BMT has developed new hybrid catamaran/SWATH hull designs that offer significantly improved vessel motions in rougher seas compared to conventional high-speed catamarans. These include the MODCAT semi-SWATH design and new eXtreme Semi-Swath (XSS) hull, both aimed at meeting client requirements for passenger comfort while maintaining high speeds. The XSS design in particular combines the motion benefits of SWATH hulls with catamaran speed through the use of waterplane area reduction and active ride control systems.

1. High Speed Catamaran Hull Development
• BMT have a large range of high speed monohull and catamaran designs
• In order to meet client requirements for good levels of comfort in rough water
conditions, the company has developed new hybrid catamaran / SWATH (Small
Waterplane Area Twin Hull) designs that offer significant improvement in vessel
motions in higher sea states.
• Hull development was initiated by extensive work undertaken for USN on
Seafighter project

2. High Speed Catamaran Hull Development
Conventional High Speed Catamaran
• Low power requirement
• Moderate seakeeping capability
• Can be susceptible to
uncomfortable pitch, roll and couple
pitch and roll motions
SWATH
• Excellent Seakeeping Capability
achieved by small waterplane area that
reduces response of vessel to wave
motions
• High power requirement
• Limited speed capability

3. High Speed Catamaran Hull Development
MODCAT – Hybrid Semi-SWATH Catamaran Design Developed for USN to offer
improverd motions in rough sea states developed by BMT Nigel Gee.
• Motions (~50% reduction)
• Powering – small power increase in calm water
• Power saving in rough water / Reduced speed loss in waves

4. High Speed Catamaran Hull Development
• New eXtreme Semi-Swath (XSS) hull form design combines the benefits of
Swath hull forms and catamarans
• Very good motions in rough sea conditions through SWATH type low
waterplane area hull form in conjunction with high speed Ride Control System
(RCS) using T-Foil and interceptor.
• Higher power than Modcat hull form but offering outstanding motions for
reduced seasickness / increased passenger comfort
• Easy maintenance of main engines and waterjet propulsion

5. High Speed Craft - What Goes Where ?
• The following looks at the performance benefits of different catamaran hull form
designs for high speed operation.
MODCAT (Semi-
SWATH Catamaran)
• Slight increase in
powering over
catamaran
• Improved
seakeeping
• Good for high
speed ops on
rougher routes
XSS (extreme Semi-
SWATH catamaran)
• Higher fuel use /
more limitation in
speed over cat’
• Exceptional
seakeeping
• Good for med’ to
high speed ops
on rough routes
Catamaran
• Good efficiency/
fuel use
• Good seakeeping
• Good for high
speed ops on
reasonably
sheltered routes