TROUBLESHOOTER
Best Practices When Under Tow
Reasons for needing a tow can range from loss of steering or loss of a propeller to engine failure, fuel issues, or a number of other problems. Most likely you have literally lost power and/or control. If you are like me, it’s easy to feel deflated and disappointed in such a situation, but remember that you still have some control over how smoothly the towing process goes. While you’re waiting for the towboat to arrive, there are a number of steps you can take to reduce the risk of complications (and thus additional costs) during your tow.
FREEWHEELIN’
Unless you lost the propeller, the force of the water across the prop during a tow will cause the shaft to spin, an effect sometimes called “backdriving” or “windmilling.” (A boat tied to a dock facing into strong current or a twin-engine boat running on one engine can experience this same effect.) The faster the tow, the higher the rotation rate of the shaft.
A spinning shaft on a dead engine can lead to trouble. Your transmission relies on a flow of oil for lubrication and a flow of seawater to cool the oil through an oil cooler (“Troubleshooter,” April 2017). Without the engine running, the gears will spin with less lubrication, generating heat in the process. The solution to this problem depends on the transmission brand and even the specific model, but all manufacturers recommend stopping the shaft from spinning as the first choice.
If your engine still runs, keeping it operational during the tow will help cool and lubricate the transmission. Make sure the transmission oil level reaches the full mark. If you cannot secure the shaft and the engine will not run, there is one more option, though it is the least desirable one. Again, this procedure varies with each transmission. With a ZF transmission, for example, you will plug the dipstick opening and fill the transmission with the proper oil to the top of the fill tube. Tag out the start key so that you remember to remove the excess oil before running the engine. Twin Disc has a slightly different approach: After overfilling, drain it back down to the full mark on the dipstick. This process will help displace any air and will provide some measure of lubrication. You must refer to the operator’s manual for your transmission to know the recommended procedures.
How do you know if the steps you have taken are working? Check the temperature with your pyrometer (“Troubleshooter,” Jan./ Feb. 2015). At temperatures of 170 degrees Farenheit or higher, damage is likely to occur. In the absence of a heat sensor, use this basic rule of thumb: If the transmission is painfully hot to the touch, it is too hot. If it’s running too hot, your only remaining option is to slow down and hope the temperature drops.
SHAFT SEALS
A spinning shaft on a dead engine can wreak havoc on your shaft seal. On most powerboats, the shaft seals rely on water injection from the engine for lubrication and cooling. Twin-screw boats often have each engine tied to only one seal—a better ar-
rangement includes a crossover tube so either engine can cool both shaft seals. If you have a single engine or twins with no crossover, you risk overheating the gland. Once again, brand and style matter. A PSS shaft seal can typically tolerate loss of flow at speeds below 12 knots. A Tides Marine shaft seal, on the other hand, can tolerate loss of water as long as the temperature does not exceed 180 degrees Farenheit. Traditional stuffing boxes are also temperature sensitive, with a threshold of about 140 degrees Farenheit before the packing starts to melt.
SECURING THE SHAFT
Based on the complexities and uncertainties described above, you can see that preventing the shaft from rotating makes the most sense. I encourage you to write us with your own clever solutions, but in the meantime I’ll offer one approach that has worked for me on more than one (humbling and painful) occasion.
It pays to keep on board a 50-foot hank of parachute cord or similar line. This small-diameter, high-strength line can be handy in emergencies (keep one in your ditch bag, too), but any small line will do. Instead of trying to tie onto the smooth and slippery shaft, turn your attention to the shaft coupling. Lace the cord around the coupling bolts, around one of the engine mounts, back to the coupling, and back to the engine mount. Keep the slack to a minimum. Once the tow gets underway, the line will tighten and the shaft will not spin.
DON’T SINK YOUR ENGINE
Securing the shaft eliminates concerns about the transmission and the shaft seal, but we still have one remaining issue. Boats with scoop strainers risk flooding the engine with seawater while being towed. These strainers pressurize the flow of water into the engine when underway. With the engine not running and the boat moving through the water under tow, the seawater can sometimes find its way past the raw water pump impeller. Once that happens, seawater will run through the cooling circuit until it dumps into the exhaust collector. In the absence of exhaust gas pressure, seawater will accumulate in the exhaust line making it possible for seawater to enter the exhaust manifold and enter the cylinders. You can easily avoid this by closing the engine intake seacock(s).
Generators face this situation every time you are underway with the generator off. That’s why external directional strainers must not be installed on generators. In other words, you need not worry about the generator when being towed.
One last prior to getting underway under tow: If your boat has fin stabilization, lock or pin the fins to prevent damage.
AFTER THE TOW
If you follow the procedures outlined above, you should arrive with no more problems than the one that stopped you in the first place. You will need to open any seacocks that you closed and drain or pump out any extra oil in the transmission. After removing the lashing from the shaft coupling, you can turn your attention to a new topic: determining which beverage you’ve earned after such a challenging day. —Steve Zimmerman
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