Re: Stability and length to beam ratios
As always in boats, you have to be careful about exactly what you are
trying to compare.
Stability works by the lever principle: moment = length * weight.
If you take a Brick and build it 12 feet long instead of 8 on the
same beam, then the length remains the same but the weight is
greater, so the stability is increased. (Weight is not just the
additional weight of the hull, but also of the larger crew you can
carry.)
Suppose you take a design and simply scale it by size, say 20%. Then
the length and beam go up by 20%, the sail area goes up by the square
(44%), and the weight goes up by the cube (73%). Just to add to the
fun, the heeling moment goes up by the cube because the center of
area is 20% higher as well as 44% greater. Confused yet?
The good news is that the form drag has not gone up as much as the
weight because the cross-section has only gone up by 44% and the fact
that the boat is longer means that the water can be pushed out of the
way more gradually. So in fact, the sail area can be reduced a
little, which means that beam and/or weight can be reduced a little,
which means that sail area can be reduced a little more, etc. The
final result is that large boats are usually proportionaly slimmer
than shorter boats and have a higher speed in a good breeze, but are
comparitively slower very light breezes due to higher wetted surface.
Just one more comment: there is a very tricky spot in sailboat design
between boats that are ballasted by crew weight (say 16' and less)
and boats that are ballasted by metal weight (say 20' and above). In
general, for unballasted boats in this middle range (approx 16'-20')
crew weight is too small in proportion to total weight to get
adequate power to carry sail, so the boats either require a lot of
crew (e.g. the Bolger Light Scooner) or have a shortish rig (e.g.
Zephyr).
Peter
trying to compare.
Stability works by the lever principle: moment = length * weight.
If you take a Brick and build it 12 feet long instead of 8 on the
same beam, then the length remains the same but the weight is
greater, so the stability is increased. (Weight is not just the
additional weight of the hull, but also of the larger crew you can
carry.)
Suppose you take a design and simply scale it by size, say 20%. Then
the length and beam go up by 20%, the sail area goes up by the square
(44%), and the weight goes up by the cube (73%). Just to add to the
fun, the heeling moment goes up by the cube because the center of
area is 20% higher as well as 44% greater. Confused yet?
The good news is that the form drag has not gone up as much as the
weight because the cross-section has only gone up by 44% and the fact
that the boat is longer means that the water can be pushed out of the
way more gradually. So in fact, the sail area can be reduced a
little, which means that beam and/or weight can be reduced a little,
which means that sail area can be reduced a little more, etc. The
final result is that large boats are usually proportionaly slimmer
than shorter boats and have a higher speed in a good breeze, but are
comparitively slower very light breezes due to higher wetted surface.
Just one more comment: there is a very tricky spot in sailboat design
between boats that are ballasted by crew weight (say 16' and less)
and boats that are ballasted by metal weight (say 20' and above). In
general, for unballasted boats in this middle range (approx 16'-20')
crew weight is too small in proportion to total weight to get
adequate power to carry sail, so the boats either require a lot of
crew (e.g. the Bolger Light Scooner) or have a shortish rig (e.g.
Zephyr).
Peter
But what about going the other way? Wouldn't a longer (and narrower)
boat also be stiffer compared to it's shorter and (proportionately)
beamier counterpart?
134 West 26th St. 12th Floor
New York, New York 10001
http://www.crumblingempire.com
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boat also be stiffer compared to it's shorter and (proportionately)
beamier counterpart?
>John Teale in his book on boat design shows the stability curves forC.E.P.
>3 boats, 33 feet long with 12.5 ft, 10.75 ft and 7.5 ft beam and 4.5
>tons displacement. Only the narrowest (7.5 ft beam) had a positive
>righting moment at all angles of heal down to 180 degrees. He
>says "a beamy boat will be stiffer that a comparable narrow rival and
>better able to stand up to a press of sail without excessive heel.
>But in the last analysis and a total knock-down the slimmer vessel is
>lakely to right herself whereas the beamier craft may remain capsized
>and upside down.
>
>He also states that lower center of gravity and higher center of
>bouancy (by higher freeboard, cabin top etc.) will give a greater
>range of stability.
>
>Reed
>
>
>
>Bolger rules!!!
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>MA, 01930, Fax: (978) 282-1349
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>
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134 West 26th St. 12th Floor
New York, New York 10001
http://www.crumblingempire.com
(212) 247-0296
L:B would be greater but so would the stability, right?
John Teale in his book on boat design shows the stability curves for
3 boats, 33 feet long with 12.5 ft, 10.75 ft and 7.5 ft beam and 4.5
tons displacement. Only the narrowest (7.5 ft beam) had a positive
righting moment at all angles of heal down to 180 degrees. He
says "a beamy boat will be stiffer that a comparable narrow rival and
better able to stand up to a press of sail without excessive heel.
But in the last analysis and a total knock-down the slimmer vessel is
lakely to right herself whereas the beamier craft may remain capsized
and upside down.
He also states that lower center of gravity and higher center of
bouancy (by higher freeboard, cabin top etc.) will give a greater
range of stability.
Reed
>Hi David,
> Doesn't stability have more to do with the relationship between the
> CG and the CB?
>
John Teale in his book on boat design shows the stability curves for
3 boats, 33 feet long with 12.5 ft, 10.75 ft and 7.5 ft beam and 4.5
tons displacement. Only the narrowest (7.5 ft beam) had a positive
righting moment at all angles of heal down to 180 degrees. He
says "a beamy boat will be stiffer that a comparable narrow rival and
better able to stand up to a press of sail without excessive heel.
But in the last analysis and a total knock-down the slimmer vessel is
lakely to right herself whereas the beamier craft may remain capsized
and upside down.
He also states that lower center of gravity and higher center of
bouancy (by higher freeboard, cabin top etc.) will give a greater
range of stability.
Reed
FBBB --
This discussion of just what a sharpie is has me wondering; it seems
that in more than one post it has been suggested that decreasing the
length to beam ratio would make more stable boat. I'm not quite sure
how this would work.
For example: if you had a brick (8'x4') and stretched it to 12 feet,
the L:B would be greater but so would the stability, right?
Doesn't stability have more to do with the relationship between the
CG and the CB?
Engineers?
YIBB,
David
C.E.P.
134 West 26th St. 12th Floor
New York, New York 10001
http://www.crumblingempire.com
(212) 247-0296
This discussion of just what a sharpie is has me wondering; it seems
that in more than one post it has been suggested that decreasing the
length to beam ratio would make more stable boat. I'm not quite sure
how this would work.
For example: if you had a brick (8'x4') and stretched it to 12 feet,
the L:B would be greater but so would the stability, right?
Doesn't stability have more to do with the relationship between the
CG and the CB?
Engineers?
YIBB,
David
C.E.P.
134 West 26th St. 12th Floor
New York, New York 10001
http://www.crumblingempire.com
(212) 247-0296