Physics of Cooling Systems

The Physics Behind a Stern Drive Cooling System

Where does the heat come from?

Before you can begin to design a stern drive cooling system, you first have to understand what is causing the heat. There are many heat sources including clutches and universal joints, but most of the heat is caused by the bearings which allow the shafts to spin. On the top half of the drive, there are two main bearings, one at the front, and one at the top. They share an equal load with one exception; depending on the type of drive you have, the front bearing is spinning 1.5 times faster for most Bravo 1s, and 1.83 times faster for Bravo 3s. Most of the heat generated by a stern drive comes from the FRONT Bearing. This led us at Simrek to ask, if the front bearing is responsible for generating 1.5 to 1.83 times the amount of heat of the top bearing, why do most drive cooler manufacturers target the top bearing cap? This increased RPM, along with the amount of extra components that it takes to spin a second propeller in the opposite direction is why the Bravo 3 is the hottest running drive in the MerCruiser line up.

What should be cooled and why?

The component inside the stern drive that is most susceptible to abuse from heat is the oil. It begins to break down at about 300 degrees. The metal components inside the drive can reach temperatures of at least three times that amount before they become affected. The only way the drive components can reach those temperatures is if the oil has broken down and the heat generating friction builds up. If you cool the oil and keep it from breaking down, it will cool and protect the components inside the drive. 

What is the best way to cool the drive oil?

If you wanted to boil water in a pot, would you rather use a pot that is one inch thick or 1/8 of an inch thick. Obviously the water would be heated much faster in the thinner pot. The same principals are true when it comes to cooling something. The top bearing cap is over an inch thick, the sides of the drive are less than 1/8th of an inch thick. The thinner the material, the easier it is to wick the temperature of the drive oil inside through the casing. So the most efficient cooling system would be one that targets the sides of the drive. So, why do most drive cooler manufacturers target the top bearing cap? The other component that is critical for cooling the drive is the amount of water the system can process and apply to the drive casing on a continuous basis. 
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