Prop fouling 101

Althorpe Island Jetty
South Australian waters are famous for their Southern Bluefin Tuna, much of which ends up in Tokyo as マグロの刺身 (tuna sashimi). Accordingly, sailing back from Port Lincoln, "Tuna Capital of the World", we were always trolling a tuna lure hoping to get a bite - a practice known as "blind trolling". Before crossing Investigator Strait bound for Kangaroo Island we decided to put in at Althorpe Island for a couple of hours. Althorpe has a small anchorage situated between a dilapidated jetty and a rocky reef (35°22.10'S 136°51.75'E). After dropping anchor, I briefly put our engines in reverse to stop our forward motion before I realized I still had the lure trailing behind. Oops, too late! I'd inadvertently tangled several meters of heavy monofilament fishing line around my port propellor. Fortunately it was calm, the anchor had set and we still had a functioning starboard engine. We promptly broke out the snorkeling gear and the "Hookah" and twenty minutes later had extricated the tangled line.

I thought that was the end of it. What I didn’t realize is that I’d damaged the prop shaft seals as well as the shaft itself. Monofilament line, esp. the hefty 130 lb. test I was using for tuna, wraps tightly around a spinning shaft and cuts into the seals in the blink of an eye. Further, it exerts a vice-like torque on the prop shaft as it seizes onto it. Finally, the heat from the friction between the spinning shaft and the nylon line can even cause the nylon to melt and fuse onto the shaft. I avoided the worst-case scenario by promptly shutting down my engine, but the damage to my shaft and seals was a fait accompli.

Although I check my engine oil religiously, I confess that in the past I was less diligent about checking my sailboat drive (s-drive) gearbox oil. Every time I did it was always full and clean, so I got out of the habit of checking regularly. Mea culpa. Alas, my port sail drive eventually started to lose oil via the damaged seal which allowed egress of oil and ingress of water. By the time I hauled Arriba out 6 months later, not only did the seals and prop shaft need replacing, but so did the top box of the gearbox - a rather expensive episode.
S-drive damage areas
Now for another confession. This is actually the second time I've fouled my port prop. The circumstances were different the first time, but still due to my neglect. I brilliantly tangled my dinghy's painter while reversing with the dinghy under tow. In my defense I was distracted having embarrassingly just run aground at my local beach. (You only make that mistake once.)

NB: Towing objects works best going forward.

Surprisingly, you don't even have to be moving to foul your prop(s). Recently I had a squid jig set at anchor and noticed that the line was taught, but heading straight down. Sure enough, I'd jigged a squid but in an effort to escape the panicked creature had wrapped the fishing line around my starboard prop, not once, but twice! Getting in the water was the only way to untangle it. I had visions of the irate squid attacking me in revenge, but it eyed me from a distance. It still ended up as salt and pepper calamari that evening.

Perhaps the scariest situation is being fouled by someone else's cray pot (lobster pot) or crab pot. There's no rhyme or reason as to where such pots are distributed. Although crab pots tend to be in shallow waters, lobster pots can be found miles off shore. You just have to maintain a good watch and keep a sharp knife handy - just in case. Getting fouled can cost you more than a prop shaft or a gearbox; it can cost you your vessel. The day before my own Althorpe Island incident the keel boat "Clare Allen" was sadly lost when it was fouled by a cray pot near Thistle Island, about 40 miles away from Arriba's position at the time.
Clare Allen (Photograph: Ron Campbell)
Nowadays I'm considerably more careful about pulling in all my lures before anchoring and I never moor with my dinghy under tow. I now grimace when I see folks anchoring with their dinghy in the water, all scrunched up against their stern, as I think they're just asking for trouble. As an added precaution, when motor sailing on one engine I usually don't fish from the "propward" side, i.e., the side with the spinning prop. Trolling under sail - with all props folded - is the best way to fish.

Most importantly, I now check the oil in my s-drive frequently, which is best done after running in gear for a few minutes. Unlike engine oil which quickly becomes dirty from engine contaminants, gearbox oil should stay clean. If the oil is milky in color, it's a telltale that water is mixing with the oil, which means the seals are leaking. My advice is to get it fixed before you ruin your gearbox!


PS You can read about more prop fouling adventures here.