Wansworth
Well-known member
What does that mean..low righting momment?
Boat described as stable and stiff with a LOW righting momment
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Bilgediver
Well-known member
Somebody was lost for words.
Chae_73
Active member
that the force exerted by the keel to return the boat to an upright position is "low". Although compared to what?
A boat with significant form stability but a low ballast ratio could still be "stiff", up to a point....
A boat with significant form stability but a low ballast ratio could still be "stiff", up to a point....
D
Deleted member 36384
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It should mean that actual value of the moment is low, a small number, hence a low force to right the boat. Moment (Nm) is the product of Force (N) and distance (m) from the fulcrum (Nm = N x m). When the boat heals, the moment increases until a maximum moment is arrived at some angle of heal. Beyond that angle of heal the moment starts to decrease until it is at zero i.e. upside down with centre of buoyancy and centre of gravity in line, same as when upright.
A low moment suggests that the boat can capsize very suddenly at some point as the buoyancy force (N) acting at a distance from the centre of gravity (m) gives a small moment, which could easily be exceeded at an additional increase in angle of heal i.e. a wave, shifting weight, a big gust.
Think of standing on a Stand Up Paddle Board, easy to capsize if you put all your weight very near the edge, but in realty quite stable as it is wide, hence stiff, when standing in the normal position.
I may be wrong in this, happy for others to comment and correct.
A low moment suggests that the boat can capsize very suddenly at some point as the buoyancy force (N) acting at a distance from the centre of gravity (m) gives a small moment, which could easily be exceeded at an additional increase in angle of heal i.e. a wave, shifting weight, a big gust.
Think of standing on a Stand Up Paddle Board, easy to capsize if you put all your weight very near the edge, but in realty quite stable as it is wide, hence stiff, when standing in the normal position.
I may be wrong in this, happy for others to comment and correct.
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D
Deleted member 36384
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An explanation with a video of the stability terms stiffness and tenderness that makes better sense:-
GZ Curves
GZ Curves
Laminar Flow
Well-known member
I think that is a misconception. Moment refers to a force times the arm or leverage it is acting upon. On a vessel this arm is commonly referred to as GZ or the horizontal distance between the centre of gravity and the centre of buoyancy. As buoyancy equals displacement it follows that RM (righting moment) is GZ x displacement.
As the location of the CB changes with angle of heel, so does the righting arm and hence the RM.
RM is the force that resists heeling.
A boat we commonly refer to as stiff has a high RM at low angles of heel and is generally the result of form stability rather than weight stability. That does not mean the boat will be stable at higher angles of heel. When G is above CB and GZ is zero, a boat is in equilibrium, beyond that she will capsize.
Conversely, a boat with low initial stability, example: narrow beam and low ballast, may have a high ultimate stability, but might not be considered stiff. She would have low RM, initially, as GZ would be small at low angles of heel since there would be little change (relatively) in GZ.
As the location of the CB changes with angle of heel, so does the righting arm and hence the RM.
RM is the force that resists heeling.
A boat we commonly refer to as stiff has a high RM at low angles of heel and is generally the result of form stability rather than weight stability. That does not mean the boat will be stable at higher angles of heel. When G is above CB and GZ is zero, a boat is in equilibrium, beyond that she will capsize.
Conversely, a boat with low initial stability, example: narrow beam and low ballast, may have a high ultimate stability, but might not be considered stiff. She would have low RM, initially, as GZ would be small at low angles of heel since there would be little change (relatively) in GZ.