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Calculations(will it float?)

Old 05-26-2007, 11:40 AM
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GreenAce92
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Default Calculations(will it float?)

Hello everyone,

I was in need of help figuring out how to calculate how much volume you should have for a boat that weighs a certain amount. Like say your electronics including battery, motor, chip, esc, ets... weigh 5 ounces how much volume should you have. And one more thing, how much bottom heavy should your boat be so it doesnt tip over when put into water?

Thanks,

Green
Old 05-26-2007, 11:42 AM
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Default RE: Calculations(will it float?)

Also what angle should the prop mounted and what COG is good for boats. Im planning on building it out of elmers foam.
Old 05-26-2007, 01:40 PM
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Default RE: Calculations(will it float?)

A gallon of water is 231 cubic inches of volume, and weighs roughly 6 lbs. That is the most I can offer.

Good luck.
Old 05-26-2007, 02:49 PM
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Default RE: Calculations(will it float?)

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Old 05-26-2007, 05:38 PM
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Default RE: Calculations(will it float?)

I will try to keep it simple here, but:

A boat floats, because the fluid in which it is floating offsets the downward pull of gravity and pushes it up. The scientific name for this force, which allows even immense objects to float in liquid, is known as buoyancy.

Archimedes principle states that an object fully or partly immersed in a liquid is buoyed upward by a force equal to the weight of the liquid displaced by that object. The buoyancy of an object is related to its density. The density of an object is defined as, its mass divided by its volume.

If the total area of an object makes contact with the water is large enough, the object floats. The object must make room for its own volume by pushing aside, or displacing, an equivalent volume of liquid. The object is exerting a downward force on the water and the water is therefore exerting a upward force on the object. Mercury for instance has greater density thus gives objects greater buoyancy. Objects of lesser density float on liquids of greater density.

This explains why wood and styrofoam float on water, whereas concrete and steel sink. It also explains why it is possible to make boats out of steel or even concrete. As long as there are portions of the boat below the surface of the water (that are hollow) the effective density of the boat can be less than that of water even though the real density of the material it is made of is greater.

Most of the space in the boat is taken up by air. The air makes the boat less dense than water.

I hope this helps,

-J
Old 05-26-2007, 08:12 PM
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Default RE: Calculations(will it float?)

I see.... Ok it makes sense thanks still what is the COG of the boat? Also what makes a boat fast? a small prop with high rpms or a boat with a large prop and low rpms.
Old 05-26-2007, 09:33 PM
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Default RE: Calculations(will it float?)

I know there are people here who know a lot more about props than I do, cause I know little if nothing about them. (One thing about this place is people sure know how to go fast!)

I never heard of the "cog" of a boat. However, I believe a COG is a small fishing boat.


-J
Old 05-26-2007, 09:47 PM
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Old 05-26-2007, 10:12 PM
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Default RE: Calculations(will it float?)

keeping it simple here, the weight of the boat cannot exceed the weight of water that would fill the volume of the hull. I'm leaving it at that.

Secondly---- cog = center of gravity, the balance point of the boat. fore to aft

third. this is a vague question, many many factors make up how fast a boat goes, from hull design, weight, power, power source, hardware, props. You could figure this out yourself in a few hours on the various boat forums. In general to make a boat go fast you want a moderately shallow vee'd hull, and a power plant that can put out enough power to make it go fast........duh.... Anyway, Let say you're using a 700 series motor, you want it to go fast as possible, you start out with a small prop for safety and work your way up prop sizes till you find the sweet spot, max speed, with the least possible damage to the drivetrain. There obviously are more variable, which you can find in the forums.

I dont mind answering some noob questions, besides lets just say, I wont remember this conversation in the morning so no biggy.
Old 05-27-2007, 12:19 AM
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Default RE: Calculations(will it float?)

GreenAce92 ,

Look, don't get discouraged and please, feel free to ask all the newbie questions you want. Everyone has to start somewhere!

Quicksilvers explanation of buoyancy is even simpler than mine and I envy his simplicity. Also, I should have recognized that COG probably stands for Center of Gravity when discussing model building....Dolt! [] (But it is a small fishing boat too.)

