How to calculate how much flotation your boats need.
The following are simplified methods to calculate the
amount of flotation needed for a boat. The notation used, such as Boat
Weight = W_{b} , is the same as used by ABYC, the Coast Guard and
NMMA, so if you have problems you can call them and they will recognize
what you're talking about. If everybody is speaking the same language it
makes life a little easier.
The first method is for outboard motor boats over 2 hp.
The procedure is basically the same for all boats. You need to find three
things;
The boat weight and how much flotation you need to
support it.
The machinery(engine) weight and how much flotation to
support it.
The persons weight and how much flotation to support it.
I have simplified the calculations but in real life
there is much more that goes into a boat than fiberglass and wood. Things
such as screws, fittings, glue, paint, windshields, consoles, seats, etc.
all add weight. There are several ways to determine the weight of each of
these. The simplest way is to weight each item before you install it and
add up the weights. But you need to know that different materials have
different weights under water. That's what the table is for. It provides
factors for determining what something weighs submerged. For instance;
Fiberglass; weighs about 1/3 under water what it weighs
dry. So the factor is 0.33.
You need to determine where everything will be when the
boat is swamped.
If it will be dry, use the full weight; if wet the
submerged weight.
The engine will not be fully submerged on an outboard so
you need to use a swamped weight. That is what it would weigh if it were
only partially submerged. For that you need to take the swamped weights
from the table.
Use
Table 4 for outboard motor weights. You can also use Table
42 at the bottom of the table page for metric weights, or use
ABYC Standard S30,Outboard Engine and Related Equipment Weights
Available from
ABYC. If using Metric
the value for water is 1000 kg/cubic meter
If you use S30, the engine weights are heavier (because they are more up
to date) than in the USCG table. S30 also includes both 2 cycle and 4
cycle engines. This results in heavier engine weights than the USCG
table. So you will come out with slightly amounts of flotation.
Additionally there is a different formula to calculate flotation for
persons weight for boats with over 550 pounds of persons capacity.
You will see this in the examples.
Method of Calculation with example:
OUTBOARD MOTORBOATS
OVER 2 HP:
14 Foot outboard
40 Horsepower motor
Maximum Weight Capacity = MC = 1400 lb
Maximum Persons Capacity = PC = 900 lb
Boat Weight = 1000 lb
Use 62.4 lb per cubic foot for specific gravity of fresh water.
Use 2 lb density Foam For flotation.
Use 60.4 lbs per cubic foot for dry weight supported by 2 lb foam.
Factors for the Specific Gravity of materials:
Factor: K
Fiberglass = 0.33
Auminum = 0.63
ABS Plastic = 0.11
Oak, white = 0.18
Oak =
0.56
Mahogany = 0.72
Plywood = 0.81
Calculate the amount of Flotation required, Ft,
for The boat using:
Fb = Flotation for boat
Fp = Flotation for people
Fm = Flotation for the outboard engine
Boat (Hull) weight(Wb) = 800 lb
Hull is submerged: use submerged weights (dry weight X factor)
Fiberglass = 600 lb
Wood = 200 lb
Fiberglass
Wood
Boat Weight submerged (600
X K 0.33 )+(
200 X K 0.81) Wb =
36
Deck is not submerged: use dry weight.
Deck weight (Wd) = Fiberglas 100 lb + Wood 100 lb =
100 + 100 Wd =
200
Maximum Persons weight (Wp)
Wp = 900
Swamped motor and controls + Battery Wt = (Wm)
(from column 2 and column 4,
Table 4, 33 CFR 183.) = 352 +
25 Wm = 377
Flotation for Boat Weight (Fb) = (Wb + Wd) / 60.4
(cu ft of 2 lb density closed cell foam)
= 36 + 200
/60.4 = 560/60.4 Fb =
3.9
Dead Weight =
Max Wt Capacity  Persons Capacity  Motor Controls Battery Wt
= Wdw
Motor Controls Battery Wt = From Column 6
Table 4) = 550
Dead Weight = 1400 
900  550
= Wdw =(If zero or neg Wdw = 0) = 0
Flotation for persons Weight (Fp) over 550 lb persons capacity
Fp = (1/2 the first 550 lbs Persons) + (.125 remainder) + .25 dead weight
Fp = (0.5 X 550)+0.125(Wp550)+.25(Wdw) / 60.4 = 275 +
350 +
0 /60.4
Flotation for persons Weight (Fp) Fp
10.35
For boats with less than 550 lb persons capacity
Fp = (0.5 X Wp)+.25 (Wdw)/ 60.4= _______ + ______ / 60.4
Fp = _____
Flotation for swamped motor (Fm)= (Wm) / 60.4
(Wm from
Table 4), 33 CFR 183 column 2)
= 143 / 60.4 = Fm
= 2.37
TOTAL Amount of 2 lb Foam Flotation (in cubic feet)
= Fb + Fp + Fm = Ft = 3.9 + 10.35 + 2.37 = 16.62 Ft = 17
Keep track of total boat weight over a production run. If total boat
weight increases significantly (say more than 50 lbs) you need to add
flotation.
