No racer here. No desire for it. If the wind.blows the boat goes. If no wind the boat...I’ve waited for over an hour for a. wind line to arrive just to get the boat moving again. If no wind line,then time to fire up the iron genny.
A little baffled by sailboat performance numbers...
- Thread starter rgranger
- Start date
I think there more information to be gleaned from looking at the two separate ratios, D/L and SA/D, rather than aggregating them into a single proxy variable such as #S.
The way I think of D/L and SA/D is as follows
D/L is an important indicator of how fast a hull can go through water if and only if it has enough power.
SA/D is an important indicator of how much horsepower the boat has compared to the water it has to push out of its way.
About D/L
D/L is an indicator of how much water the boat pushes out of its way compare to an estimate of the surface area available for planing. It is a rough indicator of the potential hull speed of the boat. When we talk about hull speed of a boat, were talking about how much water the hull pushes out of its way, how long the “standing wave” wave is and how much energy it would take for the boat to climb over the front of the standing way.
Boats make waves. Waves travel at a certain speed through the water based on wavelength and frequency. (See note 1 below).
The best know formula for the relationships between boat speed and wave lengths is the “hull speed” formula For displacement hulls:
Hullspeed(in knots)=1.34*(LWL^1/2), (where LWL is hull length at the water line).
The “constant” 1.34 works for heavy displacement boats that don’t have enough power to escape their standing wave. Anthony Deane worked this out in 1670 for British Men O'War.
Heavy boats with a large displacement to length ratio (D/L>240) must push a lot of water aside and they create big waves. They are restricted to the theoretical hull speed limit that is defined by the propagation of waves. (See note 1below)
But most modern sailboats go faster than the formula for a displacement hull predicts, provided that the sail plan is powerful enough. If your boat exceeds this old-time Hull speed limit, it has a semi-displacement or planing or wave piercing hull.
A hull is planing when it is supported by the water flow under her hull. When this happens, she is displacing less water, and this will be reflected by the wake. Watch the stern wave. When the crest of the stern wave is aft of the transom, you are going faster than "hull speed." When the stern wave flattens out, you are planing.
In 1994 , a naval architect named Dace Gerr figured out that the the hull speed equation would be more accurate if it reflected the D/L ratio. It was one of the great accomplishments of modern naval architecture.
Eq #1: D/L=(weight/2240)/(0.01 * LWL)^3
Eq#2: S/L = 8.26/(D/L)^0.311
Eq#3: Hullspeed = S/L * LWL^0.5
soooo.. why did I I just write so much stuff about D/L? Because D/L is an important indicator of how fast a hull can go if it has enough power.
About SA/D
SA/D is It’s an indicator of how much power the sail plan generates. It roughly indicates whether the sail plan is powerful enough for the boat to reach its potential top hull speed.
sail area divided by the cubic feet of water the boat displaces (preferably when loaded with the crew and stores it is designed to carry). Then the number is adjusted to reflect square vs cubic feet so it looks more linear and makes it easier to compare boats.
It’s important to know that in an age of square top and huge roach mainsails, the SA ratio is unreliable. It Doesn’t include the area of then roach or square top. It doesn’t differential between high aspect and low aspect sails.
I’m out of time. I hope this makes sense and doesn’t contain any error. I’m writing it off the top of my head.
Notes:
Note 1: For those of you who ever studied the physics of wave propagation, remember the “wave equation” that says velocity of a wave propagating through a medium is proportional to wavelength. The longer the wave length, the fast it propagates through water.
v = f • λ
Note 2. I think that #s number is an attempt to produce a single indicator that reflects D/L and SA/D. But when you look at S#, a lot of the physical properties of the boat are hidden. If you look at DL you can make several generalizations about the physical design and construction materials of the boat. Same goes for SA/D. That is NOT true for S#
It’s a truism that aggregating data points into one derived term reduces the information content. To put it succinctly, the more you aggregate observations and data points, the less information you have . It doesn’t reflect physics directly, it’s just a derivation of two terms, unsupported by data and observation.
