Boat Maintenance Tips July '12

We have seen numerous lower unit failures over the last few months and the reasons for these are varied but most, or at least 90 percent, have a common denominator. Collision with a submerged object would be the most obvious cause but, it's not. Most failures are due to severe overheating, not engine or powerhead temperature, but gear case oil reaching temperatures and pressures it was never designed to endure, with the end result being metal fatigue and catastrophic failure.

Gear oils today are better than ever. Manufacturers and aftermarket producers have developed additives that have stronger and more durable characteristics; the oil clings to gears very well, suspends contamination, emulsifies water, and has very high tolerance for heat.

Imagine for a minute an offshore boat running fifty miles at an average rpm of 4200 to 4800. Translate this to 4800 revolutions per minute, 80 revolutions per second, and all this power is being transferred to a gear the size of your fist that internally meshes with another gear to spin the props and push the load. This almost sounds abusive but is normal in an everyday fishing trip for hours and hours on end. These gear cases are operating under the water, and with the correct loading and with outside water temperature providing cooling, they are designed to be very dependable. What awesome technology.

Now imagine a center console tunnel hull boat with a jackplate crossing a flat with the lower unit actually running several inches above the surface of the water at the same 4200 to 4800 rpm level and then add digging mud and sand, thus raising the resistance values or loads to levels they were never intended to handle.

Running in the "jacked up" position, the cooling method is now via convection, a mixture of air and water, which translates to lots of potential for heat buildup. At this point the shearing taking place on the gears and oil are unbelievable and can often exceed the outboard manufacturers and gear lube design parameters .

Collision is covered by your insurance but operating beyond the outboard motor's design characteristics could have an adverse effect on insurance or warranty coverage, and can be very expensive for the owner.

Change gear oil twice a year, pull the prop and check for fishing line behind the thrust washer once a month. Monitor the condition of lower unit lube by loosening the lower unit drain plug checking for water, a white milky fluid and/or metal slivers and fragments. Having a good preventive maintenance schedule in place can add years to your outboard's life while saving you money and downtime.

Have a great season
Chris Mapp
Coastal Bend Marine - Port O'Connor, TX - 361 983 4841 -