How to calculate how much flotation you need in your...

Lesson 5: Level Flotation

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How to calculate how much flotation

you need in your boat.

For those who are math challenged, this lesson may seem a

bit daunting. Actually all you need to know is how to add,

subtract, multiply and divide. The formulas are not that

complex although at first glance they may seem so. If you

completed the lesson on displacement you can do this one.

I try to break it down into simple easy steps. (So I can

do it.) If you have problems contact me for help.

What do I use for flotation?

Most people use polyurethane foam. Some use polystyrene

foam, that is, styrofoam. There are other foams available

such as polyethylene foam but these are more expensive,

although I have seen it in some boats. Others use air

chambers. Air chambers that are integral with the hull are

not allowed for basic or level flotation. But they are

allowed on manually propelled boats, or boats with 2 HP

(1.49 KW) or less. Also, if this is a manually propelled

boat or 2 HP (1.49 KW) or less, you don't have to pass the

test with any of the air chambers punctured. Foam is a good

solution for most boats, but generally people who build

classic wooden boats don’t want plastic foam in their

boats. In that case they can use balsa, or build air

chambers into the boat or use air bags. There are other

solutions as well. Sometimes wooden boats without engines

will pass without any added flotation. However, if you add

an engine, the odds are you will need some flotation to

support the engine.

If you use foam and the foam is in the bilges, or anywhere

else where it can come in contact with oil, gasoline, or

bilge cleaners then it must meet the standards in the

following chart.


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FOAM USED IN THE FOLLOWING PLACES MUST MEET THE PRESCRIBED TESTS:

Engine Room Engine Room

Unless Open to

Atmosphere

Bilge

VAPOR TEST X

24 HR GAS X

30 DAY GAS X

24 HR OIL X

30 DAY OIL X

24 HR BILGE

CLEANER

X

30 DAY BILGE

CLEANER

X

But if you seal the foam in compartments, or in plastic or

fiberglass then you can use any type of foam.

Most builders use 2 pound (91 grams) density foam. That

means that one cubic foot weighs 2 pounds(91 grams) . All

of the examples in this lesson are based on 2 pound (91

grams) density foam.

Two pound density foam will support 62 pounds of weight in

salt water, and 60.4 pounds (27 KG) of weight in fresh

water. Salt water weighs 64 pounds per cubic foot (0.0283

cu m. 29 kg). 64 minus 2 = 62 Fresh water weighs 62.4

pounds per cubic foot. 0.0283 cu m, 28.3 kg) 62.4 minus 2

pounds = 60.4 (27.4 Kg). However, for air chambers you do

not subtract from the density of fresh water. All the

calculations here use 60.4 for fresh water. If the

flotation is calculated for fresh water it will support

more than that in salt water.

Some manufacturers like to use heavier structural foam,

that is, it has better sheer and crushing strength than two

pound, which is easily crushed or broken. In that case they

must know what it weighs, and adjust the formulas for the

density of the foam. For instance, if you used four pound

density foam you would have to subtract four pounds instead

of two from the density of water.


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However, you may achieve flotation in

any way you prefer. You are not required to use foam. You

can use other methods as long as the final result is the

same.

Milk Bottles – clever, but not recommended. The boat

failed due to metal caps on the bottles. Caps rusted and

leaked water into the bottles. Had they used plastic sealed

caps it might have passed the test.

Some builders have used flotation bags. They need to be

automatically inflating, but for really small boats this is

a good option. They also need to be fastened to the boat

in such a way that they won’t move or drift away. Actually

this is the most difficult part to accomplish.


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Two pound density closed cell foam. Recommended! A good example.


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Flotation Calculations

To calculate the flotation needed you need to divide the

calculation up into three parts.

Flotation to support the hull and deck (the boat).

Flotation to support the people.

Flotation to support the engine, batteries and machinery.

Determining the amount of flotation needed: Factors used

in calculating the amount of flotation required:

Wh = Dry weight of hull below swamped waterline

Wd = Dry weight of deck and superstructure

We = Dry weight of factory installed equipment

hardware and accessories.

