Smooth Sailing: Pros and Cons of a SWATH Vessel

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.