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Disclaimer:   I am not a spokesperson for the US Coast Guard or ABYC. For an official interpretation of regulations or standards you must contact the US Coast Guard or other organization referenced..   More.....

I developed an E-course in Capacity and Flotation for Boat Builders, for Professional Boat Builder Magazine. They no longer offer it.  In addition to these web pages  I am offering it as an e-book. Go to Ike's Store to see more
I also have a perfect companion to the Capacity and Flotation Course.  It is an excel spreadsheet that does the calculations to determine the amount of Flotation needed. Easy to use. Go to Ike's Store to see more


Capacity And Flotation For Small Craft: A course in determining values.

Lesson 1: Introduction To Capacity And Flotation

Load Capacity and Flotation for boats


Learn the rules and standards for maximum weight capacity of small boats.

Learn to calculate displacement weight.

Learn to calculate the maximum weight capacity.

Learn how to test a boat to determine the displacement and maximum weight capacity or verify the calculated capacities.

Learn the rules of flotation requirements for different boat types.

Learn to calculate the amounts of flotation needed.

Learn where to install the flotation.

Learn to conduct a flotation test.

Learn about ABYC requirements for other boats.


For small boats, that is, boats under 20 feet (6m) in length, the amount of load they can carry becomes an important factor in the safety of the boat. Overloading a small boat can result in capsizing, swamping, and if the boat is not equipped with flotation, sinking. Standards have been developed to determine the maximum capacities, and they are posted on the boat so a boat operator will know how much they can carry. 

In the United States, Canada, and in European countries, some of these standards are written into the law.  These were taken directly from the American Boat and Yacht Council (ABYC) standards, developed many years ago. They are revised and updated every five years. Generally they apply to mono-hull rowboats, outboard boats, sterndrive boats, and inboards. Additionally, ABYC has developed safe loading standards for various other types of boats, such as canoes, kayaks, pontoon boats, and inflatable boats and similar standards have been developed by the International Organization for Standardization (ISO) and are incorporated in the Recreational Craft Directive (RCD).

For this course I will refer to rules and standards.  Rules are written into law and when applicable must be followed.  Standards are voluntary but in many countries these standards are the accepted industry practice. So even though they are voluntary, a wise boat builder will use them.

When referring to Rules, I will most often cite the US Code of Federal Regulations (CFR).  The US Coast Guard web site is;

If you live in one of the other countries mentioned you should get a copy of the rules that apply in your country. In Canada, they are available from Transport Canada on their Office of Boating Safety web site In Europe and the UK the RCD is available on the web, but there is a fee.

Additionally, I will only deal with recreational (pleasure) boats.  Boats that carry passengers for hire must meet separate rules and in the US, if they carry more than six passengers, they must be inspected by the US Coast Guard. In the UK they must be certified under the MCA standards. Each country has its own governmental agency that regulates boats carrying passengers for hire.

Capacity Requirements:

The basic principles of determining maximum capacities are the same for all boats under 20 feet (6 m).  The maximum capacities are based on the displacement weight of the boat. Displacement weight is the weight of water that a boat would displace if it were pushed down until water started to enter the boat.  It is the amount of weight it would take to sink the boat.

The maximum weight capacity is determined from the displacement weight.  Depending on the type of boat, that amount will be a percentage of the displacement weight. For manually propelled boats it is one third, for outboard boats it is one fifth, and for inboard or sterndrive boats it is one seventh. Put another way, it would take three times the safe load to sink the boat, or would take five times the safe load to sink the boat, or seven times the load capacity to sink the boat.

The persons capacity measured in weight, and in number of people, is then determined based on the maximum weight capacity.  On outboard boats the weight of the engine, the battery, the controls, and a full portable (6 gal) fuel tank is subtracted from the maximum weight capacity to determine the weight of persons. In the US a formula , or in Canada a weight per person, is used to determine the number of people.  The important thing to remember is the number that should not be exceeded is the maximum weight of persons.

Flotation Requirements Introduction:

One of the most effective means of saving lives when a boat swamps, is simply to keep the boat afloat so the passengers have something to hang onto, or sit in.  This keeps them all in one place, may even keep part of their body out of the water preventing hypothermia, and provides a target for rescuers, that is, it is much easier to see a boat than a head sticking out of the water.  Countless lives have been saved by this. Why countless? Because deaths get reported, near misses don’t, so we don’t know the exact numbers.  But, the number of deaths has dropped dramatically since flotation was required so it has to be contributing in a significant way.

The amount of flotation needed is directly determined from three factors:

The weight of the boat.

The weight of the load.

The weight of the engine.

However, since things do not weigh the same underwater as they do in the air some calculations must be done to determine what they do weigh. Then the amount of flotation need to float them is determined.

A table will be provided that shows a factor for each type of material.  The weight of the material dry is multiplied times the weight of the material submerged, or in the case of an outboard, partially submerged, to determine the submerged weight.  The submerged weight is used to determine the amount of flotation.  For materials that are buoyant the factor is negative.

Flotation material is usually foam. Most of the industry uses two pound density foam, which simply means that one cubic foot weighs two pounds.  However, it doesn’t have to be foam. The rules and standards are performance based, that is, they specify what the boat must do, not how it must be done. The boat must float in such and such an attitude, etc, rather than saying you must put so much foam in the boat.  This allows other methods, such as air chambers, to be used.  I have seen everything from foam to milk bottles. However, if you are building a very nice classic wooden boat, you may not want to use foam, (ugh, plastic in a wood boat!) so you can use air chambers, or a light wood such as balsa to achieve the same results.


Displacement weight: The weight of water that a boat would displace if it were pushed down until water started to enter the boat.  It is the amount of weight it would take to sink the boat.

Displace: A boat forms a hole in the water, that is, it moves the water aside, or displaces it. The amount, or volume, of water that would fill this hole, is the amount displaced.

Mono-hull:  A boat that makes a single footprint in the water. If, when sitting at rest with its full load on board, you can draw a single continuous line around the water line of the boat it is a mono-hull. Catamarans, trimarans, pontoon boats, are multi-hulls, not mono-hulls.

The remainder of this course is in Adobe PDF Format. If you do not have Adobe Reader you can download it free from

Lesson 2 Displacement

Lesson 3

Capacity For Monohull Boats Under 20 feet more than 2 HP.

Capacity for Monohull and Manually propelled Boats under 20 feet 2HP or less.

Capacity Labels.

Questions and Answers

Lesson 4


4 Questions and Answers

Lesson 5

Level Flotation

Modified Level Flotation

Basic Flotation

Flotation Calculations

Flotation Formulas

Where do you put the flotation:

Level Flotation Problems:

Question and Answers.

Lesson 6

Other vessel types Canoes, Kayaks, Pontoon boats, Inflatables

Canoes, Kayaks: Horsepower

Pontoon Boats Worksheet


Appendix A USCG Table 4: Weights of Outboard Motors and associated equipment

NOTICE:  As of June 1, 2018, Table 4 will be updated to coincide with ABYC table S-30.  In the future S-30 will be the standard for outboard engine weights. ABYC updates S-30 every five years. It will be the boat manufacturers responsibility to check for updates.

Appendix A  ABYC Table (2005) Weights of Outboard Motors And Associated Equipment

Appendix B  Flotation Worksheets for lesson 5

Appendix C Useful Numbers  2014 09/04/2014 All Rights Reserved. Revised 01/10/2018

American Boat And Yacht Council  Boat Design Net  Wooden Boat Foundation
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