Smooth Sailing: Pros and Cons of a SWATH Vessel

SWATH stands for Small Waterplane Area Twin Hull.  Naval architects searched for a way to minimize the seakeeping motions of a catamaran.  They liked the large deck area, but those pesky waves were a problem, bouncing the ship around.  The designers noticed that the forces from those waves depended on the width of the hulls.  More specifically, the waterplane area. (Figure 1‑1)

The designers shrunk the waterplane area until it was a small strut, just wide enough to fit a crew member down.  All the buoyancy got concentrated into deep submerged hulls, which looked more like a submarine hull.  And the SWATH was born:  a vessel with excellent seakeeping because waves exert very little force on it. (Figure 1‑2)  The force of the waves depends mainly on the waterplane area.  Small waterplane meant small wave actions.

SWATHS are specialized ships with one major goal:  excellent seakeeping capability.  SWATH ships have the same massive deck area of a catamaran, with far superior seakeeping capabilities.  In Figure 2‑1, compare the motions of the monohull in the foreground to the SWATH in the background.

Figure 2‑1:  Comparison Between SWATH and Conventional Ship

These reduced ship motions are a major benefit to the crew.  In the cases of research ships, the crew may not be seasoned seafarers.  They get seasick easily; so reduced ship motions help improve crew productivity.  Even experienced seafarers appreciate the smooth gliding motion of a SWATH vessel. No more rocking around.  Just gentle swells as the ship gradually rises and slides along the waves.

When comparing a SWATH to your current monohull, be sure to clearly identify the type of motion that bothers you.  If you feel the ship jerking from side to side and knocking you around, a SWATH may not be the best option.  The side to side jerking comes from roll accelerations, which is just one small part of the total ship motions.  We have retrofit options to reduce the roll accelerations, and these retrofits are far less expensive than a new SWATH.  On the other hand, if you are more concerned about total ship motions or see a lot of pitch motion, then a SWATH is probably the best way to go.

The excellent seakeeping of a SWATH comes at a price.  First, a SWATH may be too insensitive to the waves.  Imagine that.  The wave crest swells up, rising higher and higher, but the SWATH does nothing.  That is, until the wave slams into the underside of your cross deck.  Then you get woken to a jarring bang followed by violet pitch motions.

To avoid waves hitting the cross deck, SWATH ships are often equipped with active or passive control fins.  These are small underwater versions of airplane wings. (Figure 3‑1)  They help the ship react to the oncoming waves and make sure the vessel still tracks along the larger swells.

Passive control fins are not a major cost.  They are the equivalent of adding a skeg to the vessel.  But active control fins require hydraulic machinery, motion sensors, and a control unit.  That makes a big price tag.  But all that machinery contributes to the magical smooth ride that a SWATH delivers.

SWATHS are also extremely weight sensitive.  With such slender hulls, SWATH ships sink down quickly as you add a few extra tonnes.  Looking at Figure 3‑1, you notice that the SWATH maintains a large air gap between the waterline and the cross deck.  The waves wash harmlessly in this air gap.  That is how it should work, unless you put too much weight on the ship.  Discipline over your deadweight is key.  The engineer can’t store 15 spare filters onboard.  Too much weight.  Any ship spares quickly reduce your mission weight capacity.  SWATH ships show minimal flexibility in their cargo capacity.

Speaking of weight, you will need to arrange the major ship machinery differently.  Those submerged hulls rarely have enough space for a full engine room.  The main engines typically get mounted on the main deck, which is great for maintenance.  But they require complex shafting or a diesel-electric arrangement to deliver that power to the propellers in the hull.  Not all of your main deck space goes to the mission.  Many of the spaces typically found in the lower decks now occupy your mission deck.  Arrangements on a SWATH get creative.

The seakeeping advantages of a SWATH make it valuable to many owners, despite the design challenges.  Common applications for SWATH include:

1. Research vessels
2. Pilot vessels / crew transport
3. Semi-submersible drill ships

Research vessels are some of the most frequent SWATH subscribers.  A boat full of scientists and grad students who rarely frequent the sea.  They will thank you for the days without seasickness.  But even better, they love you when the weather gets bad and you can remain on station; continue the mission.  Due to their reduced motions, SWATH ships remain effective at higher sea states.

The stable ship and wide cross deck make SWATH ships perfect as a launching and support platform.  Plenty of deck space to mount new scientific equipment.  And a very reliable platform to deploy instruments into the sea.  SWATH ships are great for subsea operations and survey vessels.

Land lubbers aren’t the only ones who love a smooth ride.  Many pilot boats also enjoy a SWATH.  You find great reassurance in the safety of the stable platform before jumping over to the inbound freighter.  And the improved ride allows the SWATH to drive much faster.  Arrive at your freighter in short time, unfatigued from the ride.

Don’t assume that the SWATH hull only works for small ships.  Some of the largest SWATH vessels are used in the offshore oil industry, called semi-submersibles. (Figure 4‑2)  These huge ships can travel out to a drill site and ballast down with their lower hulls fully submerged.  Just four columns breach the water to connect with the cross deck.  The SWATH hullform creates a nearly motionless platform that allows excellent working conditions as they drill the wells.  Combined with dynamic positioning technology, the SWATH turns a semi-submersible into a movable piece of land.  These giants carry hundreds of tonnes in drill pipe, drilling mud, and related machinery.  They stand as testament that the SWATH ship is no child’s toy.

When you think about SWATH ships, remember seakeeping.  Imagine gliding gently over waves, placidly sipping your tea.  While the nearby monohull bounces around like a cork.  That serenity is what SWATH ships deliver.  It comes at the price of several new design challenges.  But for the right mission, a SWATH is worth it.