Re: [bolger] Design ratios
At 10:16 10/05/00 +1000, you wrote:
displacement of 2m^3.
If you do the calculation my way :10^.5/2^.3333=2.51
Your way: 10/2^.666=6.3
I wonder if, at the end of the day, which ratio is more meaningful!
The point is that it is important to use consistent units, whatever they
are. This is the simple case. Hull speed is more complex and the user will
have a better understanding if he uses the Froude number.
Roger.
>Here we go again.OK. I agree. You win. Considering a boat with a sail area of 10 m^2 and a
>This relates to
displacement of 2m^3.
If you do the calculation my way :10^.5/2^.3333=2.51
Your way: 10/2^.666=6.3
I wonder if, at the end of the day, which ratio is more meaningful!
The point is that it is important to use consistent units, whatever they
are. This is the simple case. Hull speed is more complex and the user will
have a better understanding if he uses the Froude number.
Roger.
Here we go again.
This relates to
1/Roger Dewhurst orig.
#####################################
###########################
#####################
Roger,
guess I didnt make myself clear. I agree totally we must have apples and
apples, in this case we compare two areas. SA/D is an abbreviation...we are
in fact comparing our
sail area (Numerator) with another area derived from outr displacement as
Denominator.
I was commenting on this derrivation, which is the key to the ratio.
I was simply talking about what is behind the
formula, what the chap who invented it was picturing. What I picture,
sitting on my Trailer Sailer example, thinking about the fact that I have an
SA/D of 20.
1/ Obviously, its a dimensionless ratio.
2/ I am comparing my Sail area on the top line(180 ft2 in example) ,
with what must also be an area on the bottom line.
3/ The bottom line takes my Diplacement (1728lbs) and converts it to a
volume of 27 cubic feet.( by dividing by 64 the density of water).
4/ To level the dimensions, we raise this vol to a power of 2/3. This
gives us 9 square feet., our denominator.
All I'm noting is the denominator, in square feet as is the numerator of the
ratio, is the
area of one side of a cube of seawater the same vol as my boat below the WL.
It helps me to imagine what an SA/D of 20 means in itself, apart from the
usual
10-16=cruiser, 16-19=criuser-racer, 19-22=racer-cruiser, 22-30 = racer, 30
plus =.
Bring the Brandy.
Good luck on the water
Jeff Gilbert
ps More grist for the mill:
With comparisons to other boatshttp://www.image-ination.com/sailcalc.html
Able to build up your own database (fee US $20) ...
NW Marine...http://www.nwmarinedesign.com/formular.htm
This relates to
1/Roger Dewhurst orig.
#####################################
>>2. SAIL AREA / DISP RATIO = sail area/(disp/64)^.666cubic
>>2. If the displacement is in pounds division by 64 will reduce it to
>>feet. The ratio then becomes dimensionless (as it should!).2/Jeff Gilbert
###########################
>....what this ratio means.equals
>Imagine reducing your boat to a cube below the water line, and erecting a
>sail whose area equals that of one side of the cube. SA/D is how many times
>bigger in area your sail is.
> eg Your trailer sailer displaces 1728lbs or about 27cubic feet. This
>a box 3ft on edge giving a side of the box at 9 sq ft. Your sail area is180
>sq ft so SA/D is 180/9 or 20.3/Roger
#####################
>Not quite, I think. But perhaps I misunderstand you. The displacement is. SA/D= sail area/(disp/64)^.666
>equivalent to the cube of a linear dimension. The sail area is the square
>of the linear dimension. If the cube root of the volume is thus compared
>with the square root of the sail area we are comparing apples with apples,
>not figs with bloody plums!
Roger,
guess I didnt make myself clear. I agree totally we must have apples and
apples, in this case we compare two areas. SA/D is an abbreviation...we are
in fact comparing our
sail area (Numerator) with another area derived from outr displacement as
Denominator.
I was commenting on this derrivation, which is the key to the ratio.
I was simply talking about what is behind the
formula, what the chap who invented it was picturing. What I picture,
sitting on my Trailer Sailer example, thinking about the fact that I have an
SA/D of 20.
1/ Obviously, its a dimensionless ratio.
