Trou­bleshooter

Passage Maker - - Troubleshooter -

en­gine to turn over the wa­ter in the stern tube and briefly spin­ning the shafts to change the por­tion rest­ing in the bear­ings and seals will help re-oxy­genate the stain­less.

When the ox­ide sur­face film breaks down, the ex­posed metal loses its non-cor­ro­sive prop­er­ties. The ex­posed metal will break down, form­ing a small hole or pit. This pit­ting tends to de­velop in ar­eas where wa­ter flow is re­stricted, such as shaft seals and bear­ings. These hap­pen to be ar­eas hid­den from view as well, keep­ing the pit­ting con­cealed un­til the shaft is pulled out.

At what point does pit­ting threaten the in­tegrity of the shaft? That ques­tion poses a dif­fi­cult judg­ment call. How deep are the pits and how many? Will the boat be in­land or off­shore? What is the bud­get? Gen­er­ally, pit­ting less than 1/8-inch deep can be tol­er­ated, but that might be trumped by an ex­ces­sive area of pit­ting. A process known as “cladding” can re­store a pit­ted shaft. Cladding in­volves ma­chin­ing, weld­ing, more ma­chin­ing, and straight­en­ing, and must be done by a highly trained ma­chin­ist. Due to the ex­pense of cladding, it usu­ally is not cost-ef­fec­tive for shafts un­der 3 inches in di­am­e­ter. In ad­di­tion to strength con­cerns, pit­ting in the bear­ing area or the shaft seal can damage the bear­ing or seal.

What can you do: When buying a new shaft, go with 22 grade if your bud­get al­lows. If your cur­rent shaft at­tracts a mag­net, be on the look­out for pit­ting cor­ro­sion at your next haul-out. If pre­par­ing for a cruise, pull the shaft first and fully in­spect.

Al­though the chem­i­cal process for crevice cor­ro­sion does not dif­fer from that of pit­ting cor­ro­sion, the causes vary. Crevice cor­ro­sion refers to a lo­cal­ized at­tack at the in­ter­face between two sur­faces, or where a sharp edge oc­curs. This sit­u­a­tion can oc­cur, for ex­am­ple, where the for­ward end of the pro­pel­ler hub meets the shaft. On lower-grade al­loys this area will suf­fer from se­vere crevice cor­ro­sion (see be­low). On a mi­cro­scopic scale, the same process takes place where small scars or nicks in the shaft sur­face de­velop. These small im­per­fec­tions cre­ate flaws in that crit­i­cal oxy­gen film, pro­vid­ing a foothold for crevice cor­ro­sion. One study pub­lished in the Jour­nal of Fail­ure Anal­y­sis and Pre­ven­tion

looked into the fail­ure of a 2-inch di­am­e­ter shaft that failed near the for­ward end of the ta­per. Re­ly­ing on so­phis­ti­cated test­ing tech­niques, they con­cluded that ro­ta­tional fa­tigue be­gan from the sur­face of the shaft near the key­way. They de­ter­mined that the ori­gin of the fail­ure was “lo­cated on a sur­face flaw (re­cess or dent) of ap­prox­i­mately 100 mi­crons in depth.” That is merely one-tenth of a mil­lime­ter. While it might seem that a shaft fails all at once, in fact it is a process that be­gins long be­fore the break. Mi­crofrac­tures form first, even­tu­ally leading to a small crack that grows un­til the shaft strength no longer ex­ceeds the work­ing loads.

What you can do: Never use a pipe wrench or pli­ers on your shaft. Avoid any- thing that would scar the metal. Bar­na­cles can also pro­vide start­ing places. Keep the run­ning gear clean. Con­sider us­ing one of the prod­ucts de­signed to pre­vent ma­rine growth on met­als. Clean­ing a shaft with a steel brush can leave sur­face con­tam­i­na­tion that in­vites cor­ro­sion.

When­ever you have your prop pulled, your shaft re­moved, or prior to any ex­tended off­shore voy­ag­ing, re­quest a liq­uid pen­e­trant test in the high­est stress ar­eas, around the key­way and just for­ward of the pro­pel­ler hub. When cracks ap­pear, the shaft should be re­placed.

Fi­nally, we turn our at­ten­tion to gal­vanic cor­ro­sion. Gal­vanic cor­ro­sion oc­curs when dis­sim­i­lar met­als are im­mersed in a con­duc­tive so­lu­tion (sea­wa­ter). Re­gard­less of the al­loy, your shaft needs a sac­ri­fi­cial an­ode. A col­lar an­ode at­tached di­rectly to the shaft pro­vides the an­swer. If your boat has a large an­ode on the tran­som con­nected to the bond­ing sys­tem, it will pro­tect all bonded un­der­wa­ter met­als. The shaft and pro­pel­ler can­not be as­sumed to be bonded through the en­gine. The shaft con­nects to the trans­mis­sion, but the elec­tri­cal con­ti­nu­ity through the oil-bathed gears and bear­ings does not suf­fice. Bond­ing cir­cuits must have less than 1 ohm of re­sis­tance. For this rea­son, a shaft brush is re­quired to prop­erly con­nect the shaft and prop to the com­mon an­ode.

TROU­BLE-FREE CRUIS­ING

Shaft fail­ures in com­mer­cial ship­ping have prompted nu­mer­ous stud­ies and in-depth analy­ses. The forces and chem­istry at work here reach an as­tound­ing level of com­plex­ity, with many in­ter­wo­ven pro­cesses. In­stead of fo­cus­ing on any one de­tail, a com­pre­hen­sive ap­proach pro­vides the best re­sults. Start with the proper shaft al­loy, prefer­ably 22 grade, make sure the pro­pel­ler has been prop­erly in­stalled and meets the ba­sic pa­ram­e­ters de­scribed above, han­dle the shaft with care to keep it free of im­per­fec­tions, and pro­vide ap­pro­pri­ate sac­ri­fi­cial an­odes. Oh, and most im­por­tant, keep a healthy flow of oxy­gen com­ing—that is, use your boat of­ten.

n

Low- grade al­loy im­mersed in an oxy­gen-starved en­vi­ron­ment does not hold up well. Se­vere scal­ing and cor­ro­sion have de­stroyed this shaft in the seal area.

Only lower- grade al­loys will at­tract a mag­net. Se­vere crevice cor­ro­sion has de­vel­oped at the for­ward end of the key­way area.

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