Recalculate total Boat Weight
Weight of Flotation in pounds (Fw)=2 lb/cu ft X Cu Ft=2X 26 = Fw = 52
Total Boat weight = Weight of Flotation + Boat weight=
Fw+ Wb = 52 +
800 = 852
The following is the method for Modified Level
flotation:
OUTBOARD MOTORBOATS LESS THAN 2
HP & MANUAL PROPULSION:
See
Example Calculating Capacities and Making Labels
Fiberglass
Wood
Boat weight (Wb) = ( ____ X K 0.33
) + ( ____ X K 0.81 ) =_______
Deck weight (Wd) (if any) = _____ + _______
=_______
Maximum Persons weight (Wp) =_______
Swamped motor and controls (Wm) =
20 lb
(from column 2 and column 4,
Table 4, 33 CFR 183.)
Dead Weight = Max Wt Capacity  Persons Capacity = (Wdw) =
______
(If zero or negative ignore)
Flotation for Boat Weight (Fb) = (Wb + Wd) / 60.4 =
(cubic feet of 2 lb density closed cell foam)
=_____ X ___ /60.4 =________
Factors for the Specific Gravity of materials:
Factor: K =
Fiberglass = 0.33
Aluminum = 0.63
ABS Plastic = 0.11
Oak, white = 0.18
Oak = 0.56
Mahogany = 0.72
Plywood = 0.81
Note: The requirement for flotation for persons weight is 2/15 of the
persons weight or 0.1333 times the persons weight. But in the Guidelines
the USCG uses 1/4 or 0.25 times the persons weight to provide a safety
factor. This provides a safety factor of 2 or twice the amount of
flotation for persons weight. If you use 2/15 you will have a lower
amount of flotation.
Flotation for persons Weight (Fp): over 550 lb persons capacity
F p = (0.25 X 550) + 0.125(Wp550) + .25(Wdw) / 60.4
= 137.5 + ______ + _____ /60.4 =________
For boats with less than 550 lb persons capacity
Fp = (0.25 X Wp) + .25 (Wdw) /60.4 = Fp = _____ + _____ / 60.4 =_______
Flotation for swamped motor(Fm)=(Wm)/60.4 = _____ / 60.4 = Fm = _______
Amount of Flotation (in cubic feet) = Fb + Fp + Fm = Ft
Ft = _______ + _______ + _______ + _______ = ________
Weight of Flotation in lb.(Wfl)= 2 lb/cu ft x Ft = 2 X _____
=________
Total Boat weight = Weight of Flotation + Boat weight
= Wfl + Wb = _______ + ________
Keep track of total boat weight over a production run. If total boat
weight increases significantly (say more than 50 lbs) you need to add
flotation.
The following method is for Basic Flotaion:
INBOARD MOTORBOATS:
Fiberglass
Wood
Boat weight (Wb) = ( ____ X K 0.33
) + ( ____ X K 0.81 ) = _______
Deck weight (Wd) (if any)
= ________+ _________ = _______
Maximum Persons weight (Wp)
= _______
Swamped motor, drive unit + Battery Wt
(Wm) = _______
(includes motor, drive unit, propeller, shaft, battery & transmission)
Dead Weight = Max Wt Capacity  total dry weight
Wdw = _______
(If zero or negative ignore)
Flotation for Boat Weight (Fb) = (Wb + Wd) / 60.4
(cubic feet of 2 lb density closed cell foam)
=_______ X _____ /60.4 = _______
Factors for the Specific Gravity of materials:
Factor: K =
Fiberglass = 0.33
Aluminum = 0.63
ABS Plastic = 0.11
Oak, white = 0.18
Oak = 0.56
Mahogany = 0.72
Plywood = 0.81
Flotation for persons Weight (Fp) over 550 lb persons capacity
Fp = (0.5 X 550)+0.125(Wp550)+.25(Wdw) / 60.4
= 275 + ________ + _______ / 60.4
Flotation for persons Weight (Fp) Fp = _______
boats with less than 550 lb persons capacity
Fp =(0.5 X Wp)+.25 (Wdw)/60.4 = ______ + ______ / 60.4 = Fp =
_______
Flotation for swamped machinery (Fm) = (Wm) / 60.4 = _______ / 60.4=
Fm = _______
Amount of Flotation (in cubic feet) = Fb + Fp + Fm = Ft
= ______ +_______ + ______ + _______ = _______
Weight of Flotation in pounds (Fw)
= 2 lb/cu ft x Ft = 2 X ____ = _______
Total Boat weight = Weight of Flotation + Boat weight = Fw + Wb =
= ___________ + ___________ = ______
Keep track of total boat weight over a production run. If total boat
weight increases significantly (say more than 50 lbs) you need to add
flotation.
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