But hey, learning is fun too, and no one is expected to know everything. Thats why these forums are great!

As far as "malcontents" are concerned, do your self a favor and try to ignore them. This is not the first time I have seen those who feel they have cornered the "Fun" factor in the hobby business. (Your not wasting anyone's time.) Banner-ad sponsors, many fellow hobbyist and even myself (for what it is worth) welcome your participation. Who knows you may be the making of a champ someday! Or at least spend a lot of $$ trying...lol.

Hang in there, and not knowing is the first step toward learning!

-J
Old 05-27-2007, 09:00 AM
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Default RE: Calculations(will it float?)

Well alright... THanks to all, i think this will be where this forum will end.
Thanks
Old 05-27-2007, 03:33 PM
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Old 05-27-2007, 05:39 PM
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Default RE: Calculations(will it float?)

Oh Brother!

Who asked YOU?

here are alot of people on here. If you don't like the subject don't answer, or waste you supposed valuable time.

-Later
Old 05-27-2007, 06:41 PM
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Old 05-27-2007, 08:10 PM
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Default RE: Calculations(will it float?)


ORIGINAL: Quicksilver

keeping it simple here, the weight of the boat cannot exceed the weight of water that would fill the volume of the hull. I'm leaving it at that.

Secondly---- cog = center of gravity, the balance point of the boat. fore to aft

third. this is a vague question, many many factors make up how fast a boat goes, from hull design, weight, power, power source, hardware, props. You could figure this out yourself in a few hours on the various boat forums. In general to make a boat go fast you want a moderately shallow vee'd hull, and a power plant that can put out enough power to make it go fast........duh.... Anyway, Let say you're using a 700 series motor, you want it to go fast as possible, you start out with a small prop for safety and work your way up prop sizes till you find the sweet spot, max speed, with the least possible damage to the drivetrain. There obviously are more variable, which you can find in the forums.

I dont mind answering some noob questions, besides lets just say, I wont remember this conversation in the morning so no biggy.

To add to this, COG is not only fore to aft, but side to sise and top to bottom as well. If you have an engine and equipment mounted high in the boat, you center of gravity will be higher and the boat will tend to flip easier. If you have the engine and equipment off to one side, the boat will lean to that side making manuevering more difficult.

To give an example of this, we had a 18' wood speed boat with a 800 pound 5.7L motor in it. The installation resulted in the engine being mounted extremely high due to the angle of the propshaft and location of the engine compartment. The boat reached 50 mph and upon decelleration, it barrel-rolled nearly killing us. After considerable thought, we changed the angle of the shaft, moved the engine as far aft as we could get it and lowered the engine all the way to the hull. The result was no more flips.

There is another term "Center of Balance" which is used more in the fore-aft aspect of the boat. We look for a center of balance at 30% - 33% on most non-hydro boats.

The cenetr of balance could be at 12" from the transom, but the center of gravity will be 12" from the transom and somewhere above that point. It is always a good idea to get your center of gravity as low as possible in your boat to keep it stable. All my boat have the bottom of te engine less than 1/8" from the bottom.
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Old 05-27-2007, 11:01 PM
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Default RE: Calculations(will it float?)

^^^^ very true, I keep my motors at about the same height, just enough room for the cooling coil. For example my modified villain jet boat, has only flipped once, after flying through the air 5 ft over a dock. Mind you that was only because I hit the dock a bit off angle. I still need to take a video of some good ol dock jumping.
Old 05-28-2007, 02:52 AM
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Default RE: Calculations(will it float?)

Awesome Flabum,

Well not that you almost got killed, but for the info!

Surely, it may help save someone else's life. (Especially mine, as Murphy's Law follows me around like a black-cloud!)

I'm glad you and your passengers are OK and no one was seriously hurt. Getting zonked off the head with an 800 pound motor sounds like one heck of a head-ache! Your pic is worth a thousand words and your advice is priceless.

Respectfully Yours,

-J
Old 05-29-2007, 07:43 PM
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Default RE: Calculations(will it float?)

Now what flabum said is what i was asking thanks a lot. Also thanks to everyone else's input.
Also what makes the boats fast like how many rpms does the nitro boats run.

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