Note 3: here’s a link to an article I wrote in 1999 about hullspeed, based on Dave Gerrs work. Back then it was pretty revolutionary!
The way I think of D/L and SA/D is as follows
D/L is an important indicator of how fast a hull can go through water if and only if it has enough power.
SA/D is an important indicator of how much horsepower the boat has compared to the water it has to push out of its way.
About D/L
D/L is an indicator of how much water the boat pushes out of its way compare to an estimate of the surface area available for planing. It is a rough indicator of the potential hull speed of the boat. When we talk about hull speed of a boat, were talking about how much water the hull pushes out of its way, how long the “standing wave” wave is and how much energy it would take for the boat to climb over the front of the standing way.
Boats make waves. Waves travel at a certain speed through the water based on wavelength and frequency. (See note 1 below).
The best know formula for the relationships between boat speed and wave lengths is the “hull speed” formula For displacement hulls:
Hullspeed(in knots)=1.34*(LWL^1/2), (where LWL is hull length at the water line).
The “constant” 1.34 works for heavy displacement boats that don’t have enough power to escape their standing wave. Anthony Deane worked this out in 1670 for British Men O'War.
Heavy boats with a large displacement to length ratio (D/L>240) must push a lot of water aside and they create big waves. They are restricted to the theoretical hull speed limit that is defined by the propagation of waves. (See note 1below)
But most modern sailboats go faster than the formula for a displacement hull predicts, provided that the sail plan is powerful enough. If your boat exceeds this old-time Hull speed limit, it has a semi-displacement or planing or wave piercing hull.
A hull is planing when it is supported by the water flow under her hull. When this happens, she is displacing less water, and this will be reflected by the wake. Watch the stern wave. When the crest of the stern wave is aft of the transom, you are going faster than "hull speed." When the stern wave flattens out, you are planing.
In 1994 , a naval architect named Dace Gerr figured out that the the hull speed equation would be more accurate if it reflected the D/L ratio. It was one of the great accomplishments of modern naval architecture.
Eq #1: D/L=(weight/2240)/(0.01 * LWL)^3
Eq#2: S/L = 8.26/(D/L)^0.311
Eq#3: Hullspeed = S/L * LWL^0.5
soooo.. why did I I just write so much stuff about D/L? Because D/L is an important indicator of how fast a hull can go if it has enough power.
About SA/D
SA/D is It’s an indicator of how much power the sail plan generates. It roughly indicates whether the sail plan is powerful enough for the boat to reach its potential top hull speed.
sail area divided by the cubic feet of water the boat displaces (preferably when loaded with the crew and stores it is designed to carry). Then the number is adjusted to reflect square vs cubic feet so it looks more linear and makes it easier to compare boats.
It’s important to know that in an age of square top and huge roach mainsails, the SA ratio is unreliable. It Doesn’t include the area of then roach or square top. It doesn’t differential between high aspect and low aspect sails.
I’m out of time. I hope this makes sense and doesn’t contain any error. I’m writing it off the top of my head.
Notes:
Note 1: For those of you who ever studied the physics of wave propagation, remember the “wave equation” that says velocity of a wave propagating through a medium is proportional to wavelength. The longer the wave length, the fast it propagates through water.
v = f • λ
Note 2. I think that #s number is an attempt to produce a single indicator that reflects D/L and SA/D. But when you look at S#, a lot of the physical properties of the boat are hidden. If you look at DL you can make several generalizations about the physical design and construction materials of the boat. Same goes for SA/D. That is NOT true for S#
It’s a truism that aggregating data points into one derived term reduces the information content. To put it succinctly, the more you aggregate observations and data points, the less information you have . It doesn’t reflect physics directly, it’s just a derivation of two terms, unsupported by data and observation.
Note 3: here’s a link to an article I wrote in 1999 about hullspeed, based on Dave Gerrs work. Back then it was pretty revolutionary!
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