MANEUVERING TWIN-SCREW CATAMARANS

 
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Many of you are familiar with maneuvering twin-screw power boats. As good as they are to maneuver, twin screw catamarans are even better! Actually, you have every potential maneuver
available to a track vehicle.

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By Charles Kanter

At The Dock

Many of you are familiar with maneuvering twin-screw power boats. As good as they are to
maneuver, twin screw catamarans are even better! Actually, you have every potential maneuver
available to a track vehicle. The reason they are better is because their wide beam creates options
not available in a narrow boat. Twin screws provide confidence in docking. Knowing the
procedures and the theory behind those procedures allows you to quickly master the techniques
and build the skills to handle most situations. If you can handle docking, you can handle any other
maneuvering situation including tying to pilings, approaching and leaving slips and anything else
you may encounter.

The helm

The helm is not normally used in these maneuvers, all maneuvering is done with the throttles.
Simply leave your wheel with the rudders centered and perform all steering operations with the
two throttles. There are some caveats, however. This article subsumes an “average” production
twin screw catamaran with spade rudders and three-blade fixed propellers. Two-blade props,
folding props and feathering props will change the amount of thrust available. A single engine
with twin hydraulic drives has different operating characteristics since you cannot create greater
and lesser thrusts at the props. In some cases with certain propeller options there is precious little
reverse thrust to use.

I also assume single-lever engine controls. Occasionally, I come across dual lever controls, that is
the transmission and throttle on separate levers. While dual levers may be routine on single engine
installations, it complicates twin-screw installations

The theory and the practice

In most twin-screw power boats when you put one engine in forward and the other in reverse and
apply throttle to receive equal thrust to both engines, you will spin on a centerline point. In a
twin-screw catamaran, you can pivot the boat on any one of three pivot points depending upon
how you apply the power to the engines. Using equal engine thrust you will pivot from the center
of the boat. Using greater and lesser power, you will pivot from the side applying the lesser
power. [see figure 1]

For instance, if you want to pivot to the left on your port hull, you would put your port engine in
reverse and your starboard engine in forward. Then you would apply a lesser amount of power to
the port engine, just enough power to keep the boat from moving forward. With the greater thrust
coming from the starboard engine, the boat will literally swing around the port hull.

Let’s say you didn’t have enough room on the right side to clear an obstacle so it is necessary to
pivot on the starboard hull. In that case, you would put greater thrust on the port engine in
reverse which would literally pull the port side of the boat away from the forward moving
starboard, thus pivoting on the outside or in this case, starboard hull.

Not every case is identical, not every situation calls for the use of thrust exactly the way it is
portrayed in the following diagrams. A little bit of practice and the exercise of prudent judgement
and you will be an expert in no time. The same technique applies whether you are underway, at
rest or docking. This is a related technique to that used in operating a track vehicle such as a tank
or a bulldozer. The concept of greater and lesser thrust is similar, except that in a track vehicle,
the inside, or pivoting side may be held in place by a brake. Your propellor substitutes for the
dozer’s brake.

Terms

When using engines for maneuvering in the terms of this article, the terms are defined as:
1. Lesser Thrust = enough thrust to keep that hull from going forward or aft or relatively
stationary. Your dozer brake equivalent.
2. Greater Thrust = enough thrust to make that hull go forward or aft.

Thus the hull with greater thrust pivots around the center of lateral resistance of the stationary
hull. Equal and opposite thrust pivots the boat around its center point, directly under the mast.

Figure 1 Shows a twin-screw catamaran applying equal thrust forward and aft and spinning at the
center point of the vessel center.

Figure 2 shows a catamaran in a tight docking situation with strong current coming from
forward. To prevent abrasion with the dock, a fender is placed in the stern quarter. During your
learning curve, it is advisable to also use a spring line as a safety feature.

To prevent rolling the fender out, the technique is to literally pull the starboard hull around until
the bows are clear of the forward boat. The port engine maintains just enough power to keep the
boat from going backwards into the dock. At the point that the bows clear the boat in front, the
starboard engine is taken out of reverse and put in forward, more power is applied to the port
engine and the boat moves quickly out of the slip, the current keeping the vessel clear of the
docked boat

Additional help may come from a crew member who stands on the transom and holds the fender
in the appropriate location. That person will whip the line around a stern rail or stanchion or
whatever is handy to increase holding power. No one should hold it directly to reduce the
possibility of rope burns.

Figure 3 shows the identical technique except the current is reversed making it expedient to go
out backwards. Twin screw catamarans are equally maneuverable forwards or backwards.
(exception to that if you have folding two-bladed props)

Figure 4 shows the most difficult situation for any sailboat, being pinned to the dock by current
or wind or both. The technique is similar to figure 2, but you must be prepared to either walk the
fender forward or have several fenders available. Preventing fenders from tangling or jamming in
pilings is the major problem. Use more thrust on the port side engine as the cat turns into the
current. Once the bows are clear of the boat ahead of you and the stern is clear of the dock,
continue enough reverse thrust on the starboard engine to assure the bows continuing to turn into
the current but not so much that you force the boat backwards. often, you can just slip the engine
out of gear at this point so as to lessen the aft-pulling force. It’s a judgement call depending upon
the strength of the current or wind.

As soon as the port bow passes clear of the boat ahead of it, give more power on the port engine
and less on the starboard to speed the pivot the boat into facing directly up current. Keep pivoting
until you are facing into the current to make sure you do not get swept into the boat ahead of you.
The figure shows the final position when both engines are in forward but the port (leeward)
engine is using greater thrust to compensate for the current and bias the boat towards the favored
side.

Figure 5 step 1 through 3, shows a procedure to leave a crowded dock when there is no
significant wind or current. This is the skill area to practice at every opportunity. It is possible to
do this without any fenders against the dock and it a similar procedure that actually allows you to
“walk” a boat into or out of a tight slip sideways. In the diagram, first tip the bows in towards the
dock to pull the stern out. Then reverse the engine directions to pull the bows away. If your first
try pulled the stern far enough away so you can clear the dock, just keep turning until clear.
Otherwise, repeat the first procedure to “walk” the boat out a little further as shown in figure 5a,
5b and 5c.



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