Boat Speed in Small Boats: The Physics of Going Faster

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Boat Speed in Small BoatsThe Physics of Going
Faster

• Paul Miller
• Naval Arch Ocean
• Engineering Dept.
• US Naval Academy

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The Big Picture in Winning Races
Boatspeed A useful application of what you
learned (?) in physics!
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FactDinghy sailors win in more types of boats
than big boat sailors?
Background
Why? Assuming you learned something more than
starts and tactics in college!
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Warning

• Boatspeed Blindness can be detrimental to your
  racing success!

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1996 Intl Canoe WorldsLemon Tree Passage,
Australia
The Start!
The Finish...
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The Key Measurement of Racing Boatspeed

• V Velocity of boat
• Vmg Velocity of boat made good to the next mark
  (sometimes V to windward)
• Which wins boat races?
• They are related by Vmg V cos(f)

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Example Two Boats Beating
In Point Mode V 5.24 knots, f 37 degrees
In Foot Mode V 5.40 knots, f 40 degrees
Which gets to the weather mark first?
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Solution
In Point Mode Vmg 4.18 knots In Foot Mode
Vmg 4.13 knots
On a 1/2 mile beat, the pointer is 6 seconds (3
boatlengths) ahead!
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How do you find the optimum V and pointing angle,
f?
1. Experiment, measure and record (could be
seat of the pants)
2. Two-boat-test for relative improvement (race
experience or practice)
3. Predict using a Velocity Prediction Program
(VPP) (IMS and IRM use a VPP to get ratings)
4. Switch to Naval Architecture as a major...
(My chance to put in a plug!)
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VPP Polar
Provides predicted speeds for all points of sail
for common wind strengths. VPPs are often
customized for different boats types (ex. IACC,
IMS, 12m)
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Basic Physics of Boat Speed

• Fma !
• The sum of the forces equals zero ?F0
• The sum of the moments equals zero ? M0
• or,
• For every action there is an equal and opposite
  reaction.

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Sail Force
Where does the force in the sails come from and
where does it go?
Lift
Drag
Wind
Note the wind is deflected by the sail!
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In Detail
Force Generated by the Sails Mass of Wind x the
amount the wind is decelerated by the
sails versus
Force Generated by the Sails Mass of the boat x
the amount the boat is accelerated,
(Thrust) plus the mass of the water x the
amount the water is accelerated, plus the mass of
air x the amount the air is accelerated (Drag)
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Why is Acceleration Important?
Velocity ?Acceleration dt
Distance ?Velocity dt
And the one that goes the farthest in a given
amount of time, or covers the same amount of
distance in the shortest time wins the race!
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The Goals From Physics Are

• Take as much from the wind as you can
• Reduce the mass of the boat as much as possible
• Disturb the water and wind as little as possible
• All the while making sure you are maximizing Vmg
  rather than V!

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It isnt quite that simple(but its close)!Quiz
1Which is faster?
Boat A 22 feet 4200 lbs 300 sq ft
Boat B 24.5 feet 4200 lbs 300 sq ft
Waterline Length Weight Sail Area
177 PHRF Rating 129 Boat B is 48 seconds per
mile faster!
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Boat A and Boat B
J/24
Express 27
If everything else is equal, the longer boat is
faster!
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The Big Picture in Winning Races
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Sail Force

• Recall that For every action

As the fluid is deflected past the sail, the sail
is deflected the opposite way.
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Sail Force

• The Magnitude of the force is approximated by
  Bernoullis Equation
• F½?(air density)?(wind velocity)2?(Sail
  Area)?(Coef. of Lift)

To get more sail force you can increase any of
these terms!
1. Sail for the puff, or put up more sails... 2.
For most sailors the only legal option is to
adjust the Coefficient of Lift This is
accomplished through sail trim.
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Sail Trim

• The Direction of the Sail Force depends on how
  much Lift and Drag the sail is producing.
• Lift is the force produced perpendicular to the
  wind
• Drag is the force parallel to the wind.

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Quiz 2Which contributes more to boatspeed Lift
or Drag?
Answer Both!
Upwind Goal High Lift Low Drag
Downwind Goal High Lift High Drag
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Upwind Sail Trim

• High Lift
• Full sail
• High Angle of Attack
• Even twist
• Low Drag
• Flat sail
• Low Angle of Attack
• Even twist

Highest Lift
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Downwind Sail Trim

• High Drag and Lift
• Full sail
• High Angle of Attack (near stall on reach,
  stalled on run)
• Even twist

Highest Drag
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Tell-Tales(Results from Wind Tunnel Tests)
High Lift/ Low Drag
High Lift/ High Drag
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Other Sail Controls

• Vang (twist, forestay tension, mast and boom
  bend)
• Outhaul (lower part of the main lift/drag
  control)
• Luff adjustment (flow attachment and lift
  coefficient control)
• Mast bend (spreaders, shroud tension)

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How do you know when to adjust the controls?

• Is the twist even?
• Boom and top batten roughly parallel
• Is the boat overpowered?
• Cant keep it flat, luffing sails
• What are the faster boats doing?
• If they are going faster than you, find out why!

