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Forms to Calculate Flotation Needs |
The following are simplified forms to caculate the amount of flotation needed for a boat. The notation used, such as Boat Weight = Wb , 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 everbody is speaking the same language it makes life a little easier.The first form 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 forms, 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 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 the table for outboard motor weights. Additionally There is a different calculation for boats with over 550 pounds of persons capacity. Watch for that on the form. OUTBOARD MOTORBOATS OVER 2 HP Fiberglass
Wood
Boat weight(Wb) ( 600 X K 0.33 )+( 200 X K 0.81 ) = 360
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
Deck weight (Wd) = 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
Dead Weight
= Max Wt Capacity - Persons Capacity - Motor Controls Battery Wt
=Wdw(If zero or minus ignore)From Column 6 Table 4)1400 -900-550 Wdw= 0
Flotation for Boat Weight (Fb) = (Wb + Wd) / 60.4
(cu ft of 2 lb density closed cell foam)
= 360 + 200 /60.4 = 560/60.4 Fb = 9.27
Flotation for persons Weight (Fp) over 550 lb persons capacity
Fp = (0.5 X 550)+0.125(Wp-550)+.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) = 352 / 60.4 = Fm = 5.8
TOTAL Amount of Flotation (in cubic feet)
= Fb + Fp + Fm = Ft = 9.27 + 10.35 + 5.8 = 25.42 Ft = 26
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
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.
OUTBOARD MOTORBOATS LESS THAN 2 HP & MANUAL PROPULSION
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 =________
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
F p = (0.5 X 550) + 0.125(Wp-550) + .25(Wdw) / 60.4
= 275 + ______ + _____ /60.4 =________
For boats with less than 550 lb persons capacity
Fp = (0.5 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.(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.
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 = _______
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(Wp-550)+.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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