Question about Center of Gravity

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Discussion Overview

The discussion revolves around the center of gravity (CG) of two different speed boat designs, specifically comparing configurations with one fuel tank versus two fuel tanks. Participants explore the implications of CG on stability, theoretical considerations, and practical applications in boat design.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • Some participants suggest that the first boat design, with one fuel tank, may have a lower CG and therefore better stability.
  • Others argue that the second design, with two fuel tanks, could provide more rotational inertia, potentially reducing rolling motion in rough waters.
  • A participant questions the relationship between arm positioning for balance and the concept of CG, seeking clarification on its relevance to boat stability.
  • Some contributions emphasize that the weight of the fuel is relatively insignificant compared to the total weight of the boat, suggesting that design comfort may take precedence over CG considerations.
  • There is mention of the complexities of forces acting on a speed boat in uneven sea conditions, indicating that stability is influenced by multiple factors beyond CG alone.
  • A participant references the concept of metacentric height and its relation to ship stability, suggesting that CG must be considered in conjunction with other stability factors.
  • Some participants express uncertainty about the applicability of theoretical concepts to real-world scenarios, particularly in maritime contexts.
  • There are requests for simplified calculations to compare the two designs, indicating a desire for quantitative analysis.

Areas of Agreement / Disagreement

Participants express differing opinions on which boat design has the better CG for stability, with no consensus reached. The discussion includes multiple competing views and theoretical considerations, leaving the question unresolved.

Contextual Notes

Participants note that the discussion is theoretical and may not account for all practical factors affecting boat stability, such as environmental conditions and design choices. There are also references to the need for calculations that remain unaddressed.

Who May Find This Useful

This discussion may be of interest to individuals studying boat design, stability in engineering contexts, or those involved in maritime engineering.

Harald 1966
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Hi!
I am new to the forum.
I have a short question:
I have uploaded drawings of two "speed boats" with fuel systems.
Which boat got the best CG?
The one with one, or the one with two fuel tanks?

What do you think.
 

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Best meaning lowest?

Well for the CoG weight is very important and missing in the pictures. Still i would say the first ( personal opinion, that has to come out).
 
Yes, I mean lowest! (Best stability.)

Everything is off course theoretical.

The weight of the boat is the same in both examples.
The weight of the fuel is also the same, 2 lb. (Small RC boat)
The area of one big tank is the same as the area of the two small combined, meaning length of all tanks will be the same.

If you ballance on a thin rope, why do you lift arms up and out if best CoG is keeping arms down close to your boudy?

What do you think?
 
Harald 1966 said:
Yes, I mean lowest! (Best stability.)

Everything is off course theoretical.

The weight of the boat is the same in both examples.
The weight of the fuel is also the same, 2 lb. (Small RC boat)
The area of one big tank is the same as the area of the two small combined, meaning length of all tanks will be the same.

If you ballance on a thin rope, why do you lift arms up and out if best CoG is keeping arms down close to your boudy?

What do you think?

In this case the first boat layout is best ( no more personals)
Cause the lower the fuel is the lower the CoG is.
Btw a 2 lb fuel tank is insignifiant considering that the boat is probably 100 times heavier.
Best to just make the fuel tank as you wish ( comfort and overall design where the second layout might be better) and sit in the boat in the lowest position as your 160 lb really make a difference.
 
The total boat is 25 lb included 2 lb fuel,

but why do you take arms up and out to gain ballance and stability?
 
Can anyone please help me compare them by doing a simplyfied calculation?
 
Harald 1966 said:
but why do you take arms up and out to gain ballance and stability?

I don't really understand this question.
 
Theoretically speaking, the first one(fuel being in the center) will be more stable than the second.

However, my answer could be wrong due to the fact that condition may be different on sea. Many other factors must also be take into consideration, one ought to study 'maritime engineering' to have a total grasp on it.

But then again, is there such things as an absolute? I believe not.
 
Harald 1966 said:
but why do you take arms up and out to gain ballance and stability?

If you're walking on a rope your center of gravity will be above your feet, so you
will be unstable anyway. If you stand still you'll fall. To keep upright you have to
move your arms.

If you start falling to the right, you can stick an arm out to the right and move the
rest of your body somewhat to the left. This will move your center of gravity to the
left, so you can stop falling.
 
  • #10
willem2 said:
If you're walking on a rope your center of gravity will be above your feet, so you
will be unstable anyway. If you stand still you'll fall. To keep upright you have to
move your arms.

If you start falling to the right, you can stick an arm out to the right and move the
rest of your body somewhat to the left. This will move your center of gravity to the
left, so you can stop falling.

But does the theory apply on water? That explain why I choose the center one instead
 
  • #11
Consider also the resistance towards a rolling motion of the boat, caused e.g. by a wave pushing it up only on one side. The divided tank will provide more rotational inertia and the boat will be less likely to roll around its axis of motion.

A speed boat experiences very complicated/chaotic forces on an uneven sea. It's probably best to go for a tried and true design, so as not to cause any uneccesary risk. If innovation is not an important design goal, that is.

Torquil
 
  • #12
The article below article discusses stability of ships (and boats). The center of gravity may be above the center of buoyancy, but must be below the metacenter. The distance between the center of gravity and the metacenter is related to the stability of the ship.

http://en.wikipedia.org/wiki/Metacentric_height

Bob S
 
Last edited:
  • #13
Harald 1966 said:
Can anyone please help me compare them by doing a simplyfied calculation?
Why do you need a calculation? Is this a homework problem?
 

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