Towing an Inflatable Dinghy

Necessary Cruising Companion

by Jim Hebert, jimh@continuousWave.com

One necessary item for a cruising sailor is a dinghy. This is especially true in the North Channel of Lake Huron, where visits to marinas are generally only once-a-week occurrences, and the rest of the time is spent anchoring in pristine wilderness harbors. Most of the cruising sailboats seen up there tow eight-foot to eleven-foot inflatable dingies.

Tow Properly

According to a local vendor of inflatables, the most common mistake made when towing is improper use of the towing bridle. Many boaters knot the tow bridle into a loop at the center, and attach a towing line there. This is wrong. The towing line should not be fixed to the bridle, but instead should end in a small bowline loop, through which the bridle passes. This allows the tow line to slew back and forth in the bridle, and provides for tension in both sides of the bridle. Not rigging the tow line this way is the most common cause of failure of the pad eyes on the dinghy to which the tow bridle is attached. It is inevitable that during towing, the dinghy will rock a little from side to side, shifting the strain on the bridle from one side to the other. If the tow line is rigidly fixed to the center of the bridle, one pad eye will be momentarily required to sustain the entire tow load. If the tow line can shift its position in the bridle, both sides of the bridle will remain in tension, and the load will be distributed to both pad eyes on the dinghy.

Dinghy Outboard On or Off?

Many cruising sailors have small outboard motors for use with their inflatables, which creates a dilemma when it's time to tow: should the motor be left on the dinghy while towing? The tendency to do this is proportional to the weight of the outboard and the difficulty in removing it and storing it on the boat. There are a number of reasons for recommending against towing with the outboard on the dinghy.

First, there is the hazard to the outboard. In large seas the dinghy might flip over, submerging the motor. It's also possible that violent motion in a seaway could cause the motor to fall off the dinghy transom. These things always seem to happen when you'd least like them, usually when a ten-foot sea is running!

Another consideration is the additional tow load the extra weight of the motor will create. A completely empty inflatable is usually a very light craft and hence a very light tow load. When you add 70 pounds or more of motor and gas tank, you could very well be doubling the tow load from a small inflatable.

Of course, I am assuming that the motor is being towed with its lower unit out of the water. If you want an appreciation of how much horsepower it takes to drag the lower unit of an outboard through the water, just observe the very significant increase in tow rope load when your dinghy outboard's lower unit is in the water. The jump in load is very noticeable, especially if you are towing at speeds above a few knots. My experience is that you might begin your tow with the dinghy motor tilted out of the water, but eventually, given enough time, motion, and vibration, it will work its way back into the water. When it does, it is likely to be just at the moment you'd least like to have to go aboard and remedy that problem.

Just Add Water

The problem most likely to occur in towing the dinghy is shipping water aboard. While these little craft have plenty of reserve buoyancy and probably won't sink even if filled, towing a dinghy full of water increases the tow load enormously. It can quickly lead to failure of the bridle pad eyes, especially in those instances where the load is not being equalized. Even just six inches of water in an inflatable can weight over a thousand pounds.

In 1996 I found out what happens when you get a load of water in your dinghy. We were just heading back to our home port of Little Current after a leisurely week of cruising. The waves were not large, perhaps three footers with occasional ones twice that. We were under sail, towing the dinghy without the outboard attached, when I suddenly noticed the boat speed was down. The cause was the dinghy tow load, which had just gone through the roof.

A big wave had come aboard the dinghy, filling it with about six inches of water. When this happens the dinghy's trim shifts quickly, and she will ride down by the stern. That makes it easy for more water to come aboard, and you are soon locked in a tightening loop of worse trim and more water coming over the transom. In my case, the load on the tow line was so high I was afraid I would pull the pad eyes out of the dinghy fabric. We had to slow down to a crawl and nurse our way into calmer water, where I could go bail out the inflatable.

The interior of the inflatable was roughly four feet wide by eight feet long. There were six inches of water in her. That is about sixteen cubic feet of water. Since water weighs about 62 pounds per cubic foot, that means there was almost a thousand pounds of water in the darn dinghy! No wonder the load on the tow rope was about 20-times normal!

Remember, this all happened on a nice sunny day, in a moderate breeze, with only three-footers. Luckily, we were just a mile or two from Little Current, so we could avoid attempting to bail the water out while offshore.

Gain a Knot

Even when everything goes right, the load of towing the dinghy has to subtract from your speed under sail. If you are making a long passage, an extra quarter or half a knot can add up to a considerable time savings. If you are making a 50 mile passage at 5.0 knots, you'll arrive an hour and ten minutes ahead of someone only making 4.5 knots and towing his dinghy. Those extra hours come at the end of the trip, just when you wish you were there already.

Lose a Knot

Just as towing the dinghy slows you down a little, not towing it will speed you up. This handy relationship was proven to me one afternoon in Northern Georgian Bay. We were on a long beat to weather, heading for the Bustard Islands, when I happened to notice the boat speed was up a bit. I turned around to gauge our distance off Green Island, which we had recently passed, when I noticed a small boat dancing in the waves just on the horizon behind us. "Good Heavens", I thought to myself, "what kind of an idiot would be out here in that little boat!" It took a few more seconds for me to realize that what I was looking at was our brand new inflatable bobbing by itself in the waves about a half mile behind us! The constant tugging on the polypropylene tow line had worked loose my best knot, and the blue-grey dinghy had made off on her own about five minutes ago. I spotted her just in time.

Docking

Another motivation for not towing the dinghy behind you, especially on a long tow line, occurs when you approach a dock after a long passage. By then, you've more or less forgotten about the dinghy--assuming it hasn't swamped-- so when you begin to make stern propulsion, the suction pulls the dinghy painter under the boat and wraps it around the prop! Remember to always shorten up the dinghy tow line before you approach the dock.

Vows Made In Storms

"Vows Made in Storms Are Forgotten in Calm Waters," says a motto that hangs over the bar at a local watering hole with a nautical motif. It is a fairly accurate assessment of behavior at sea. The lure of fair weather will often overcome the best of intentions and the inertia of a heavy dinghy at the end of a long towline. But keep these suggestions in mind before you depart on your next long passage. It might pay handsomely for you to hoist that inflatable aboard and stow its outboard below deck.