Towing little things with a big boat is pretty straightforward. There are plenty of nuances and details, of course; one sure way to spark a lengthy debate among cruising sailors is to ask about the best way to tow a dinghy. But, ultimately, towing something smaller than your own boat boils down to "tie it up, fiddle with the tow line, then drive normally".
Towing big things with a little boat is somewhat trickier, not to mention riskier. It is, nevertheless, important to be able to do it. You never know when you might have to press the dinghy into service to get a crippled mothership into harbour, or when you might need to help someone out in an emergency.
I'll assume, for the present purposes, that our tugboat is an outboard or sterndrive powered vessel of roughly 3 to 6 m (10-20 ft).
Towing from a Bollard
The ideal place to attach the tow line is near the tow boat's natural pivot point, which for our little powerboats is about 1/4 to 1/3 of the boat's length ahead of the transom. (If you take a close look at a ski or wakeboard boat, you'll see a towing pole or arch right around this point; on a big tugboat, the bollard is in the middle of the aft working deck, just above the rudder post.) This configuration keeps the tension on the tow line from having much of an effect on the tow boat's manoeuverability.
Let's accelerate to four knots or so, then put the helm over to port. The tension on the tow line is acting through the tow boat's natural pivot point, and therefore doesn't affect the tow boat's heading.
The vectored thrust causes the tug to yaw counterclockwise (to port) and slide to starboard. The tow, being larger and heavier, has not yet had time to respond.
Our tug is now on the desired heading, but is still more-or-less in line with the tow. When we straighten the helm, the tug takes off in the new direction, and the tow line follows.
Now that the pull is off-axis, the bigger boat yaws around to follow, and we continue on our new course.
Towing from a Bridle
Unfortunately, mid-mounted towing bollards are very rare on small powerboats. (They do, after all, take up valuable space in the cockpit.) On most dinghies and small outboard powerboats, towing must be done with a transom-mounted line or a bridle.
Sterndrive powered boats, along with many V-drive, inboard and jet types and some twin-outboard boats, can have a centreline towing eye mounted on the transom. This is rather more convenient, and provides somewhat better handling, than a bridle. With a single outboard, though, you're often stuck with a bridle arrangement, the tow line being attached to a floating pulley (block) at the apex of the bridle.
This is quite a workable arrangement, once you get used to it, but there is one very peculiar handling trait that takes some getting used to. Not unlike backing the boat trailer down the ramp, helm control (if the tow is big enough) starts out reversed.
We'll perform the same manoeuver as before, putting the helm over 30 degrees to port. This yaws the tug to port and slides her to starboard, but in this case, the stern-mounted bridle- attached well aft of the tug's natural pivot point- fights back.
Exactly how the boat will react depends on the particular boat involved, but in many cases, this configuration shifts the tug's effective pivot point way aft, to somewhere in the V of the bridle. Putting the helm over to port results in a moment around the effective pivot point that, once all the forces are considered, moves the tug to starboard. (In some cases, she may yaw to port but slide far over to starboard, a confusing situation indeed.)
An easy way to resolve this situation is to realize that when you set the helm back to dead ahead, the complete system (tug, line and tow) will try to form a straight line. If you imagine a line from the tow's pivot centre through the tug's pivot centre and extending onward, that line will be (approximately) your new course.
So, if you're towing something much bigger than you on a bridle, try this: steer opposite the direction you want to go, hold that rudder until the tug and tow are aligned on the new course, then straighten out.
Tow Rope Length
I've drawn the tow rope quite short so far, simply to make it fit nicely on the screen, but you shouldn't actually tow a boat that close. Crippled boats are cranky, cantankerous beasts and will waste no opportunity to turn and bite you.
If the rope is too short, it's easy to get caught in this situation:
You can probably see what's going to happen here. The tug slows down to climb the wave, while the tow starts surfing down the next wave. The tow rope goes slack, and:
- The rope gets wrapped up in the tug's running gear, or
- The tow picks her own course to surf on and broaches, or
- The tow holds the same course and rear-ends the tug.
- If the tug makes it over the wave, the rope suddenly snatches tight, yanking the tug's stern and the tow's bow down into the wave.
If you're towing in this fashion, you're going to spend a lot of time adjusting the length of the rope to keep both boats in phase with the waves. You'll likely find that you need to tow at least one wavelength back; two wavelengths may be better. And if the waves get big, you'll want a way to cast off the line without putting your crew at risk. (Any rope, even a low-stretch one, will snap back HARD if it breaks or is cut under tension.)
Pushing from the Side
If the configuration of the two boats allows it, this is probably my favourite way to tow. You break out the fenders- all of the fenders- and pack them between the two boats. The tug then gets snugged up close with mooring lines.
The spring line, in this configuration, takes a substantial fraction of the thrust, so you'll need seriously beefy cleats and lines to make this work. The bow and stern lines are mainly to stabilize the whole contraption and allow the tug to sort-of steer the bigger boat. Much like the spaceships Leonov and Discovery in 2010: The Year We Make Contact, the thrust here is highly asymmetrical and you'll need constant port rudder on both boats to hold a straight course.
Despite being a bit tricky to set up, this configuration is relatively safe and can provide enough control to get the disabled boat right into a marina.
Pushing from Astern
From a control standpoint, this is by far the best option, and virtually all of the commercial tugboats in our area use this configuration when moving big barges.
Not all powerboats can push like this; you need a bow that's designed to handle it. The thrust is transmitted directly from stem to transom, and the mooring lines are to hold everything in line and allow for some semblance of steering control.
Catamarans are particularly amenable to being pushed in this manner, but there is a significant danger involved: if anything fails, the tug will shoot under the cat's bridgedeck at high throttle. Anyone riding in the tug when that happens would likely be injured.
Practice Makes Perfect
I've towed perhaps half a dozen other boats out of danger, been towed once or twice (friggin' high-strung outboard...) and hauled some rather large fallen trees around using a rather small 10-horse metal skiff. It gets a little easier every time, but it almost always takes a few tries to find a configuration that'll work for any particular situation.
If you're fortunate enough to have a tender and mothership, getting a bit of practice in calm weather and a controlled environment is certainly a good idea. You don't want your first experience using the 11' RIB as a tugboat to happen when your main engine quits in a remote area and a wind shift puts you on a rocky lee shore.
The rest of us may go our entire boating careers without having to serve as a tug, or- as is the case up in Northern Ontario- we may be called upon to help out a neighbour on a somewhat regular basis. Even without any emergencies, there's always a fallen tree to haul off or a replacement dock to tow in. Being familiar with how your boat handles with a tow is, I think, just part of good seamanship.