2/ I am comparing my Sail area on the top line(180 ft2 in example) ,
with what must also be an area on the bottom line.
3/ The bottom line takes my Diplacement (1728lbs) and converts it to a
volume of 27 cubic feet.( by dividing by 64 the density of water).
4/ To level the dimensions, we raise this vol to a power of 2/3. This
gives us 9 square feet., our denominator.
All I'm noting is the denominator, in square feet as is the numerator of the
ratio, is the
area of one side of a cube of seawater the same vol as my boat below the WL.
It helps me to imagine what an SA/D of 20 means in itself, apart from the
usual
10-16=cruiser, 16-19=criuser-racer, 19-22=racer-cruiser, 22-30 = racer, 30
plus =.
Bring the Brandy.
Good luck on the water
Jeff Gilbert
ps More grist for the mill:
With comparisons to other boatshttp://www.image-ination.com/sailcalc.html
Able to build up your own database (fee US $20) ...
NW Marine...http://www.nwmarinedesign.com/formular.htm
Bill:
[displacement / waterplane area ^(3/2)] while still reflecting his
subjective sense that beam (thus indirectly metacentric height) has a
greater (negative) impact on comfort than length. He also incorporates some
of the effect of overhangs by weighting LOA and LWL.
The whole thing is really tricky because "comfort" and "corkiness" are only
of interest when there is a sea running. Under such condiditions, the
designed waterplane has very little to do with the dynamic waterplane, and a
vessel's displacement varies continuously. Even sailing in smooth water, a
boat's wave train changes these characteristics (waterplane loading) pretty
significantly. Perhaps I think about this stuff too much...
If I were doing my own "comfort ratio" I would probably do a non-dimensional
waterplane loading [displacement^(2/3) / waterplane area] and include a
factor for the ratio of dampening to stiffness [lateral plane area /
metacentric height^2].
Comfort = (D^(2/3) * LPA)/(WPA * GZ^2)
where: D = displacement in cubic feet, LPA = lateral plane area in square
feet, WPA = waterplane area in square feet, and GZ = metacentric height (I
don't recall if this is the right convention or not, my texts are at
home...perhaps GZ is righting moment?)
I think the weakest part of this is the use of lateral plane for the
dampening proxy, anyone have any bright ideas to improve it?
> On the "comfort factor", Weston Farmer, "From My Old Boat Shop", 1996,(like
>Boat House, pp. 91-94, proposes a pound per square foot of waterline area
>(i.e. the area of the plane of the hull's intersection with the waterline)
>criteria; I really don't grasp the concept, but the idea is that a boat
>an ELCO 34' "CRUISETTE", to give his example) should have a disp/ft^2 valuethe
>of 64 (i.e. # water/ft^3, I think(?))...
>
> On a minor mathematical note that I'm sure would be a piece of cake for
>you - Would dividing the displacement in cubic meters by the cube root of
>LWL (in meters?), as suggested by Roger yield the same value of D/L as theBrewer's formula is an attempt to get close to a non-dimensional ratio
>conventional formula?
[displacement / waterplane area ^(3/2)] while still reflecting his
subjective sense that beam (thus indirectly metacentric height) has a
greater (negative) impact on comfort than length. He also incorporates some
of the effect of overhangs by weighting LOA and LWL.
The whole thing is really tricky because "comfort" and "corkiness" are only
of interest when there is a sea running. Under such condiditions, the
designed waterplane has very little to do with the dynamic waterplane, and a
vessel's displacement varies continuously. Even sailing in smooth water, a
boat's wave train changes these characteristics (waterplane loading) pretty
significantly. Perhaps I think about this stuff too much...
If I were doing my own "comfort ratio" I would probably do a non-dimensional
waterplane loading [displacement^(2/3) / waterplane area] and include a
factor for the ratio of dampening to stiffness [lateral plane area /
metacentric height^2].
Comfort = (D^(2/3) * LPA)/(WPA * GZ^2)
where: D = displacement in cubic feet, LPA = lateral plane area in square
feet, WPA = waterplane area in square feet, and GZ = metacentric height (I
don't recall if this is the right convention or not, my texts are at
home...perhaps GZ is righting moment?)
