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 ½” by 21 ½”
10” = 0.8333 ft
18 ½ “ = 1.54 ft
21 ½ = 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 ½ by 18 ¼ by 11 ½. 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 ½ = 32 ½
32 ½ / 2 = 16 ¼ = 1.354 front area
the rear side = 28 ¾ by 22 by 18 ½
28 ¾ + 22 = 50 ¾
50 ¾ / 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 ¼ = 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).
Content Continues after Ad 
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


This Web site may contain copyrighted material the use of which has not always been specifically authorized by the copyright owner. I am making such material available in my efforts to advance understanding of educational, economic, and scientific issues, etc. I believe this constitutes a "fair use" of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this Web site is distributed without profit to those who have expressed a prior interest in receiving the included information for nonprofit educational purposes. For more information see: www.law.cornell.edu/uscode/17/107.shtml. If you wish to use copyrighted material from this Web site for purposes of your own that go beyond "fair use", you must obtain permission from the copyright owner. 