OK – the less of the lower unit in the water, the less drag. I often run a 16"+ transom height on a light boat. The limiting factor is the propeller. It must be deep enough to get traction. It needs the most traction getting on plane. Turning pushes the back of the boat at an angle. This lifts the outside edge of the boat and brings the propeller closer to the surface. For this reason, it is apt to break loose on turns if raised too high. The propeller has a great deal to do with the speed. Thin blades have less drag, but for the same material have less strength and are more easily dammaged. Brass blades are stronger, more rugged and easily repaired. They are also heavier and may hasten shifter damage. Stainless steel is strongest, but expensive to build and repare. A damaged propeller on an outboard is like a flat tire on a truck. The more damage the less efficiency.
What would cause a loss of power in an outboard? Stuck piston rings and scored (Worn) pistons has been discussed and these are usually caught by a compression check. Chrysler published a compression figure for it’s outboards, OMC never did. The figures we guesstimate have been found by trial and error. (Probably more error than necessary.) This cannot guarantee a good crank case compression. Any leaks from loose bearings, dammaged piston skirts, Broken reed valves, bad seals or leaking seams may not show up on a compression test. Bad bearings or components out of alignment may cause horse power robbing friction. Water pump damage or gear case problems may be at fault. Skeg damage may also damage the drive shaft alignment causing excessive friction. The drive shaft breaking at the water pump drive pin hole is a bad sign.
I am sometimes amazed at the amount of damage an outboard can sustain and still run. I had one Big Twin come in with complaints of fouling spark plugs. He had replaced a box of spark plugs over a weekend of fishing. It turned out it was broken bits of needle bearings that were doing the dirty work, but the motor still brought him back to his dock many times. Another was brought in with complaints of an oil leak. The motor seemed to run OK but had a rough idle. I found a broken connecting rod screw head had pushed through the crankcase leaving a 1/2" hole. The hole was covered by the counterweight during crank case compression so it still ran. I was amazed it held together with only one screw holding. Both these powerheads were replaced under warrantee. . . 😉