I think the weakest part of this is the use of lateral plane for the
dampening proxy, anyone have any bright ideas to improve it?
At 14:16 9/05/00 +1000, you wrote:
equivalent to the cube of a linear dimension. The sail area is the square
of the linear dimension. If the cube root of the volume is thus compared
with the square root of the sail area we are comparing apples with apples,
not figs with bloody plums!
Roger.
>Not quite, I think. But perhaps I misunderstand you. The displacement is
>----- Original Message -----
>From: Roger Dewhurst <dewhurst@...>
>mentions, among other perf. ratios/ comments,
>
>>2. SAIL AREA / DISP RATIO = sail area/(disp/64)^.666
>>2. If the displacement is in pounds division by 64 will reduce it to cubic
>>feet.
>>The ratio then becomes dimensionless (as it should!).
>
>Roger,
> thanks mate you've finally given me the clue as to what this ratio
>means.
>Imagine reducing your boat to a cube below the water line, and erecting a
>sail whose area equals that of one side of the cube. SA/D is how many times
>bigger in area your sail is.
> eg Your trailer sailer displaces 1728lbs or about 27cubic feet. This equals
>a box 3ft on edge giving a side of the box at 9 sq ft. Your sail area is 180
>sq ft so SA/D is 180/9 or 20.
equivalent to the cube of a linear dimension. The sail area is the square
of the linear dimension. If the cube root of the volume is thus compared
with the square root of the sail area we are comparing apples with apples,
not figs with bloody plums!
Roger.
>
>>5. COMFORT FACTOR = disp/(65*(.7*lwl+.3*loa)*beam^1.33)
>>5. Oh shit!
>
>Well said. This one comes from Ted Brewer, who says (in "Understanding Boat
>Design")... "I dreamed this one up, tongue in cheek, for a magazine article
>some years ago. However, it is now accepted by many...."....."It is based on
>the fact that the quickness of motion or corkiness of a hull in a choppy
>sea is what causes discomfort and seasickness. That corkiness is determined
>by two main factors, the beam of the hull and the area of the waterline"
>Actually the logic you applied to 2/ works here, its basically comparing 2
>displacents, but to be neutral dimensionally it would need a power of 1.5
>methinks (not 1.333).
>Cheers
>Jeff Gilbert
>
>
>
>
>
>
>
>------------------------------------------------------------------------
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>
In a message dated 5/8/00 11:20:10 PM Central Daylight Time,
jgilbert@...writes:
<< >5. COMFORT FACTOR = disp/(65*(.7*lwl+.3*loa)*beam^1.33)
Design")... "I dreamed this one up, tongue in cheek, for a magazine article
some years ago. However, it is now accepted by many.... >>
Hi Jeff!
I was hoping you would respond to this posting! Nice to have someone who
is numerate look at this stuff. I haven't read Brewer's book, but will look
it up soon.
On the "comfort factor", Weston Farmer, "From My Old Boat Shop", 1996,
Boat House, pp. 91-94, proposes a pound per square foot of waterline area
(i.e. the area of the plane of the hull's intersection with the waterline)
criteria; I really don't grasp the concept, but the idea is that a boat (like
an ELCO 34' "CRUISETTE", to give his example) should have a disp/ft^2 value
of 64 (i.e. # water/ft^3, I think(?)); he indicates that different types of
vessels should have different loads, but says anything lighter than this will
feel "corky" and anything heavier the opposite, regardless of metacentric
height. I can't say that I'm persuaded. I have no idea whether there is any
congruence between this formulation and Brewer's tongue-in-cheek formula, but
I think I'll try to run some brute force computations on some examples, if
only to see if they diverge as rapidly and implausibly as Dave Gerr's
displacement SHP curves diverge from Crouch's planing SHP curves for Bolger's
"SNEAKEASY" and its progeny.
On a minor mathematical note that I'm sure would be a piece of cake for
you - Would dividing the displacement in cubic meters by the cube root of the
LWL (in meters?), as suggested by Roger yield the same value of D/L as the
conventional formula?
