Dealing with prop problems

When internal combustion grabbed hold of the waterfront, propellers upstaged paddles, oars and stern wheels. The nifty screw like device, rotating on a central axis, harnessed Newton’s second and third laws of motion and demonstrated that thrust could indeed accelerate a mass, and that equal and opposite reactions do prevail.

Early proponents noted the prop’s helical wake, and many began to refer to the device as “the screw.” Pitch or blade angle was often shown in the context of an auger forcing a vessel through the water column. But like most over simplifications, a closer look introduces more factors into the mix. The screw analogy suffered a little from the hydrodynamics of slip, tip vorticity and the lift generated by the fluid circulation around the blade &mdash all factors that influence performance and propeller shape. The art and science of propeller design yields an array of shapes and structures based upon either an obsession with efficiency (the relationship between thrust and drag) or the recognition that durability and cost are also vital concerns. In every case, the prop is the sequel to the shore side slogan “where the rubber meets the road,” and as a spinning thrust generator, a prop delivers the goods, but it must also stand up to the flotsam and jetsam of our waterways.

Designers have come up with mini keels, bipod struts, half wells and even nozzles to hide prop blades from contact with unintended obstructions. Regardless of the protection however, there seems to be a way that a blade bending offender can find its target. Prop damage is modern motor boating’s torn mainsail, and what it means to a boat owner depends upon the severity of the damage. With some encounters all that may happen is a minor blade tip ding or bend, and by throttling back to eliminate vibration, the vessel and crew may be able to limp all the way home without assistance.

During design and manufacture, props are carefully balanced in both a static and dynamic context. Care is taken by the manufacturer to distribute metal evenly around the central axis and make sure that blade pitch, shape and skew are identical. If this isn’t done properly, one blade may actually generate a little more or less thrust than the others, resulting in an annoying harmonic vibration. More egregious imbalances can turn into loud rumbles and serious fatiguing vibrations. Among the most vulnerable are performance oriented “skewback” prop shapes with high aspect ratio blades, and the new genre of counter rotating forward facing props. With these, any thud may cause the crew to worry about prop damage, but there’s often a lot more in play.

The propeller is the distal end of the drivetrain, and impacts that involve prop blades radiate up the chain of running gear components all the way to the engine itself. Full speed groundings, by vessels on a plane, do much more than rosebud (fold up) the propeller. The damage can extend well beyond the rotating components in the bilge. And as soon as the engine(s) is shut off, the cabin sole floor boards should be lifted and the hull integrity checked. Point loads stress strut attachment points, stern tubes and stuffing boxes. In dramatic full stop groundings, engines can even be pulled off their mounts and prop replacement may become a minor item on the overall yard bill.

In less traumatic contact situations, damage is usually much more localized. For example, those who inadvertently come across an errant, old water-logged pier timber &mdash dubbed a “wheel inspector” by many tug captains &mdash may get away with only a bent blade tip or two. However, these timbers can be three to 30 feet long with only a barely buoyant 12″x12″ end bobbing near the surface.

A short haul and close inspection will tell all, and with a bit of luck plus the local shop’s reconditioning service, the result is a quick turn around. A skilled prop shop can repitch blades, electric rod or inert gas weld tip tears and turn minor damage into a good-as-new prop. NiBrAl props are harder and seem to hold up better than more traditional manganese-bronze props, and consequently they have been gaining market share. But regardless of the material used, more extensively damaged propellers should be sent to the manufacturer for reconditioning, or if necessary &mdash replacement.

Most experienced mechanics automatically turn to the shaft to see if the offset pressure that damaged the prop also bent the shaft. In cases where only a minor tip bend or tear is causing a vibration, it may only take a bit of prop reconditioning to get the show back on the road. If after reinstallation and engine alignment all runs smoothly, the skipper can breathe a sigh of relief. If noise and shaft rumble that wasn’t present prior to the prop damage persists, more components need checking. This is why it’s usually wise to send the shaft and coupling along with the prop. The crew at the shop can “mic” the shaft and make sure it’s straight. They also check for keyway distortion and set up the prop and coupling for optimum fit.

One of the most often missed bits of damage occurring when the prop strikes a submerged or floating object lies at the opposite end of the drivetrain. It’s a piece of torque linking hardware that connects the engine’s flywheel with the reverse gear. Designed to take the harsh thud out of the shifting process, the damper plate (aka drive plate, flex plate) is also an intentional weak link between the engine and the drivetrain. A brutal impact, such as what happens when a prop hits a rock, can damage both drivetrain and engine components. So, rather than bending a connecting rod or damaging the gears in a marine transmission, the damper plate acts like a fuse and its spring assembly or the entire structure shears, saving key components but leaving the drivetrain in perpetual neutral. The repair bill is less than what it would be for a new reverse gear or engine rebuild, but it does require shaft and reverse gear removal in order to replace the damper plate. In many cases the plate partially fails when serious prop damage occurs, but it doesn’t completely shear. Often continued fatigue linked to normal operation drives the damaged damper plate to the critical brink. And just as you’re coming alongside and are counting on a resounding display of reverse thrust, the part fails. A good mechanic may be able to hear or feel plate problems when shifting, and replace the part before it reaches the end of the road.

Power voyagers headed off the beaten path benefit from hull shapes with protective keels and props tucked into hull indents. For those out for prolonged periods, carrying a replacement prop makes a lot of sense, especially when there’s potential to find a boatyard able to hoist your vessel for a repair. Otherwise, dive gear may be a viable option, especially if the vessel is low powered, sports a modest sized shaft and prop, and a simple mechanical prop puller can do the job.

Many boaters pamper their fuel systems and follow the engine maintenance mandates explicitly, but lose sight of the all important drivetrain. So give some thought to just what happens as the normal torque of the engine momentarily spikes and, in addition to prop deformation, a pulse of metal bending and fiberglass cracking energy works its way from the prop(s) to the engine mounts themselves. If substantial energy has been absorbed by the drive train and the damage is severe your best friend is the most competent marine surveyor in the region. It’s easy to spot the major damage, but it’s the ancillary cracked laminate and hidden damage to the drivetrain that’s harder to pinpoint.

In addition to the designer’s best efforts to hide props from damage and use more durable materials in the drivetrain, every experienced skipper recognizes that the bottom line remains good navigation and attentive watch keeping.

Ralph Naranjo is a freelance writer and photographer living in Annapolis, Md.

By Ocean Navigator