Wm = Dry weight of motor, batteries, & machinery

K = conversion factors for material from Table I in

ABYC H-8. All materials weigh less when in the

water, and some are actually buoyant, so this needs

to be accounted for. For instance wood adds to the

buoyancy, so if your fiberglass boat has wood in it

then the amount of flotation need is less. The table

gives you the factors to determine this.

B = Buoyancy of flotation material in pounds per cubic

foot. (or kg per cubic meter)

G = A percentage of the weight of Machinery (engines,

sterndrives, outboards, etc., usually 75%) For Outboards

See Appendix A at the end of this lesson.

Fb = Amount of Flotation need to support the swamped boat

Fp = Flotation needed to support propulsion equipment

Fc = Flotation needed to support persons and dead weight

B = 62.4 For Air Chambers (28.3 KG)

B = 60.4 For 2 pound density foam (fresh water)(27.4 KG)


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© 2004 American Boat & Yacht Council, Inc.


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Formulas for determining the amount

of Flotation For Basic Flotation

Fb = (Wh x K1) + (Wd x K2) + (0.69 We) / B

B = 62.4 For Air Chambers

B = 60.4 For 2 pound density foam

The boat is an inboard with Fiberglass Hull and some

plywood floors and other parts.

The fiberglass portion of the boat weighs 600 lb (272.16

kg). The plywood going into the boat weighs 120 lbs (32.66

Kg). Various pieces of hardware weigh 50 lbs (22.7 kg).

The engine and battery weigh 900 lb (408.23 kg). The

maximum weight capacity is 2400 lb (1088.6 Kg) and the fuel

tank holds 40 gallons (151.4 L) of gasoline.

So how do you know what these weights are? There are two

ways to determine this. One is to weigh everything before

it goes into the boat. The other is to calculate the

weights based on the known weights of various materials.

For instance if you know that plywood weighs one half pound

per cubic foot, and you put 240 cubic feet into the boat

then you have 120 lb of plywood. Other materials can be

determined the same way. The weight of the engine comes

from the manufacturer’s engine specifications, and the same

for the battery and other equipment. But by far the

simplest way is to simply weigh everything before it goes

in the boat. Also you should weigh the completed boat and

keep a record of the weight. If you do this and over a

period of time ( a year is a good time) see an increase in

weight then you need to add flotation. Boats rarely get

lighter over a model run. Generally they get heavier as

you add equipment and options.

When calculating weights remember that some of the boat

will be submerged and some won’t. For those parts that are

not submerged use the full weight, do not multiply by the

factor. For parts that are submerged use the factor to

calculate the submerged weights.

BASIC FLOTATION EXAMPLE

Wh = 600 lb (272.16 kg) (Fiberglass)

Wd = 120 lbs (32.66 Kg) (Plywood)


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We = 50 lbs (22.7 kg)(Equipment

hardware and Access.)

Weight of the engine, battery, machinery = 900lb(408.23 kg)

K = Factor for each type of material to find submerged wt.

(see Table 1 H-8)

Fb = Flotation to support the swamped boat

Fb = (Wh x K1) + (Wd xK2) + (0.69 x Wp)

B

Fb = (600x0.33)+(120x -0.81)+(0.69x50)

60.4

Fb = (198 - 97.2 + 34.5)

60.4

Fb = 135.3

60.4

Fb = 2.24 Cubic Feet 2 Lb Density Foam (63.4 cu cm)

Fp = Flotation needed to support propulsion equipment

Engine: Weight of Propulsion System including sterndrive

and battery = 900 LB

Fp = G / B

G = 0.75 x installed weight of sterndrives & inboards

Example: Fp = G/B = (900 x.75) /60.4 = 675/60.4

Fp = 11.18 Cubic Ft 2 lb density foam (317 cu cm)

Fc = Flotation needed to support persons and dead weight.

Maximum Weight Capacity = 2400 lb.

Fuel tank = 40 gals = 240 lbs. (gasoline weighs about 6 lb

per gallon)

Fc = .25(Fc1 + Fc2)

B

Fc1 = Maximum Persons Capacity – Fuel Wt. = 2160 lb.

