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Boat Speed in Small Boats: The Physics of Going Faster

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Boat Speed in Small Boats:The Physics of Going Faster

Paul Miller

Naval Arch & Ocean

Engineering Dept.

US Naval Academy

Boatspeed:

A useful application of what you learned (?) in physics!

Background:

Fact:Dinghy sailors win in more types of boats than big boat sailors?

Why? Assuming you learned something more than starts and tactics in college!

Warning:

“Boatspeed Blindness” can be detrimental to your racing success!

The Start!

The Finish...

- 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)

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?

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!

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!)

Provides predicted speeds for all points of sail for common wind strengths.

VPP’s are often customized for different boats types (ex. IACC, IMS, 12m)

- F=m*a !
- The sum of the forces equals zero F=0
- The sum of the moments equals zero M=0
or,

“For every action there is an equal and opposite reaction.”

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!

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”)

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!

- 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!

Boat “A”

22 feet

4200 lbs

300 sq ft

Boat “B”

24.5 feet

4200 lbs

300 sq ft

Waterline Length

Weight

Sail Area

177PHRF Rating129

Boat B is 48 seconds per mile faster!

J/24

Express 27

If everything else is equal, the longer boat is faster!

The Big Picture in Winning Races

Recall that “For every action…”

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

The Magnitude of the force is approximated by Bernoulli’s Equation:

F=½(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”.

- 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.

Answer: Both!

Upwind Goal:

High Lift & Low Drag

Downwind Goal:

High Lift & High Drag

- High Lift
- Full sail
- High Angle of Attack
- Even twist

- Low Drag
- Flat sail
- Low Angle of Attack
- Even twist

Highest

Lift

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

Highest Drag

High Lift/ Low Drag

High Lift/ High Drag

- 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)

- Is the twist even?
- Boom and top batten roughly parallel

- Is the boat overpowered?
- Can’t keep it flat, luffing sails

- What are the faster boats doing?
- If they are going faster than you, find out why!

Cogito

Current holder of the

“Little America’s Cup”

Routine speeds of 20 knots

in 15 knots of breeze!

“World’s Fastest Raceboat”

The Big Picture in Winning Races

- F=0
- So Side Force generated by the sails is balanced by the side force (Lift) of the Foils (Centerboard and Rudder)

- The same concept as sails
- Bernoulli’s 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

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

The Drag Equation from Bernoulli’s is:

Fdrag=½(water density)(boat speed)2(Foil Area)(Coef. of Drag)

The two easily-changed variables are area and Cd!

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

Polished

Sanded with 180 grit

Cl

Angle of Attack

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 don’t increase rudder drag due to loss of steering control!

- 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

“Steer with your weight”

“Steer with the sails”

This minimizes the foil drag.

Think of the rudder as a brake.

The Big Picture in Winning Races

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

420

Int’l Canoe

- 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)

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

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!

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!”

The Big Picture in Winning Races

Saving the best for last!

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!

StabilityThe most important factor in speed?

Effect of heel on thrust

- Reduced sail area
- Reduced rig efficiency

F=0, M=0

h x SF = weight x t

Thrust=SF x sin(B)

B=sail trim angle

So,

Thrust =(w x t x sin(B))/h

There will not be a quiz at the end!

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 3’3”, 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!

Crossover at about

the point when

whitecaps start

w*t

1885

vs

1995

Sliding seat

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

From the basic equations...

F=½(air density)(wind velocity)2(Sail Area)(Coef. of Lift)

h F = w t

Thrust =(w t sin(B))/h

Canting

ballast

best uses

available

weight

Note also bow

and stern

rudders!

V=28+ kts!

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

- Proper twist
- Hike harder, sail flatter
- “Flat is fast and fast is fun!”