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The Ultimate Sail?
Cogito Current holder of the Little Americas
Cup Routine speeds of 20 knots in 15 knots of
breeze! Worlds Fastest Raceboat
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IACC/Intl Canoe Mast Project
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The Big Picture in Winning Races
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Foil Basics

• ?F0
• So Side Force generated by the sails is balanced
  by the side force (Lift) of the Foils
  (Centerboard and Rudder)

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Foil Lift and DragCenterboard and Rudder

• The same concept as sails
• Bernoullis Eqn for force (Lift or Drag)
  magnitude
• Vector addition of lift and drag components for
  direction
• Goal is high efficiency
• (High Lift/Drag ratio)

Lift and Drag on Foils
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Foil Drag Components

• Friction (Viscosity)
• Pressure (Lift induced, eddies)
• Aspect Ratio (Span2/Area)
• Planform

The Drag Equation from Bernoullis
is Fdrag½(water density)(boat speed)2(Foil
Area)(Coef. of Drag)
The two easily-changed variables are area and Cd!
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Foil Frictional Drag

• Two things for sailors to think about
• Smoothness (1/c Huffman EN245A)
• Smoother the better
• Laminar vs Turbulent
• Min sand w/400 grit
• All coatings were worse
• Area

Polished
Sanded with 180 grit
Cl
Angle of Attack
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Example of Area Reduction
Centerboard area is approximately 10 of the
total wetted surface.
In light air wetted surface drag is
approximately 80 of total drag.
A 420 Running
Raising the board 90 of the way will reduce drag
7! Giving 0.14 kt!
This assumes you dont increase rudder drag due
to loss of steering control!
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Foil Pressure Drag

• Keep angles of attack small so as to stay in low
  drag area of foil performance. (High Lift/Drag
  ratio)

High Drag 8o
In a 420, increasing the rudder angle from 2o to
6o will cost 0.1 kt!
Low Drag 0-2o
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Example of How to Minimize Angle of Attack
Steer with your weight Steer with the
sails This minimizes the foil drag. Think of
the rudder as a brake.
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The Big Picture in Winning Races
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Hull Resistance

• Friction
• Pressure (eddies)
• Wave Making
• Spray

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Typical Dinghy Resistance Curve
420
Intl Canoe
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Hull Friction Drag

• Like foils, make it as smooth as possible! (Min
  400)(Benefit is not as great as foils)
• Reduce area by heel or trim (flat areas out,
  round sections in)

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Hull Pressure Drag

• Reduce eddies by not letting transom drag (look
  for clean flow off stern)
• Move forward if possible

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Hull Wave-Making Drag
Vmg

• To make waves takes a lot of energy!
• Energy used in making waves is based on
• Wave length
• Volume of water displaced

When beating in a 420 in light air, the lighter
crew (50 lbs) is 0.15 knots faster!
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Example of Weight/Length Effect
Cal 20 and Moore 24 (originally) Same Weight and
Sail Area Different Length
Moore 24 is 1.5 minutes a mile faster! Moral is,
Think Light!
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Research in Length
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New Navy 44 Research
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The Big Picture in Winning Races
Saving the best for last!
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StabilityThe most important factor in speed?

• Effect of heel on drag
• Increased yaw moment
• Increased leeway
• Increased rig drag
• Increased wave making

Except in light air and flat bottomed boats, heel
is slow!
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StabilityThe most important factor in speed?

• Effect of heel on thrust
• Reduced sail area
• Reduced rig efficiency

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How stability fits with physics

• ?F0, ? M0

h x SF weight x t
ThrustSF x sin(B) Bsail trim angle
So, Thrust (w x t x sin(B))/h
There will not be a quiz at the end!
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Example Effect of Hiking
How much more sail force can we develop if we
hike just 3 farther out on a 420?
Thrust (w x t x sin(B))/h
If t goes from 3 to 33, then Thrust goes up
1! That gives us 2 boatlengths/beat on a short
course!
If t goes from 3 to 6, Thrust is
doubled! Hence the value of a trapeze!
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Effect of Crew Weight on Speed
Crossover at about the point when whitecaps start
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Nothings new in Naval Architecture
wt
1885 vs 1995
Sliding seat
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So what do you do when you have too much wind,
knowing that heeling is slow?!
From the basic equations... F½(air density)(wind
velocity)2(Sail Area)(Coef. of Lift)
h F w t
Thrust (w t sin(B))/h

• Options
• Decrease Sail Area or Cl- Smaller sail, reef ,
  twist or flatten
• Increase weight or t - Bigger crew or hike
  farther out
• Decrease h - Lower sail or raise centerboard
• Increase B - Lower traveller, barber haul, ease
  sheet, twist sails

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Something new in naval architecture(Actually
proposed by L. F. Herreshoff in 1947)
Canting ballast best uses available weight Note
also bow and stern rudders! V28 kts!
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The Big Picture in Winning Races

• Key points to remember about boatspeed
• Reduce drag of sails, hull and foils
• Wetted surface, rudder angle, sail fullness,
  total boat weight
• Adjust power to match righting moment
• Proper twist
• Hike harder, sail flatter
• Flat is fast and fast is fun!

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Just for fun, what would happen if you got in the
way of a Navy 44?
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Have fun and think fast!