Level Flotation

Level Flotation:

Applies to:

Monohull outboard boats (not catamarans, trimarans, or pontoon boats.) Mono-hull means that

if you can draw a continuous line around the hull at the waterline when the boat is at rest it’s a

mono-hull. If it makes two or more footprints in the water it’s a multi-hull. Greater than 2

Horsepower (1.49 kw): Less than 20 (6 m) feet (measured on the center line, including rub rails):

It does not include:

♦ amphibious vehicles

♦ canoes

♦ inflatables

♦ kayaks

♦ multihulls

♦ race boats

♦ sailboats

♦ submersibles

♦ surface effect vehicles

This category includes most outboard motor boats.

Requirements: The boat has to float when swamped—that is, full of water—in an upright level

attitude. See the diagrams in each test for specific requirements.

At the end of this lesson in Appendix B, are tables of weights for outboard motors, batteries and

portable fuel tanks. Table 4 is from The US CFR 183, and Tables 1a and 1b are from ABYC Standard S-30.

These tables must be used for calculating capacity, and flotation, and for doing the flotation test. Do not

use the actual weight of any particular outboard. The weights of the heaviest outboards are included in

the tables. This is so no matter which brand the owner of the boat uses, they will have enough flotation

to float level.

To test the boat you need to know the following:

1. The maximum weight capacity (the safe load – covered in Lesson 3) your boat will carry.

2. The weight of the persons. This comes from the persons weight on the capacity label or your

capacity calculations.

3. The machinery weight: the weight of the engine from Table 4 or the ABYC tables

4. These will be used to determine the amount of weight to load into the boat to test it for

flotation.

Level Flotation

Then measure the following reference areas on your boat. Reference areas are specific areas on the

boat where weights representing the load are placed.

The cockpit area. 40% reference area. (The following photos illustrate measuring the 40%

reference area)

Photo #1

Level Flotation

Photo #2

The passenger carrying area: 70% reference areas. The two foot (61 cm) reference areas fore

and aft.

There are three tests to do.

The first is the level flotation test, after an 18 hour soak.

1. Put your boat in the water.

2. Put half the persons weight in the boat assuming the maximum persons weight is less than

550 pounds. If over 550 pounds (249.5 kg), add 0.125 (1/8) times persons weight minus 550) in

the 40% reference area. The Center of Gravity of all the weights needs to be in the 40%

reference area. That doesn’t mean all the weights have to be in the 40% area, just the Center of

Gravity of all the weights.

3. Place weights to simulate the engine in a basket on the transom. You get this weight from

table 4 column 2, Swamped Engine weights, or column 4 from the ABYC table. Why swamped?

Because only the lower portion of the engine will be underwater. The center of gravity of the

engine weights must be no more than three inches aft of the transom and no more than three

inches above the top of the transom.

Level Flotation

4. Place weights to simulate the battery in the position where the battery is normally mounted.

This weight comes from Table 4 Column 4, Submerged Battery Weights, or ABYC table column 8.

If no position for the battery is indicated, put them on the centerline of the boat.

5. Place weights to simulate the full portable full tank, also from Table 4 or the ABYC table, in

the position where the tank would be placed or if no position is indicated, on the centerline. If

the boat has a permanently installed full tank, either fill the tank ¾ full of water, or use weights

to simulate the weight of the tank. These weights should be place so their center of gravity is a

close to where the tanks center of gravity would be.

6. Pull the plug! Let the boat fill up with water.

7. Leave it that way for 18 hours.

Fig 3

The 18 hour soak is primarily designed to get trapped air out of areas on the boat.

To test the boat properly you should drill small holes in areas where air may be trapped, open up the

backs of upholstered seats and open hatches or covers to compartments.

All areas of the boat should be flooded.

Level Flotation

The easiest way to do the 18 hour soak is to prepare the boat in the afternoon, put it in the water at

four or five pm, and let it sit overnight.

You can then test it first thing in the morning.

TEST 1. After the 18 hour soak the boat should float in the following way:

One end out of the water.

The other end 6 (15.24 cm) inches or less under water (or out of the water too)

measured at the 2 foot (61 cm) reference area.

No more than a 10 degree heel angle. (You can buy a device at a hardware store that

measures this. See Photo #4).

TEST 2. The second test is the stability test. You need to do this one twice, once on each side of the

boat.

Place half the weight for persons on one side of the boat. The center of gravity of these

weights has to be in the 70% reference area on that side of the boat. You have a lot of

lee way here. You can move the weights from front to back, or back to front to get the

boat to pass as long as the Center of Gravity is in the 70% area, and 30% of the total

passenger weight is in the 70% area

Take the other half of the passenger weight out of the boat.

Leave the engine, batteries, fuel and control weights where they are.

WARNING! Make sure you have some ropes or straps attached to the boat to keep it from rolling over. If

you don’t have enough flotation to keep it upright, that’s what it will do. Then the weights will fall out

and go to the bottom of the pool or lake. When it settles down it should float like this. (See diagram

below.)

Level Flotation

The final results should be:

One end out of the water.

The other end 12 inches (30.48 cm)or less under water (or out of the water too)

measured at the 2 foot (61 cm) reference area.

No more than a 30 degree heel angle.

Experience has shown that if it goes 30 degrees, it will probably roll all the way over. When finished,

repeat the test on the other side.

Level Flotation

Photo #3 above, Photo #4 below

Level Flotation

TEST 3: The final test is without the passenger weights.

Take all of the passenger weight out of the boat.

Leave the engine batteries and gear weights where they are. Let the boat settle

Fig 5

The final result should be:

One end out of the water.

The other end 6 inches (15.24 cm)or less under water (or out of the water too)

measured at the 2 foot reference (61 cm) area.

No more than a 10 degree heel angle.