A sailboat prop can have two blades or three. A 2-blade prop is more efficient (because there is less interference between the blades) and creates less drag while sailing. A 3-blade prop gives somewhat less vibration and better control while docking.
Diameter
and pitch
Props are sized first by diameter and then by pitch, both measured in inches. The pitch measurement is the theoretical forward travel during one full revolution, neglecting slippage. A 14 x 9 prop, for example, is 14 inches in diameter and has 9 inches of pitch. A prop should be sized such that the engine just reaches max RPM at full throttle. The other critical dimension is, of course, shaft diameter.Prop rake
and cup
Rake is the angle between the center line of the prop blades and the shaft, expressed in degrees aft of a line perpendicular to the shaft. Cup is extra curvature added at the trailing edge of a powerboat prop blade, or at the tip, primarily to reduce cavitation at high RPM. Cupping increases rake, when added at the tip, and it increases pitch, when added at the edge. It is usually described as low, medium, or high cupping. Most sailboat props have no cup except for some folding and feathering props.Direction
of rotation
Like engines, almost all sailboat props are right-handed and rotate clockwise when viewed from astern, which means that their prop walk turns the stern right (facing aft), or to port, in reverse. Left-handed props do exist, however, because they are often paired with a right-handed prop in boats with two engines.A propeller shop can change the pitch, rake, and cup of a bronze prop by heating it and beating the blades against formed cones or mandrels. The shop can also increase the shaft size and reduce the prop diameter. As a rough approximation, one inch of diameter equals two inches of pitch and medium cupping adds one inch of pitch. Online calculators are handy for playing with the numbers.
Inspect the prop at every haul-out for nicks, bent blades, curled blade edges, pits, corrosion, or marine growth, any of which will detract from performance.
Stuffing
box
A stuffing box (also called a packing gland) seals the place where the propeller shaft leaves the boat through the hull. It allows the shaft to rotate while preventing seawater from flowing around it and into the boat. Where the shaft passes through the hull there must be enough clearance so the shaft can rotate freely, together with enough of a seal to keep water out. A stuffing box does this by squeezing soft packing material around the shaft.From the propeller moving forward, the shaft passes through a rigid tube, called the shaft log, that is bonded to the hull. Inside the hull is the stuffing box containing several turns of packing material held in by a packing nut. Tightening that nut compresses the packing against the shaft. A lock nut then keeps the packing nut from working loose.
Traditional packing material was flax impregnated with wax or grease. Modern packing often contains graphite, Teflon or carbon fibers. The prop shaft, usually stainless steel, rotates within the packing, which does not create a perfectly dry seal, but instead is compressed just enough to restrict all but a small amount of water to lubricate and cool the packing. Without it, friction would overheat the shaft.
Traditional flax packing is compressed with the packing nut to admit up to three droplets of water a minute while underway. (Some accept 5 to 10 drops a minute.) At the dock, ideally there should be no dripping or perhaps one droplet every few minutes.
To adjust the stuffing box, first loosen the lock nut and then tighten the packing nut only a small amount, about 1/8 turn. Re-tighten the lock nut and run the engine in gear. After a few minutes the stuffing box should still be cool or only slightly warm. If too hot to touch, the packing nut is too tight. Besides heating up, an over-compressed packing can score the shaft.
Packing wears out eventually, every 5 to 10 years, and must be replaced. It might need replacing earlier if tightening the packing nut no longer controls dripping. To replace it, remove the lock nut and the packing nut, then pull out the old packing with a pick or small screwdriver. Wrap three or four turns of new packing around the shaft. If using rings of packing material, stagger the joints.
Packing size is based on the shaft diameter and the inside diameter of the stuffing box. Modern graphite or Teflon packing runs cooler, lasts longer and can often be set for almost no visible drip. Even so, it is best to allow a slight leak for lubrication unless the packing is specifically designed for near-dry operation. Adjustments might require an especially large wrench, or even two.
Dripless
shaft seal
A dripless shaft seal blocks the opening where the propeller shaft exits the hull, keeping seawater out while allowing the shaft to rotate freely and keeping the bilge totally dry. Among the most common designs is the PSS Shaft Seal. It replaces the traditional stuffing box, which allows a small amount of water to drip for lubrication.The PSS seal has four main components. The stator is a stationary ring made of highly polished carbon and attached to the stern tube, or shaft log, by a rubber bellows. The carbon is self-lubricating and extremely wear-resistant. A stainless-steel rotor is clamped tightly to the propeller shaft and rotates with the shaft. Its face is precision-machined to a mirror finish. Spring-lined rubber bellows, under adjustable tension, connect the carbon stator to the stern tube. They keep the stator pressed tightly against the rotor. Hose clamps and set screws hold everything in place.
The dripless seal occurs only at the two polished faces pressed together by the spring-loaded bellows. When the shaft rotates, the stainless-steel rotor face turns while the carbon face is stationary. A microscopic film of water between these faces lubricates and cools the seal while preventing drips. Unlike in a stuffing box, there is no packing material around the shaft.
At low shaft speed, water naturally fills the stern tube. At higher speeds—especially on larger boats or boats with long stern tubes—the seal needs a water injection line that usually runs from near the exhaust elbow. This water lubricates and cools the seal faces and removes trapped air. There might also be a small vent hose to ensure that the seal stays flooded.
Bellows must be replaced from time to time, typically every 6 to 10 years. This requires hauling the boat and pulling the shaft. A stuffing box, in contrast, can be maintained easily at dock or even while underway if not motoring.