Achilles’ Heel

IS THE TYPICAL SINGLE-SCREW’S PROP PROTECTION ABSOLUTELY FAIL-SAFE?

You’ve probably heard it said many times over the years, by dockwalkers who either own or are contemplating owning a single-screw trawler or cruiser: Propulsion configurations that put a single prop in a protective aperture, with a beefy, steel- or steel-shod grounding shoe underneath, constitute the safest type of running gear that a builder can install on a boat. Indeed, you’ve probably even heard people claim—and I must admit I’ve subscribed to this viewpoint for many years myself—that one prop in a fully found aperture is virtually impervious to damage in a grounding situation and, for that matter, in many other situations where props get torn up. After all, wrapping the typically thick structure of a keel or skeg around the forward and upper parts of a propeller and then following suit with an equally substantial chunk of fiberglass or steel under the lower part should virtually guarantee propeller health and safety, right? Well...

Recently, a friend of mine and his brand-new, single-engine trawler were headed east on the Intracoastal Waterway not far from where the wide brown vistas of Mobile Bay squeeze into the “ditch” that crosses a good bit of the Florida Panhandle. All was fundamentally right with the world at the time—it was broad daylight, not a cloud in the sky, and a powerful little diesel whirred smoothly at about two-thirds throttle in the engine room.

“But something started to feel a bit off,” my friend says. “A couple of the markers on the starboard side seemed unusually far apart. In fact, as I kept going, they seemed so far apart I started to get nervous, not being able to locate the marker I thought should be between ’em. What I was seeing wasn’t matching up with my chart.”

Of course, then it happened—a thud of monstrous proportion, followed by such a screeching and heaving as only a full-bore catastrophe can visit upon a semi-suspicious skipper. An abrupt and ominous cessation of the whirring from the diesel put the final touches on the incident.

Thanks to my friend’s long experience with boats and operating them in all kinds of situations, an explanation materialized as soon as the ragged, hacked-off end of an upright piling showed itself briefly abaft the swim platform.

Apparently, the night before, one of the many tug-and-barge combos that transit Florida’s portion of the ICW had gotten crosswise in the channel and sheared off, just below the waterline, a piling that supported a navigational marker. So while my friend had had no luck predicting the incident by eye or experience, he had certainly discovered it by feel, slamming the stub-end of the piling with the stem of his trawler at a rousing clip.

The screeching, heaving, and ominous cessation that followed? After encountering the stem, the top of the piling had begun scraping torturously aft along the side of the keel, thumping, bumping, and gouging as it went, finally nailing the prop from the only vulnerable quarter, dead ahead. This stalled the diesel.

“You know,” my friend says, “I’ve always believed that if you compare single-screw boats that have protected props with twinscrew boats that have exposed props, the single’s gonna be a hundred percent safer. The prop on a single is just so structurally enclosed and, often, so are the struts and shafts. But now, after this deal, I’m revising my thinking just a little.”

I’m doing some revising as well. Is singlescrew-type running gear less damage-prone, and therefore safer, than the typical twinscrew inboard configuration, where the inboard’s props are not housed in anything like a set of protective apertures? For sure! But can an underwater obstruction, lying dead ahead of a single-screw boat, cause a seldom-acknowledged Achilles’ heel to surface—with a vengeance? Again, for sure!

“Darn thing did a lotta damage,” my friend reflects. “Prop included.”