Bill in MN, still wondering about Scarper Flo's sealing arrangements for the
swing keel.
jgilbert@...writes:
<< >5. COMFORT FACTOR = disp/(65*(.7*lwl+.3*loa)*beam^1.33)
>5. Oh shit!Well said. This one comes from Ted Brewer, who says (in "Understanding Boat
Design")... "I dreamed this one up, tongue in cheek, for a magazine article
some years ago. However, it is now accepted by many.... >>
Hi Jeff!
I was hoping you would respond to this posting! Nice to have someone who
is numerate look at this stuff. I haven't read Brewer's book, but will look
it up soon.
On the "comfort factor", Weston Farmer, "From My Old Boat Shop", 1996,
Boat House, pp. 91-94, proposes a pound per square foot of waterline area
(i.e. the area of the plane of the hull's intersection with the waterline)
criteria; I really don't grasp the concept, but the idea is that a boat (like
an ELCO 34' "CRUISETTE", to give his example) should have a disp/ft^2 value
of 64 (i.e. # water/ft^3, I think(?)); he indicates that different types of
vessels should have different loads, but says anything lighter than this will
feel "corky" and anything heavier the opposite, regardless of metacentric
height. I can't say that I'm persuaded. I have no idea whether there is any
congruence between this formulation and Brewer's tongue-in-cheek formula, but
I think I'll try to run some brute force computations on some examples, if
only to see if they diverge as rapidly and implausibly as Dave Gerr's
displacement SHP curves diverge from Crouch's planing SHP curves for Bolger's
"SNEAKEASY" and its progeny.
On a minor mathematical note that I'm sure would be a piece of cake for
you - Would dividing the displacement in cubic meters by the cube root of the
LWL (in meters?), as suggested by Roger yield the same value of D/L as the
conventional formula?
Bill in MN, still wondering about Scarper Flo's sealing arrangements for the
swing keel.
----- Original Message -----
From: Roger Dewhurst <dewhurst@...>
mentions, among other perf. ratios/ comments,
>2. SAIL AREA / DISP RATIO = sail area/(disp/64)^.666
>2. If the displacement is in pounds division by 64 will reduce it to cubic
>feet.
>The ratio then becomes dimensionless (as it should!).
Roger,
thanks mate you've finally given me the clue as to what this ratio
means.
Imagine reducing your boat to a cube below the water line, and erecting a
sail whose area equals that of one side of the cube. SA/D is how many times
bigger in area your sail is.
eg Your trailer sailer displaces 1728lbs or about 27cubic feet. This equals
a box 3ft on edge giving a side of the box at 9 sq ft. Your sail area is 180
sq ft so SA/D is 180/9 or 20.
>5. COMFORT FACTOR = disp/(65*(.7*lwl+.3*loa)*beam^1.33)
>5. Oh shit!
Well said. This one comes from Ted Brewer, who says (in "Understanding Boat
Design")... "I dreamed this one up, tongue in cheek, for a magazine article
some years ago. However, it is now accepted by many...."....."It is based on
the fact that the quickness of motion or corkiness of a hull in a choppy
sea is what causes discomfort and seasickness. That corkiness is determined
by two main factors, the beam of the hull and the area of the waterline"
Actually the logic you applied to 2/ works here, its basically comparing 2
displacents, but to be neutral dimensionally it would need a power of 1.5
methinks (not 1.333).
Cheers
Jeff Gilbert
In a message dated 5/7/00 7:02:58 PM Central Daylight Time,
dewhurst@...writes:
<< I found the following design ratios.
1. DISPLACEMENT / LENGTH RATIO = disp/2240/(.01*lwl)^3
2. SAIL AREA / DISP RATIO = sail area/(disp/64)^.666
3. VELOCITY RATIO = 1.88*lwl^.5*sail area^.33/disp^.25 / (1.34*lwl^.5)
4. CAPSIZE RISK = beam/(disp/(.9*64))^.333
5. COMFORT FACTOR = disp/(65*(.7*lwl+.3*loa)*beam^1.33)
6. SPEED/LENGTH RATIO = lwl (feet)/(V (kts))^.5 >>
Roger: I'm innumerate, but my unwonted two cents:
1) Displacements for water craft are normally given by weight, not volume in
cubic meters. Undoubtedly one could convert displacement in pounds into
displacement in cubic meters as readily as this formula converts from pounds
to tons, but I don't see how that would simplify things. Whether
displacement in cubic meters divided by the cube root of the lwl produces the
same constant for D/L ratio as this formula seems doubtful to me, but that's
beyond my mathematical competence - but if it doesn't, another conversion
factor would have to be entered to use the derived value in other formulae
and data which are numerous and use the constant as calculated above.
