SWATH work platform

[waterballast]

Does anyone know of a SWATH type vessel that has its totally submerged hulls in much deeper water than just immediately under the conventional hull.( 6 metres) Similar to an oil drilling platform but smaller. The struts from the platform/ work deck would go down to what would essentually be two remotely operated shallow water submarines in tandem.

[sharpii2]

WB:

From my understanding SWATH stands for Small Watertplane Area Twin Hull.
The basic concept is two catamarans stacked on top of each other. One is much wider and bouyant than the other. The wider one rides at or below the surface like a submarine. The narrower ones' bottomom is at or slightly above the surface. The two can be melded together to make a single catamaran with hulls that in effect have very bouyant keels which are much wider and have much more volume than the topsides.

This confers three major advantages:

1.) A vast reduction of wave making. It is the part of the hull at and above the WL that makes most of the waves.
2.) An increase in top speed. This is due to the fact that most of its drag is friction and not wave making.
3.) A vast reduction of pitching and rolling due to the fact that most of each is a factor of the waterplane area which, in this case, is very small in proportion to the hull's displacement.

It also confers three major disadvantages:

1.) The deck needs to be built much higher off the water than with a conventional cat. This is bcause it will tend to go through waves rater than over them.
2.) The friction drag is much higher than even a conventional cat because th much greater hull surface area.
3.) Loading it with a cargo that is a significant portion of its total displacement is very iffy because cargo capacity is also tied to the actual Waterplane Area. The fat bottoms of the SWATH may have to have ballast tanks in them much like a submarine, so that it can always maintain its ideal displacement and trim during a wide variety of load conditions.

Such a vessel would be great for work that needed a steadier than usual platform and required speeds significantly above hull speed.

I imagine such a craft would be ideal for offshore light to medium salvage work. Work such as raising sunken yachts, perhaps. Marine archaeolgy would be another practical use. As well as hunting for sunken treasure.

The vessel would be much more expensive to build and maintain, though.

Bob

[pbury]

WB: I have a design for a similar vessel to what you are describing. Perhaps a little longer, closer to 10m but designed as a versatile workboat/workplatform. Send me an email if you want more info: paul@bury.com.au

[waterballast]

Yes I would be very interested in your design but when I mentioned 6 metres I was talking about the distance between water level and the submerged hull.
The platform itself could be 20mx20m ...the submerged hulls 50m.

Anyhow please let me know about your ideas on this type of craft. I will look forward to your reply.

[jehardiman]

Waterballast;

My senior design study (1984) was a SWATH. There are complications when the submerged depth of the demi-hull becomes too great, the first being the draft itself (yes you can deballast but stability limits this). Second is that the powering requirements loose all the powering advantages of the SWATH hullform (i.e. the hollow reductions) due to wetted surface increases. Finaly, the structural requirements for strut depth and demi-hull strength grow until the structural weight becomes a limiting factor.

Most optimum SWATH hull forms have already been defined. See

SWATH ship design state-of-the-art;KERR, G. D.ANDERSON, T. A.KENNELL, C. G. (U.S. Navy, Naval Ship Engineering Center, Washington, D.C.); AIAA-1978-737, American Institute of Aeronautics and Astronautics and Society of Naval Architects and Marine Engineers, Advanced Marine Vehicles Conference, San Diego, Calif., Apr. 17-19, 1978, 11 p.