

Okay, so how do you do that?

If it capsizes....add...if it sinks...subtract! :p

Capt. Slick

[steveciambrone]

Just put it in the tub and add weight.

Steve

[tsenecal]

theoretically, this should work for anything that displaces water...

but it doesn't alway work...

the real alvin supposedly weighs 30,000 lbs.

my 1/8th scale alvin should therefore weigh 58.6 lbs.


it weighs about 22 lbs.

why the 37 lb difference?

if you ask kindly, I might answer :)

(and yes, I know why its so far off.)

[Ghost 2501]

to be honest, when you are about to ballast a model for the first time, simply put radio gear in, batteries into required position and add weight until the model sits level in the water and on its waterline

[johnmca72]

to be honest, when you are about to ballast a model for the first time, simply put radio gear in, batteries into required position and add weight until the model sits level in the water and on its waterline
That's my method. I use lead shotgun pellets in small Ziploc bags strategically placed. Once I've made a final determination, I (usually) dump out the shot & epoxy it into place.

JM

[Kmot]

Okay, the question was:

"how do you calculate ballast?"

The question was not "how do I ballast a model boat?"

I know how to ballast a boat. Like you said, keep adding weight till you get to the waterline. Simple.

The reason I asked about how to calculate ballast was because I was curious how much dead weight will be needed before I ever add a bag of shot, a box of bricks, or whatever.

Sheesh! :p

[patmat2350]

It's a good question though. Many models, especially smaller ones and quicker ones, build close to their finished weight... converted plastic kits especially. Before you go too far in, you want to know what the weight budget is, can make the difference in what kind of motor and batteies you select.

My notes above are fine if you have data on the original and the model is geometrically correct.
For a Springer, easy enough to estimate the volume of water displaced.
For plastic kits, the very first thing I do is mark the bare hull's waterline, float it, and pour in weight until it sits right. Then I weigh the rest of the plastic, and hopefully there's something left over for rc gear. On Lindberg's Robt. E. Lee, there isn't!

PM

[tigerbay]

[This may be the calculation you are looking for.
From the book by Martin Becker.

First find hull dispalcement
There are many complex ways to do this precisley. But they are hard work.

If you know the dispalcement of the orinal vessel you can scale this down.
V (model) = V (original) : m3 (meter cubed)


If you don't then simplest is using the 'block coefficient'. this will gve you an approximate figure.

Do all measurements in centimeters (cm)
Length of the construction waterline (Lcwl)
Maximal breadth at cwl (Bcwl)
total draught (D)
Corrective number (CN) approx 0.65 for fat boats (barges, square riggers), 0.35 for fine boats. This is only and estimation. If you ahve a long sleek deep hull (e.g. J class) you cannot use this method.

Lcwl x Bcwl x D x CN = displacement volume in liters.
I liter of water weighs 1 kg.

Once you have displacement, subtract the total weight of model (hull, superstucture, rigging, batteries, motors, RC gear, etc).

The remaining figure is the amount of ballast you will need.

If you need a more accurate method you can calculate displacemtn from the frame plan. But it is too much like hard work for me.

[woodybob]

I thought we were building model boats here, not rockets... :eek:

[Kmot]

Lcwl x Bcwl x D x CN = displacement volume in liters.
I liter of water weighs 1 kg.


That'll work! Thanks tigerbay. :)

[Kmot]

I have measured my 3X Springer and it is estimated that the hull will displace 99.71 kg (220 lbs) at the water line. :eek:

[nick_75au]

Calculating Springer, The answer is right in front of you :D , full size displacement divided by cube of scale. Going bigger, displacement times cube of scale. Kmots will be 3 * 3 * displacement of average Springer 8lbs = 216 lbs. I know he's already answered it I just couldn't help myself ;)

[tigerbay]

I thought we were building model boats here, not rockets... :eek:

You should see the other more complex calculations mate. :censored: :confused:

[RhumbLine]

Kmot, didn't I see pictures of a boat ramp at hansen? :D

[CG Bob]

I thought we were building model boats here, not rockets... :eek: We are, but it's nice to know how much the finished model will weigh before we start designing and building. Knowing the model weight sometimes stops people from building a model thats way too big for them to lift alone. I was asked to build a 1:10 scale rc model of an Elco 80' PT boat - 8' of model. The ballast/displacemnet/weight calculations show that the model will weigh over 100 pounds to float at the DWL. When I informed the person who wanted the model of the weight, he decided a smaller model was needed and that he would build it himself.

[Kmot]

Kmot, didn't I see pictures of a boat ramp at hansen? :D
Yes indeed! :cool:

[steveciambrone]

Don't always have to ballast to the waterline, real boats and ships vary there displacment every day. Depends on the fuel loaded/consumed, ballast, and weight of the equipment/freight carried. Naval ships are probably pretty consistant but commercial vary quite a bit.
Our research vessel changes reqularly dependent on the testing were doing at the time.

Thanks
Steve

[Kmot]

Yeah, what Steve said! I just lightened my load by 50 lbs! :D