Cooling Systems

Most engines use "open" cooling systems. The engine draws water in, circulates it to cool the engine, and empties the water through the exhaust system or through a small opening above the water line. If the intake is clogged with debris, or the water pump fails, you will not see a stream of water coming from the opening while the engine is being used.

Some inboard and stern drives operate with a "fresh water" cooling system. This is a closed system that works like the cooling system in an automobile. A heat exchanger cools the water, working like a car's radiator. This cooling system can reduce corrosion when the boat operates in salt water.

Hull Designs

Powerboats have two kinds of hulls—displacement or planing hulls. Powerboats with displacement hulls move through the water, and require more power to push through the water. Powerboats with planing hulls skim over the water and travel at higher speeds. Planing hulls work best when boats operate on calm or flat water. All hulls are displacement hulls when boats run at low speeds.

Powerboats also have five types of hull designs. The following chart outlines advantages and disadvantages of each design:

Types of Hulls




Flat bottom

shallow draft plane easily

excessive pounding at high speeds

jon boats, small utility boats, racing runabouts

Round bottom

move easily through water at slow speeds

somewhat unstable

sailboats, canoes, some trawlers


smooth ride in choppy water

require more power to move at same speeds as flat bottom

some small utility boats and runabouts

Deep vee

smoother ride in choppy water

require even more power than vee hulls

most runabouts cruisers and ships


provide great stability in most conditions

some multi-hull boats have reduced maneuverability

catamarans, trimarans and houseboats

Powerboats come in many types. Utility or jon boats are widely used for fishing and hunting in protected waters. Runabouts are commonly used for fishing, water skiing, and cruising. Cruisers offer more room and special features, such as cuddy cabins, berths, heads, and galleys. Personal watercraft (PWC) are for recreation or light duty.


Be careful of debris in the water. If you clog the cooling water intake, the engine will overheat. You should check cooling systems that empty the water into the exhaust if you accidentally drive the boat through weeds or kelp. To clear the intake, run the engine in reverse gear when in clear water.

Personal watercraft and other jet-drive engines clog easily when you operate them in shallow water.

Prevent internal engine corrosion by flushing out the cooling system with fresh water after you operate your boat in salt water.

Choose your boat carefully

You should select a boat by how you plan to use it. For example, a flat-bottomed boat is not safe on the ocean, and boats with a deep vee hull will have difficulty navigating in shallow water. Choose a boat according to:

• The intended boating activities or special purposes

• The bodies of water where you'll operate the boat

• Your skill level

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