Fc2 = Maximum Weight Capacity - Maximum Persons Capacity =

2400 -2160 = 240 (Correct to 2400-2400 = 0


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Fc = 0.25 (Fc1 + Fc2) = .25((2400-240)

+ (2400 – 2400)

B 60.4

Fc = 540/60.4 = 8.94 Cubic ft 2 lb density foam (253 cu cm)

Total Basic Flotation Requirements:

Ft = Fb + Fp + Fc

2.24 Cu Ft - Round to 2.3 To float hull and deck

11.18 Cu Ft - Round to 11.2 to support engine

8.94 Cu Ft - Round to 9 to support people & dead wt

Ft = 2.3 + 11.2 + 9 = 22.5 Cubic Ft (637 cu cm)

LEVEL FLOTATION Example

See H-8 page 5, 8.8.2

Outboard Powered boat

HP = 140 (104.39 Kw)

MC = Maximum Weight Capacity = 1600 LB (725.75 kg)

PC = Maximum Persons Capacity = 887 LB (402.34 Kg)

Wh = Dry Weight of Hull Material

Whf = Weight of Fiberglass Hull = 500 Lb (226.8 kg)

Whp = Weight of Plywood in Hull = 220 Lb (99.8 kg)

Wd = Weight of deck & equipment = 185 lb 71.7 Kg)

K = Factor for each material to find submerged weight

Dead weight = Maximum Weight Capacity - Persons Capacity –

Engine weight, battery Controls and fuel tank weight.(from

Table 4 col. 6 or S-30 col. 10)

Fb = Flotation to float the swamped boat

Fb= (Wh x K) + Wd)

B

Where Wh x K = (Whf x Kf) + (Whp x Kp)

Fb= (500x 0.33) + (220 x -0.81) + (185)

60.4

Fb= (165-178.2+185)

60.4

Fb= 171.8/60.4 = 2.85 Cu Ft 2 lb density foam (80.7 cu cm)


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Calculation for flotation to support swamped engine.

HP = 140: see Appendix A for Table 4 33 CFR and ABYC tables

G = swamped weight from ABYC S-30 Table 1a Col. 4, for

outboards = 453 Pounds

Submerged Battery = 25 LB(S-30 Table 1a, Col. 8) (11.3 kg)

Engine:

Fp = G/B

Fp = (453 + 25) / 60.4

Fp = 478 / 60.4

Fp = 7.91 Cu. Ft. 2 lb density foam (224 cu cm)

Calculation for flotation needed to support persons

capacity and deadweight.

MC = Maximum Weight Capacity = 1600 lb.

140 HP Outboard (From table S-30 Col 10) = 713 lb.

PC = Persons Capacity = 887 lb.

Fc = 1/2(First 550 lb of PC) + 1/12 (Persons Capacity -

550) + .25 (Dead Weight) Divided by 60.4. If the Dead

weight is less than zero, that is, negative, use zero (0).

Fc = 1/2 (550)+ 1/12 (887-550) + .25 (deadweight)

60.4

Fc = 0.5(550) + 0.125(887-550) + (0.25(1600-887-740)

60.4

Fc = 275 +42.125 + 0

60.4

Fc = 317.125 /60.4 = 5.25 Cu Ft (148.7 cu cm)

Total Flotation Requirement

2.85 Cu Ft - round to 3.0 - to float deck & hull

7.91 Cu Ft - round to 6.3 - to float engine & equipment

5.25 Cu Ft - round to 5.3 - to float persons and deadweight


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Ft = Fb + Fp + Fc = 3 + 6.3 + 5.6

Ft = 14.6 Cu Ft. (413.4 cu cm)

All of the preceding examples were taken directly from ABYC

H-8 and the engine weights were taken from Table 1a in S-

30.

The amount of flotation for a modified level flotation boat

is calculated the same way as a level flotation boat,

except, there is no battery or fuel tank weight. The

engine weighs 25 lbs.

