Building the FL12 Page 2
Calculating Capacities and Making labels
One of the more important tasks a builder must perform for a small boat
is determining weight and persons capacity, calculating how much flotation
is needed and making a label for the boat. Also attaching a Hull
Identification Number (HIN) is required.
For back yard boat builders in the US, who are building a boat for
themselves, an HIN can be obtained from the state office that registers
boats. When registering the boat tell them it is a home built boat.
They will have you fill out a form declaring that you built the boat, and
assign an HIN.
In Canada this number is available from the Canadian Office of Boating
Safety:
Canadian Office of Boating Safety Regulations
Calculating Capacities and Flotation:
Many people consider
calculating capacities and flotation as some mysterious scientific process
beyond the capabilities of the average boat builder. This is definitely
not true. Anyone with a basic education can do it. If you can add and subtract, multiply and divide, you can calculate
boat capacities and flotation. For a boat like the FL 12 it is even simpler because the shapes are all
squares, rectangles and triangles. You need to be able to find area and
volume.
Area
of any rectangle is the length times width.
Area
of a square is one side times itself (or length times width but in this
case the length equals the width).
The
area of a triangle equals the base of the triangle (the bottom width)
times the height, divided by two.
Volume
involves one more dimension, the thickness or depth. So the volume of a
box is the length of the box times the width times the depth.
The
same applies to a triangular shapes but it depends on whether it is a
three sided shape or a four sided shape.
Any
boat like the FL12 that has a flat bottom, or even a vee bottom can be
divided up into these shapes and the volume of each shape is found. Then the volumes are added up. The total volume times the weight of water (62.4 pounds per cubic
foot) equals the displacement.
Displacement
is the weight of the water that is moved out of the way when you make a
hole in the water by putting a boat in it. Displacement
divided by five is the maximum weight capacity. If you subtract the weight of the engine, battery, and a
portable fuel tank, you get maximum persons capacity. That weight plus 32 and divided by 141 equals the number of person.
Of
course the simple way to do this is put weight in the boat. Add weights until water is about to come in. The total weight is
the displacement weight. Divide the total by five, and that gives you
maximum weight capacity.
Another
easy way to find persons capacity is to put weight on one side of the boat
as far outboard as possible. Add weight until the boat heels to the point where water comes in.
Divide that weight by 0.6 (or multiply by 1.6667) and that is the
maximum persons capacity.
For
the FL 12 I divided the boat into 10 equal sections. Each section forms a
box. Then I measured the
dimensions of each box to get the volume.
Starting
at the bow. The bow section forms a triangular shaped box. Box 1 is 10”
by 18 Â½” by 21 Â½”
10” = 0.8333 ft
18 Â½ “ = 1.54 ft
21 Â½ = 1.79 ft
the formula is 0.8333 X 1.54 / 2 = 0.642
then multiply 1.79 X 0.642 / 2 = 0.5746 = volume of Box 1
Box number two is a little more complicated because one side is large than the
other side. That is the rear side is bigger than the front side.
The
front side is 21 Â½ by 18 Â¼ by 11 Â½. It is not a rectangle, but since two sides are the same length we
can treat it the same as a rectangle by using the average of the length of
the two unequal sides.
21
+ 11 Â½ = 32 Â½
32 Â½ / 2 = 16 Â¼ = 1.354 front area
the
rear side = 28 Â¾ by 22 by 18 Â½
28Â¾ + 22 = 50 Â¾
50 Â¾ / 2 = 25 3/8 = 2.115 rear area
2.115 + 1.354 = 3.469
3.469 / 2 = 1.7345 Avg. area
1.7345 x 18 Â¼ = 1.756 X 1.54 = 2.671 volume of box 2
And so on.
I did it both ways to get a comparison between
using Simpsons Method, (A method engineers use for calculating volumes of
irregular shapes, derived from Calculus)and doing it the simple way by
adding up the shapes of each section of the boat. See
Flotation: Calculation Methods For the FL12 the
Simpson calculations look like this (I did this in an Excel Spreadsheet)
Look at the middle of the table where it is
outlined in red. On the far right are the displacements calculated both
ways and they are within 2 lb. of each other. This is not a significant
difference and makes no difference in the capacities and amount of
flotation.
By Calculating volume; volume equals 2545
lb. = displacement weight
Maximum Weight Capacity = 2445 times 3/10 = 733.5
lb.
Maximum Persons weight = (733.5  25) x 0.90 =
637.5 lb.
Maximum Persons = 638 + 32 / 141 = 4.75 persons
Note: These are calculations for a boat with
2 HP or less. The formulas for a boat greater than 2 HP or an inboard with
level flotation are different but the procedure is the same. see
Calculating Flotation Needs.
Calculations for
Areas of Stations 











Calculations for
displacement weight Using Simpsons Method 
Alternate Method 
Station No. 
Half Area 
Simpsons Function 
Functions 

volume by blocks 
0 
0.3120 
1 
0.312 

sta 

1 
1.0420 
4 
4.168 

bow 
0.2535 
2 
1.5860 
2 
3.172 

01 
1.3940 
3 
1.9900 
4 
7.960 

12 
2.6700 
4 
2.2730 
2 
4.546 

23 
3.5800 
5 
2.4060 
4 
9.624 

34 
4.3570 
6 
2.4450 
2 
4.890 

45 
4.7700 
7 
2.3550 
4 
9.420 

56 
4.8370 
8 
2.0060 
2 
4.012 

67 
4.7990 
9 
1.9570 
4 
7.828 

78 
4.5380 
10 
1.6875 
1 
1.688 

89 
4.1670 





910 
3.6400 
Total Simpsons functions 

57.620 

10T 
1.7483 
volume from simpson 


38.413




vol of transom 


0.375 



stern piece 


1.748 



bow piece 


0.254 



total vol 


40.79 

Total Vol 
40.75 

Distance btwn 





Stations= 1 
Vol 
Added Vol 
Total Vol 
Buoyancy 
Displ 
Simpsons 
0.67 
38.413 
2.377 
40.79 
62.4 
2545.30 
Alternate
Method 


40.75 
62.4 
2543.04 







Weights are in pounds 

Simpsons 

Blocks 

Boat weight 


100 

100 

Max disp boat weight 

2445.30 

2443.04 

Max Weight Capacity 

733.59 

732.91 

Max Persons Capacity 

637.73 

637.12 

Max Persons 

4.75 

4.75 







K for Ply 
HP = 2 
Engine Weight = 25 lb 
Buoyancy = 60.4 
Hull Weight = Wh 
= 0.81 



Wh 
K 

cu Ft 
Flotation for Boat 
Wh X K / 60.4 
100 
0.81 

1.34106 



Engine Wt 



Flotation For Engine 

25 


0.413907 



Persons Wt 



Flotation for Persons 

637.73 


2.639611 







Total Flotation 




1.712458 







Actual installed flotation 2
ft X 8 ft X 0.167 ft = 



Aft 
Mid 
forward 
Total 



0.75 
1.2 
0.722 
2.672 



Making a label. You can create the label on
any inkjet printer. You first create it in Microsoft Word, or other
word processor and then print it out. See what the labels look like at
Labels.
You can download a word doc that has a blank label here.
1. Replace the numbers with the correct
capacities.
2. Save Document
3. Print
4. Cut out Label
5. Laminate in plastic so it is waterproof.
6. Glue or fasten to boat. It must be permanently affixed.
It should look like this:
REVISED 01/13/2011 © newboatbuilders.com 2010 All rights reserved