6) In the form given this formula is simply incorrect, IMHO - S/L is properly
the speed in Kts (a non-metric, arbitrary and perhaps inconvenient unit, to
be sure), divided by the square root of the LWL (in feet, an arbitrary, but
sometimes convenient unit) (i.e. Kts/LWL^.5) This formula is one where the
arbitrary units have great beauty for the innumerate (like me) - e.g. - for a
16' boat, LWL^.5 = 4, ergo 4 Kts = S/L 1.0, a displacement sailboat is
unlikely to exceed S/L 1.3 or so, so without even pencil and paper we can
estimate that a 16' displacement sailboat probably cannot exceed 5.2Kts. As
given, LWL divided by the square root of Kts., the quotient is definitely not
the quantity used in the literature as S/L.
I've never seen formulae 3), 4) or 5) and am curious as to what they purport
to represent under what circumstances.
Is it just my perception, or is there some Internet etiquette rule against
citing sources? If not, please share where you found these. Perhaps because
I'm innumerate, I enjoy crunching these numbers on a Lotus spreadsheet and
I'm curious to learn more about what virtues are claimed for these three
formulae.
Bill in MN, 90 F yesterday, 60 F today and no water in Minnehaha Creek!
dewhurst@...writes:
<< I found the following design ratios.
1. DISPLACEMENT / LENGTH RATIO = disp/2240/(.01*lwl)^3
2. SAIL AREA / DISP RATIO = sail area/(disp/64)^.666
3. VELOCITY RATIO = 1.88*lwl^.5*sail area^.33/disp^.25 / (1.34*lwl^.5)
4. CAPSIZE RISK = beam/(disp/(.9*64))^.333
5. COMFORT FACTOR = disp/(65*(.7*lwl+.3*loa)*beam^1.33)
6. SPEED/LENGTH RATIO = lwl (feet)/(V (kts))^.5 >>
Roger: I'm innumerate, but my unwonted two cents:
1) Displacements for water craft are normally given by weight, not volume in
cubic meters. Undoubtedly one could convert displacement in pounds into
displacement in cubic meters as readily as this formula converts from pounds
to tons, but I don't see how that would simplify things. Whether
displacement in cubic meters divided by the cube root of the lwl produces the
same constant for D/L ratio as this formula seems doubtful to me, but that's
beyond my mathematical competence - but if it doesn't, another conversion
factor would have to be entered to use the derived value in other formulae
and data which are numerous and use the constant as calculated above.
6) In the form given this formula is simply incorrect, IMHO - S/L is properly
the speed in Kts (a non-metric, arbitrary and perhaps inconvenient unit, to
be sure), divided by the square root of the LWL (in feet, an arbitrary, but
sometimes convenient unit) (i.e. Kts/LWL^.5) This formula is one where the
arbitrary units have great beauty for the innumerate (like me) - e.g. - for a
16' boat, LWL^.5 = 4, ergo 4 Kts = S/L 1.0, a displacement sailboat is
unlikely to exceed S/L 1.3 or so, so without even pencil and paper we can
estimate that a 16' displacement sailboat probably cannot exceed 5.2Kts. As
given, LWL divided by the square root of Kts., the quotient is definitely not
the quantity used in the literature as S/L.
I've never seen formulae 3), 4) or 5) and am curious as to what they purport
to represent under what circumstances.
Is it just my perception, or is there some Internet etiquette rule against
citing sources? If not, please share where you found these. Perhaps because
I'm innumerate, I enjoy crunching these numbers on a Lotus spreadsheet and
I'm curious to learn more about what virtues are claimed for these three
formulae.
Bill in MN, 90 F yesterday, 60 F today and no water in Minnehaha Creek!