Sample Problems:

Basic Flotation example

19 foot inboard with fiberglass hull

Wh = Dry weight of hull (fiberglass) = 500 lb (226.8 kg)

Wh = Dry weight of hull (wood) = 200 lb (90.7 kg)

K = conversion factor for fiberglass = 0.33

K = conversion factor for wood = -.81

Wd = Dry Weight of deck = 200 lb (90.7 kg)

Wc = Equipment hard ware and accessories = 62 lb (28.1 kg)

B = Buoyancy of flotation in lb per cubic foot = 60.4

PC = Max Persons Capacity = 1000 lb(453.6 kg)

MC = Max Weight Capacity = 1400 lb(635 kg)

C = Max Weight Capacity–dry wt of engine=700 lb(317.5kg)

Engine (including batteries, drive unit, transmission,

shaft, and propeller) 165 hp inboard

Fuel Capacity = 40 gallons (240 lbs) (151.4 L, 108.9 kg)

There are four steps:

Divide up the boat into three parts, the hull, the

machinery weight, and the persons weight. Calculate the

flotation required for each. Then add up the amounts of

flotation.

Step 1: determine amount of flotation need to support the

boat.

Fb = (Wh X k) + (Wh X K) + Wd + 0.69 W(e)

60.4

Fb = (500 x .33) + (200 x -.81) + 200 + 42.78

60.4


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Fb = 203/60.4

Fb = 4.06 cu ft rounded up to 4.1 cubic ft. (116.1 cu cm)

Step 2: determine the amount of flotation to support the

swamped motor.

G = For inboards and sterndrives, 75% of the installed

weight of the engine, drive and battery (inboard), or the

engine, outdrive and battery (sterndrive) - in pounds to

the nearest whole number;

G = swamped weight of the motor = (.75 X 700)

Fp = G / B

Fp = (.75 X 700)/60.4 = 525/60.4=8.69 = 9 cu ft(256 cu cm)

Step 3: Determine flotation to support persons capacity and

dead weight.

PC = 1000 lb

Fc= .25 ( Fc1 + Fc2) /B

Fc1 = the maximum persons weight capacity minus the fuel

weight (fuel weight = six pounds/gallons times the tank

capacity in gallons).

Fc1 = 1000 -240 = 760

Fc2 = dead weight = maximum weight capacity minus maximum

persons weight. A negative number shall be raised to zero.

Fc2 = 1400 -1000 = 400

Fc = .25 ((1000-240) + (1400 – 1000))/B

= .25(760 + 400)/60.4 = 1160/60.4

= 4.8 Cu ft. round to 5 cu feet.

Step 4. Determine the total flotation material.

Ft = Fb + Fp + Fc = 4.1 + 9 + 5 = 18.1 Cu ft(512.5 cu cm)


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Level Flotation sample.

17 Foot Fiberglass Outboard Runabout

W(h) = Hull Weight = 800 lb (362.9 Kg)

Wd = Deck Weight = 200 lb (60.6 Kg)

(deck weight includes everything above the gunwale

including windshield, deck hardware and accessories)

W(hp)= Weight of wood in the hull = 0

PC = Maximum Persons Capacity = 900 lb (362.9 Kg)

MC = Maximum Weight Capacity = 1400 lb (635 kg)

HP = Maximum Horsepower Rating = 85 HP (63.38 Kw)

K(f) = conversion factor for fiberglass = 0.33

K(w) = conversion factor for wood

B = Buoyancy of flotation in lb per cubic foot = 60.4

C = Maximum Weight Capacity – Dry weight of engine

battery and portable fuel tank weight, see col 6 in Table

4, or col 10 in Table 1a and 1b.

Step 1: determine flotation to support the boat.

K = conversion factor for fiberglass = .33

Fb = (Wh x K) + Wd / 60.4

Where (Wh x K) = ((Whf x Kf) + (Whp x Kp))

There is no wood so Whp X kw = 0

(Wh x k) = (800 x .33)

This is a level flotation boat so the deck is not

submerged. Therefore use the dry weight of the deck.

Wd = 200 lb (60.6 Kg)

Fb = (800 x .33) + 200 / 60.4

Fb = 264 + 200 / 60.4 = 7.68

Flotation to support the boat =7.7 cu ft (218 cu cm)

Step 2: determine flotation to support the swamped motor.

