[PCW] Convert sail cat to power part 2 - was hulls

[Candy Chapman and Gary Bell]

Part 2:

Two other major sorts of monohull boats are seen in today's marinas.  
The full displacement monohull does not have a clear derivative in 
modern power catamarans, which all utilize planing to one extent or 
another.  The monohull sailboat contributed sailing rig and other 
traditional attributes to the far more recent development of sailing 
catamarans, but it's hull form was generally abandoned (with the 
possible exception of daggerboards).  These sail catamarans then evolved 
into today's second type of power catamarans, retaining virtually none 
of the monohull sailboat hull features. 

Finally, to converting sailing catamarans to power catamarans.  Why?  
Sailing catamarans have, by virtue of their sail rigs, virtually 
unlimited range.  Modern sailing catamarans are commonly used for 
coastal cruising as well as routinely crossing oceans.  By virtue of 
their high sailing speeds, limitless range and their unique 
maneuverability under power they are particularly well suited to do 
so.   In coastal and local cruising, sailboats of all sorts use their 
engines -- and don't use their sails -- maybe 90 percent of the time.   
Sailing catamaran hulls are similarly as slender as my second sort of 
power catamaran, or even more so, and use the slender hull minimized bow 
wake approach to breaking out of the 'hull speed wave trap.'  Note 
please that they lack the modern power catamaran's 'semi-displacement' 
planing features of flat bottomed aft sections and powerful enough 
engines to achieve significant hull lift.  They operate in strictly 
displacement mode.  The particularly small engines needed to only 
approach displacement hull speed are designed into the hull with oodles 
of compromises and trade-offs which have evolved over a considerable 
time to uniquely suit the hull and engine combo.  Adding a larger engine 
will be very challenging in terms of the strength of various hull 
components, size and shape of the spaces available, etc., and without 
planing capabilities in the hull shape, will not give spectacular 
improvements in top speed.   Interior space utilization in sailing 
catamarans has evolved into very efficient and effective designs.   
Significant modifications to add larger engines or much larger fuel 
tankage will force some very unhappy compromises.  Primary among these 
are considerations of weight and balance.  Catamaran performance, both 
sail and power, is incredibly sensitive to both total weight, and to the 
presence of significant weight far from the center.   Orders of 
magnitude more sensitive than the common monohull.  Adding significant 
weight in upgraded engines, and far worse in increased fuel tankage to 
an existing evolved sail catamaran hull will cost large penalties in 
sail performance and boat motion, for small gain in top speed under 
power and/or greater range under power alone.  The main fuel tank in my 
PDQ is probably the heaviest item aboard, and occupies the bridgedeck 
space where the center of gravity of the whole boat resides.  Adding a 
large fuel tank to a sailing catamaran would severely compromise its 
behavior if it could not be correspondingly placed.  Sailing catamarans, 
and my second type of power catamarans derived from them are thought to 
have some difficulty in finding comfortable marina berths.  In practice, 
I find that my anticipations far exceeded the reality, and I have had a 
little increased cost, but no particular difficulty finding dock space, 
and that the catamaran hull is particularly better suited for anchorage, 
by virtue of its roll resistance and the level of comfort aboard.  
Anchoring out is a particular treat in my book. 

In short, if you took the considerable trouble to 'convert' a sailing 
catamaran into a power only catamaran, you would compromise off some of 
the finest advantages of the sail cat, while gaining little of the 
advantages of the modern power cat.  A modern sailing catamaran cannot 
go very fast under power alone, but screams along under sail.  Sailboats 
of all shapes have interior spaces already compromised for sailing, like 
low overheads and sail handling cockpits.  These would be difficult to 
reconfigure to match the interior space utilization of a power boat.  
The difficulties of sailing are that you cannot ever count on favorable 
winds, and sailing catamarans are variously compromised in their upwind 
performance.  If you can loosen up your schedule, shed your case of 
'get-there-itis' and just go with the flow, the sail performance alone 
will get you wherever you would like to go, and if you are not after the 
speed of a planing power boat, catamaran or monohull.  When the 'diesel 
wind' is used either because the wind is wrong or the owner is lazy, he 
gets the performance of a full displacement 'trawler' sort of boat, not 
such a bad way to go either.  The tiny diesel engines already found on 
these boats can be given significantly more range by temporarily adding 
a modest amount of fuel bladder or gerry can storage for a particularly 
long jaunt without taking a huge penalty in handling and boat motion.   
With an unmodified, or slightly modified sail catamaran one can go 
anyplace today's power catamaran can go, albeit not on precisely the 
same schedule.  The long deep ocean passages that only the larger power 
catamarans can manage are readily available to almost any modern sail 
catamaran.