SEI Sterndrives

 

Marine engineer Charles D. Strang Jr. is credited for creating the stern drive in 1948 while working as a Research Associate in the Department of Mechanical Engineering at the Massachusetts Institute of Technology, where he first designed a sterndrive on paper. The introduction of the sterndrive gave boaters the opportunity to own a powerboat without the headaches of driveline issues that plagued pleasure boating at that time. The sterndrive offered multiple advantages, and because it was a complete unit there was now no need for the time-sucking engine alignments that boat manufacturers were eager to drop.

 

When discussing sterndrives in mixed company with non-boaters, or even some new boaters whose boats are powered by outboard motors only, the question, "What does 'sterndrive' mean?" often comes up. That's a good question, and one that folks shouldn't feel silly asking; after all, no one exits the womb knowing what sterndrives are. Besides, they're known by multiple names which only adds to the confusion.

 

As for the names, sterndrives are also referred to as inboard-outboards (abbreviated as I/O) or outdrives. The sterndrive consists of two main components, the motor and the outdrive. The outdrive is attached to the outside of the vessel’s transom with the motor attaching to the transom from the inside; an advantageous setup because it allows for more space onboard than an inboard only boat.

 

How Does a Sterndrive Work?

 

All well and good, you may say, but how does a sterndrive work? It's an interesting creature, to say the least, one reason being that the drive unit is both the transmission and propulsion. When the driver of the boat turns the steering wheel the entire drive turns like an outboard motor to direct propeller thrust, all without a rudder, which is not needed.

 

Sterndrives can be raised and lowered and thus provide a range of trim. Being able to raise it when loading and unloading is a good thing for obvious reasons, but being in shallow water is a prime time to raise it as well. Also, when trimmed up the ride will be smoother, and trimmed down means you'll accelerate faster.

 

The sterndrive is exposed to the elements and consists of three main sections; the gimbal housing, the upper unit, and the lower unit. All three sections have parts that wear, can break, and require routine inspection and maintenance. Doing so without fail will literally help keep your boat afloat.

 

Sterndrive Maintenance

 

One of the main causes of dockside sinkings is failure to inspect and/or maintain or replace the sterndrive bellows. The corrugated black rubber stuck to the transom and sterndrive may not look like much, but it's of utmost importance that they are kept free of marine growth, corrosion, and the like.

 

Barnacles and mussels can attach themselves and cut the bellows to the quick, allowing water to get in. The rubber can become brittle over time and crack. According to many manufacturers, bellows should be replaced at least every five years, even if they've had good care and look good because at that age, they could easily fail--and it's not a matter of if but of when.

 

The units with the working parts are quite durable but when they break, housings, parts, and labor can be costly irrespective of the manufacturer. Cost aside, parts availability is a problem for many of the legacy I/O models and so a boat owner's choice might be a completely new replacement, which often outvalues the entire boat.

 

Great. Something Broke. Now What?

 

Fortunately, there is aftermarket support from a company called Stern Drive Engineering or SEI Marine Products for some outdrive and outboard components.Lower units are more susceptible to damage than the upper units. Even a minor incident such as a prop strike can bend a shaft or crack the lower unit housing.

 

SEI offers replacement lower units for some types of outdrives as well as for some outboard motors. Although the upper units are not as exposed as the lower units during operation and are less likely to suffer damage, there are moving parts within and they sometimes meet their fate as well and need replacing. If the entire outdrive is toast, you may well find an SEI outdrive to fit, as there are complete units available. 

 

Replacement parts and housings for upper and lower outdrive units are also available from aftermarket supports such as SEI and others. Like propellers, for instance. An outdrive unit is not complete without a propeller or two. Props that are damaged, bent, or unbalanced will reduce the life of an outdrive considerably, and using them should be completely avoided.

 

Cruise around our site! Whether you need an entire outdrive, upper or lower unit, propeller, or other sterndrive parts, we have something for you here.

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