Fp = (S/B)

S = Swamped weight of engine(Table 4 col 2)=352 lb 159.7 kg


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Fp = 352/60.4 = 5.8 cu Ft = 6 cu ft (170 cu cm)

Step 3. determine flotation to support the persons capacity

PC = 900 lb

C = MC – Engine Weight = 1400 – 550 = 850 lb (385.6 kg)

Fc= .5(first 550 lb of PC) +.125(PC-550) + .25(MC-PC-Eng)

60.4

Fc= .5(550) + .125(350) + .25(1400-900-602)

60.4

Fc= (275 + 43.75 + 0)/60.4 = 318.75/60.4 = 5.27

Flotation to support persons capacity= 5.27 cu ft(149 cu m)

Step 4. Determine total Flotation Needed

Ft = Fb + F(m) + F(pc)

Ft = 7.7 + 6 + 5.7 = 19.4 cubic feet (550 cu cm)

Flotation Calculation Data Sheets:

In Appendix B at the end of this lesson are three data

sheets for calculating flotation for outboard boats,

inboard boats, and manually propelled or less than 3 hp

boats. These are intended only as an aide in making

calculations. There may be material on the boat that is

not included on these sheets but that needs to be included

in the calculations.

If you have questions, please contact me.


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Problems that cause the boat to fail

flotation requirements

Use the heaviest production tolerances; Boats grow in

weight. If the boats coming off the line are heavier than

that used in the calculations then the boat will fail.

Watch out for too much flotation. This sounds odd but is

true. Too much flotation in the wrong place can cause the

boat to heel too much, or in some cases roll over.

Be careful of the placement of flotation:

Too much in the bottom can cause the boat to roll over

Too high (out of the water) in the boat. If the boat has

too much flotation down low then flotation that is placed

high on the sides or under the gunwales will not even be

submerged and will provide no buoyancy at all.

Beware of changes in a model without looking at the amount

of flotation. This can result in increased weight or a

different weight distribution which affects the amount and

placement of the flotation.

Problems with foam Flotation material.

Water soaked foams

Foams that don’t foam properly

Too hot

Too cold

Humidity

Dirty &/or un-calibrated guns

Wrong ratios (two part foams)

Follow the foam Manufacturers Instructions


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Flotation Foam The Good, The Bad, the

Ugly


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Photos 1 & 2 are the good foam. The foam has expanded

evenly, formed a nice skin on the surface and is uniform

throughout.


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This is the bad. This foam was too cold. More than likely

it initially foamed up nicely but being against the skin of

the hull (the white shell) it cooled off quickly and shrunk

back down. So the foam is now about half the cubic footage

that it should have been.


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.

This is just plain ugly. This foam got way too hot during

the exotherm, that is, when the chemical reaction occurs it

gives off heat (exotherm). If it gets too hot then the

cells get very thin walls, get brittle and start breaking.

They expand too much, which leaves foam that looks like

broken glass and it crumbles very easily. It also absorbs

water.

Problem solutions

Do a test shot before putting in the foam.

Check temperature of the shop, the chemicals and the boat.

Follow the foam manufacturer’s instructions.

Clean and calibrate the foam gun every single day.

Use block foam if possible.

Encase the foam.

RECALLS: If you get a recall you will have to make the

following choices.

How do you fix the problem?

More foam?

Or less capacity?

Change capacities and the label or add foam?


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Appendix A

Table 4 from 33CFR183

Weights of Outboard Motors and Related Equipment For

Various Boat Horsepower Ratings

Boat Horsepower

Rating

Motor and Control

Weight Battery weight

Full

Portable

Fuel Tank

Weight

column

Dry Swamped Dry Submerged 1+3+5

1 2 3 4 5 6

.01 to 2 25 20 - - - 25

2.1 to 3.9 40 34 - - - 40

4.0 to 7 60 52 - - 25 85

7.1 to 15 90 82 20 11 50 160

15.1 to 25 125 105 45 25 50 220

25.1 to 45 170 143 45 25 100 315

45.1 to 60 235 195 45 25 100 380

60.1 to 80 280 235 45 25 100 425

80.1 to 145 405 352 45 25 100 550

145.1 to 275 430 380 45 25 100 575

275.1 and up 605 538 45 25 100 750

Transoms Designed for Twin Motors

50.1 to 90 340 286 90 50 100 530

90.1 to 120 470 390 90 50 100 660

120.1 to 160 560 470 90 50 100 750

160.1 to 290 810 704 90 50 100 1000

290.1 to 550 860 760 90 50 100 1050

550.1 and up 1210 1076 90 50 100 1400

Below are tables 1a and 1b published in ABYC Industry

Conformity Standard S-30, Outboard Engine and Related

Equipment Weights. It is updated on a five year cycle and

contains the weights for four stroke outboards..


