Catamarans and Twin Drives - Getting the Feel

In this section, we discuss catamarans and monohulls with twin drives together because they have similar maneuvering and docking abilities. This similarity arises from the fact that both types of boats use twin drive systems—catamarans have one drive in each hull, while twin-drive monohulls have two engines mounted on opposite sides of the boat.

The main advantage of catamarans in terms of maneuverability is the wider spacing between the drive units, which provides greater leverage for turning. This wider spacing allows catamarans to have a tighter turning radius and better spinning ability compared to a twin-drive monohull, where the engines are closer together.

Some people compare maneuvering a catamaran or a twin-drive monohull to driving a bulldozer because both sides can operate independently. While you may not have driven a bulldozer, you might remember how its tracks work: each track can move independently to allow for sharp turns or spinning in place. Similarly, with twin drives, the farther apart the drives are mounted across the width of the boat, the more effectively the boat can turn or spin.

In catamarans, the propellers are positioned at the rear of each hull, placing the thrust forces far apart, which significantly enhances their turning and spinning capabilities compared to a twin-drive monohull. However, twin-drive monohulls are still very effective at maneuvering. Just like a bulldozer’s levers, which control each track independently, the throttles of the two engines on these boats allow for precise control. By pushing one engine forward and the other in reverse, the boat can spin on its axis, making it highly maneuverable.

If you can imagine a bulldozer when you push the left track lever forward and the right track lever backward, which way does the bulldozer spin? Clockwise right? The left track goes forward and the right track goes back; this acts to turn the bulldozer clockwise.

Both tracks forward—bulldozer goes forward.
Both tracks back—bulldozer goes backward.
Left track back and right track forward—bulldozer turns counterclockwise.
Right track back and left track forward—bulldozer turns clockwise.

And bulldozers are cool because they can turn inside their own length.

Rule of Thumb

If you’ve not driven a bulldozer or the like, no worries! Just remember that the cat turns by a rule of thumb. That is … the cat turns in the direction of your thumb on the throttle that you have engaged in forward.

See this

Fortunately, the Power catamaran industry had their heads screwed on properly when they decided that power cats were a good idea.

First off, they put the rudders aft of the propellers.

As surprisingly obvious that is, some sailing catamarans have the rudders in a position forward of the propellers. STUPID idea. It means that you have no wash of propeller water over the rudder for maneuverability in forward gear. Again STUPID. Only if you have to have the engine in reverse do you get propeller wash over the rudder. Did we say stupid already?

Power cats don’t suffer from this stupid idea.

And in addition, most power cats have counter-rotating propellers so that they do not suffer from prop walk. The wash lift effect from one hull is canceled from the opposite wash lift effect on the other hull

On top of that, some catamarans use azimuth drive systems – cheating!

Simple Imaginary Exercise

With the helm locked in the center, try this:

Both throttles forward—cat goes forward.
Both throttles back—the cat goes backward.
Left throttle back and right throttle forward—cat spins counterclockwise.
Right throttle back and left throttle forward—cat spins clockwise
Left throttle back a little and right throttle forward full—slow forward turn counterclockwise.
Right throttle back a little and left throttle forwards full—slow forward turn clockwise.

Play with the animation below by clicking on the arrows. Note that for broad turning, the reverse throttle is not all the way back.

As ridiculous as it seems, we want you to get into your favorite armchair right now, clench each of your fists around a screwdriver or chopstick, and practice moving those bulldozer levers alternately left-back/right-forward and vice versa. (Feel free to make brrrmmm brrrmmm noises). Come on do it – this is building muscle memory. You’ll intuitively feel which way the bulldozer will turn. Do it a couple of dozen times and you’ve probably got enough skill to get you out of the marina—well, maybe not.

Notice that at this stage we have not talked about the wheel/rudders. You can perform the maneuvers in the above animation with the wheel locked in its center position.

Momentum

Become confident in how quickly or slowly the boat will stop. You’ll notice that the forward momentum of the boat is hard to quell with the engines. This is because the propellers are set up to be efficient in forward thrust, but not in reverse thrust. Thus, a significant understanding of killing off boat momentum should be practiced—away from anything. You’ll be surprised how long it takes some of these boats to stop. No problem, however; you’re gaining the feel with all the exercises above. In reverse, the story is different. Reverse momentum, as with a monohull, can be killed off quickly with just a little (efficient) forward thrust. Ha ha – don’t practice reverse thrust while towing a dinghy.

Practicing is important – not just for embarrassing reasons but for your credit card deposit or insurance deductible reasons. Collisions in boats this expensive are just not necessary. Simply spend a little time practicing and gaining an appreciation of the maneuverability of the boat. But do your practicing out of the marina.

Exercise CAT1 – General Maneuvering:

Practice all the monohull examples given in this book on your cat once you’re out of the marina, clear of everyone, clear of sight of the charter company, and before you set the sails.

With the boat moving forward, put the boat in reverse and try to stop it
With the boat moving in reverse, put the boat in forward and try to stop in
Spin the boat clockwise
Spin the boat counterclockwise
Do figure 8’s in forward and reverse
Engage just one throttle in forward and see what the boat does – add a little opposite reverse
Engage just one throttle in reverse and see what the boat does – add a little opposite forward thrust.
Try various combinations of forward and reverse thrust

What You Learned

As with the monohull exercises, you’re getting the hang of the boat for its turning capacity, momentum, and reversing ability.

There is a temptation to forget about the rudders when maneuvering because the engines are extremely effective. In the next section, you’ll see the added dimension of the rudders and what they can do to assist.