Physics

The Physics behind Cooling the Drive - Where does the heat come from? Before you can begin to evaluate a stern drive cooling system, you first have to understand what is causing the heat. If you can start a fire by rubbing two sticks together, it must be true that friction causes heat. If the faster you rub the sticks together the more heat you create, then it’s the speed or the RPMs, which is causing the greater heat. How does that equate to a stern drive? 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. It doesn’t take a physics professor to figure out that most of the heat generated by a stern drive comes from the FRONT Bearing. SO, 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 you be cooling 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. Now you should be asking, why do most drive cooler manufacturers target the top bearing cap? There is still one more reason to ask the same question. What is the best way to cool the drive oil? - Think of it like this; 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 again, why do most drive cooler manufacturers target the top bearing cap? Why it doesn't make sense to target the top cap but also force the water down the sides? - If you wanted to cool a glass of water you would be better off putting it in the freezer than in the refrigerator. The same is true of cooling your drive, the colder the water the better. Why would you want to pre-heat the water by first passing it over the top bearing cap? - Anytime you force water into a chamber, as many coolers do, you will “break its stream”. Anyone who has used a garden hose has seen the effects of water streaming. Once water has taken a shape it has a tendency to retain that form. That’s why the water from a hose keeps the shape and diameter of the hose until gravity or some other force interrupts it. Once the water is inside this chamber and is forced to exit through a hole or series of holes or even simply by turning 90 degrees and then downward by 90 degrees, the flow has been interrupted twice. This is called “resistance to flow” or “back pressure” and reduces the amount of water that the system can process. The smaller the amount of the water flow, the less efficient the system will be at cooling the drive and if enough back pressure is created, the pick-up port may cause enough drag to slow the boat down. 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. So the better the water pick-up system is the more cooling there will be. Check out the Water Volumes page to see what a drive coolers water flow needs to be.