Sterndrive Cooling Systems
Originally popular in the United States, sterndrives have become more popular in Australia in recent years as the market has grown. It's not just the market that has grown; so has the size of the engines. Many sterndrives feature quite large V8 engines, and so they can generate a lot of heat in a closed engine bay, which is why cooling systems are so important. Motor components have to do more than just generate heat, they also have to take it away.
Understanding SterndrivesAt base, a sterndrive is a cross between a traditional inboard and an outboard motor. The engine mounts inside the hull of the boat, while the propeller mounts a moveable leg, much like the bottom of an outboard. It's more stable than an outboard, because the engine goes lower down, but it also offers the low speed handling of vectored thrust, unlike an inboard, which needs steerage way for the rudder to operate.
Open Loop CoolingSterndrives originated in the United States, and engineers primarily developed them for use in freshwater, not saltwater. This meant that designers were able to take advantage of open loop cooling which draws cooling water from its surroundings, and then expels it along with excess heat. It's a simple and effective system, but it was designed without thought for the corrosive effects of saltwater on sterndrive motors and components. Even with freshwater it's important to flush the cooling system regularly, but with saltwater it's vital.
Closed Loop CoolingOne way to deal with the corrosion was the shift to stainless steel and other corrosion-resistant parts, and another was the move to closed loop cooling systems. These systems rely on heat exchangers to transfer heat from the cooling system to the outside water. They are less efficient than open loop systems, but much easier on your engine.
Choosing EnginesWhen looking for complete sterndrive outdrives, it's important to check the cooling system. Open loop cooling systems often have the inlets in the outdrive, so check the condition. You can run an open loop system in saltwater, it simply takes more maintenance.