Powered Ventilation 

TABLE III - 183.610

Columns 1 and 2 were discussed under 183.610(b).

COLUMN 3 - Blower System Output

A blower system includes the items and devices used to convey ventilation air flow into and out of a boat. Examples of such items and devices, but not excluding others, are as follows (NOTE: other items and devices may be part of a blower system):

• Blower(s)
• Ducting
• Terminal fittings
• Cowls, scoops, funnels

The air flow required (Fo) for the exhaust blower system, like the rated blower capacity, is based on the net compartment volume, V, shown in Column 1 of Table 183.610. The blower system output is stated in Column 3 of Table 183.610. These blower system output requirements establish the minimum efficiency permitted (40%) for an exhaust blower system design. Each item or device used in a blower system offers resistance to the air flow available at the blower. Consider the following when designing a blower system:

• Duct resistance
• Duct bend resistance: the tighter the bend, the higher the resistance
• Terminal fittings: end brackets, Y fittings, adapters, etc.
• The distance of the duct opening away from a surface that could obstruct air flow.
• Cowl, scoop or funnel resistance
• Screen resistance
• Dorade box resistance

The above list is not intended to exclude any item or device in the blower system that might offer resistance to air flow.

If more than one blower is used, the blower system output is the total quantity of air from all blowers, in cubic feet per minute, exhausted from the boat. Figure 6 is a graph of the minimum blower system output required versus net compartment volume.

FIGURE 6 - Minimum Blower System Output

EXHAUST BLOWER SYSTEM AIR FLOW DETERMINATION

During the process of rating a blower, curves of blower performance are usually developed and are required to be tested in accordance with the UL 1128 Marine Blower standard. The curves show air flow for various static pressures and record the current and RPM of the blower at these air flows. See Figure 7 for typical blower curves.

VENTILATION SYSTEM DESIGN

In designing a powered ventilation system, it is helpful to have an idea of what the system output might be before the boat is built. The rules of thumb presented here are based on data accumulated from a number of isolated tests. The data is not to be used to determine compliance with the regulation but only to estimate the blower system output. See Table II below.

TABLE IV - Estimated Effect of Blower System Components

NOTE:
Lower resistance items may be selected resulting in an improved system efficiency.

Example: A contemplated blower system has a 5 foot duct (10%), one 90° bend (10%), a clamshell (20%) and a screen (10%). Therefore, the estimated blower system output is 50% less than the blower rated capacity.

Because the performance of axial flow, in-line blowers are highly dependent upon the propeller design and other factors selected by the blower manufacturer, the estimated effects of the airflow resistances in Table II will be unpredictable. If an axial flow blower is installed in the output system, an airflow meter, pitot tube or other system recommended by the blower manufacturer must be used to check the actual output of the ventilation system as installed in the boat.

FIGURE 7- Typical Blower Performance Curves

Obtaining the air flow of an exhaust blower system, using the blower performance curves, is an easy task. The following outlines three methods which may be used on an installed system:

METHOD 1 - Current Measurement.

See Figure 8.

Step 1: Connect an ammeter into the wiring going to the blower.

Step 2: Energize the blower at its nominal voltage; e.g. 12, 24, 32 volts, etc. A rheostat may be needed to control the voltage.

Step 3: Read the current draw in amperes. The engine should not be operating while taking the reading.

Step 4: Enter the performance curves at the determined amperage and read the air flow in cubic feet per minute (c.f.m.)

NOTE:
This current measurement method does not provide accurate results for in-line, axial flow blowers.

FIGURE 8 - Method 1 - Current Measurement

METHOD 2 - RPM Measurement.

See Figure 9

Step 1: Energize the blower at nominal voltage; i.e. 12, 24, 32 volts, etc. A rheostat may be needed to control the voltage.

Step 2: Determine the RPM of the blower. A stroboscope is one instrument that is used to read RPM of rotative machinery. The boat’s engine should not be operating during the testing process. Carefully follow the test procedures supplied with the instrument to avoid a false reading.

Step 3: Enter the performance curves at the determined RPM and read the air flow in cubic feet per minute (c.f.m.)

FIGURE 9 - Method 2 - RPM Measurement

The above are accurate for determining the effective air flow of an exhaust blower system with a squirrel cage (radial fan). There are many instruments that measure air velocity in feet per minute.

METHOD 3 - Air Velocity Measurement

To obtain air flow in cubic feet per minute:

Step 1: Determine the cross section of the duct, at the measuring point, in square feet.

Step 2: Determine the average air velocity across the duct at the measuring point. The air velocity varies from the duct surface to the center of the duct. See Figure 10.

Step 3: Multiplication of the cross section in square feet by the average air velocity in feet per minute will provide the air flow in cubic feet per minute (c.f.m.).

This method depends on the ability to determine an accurate average air velocity. This is probably the only method that will work for axial flow type blowers. It is possible to develop a correlation between this method and Methods 1 or 2 in which case this method may prove satisfactory. If you are familiar with duct air flow theory and the associated formulas, all the above could be used.

FIGURE 10 - Method 3 - Air Velocity Measurement

The purpose of exhausting air is to remove potentially explosive or flammable vapors that accumulate in the engine compartment during normal operation of the boat. It is intended that the ventilation required by this regulation be sufficient to maintain safe operating conditions under normal circumstances. Ventilation cannot be relied upon to remove liquid fuel or all of the vapors that may be present if there is a leak in the fuel system.

It is important to evaluate each engine compartment design and locate the intake opening of the exhaust blower duct so it will be in the best position to remove any collected vapors.

The vapors that occur during normal operation are associated with carburetor boil-off after the engine is turned off. These vapors will flow to and collect in the lowest part of the compartment. The duct connected to the intake side of the blower is used to select the point in a compartment where the compartment air will be exhausted. The regulation requires that the exhaust blower duct opening be located in the lower one-third of the compartment. Refer to 183.630(b). Usual locations include:

• under an engine
• between engine stringers
• at a sump, possibly provided as a bilge water collecting point.

Consideration must be given to the possibility of normal bilge water accumulations covering the intake opening. Normal accumulations of bilge water occur from propeller shaft stuffing box seepage, spray while operating the boat, and rainwater. Water remaining in the boat after a bilge pump completes its normal pumping cycle would be considered normal. The opening of the exhaust blower intake duct must be above this normal level of accumulated bilge water.

The lower end of the ducts should be securely fastened to ensure the system’s continued operation as intended. Normal operation of the boat or routine maintenance on the engine may result in the duct being removed from its intended pickup point.

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