Lesson 3 Outboard Boats Under 20 feet (6m) greater than 2 HP
Outboard Boats under 20 feet (6m) rated for greater than 2 HP. (1.49 kw)
Sec. 183.35 Maximum weight capacity: Outboard boats.
(a) The maximum weight capacity marked on a boat that is designed or intended to use one or more
outboard motors for propulsion must be a number that does not exceed one-fifth of the difference
between its maximum displacement and boat weight.
US: Maximum Weight Capacity = maximum displacement - boat weight /5 = W
*Note in Canada there is a slight variation in this formula. Look up the rules.
Maximum Persons Capacity = Maximum Weight Capacity - Col 6 Table 4.
If this boat has a maximum persons capacity of less than 550 lbs then the test must be performed to
determine if the maximum persons capacity is correct.
Float the boat with all its permanent appurtenances;
(ii) Add, in normal operating positions, the dry motor and control weight, battery weight, and
full portable fuel tank weight, if any, shown in table 4 of Subpart H of this part for the maximum
horsepower capacity marked on the boat. Permanently installed fuel tanks shall be full of fuel;
(iii) Gradually add weights along one outboard extremity of each passenger area, at the height
of the seat nearest the center of that area, but no higher than the height of the gunwale, and
distributed equally forward and aft of that center in a plane parallel to the floorboards until the
boat assumes the maximum list or trim, or both without water coming aboard;
(iv) Compute the persons capacity in pounds using the following formula: Persons capacity =
A/0.6 where A is the total of the weights added in paragraph (a) (2) (iii) of this section.
What is col. 6 Table 4?
Table 4 is contained later on in this lesson. I have taken it from the US Code of Federal Regulations. It is a
compilation of the weights of outboard motors, batteries, and portable fuel tanks. I have also, courtesy
of ABYC, included the ABYC version of these tables which are in both pounds and Kg and much more up
to date.
Every five years, ABYC surveys the engines available in the market and collects data on their weight. The
heaviest engine weights are entered into the table. This ensures that no matter what outboard you put
on a boat, the capacities and flotation will be in compliance. The flotation will be enough to support the
heaviest engine available. Column six is the weights of the engine, battery(s) and full portable fuel tank
added together.
This table also has columns for dry weight, submerged weight and swamped weight.
Dry weight is the actual weight out of the water.
Submerged weight is the weight of the item fully under the water.
Lesson 3 Outboard Boats Under 20 feet (6m) greater than 2 HP
Swamped weight is the weight only partially under the water. This is particularly important for outboard
engines. If the boat has the correct amount of flotation then the head of the outboard should be out of
the water and only the lower unit is submerged. So the term swamped is used, implying that it is not
fully sunk. However, a swamped engine weighs more than a fully submerged engine, so swamped
weights must be used. The same applies to batteries, but not to full portable fuel tank weights because
it is assumed that the tank will float free.
What about boats that have permanent tanks? They are considered part of the boat weight, but since
gasoline does not weigh as much as water the weight difference must be taken into account.
The US Coast Guard has developed what is called a protocol for testing boats for capacity and flotation.
The test protocol is simply the instructions you follow to perform the test correctly. The test method is
not clearly explained in the CFR so a separate protocol had to be developed for conducting the test. The
protocol includes things such as, how to measure the boat, what holes should be plugged, where to drill
holes to allow trapped air to escape, where to place battery weights, and how to handle permanently
installed tanks. There is also a explanation of the Test procedure in ABYC Standard H-5 Boat Load
Capacity and in H-8 Buoyancy in the Event of Swamping.
When the test was first devised most small boats were outboards with portable fuel tanks. Today most
of these boats have permanently installed tanks, and the portable tank is just used as a reserve. The first
protocol, developed by Underwriters Laboratories in the early 1970’s, had the tank filled with gasoline.
This was found to present safety and environmental hazards.
Various alternatives have been developed.
One is to simply remove the tank and replace it with weights. How much weight? Gasoline weighs less
than water, so it actually provides some buoyancy, but still adds weight to the dry boat. So a calculation
must be done to determine the weight. Also the location of the tank has some bearing on the weight. Is
the tank fully submerged, partially submerged or completely out of the water? In the maximum weight
capacity test, and persons capacity tests it is dry, so the full weights are used. But in the flotation tests
any one of the above may occur.
Removing the tank may not be a good alternative. Another alternative is to fill the tank with water. But
water weighs more than gasoline, so how much? Gasoline weighs about 6 lb/gal (750 g/l) but water
weighs 8 lb/gal (1 kg/l). So you have to fill the tank to about three
fourths, or 75% full. Then the problem becomes how to get all the water out after the test is over? This
is more of a problem than it sounds like, so some testers don’t like this idea.
Yet another alternative is to place weights around or on the tank with the center of gravity of these
weights at approximately the same location as the full tank. This is the method most often used.
Lesson 3 Outboard Boats Under 20 feet (6m) greater than 2 HP
The Coast Guard policy on this is:
“Weights equivalent to the weight of fuel are placed on the deck over the center of gravity of
the fuel tank. If the boat fails to comply with flotation requirements, then the fuel tank is filled
to ¾ capacity with water. If the boat still fails to comply, then the tank is completely filled with
water. If the boat still fails to comply, then a failing report is sent to the manufacturer.”