Pitch, diameter and what the tachometer tells you
Diameter is the circle the blades sweep. Pitch is the theoretical distance the propeller would travel forward in one revolution. Together they decide how hard the engine has to work. Too much pitch and the engine cannot reach the top of its rated RPM range at wide open throttle: it labours, runs hotter, and lives permanently under a load it was never designed to carry. Too little pitch and it over-revs, which the limiter catches but which wastes both fuel and speed.
The test is the tachometer, not the top speed. Every engine has a wide-open-throttle RPM range published by its manufacturer, and the correct propeller puts the engine near the top of that range with a normal load aboard. Roughly every inch of pitch changes engine speed by something like 150 to 200 RPM. That is the arithmetic behind a prop change, and it is why the right propeller for an empty boat is the wrong one for the same hull loaded with six people and gear.
Dings, bent blades and where vibration comes from
Damage starts small. A ding on the leading edge, a curled trailing edge, a blade tip a few thousandths of an inch out of line with the other three: none of it looks like much sitting on the bench. Underwater it matters a great deal, because each blade is supposed to load identically once per revolution. A blade that is out of true loads differently from its neighbours, and that difference becomes a vibration you feel through the deck and the wheel. It gets worse with speed.
Damaged blades also cavitate. Water flow separates off a rough or bent edge, vapour bubbles form and collapse against the metal, and the surface erodes into a pitted, chalky grey patch. Cavitation off a damaged blade costs thrust, makes noise, and slowly eats the propeller. Missing material at the tips reduces effective diameter, which raises engine RPM and hides the original fault behind a tachometer reading that has quietly changed. Blades should be checked with the propeller off, where all of it is visible.
A spun hub, and how it is diagnosed
Most propellers drive through a rubber or plastic hub bonded between the propeller barrel and the splined insert. That hub is a deliberate sacrificial part: it absorbs shock so an impact does not go straight through into the gearcase. When the bond lets go, the engine revs freely and the boat barely moves, most obviously under hard acceleration or in reverse. The classic report from an owner is that the boat suddenly feels as though it is slipping rather than driving.
A spun hub is diagnosed by marking the propeller barrel and the insert with a line across both, running the boat, and then looking at whether those marks still line up. Partial spin is common and intermittent, which is why it gets misdiagnosed as a drive or transmission problem. A hub can be pressed out and a new one fitted in most propellers, which costs far less than a new propeller. The important part is not blaming the gearcase for a rubber part.
Stainless or aluminium, repair or replace
Aluminium propellers are softer, cheaper and easier to repair; they also flex under load, which costs a little performance, and they bend rather than resist when they hit something. Stainless blades are thinner, stiffer and hold their shape, so they deliver more of the engine's output, but stainless does not give way in a strike and what gives way instead is the hub, the shaft or the gearcase. Neither is the right answer everywhere. On a lake with rock, plenty of owners keep an aluminium spare aboard.
Repair is usually the better economics. Straightening, rebuilding a leading edge and re-pitching to a measured specification restores the geometry, and the propeller is balanced afterwards so the blades load evenly again. Repair stops making sense when blade material is badly eroded, when a blade has been welded repeatedly, or when the barrel itself is damaged. Anodes on the drive still matter in fresh water: there is no salt here so corrosion is slower, but stray current in a marina does not care about salinity.
What vibration does to everything downstream
Vibration is not a comfort problem. An out-of-balance propeller applies a rotating side load to the prop shaft every single revolution. That load works the prop shaft seal until it starts weeping, and water then finds its way into the gear lube, which is covered in full on the lower unit page. It works the shaft bearings. Over a season it can bend a shaft that was straight, and on a sterndrive the same load passes back into the gimbal bearing and the couplers.
The strikes that cause all this are ordinary here. Winnipesaukee has granite ledge and boulders close to the surface, particularly in the shallow water around its islands, and the lake holds at least 264 of them. A strike at speed can bend blades, spin a hub and shock-load the gearcase all at once. After any contact the right sequence is to pull the propeller, inspect the blades and the hub, check the shaft for runout, and then look at the gear oil.