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Appendix B:

Outboard Motorboats Over 2 HP (1.49 KW) Fiberglass Wood

Wh = Boat Weight ________ + ________

Wd = Deck Weight ________ + ________

PC = Maximum persons weight ________

Wp = Swamped Motor & Controls + Battery ________ + ________ = ________

From Table 4 33 CFR or ABYC Tables 1a or 1b

Dead Weight = Maximum weight Capacity – Persons Capacity – Motor Controls Battery

Wdw = MC – PC – Wm Ignore if zero or less.

Fb = Flotation for Boat Weight (Wh + Wd) X K / 60.4 = Fb

Factors: Fiberglass 0.33 (cubic feet of 2 pound density

Aluminum 0.63 closed cell foam)

Plywood -0.81

ABS Plastic 0.11 ______ x _____ x K /60.4 = ________

Oak, White -0.18

Oak -0.56

Mahogany -0.72

Fc = Flotation For Persons Weight

Fc For boats over 550 lb PC = (0.5 X 550) + .125 (PC-550) + .25(Wdw)/60.4

275 + _______ + _______/60.4 = ________

Fc For boats with less than 550 lb PC = (0.5 x PC) + .25(Wdw) / 60.4

________ + ________ / 60.4 = ________

Fp = Flotation for swamped motor (Wp ) / 60.4

Wp comes from Table 4 or ABYC ________/ 60.4 = ________

Tables 1a or 1b

Amount of Flotation in Cubic feet Ft = Fb + Fc + Fp

_______ + _______ + ______ = ________


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Inboard Motorboats Fiberglass Wood

Wh = Boat Weight ________ + ________

Wd = Deck Weight ________ + ________


Wp = dry weight motor + drive + battery = ________

(includes motor, drive, propeller, shaft, battery and transmission)

Fb = Flotation for Boat Weight (Wb + Wd) X K / 60.4 = Fb



Plywood -0.81

ABS Plastic 0.11 ______ x _____ x K /60.4 = ________

Oak, White -0.18

Oak -0.56

Mahogany -0.72


Fc = 0.25 (Fc1 + Fc2) / 60.4

Fc1 = PC – fuel weight 0.25 ( _____ + _______ ) = ________

Fc2 = MC – PC ( ______- _______ ) = ________

Fc = 0.25 (Fc1 + Fc2) / 60.4 0.25 ( _____+ ______ ) /60.4 = ________

Fp = Flotation for swamped motor (0.75 x Wp ) / 60.4

________/ 60.4 = ________


_______ + _______ + ______ = ________


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Outboard Motorboats 2 HP (1.49 KW) or less. Fiberglass Wood

Wh = Boat Weight ________ + ________

Wd = Deck Weight ________ + ________


Wp = Swamped Motor & Controls = ________

From Table 4 33 CFR or ABYC Tables 1a or 1b

Dead Weight = Maximum weight Capacity – Persons Capacity

Wdw = MC – PC Ignore if zero or less.

Fb = Flotation for Boat Weight (Wb + Wd) X K / 60.4 = Fb



Plywood -0.81

ABS Plastic 0.11 ______ x _____ x K /60.4 = ________

Oak, White -0.18

Oak -0.56

Mahogany -0.72


Fc For boats over 550 lb PC = (0.5 X 550) + .125 (PC-550) + .25(Wdw)/60.4

275 + _______ + _______/60.4 = ________

Fc For boats with less than 550 lb PC = (0.5 x PC) + .25(Wdw) / 60.4

________ + ________ / 60.4 = ________

Fp = Flotation for swamped motor (Wp ) / 60.4

Wp comes from Table 4 or ABYC ________/ 60.4 = ________

Tables 1a or 1b


_______ + _______ + ______ = ________

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