[PCW] Convert sail cat to power part 1 - was hulls

[Candy Chapman and Gary Bell]

On 12 Mar 2008, at 13:31, Roger Jacob wrote:


>> I know this is na over discussed topic, but can I get a 46'sailing cat
>> and make of it a power cat? (changing engines from 4 to 6 cilinders,
>> biggers fuel tanks...)Will it be a safe and seaworth boat? What are  
>> the downsides and upsides?
>>
>> Roger
>  
>

Is it over-discussed? I would be interested in reading some of the  
comments myself.

rupert.


Part One:

OK, here goes.  These are my thoughts.   I encourage a lively discussion 
of them, particularly by folks who disagree in general or specific 
ways.  Disclaimer One:  I am speaking in broad generalities here, and in 
no way am I baiting advocates for any specific boat design to loudly 
proclaim their loyalties or the unique ways in which they exploit the 
advantages or dodge the shortcomings of their type of design.  There are 
probably a number of boats which do not entirely fit my general types in 
one way or another, although I can't think of any which are clearly 
midway between them.  Disclaimer Two:  I own and love my PDQ 34 
(described here as my second type), and claim that I have some 
experience and insight into its design and the ways it contrasts with 
others.   I am not a naval architect, and have no association with any 
commercial boating interest, apart from being a happy consumer. 

First, even a casual survey of the power catamarans available today will 
show two distinctly different sorts of boats.  Give particular attention 
to pictures of them underway.  One throws big splashy wakes typical of a 
power boat, and the other throws much smaller and less splashy wakes 
associated with a sailing catamaran.  This first one is derived from 
planing monohull designs, and gains power catamaran status with the 
addition of a relatively narrow and shallow tunnel down the middle of an 
otherwise familiar hull.  This provides significant improvements in 
handling, comfort, stability and internal space while retaining the 
basic planing boat hull shapes and large lusty (also means thirsty) 
engines.   The majority of modern power catamaran designs available 
today (under about fifty feet) fit this general pattern.   These boats 
trade off cruising range for speed, and exhibit typical planing 
behavior, namely a range of less useful speeds where the boat is 
climbing up and over its bow wake (getting 'up on the step') before it 
succeeds in skimming over the surface of the water supported largely by 
lift.  This planing lift is achieved by providing the hull with large 
flat bottom surfaces aft, and the aforementioned lusty engines.   Thanks 
to the typical 'deep vee' style bows, among other factors, these boats 
handle rough water rather well, although their limited range means that 
they dare not go far enough offshore for ocean crossing passages.  They 
are excellent coastal cruisers, although by virtue of their higher top 
speeds they probably are more used for shorter and faster trips typical 
of planing monohulls -- like a run out to the hottest fishing hole.   
They have achieved long coastal passages, although their general use is 
the shorter faster cruising.  I don't think there are very many of this 
design among the over fifty foot sized boats. 

The other sort is derived from sailing catamarans and is recognized by 
its slender hulls, its dramatically wider and taller 'tunnel' between 
the hulls (where water seldom contacts the bridgedeck -- ideally!), and 
its much smaller engines - by a factor of three or more.  These boats 
generally have beams around half their length, and the individual hulls 
width in the water compared to their length is on the order of one to 
ten or more.  These boats feature somewhat lower speeds, depending upon 
the slender hulls making a correspondingly smaller bow wake, and what is 
loosley called 'semi-displacement' behavior where a small flat bottom 
section aft gives just enough lift to to allow the much smaller engines 
to push through the bow wake.  Note that I said through in place of 
over.  The large bridgedeck provides abundant space for very comfortable 
interiors, and the various designs cleverly use the narrow hull space to 
compliment that, again derived directly from sailing catamaran 
practice.   The huge beam means means wonderful freedom from harmonic 
rolling while the large separation of the props, provides fabulous 
maneuvering.  The slender fine entry hulls unhappily are more sensitive 
to harmonic pitching, although that is generally easilly remedied by 
changing speed and angle of attack to the offending waves.  This pitch 
sensitivity, with a couple of other common design choices limit their 
heavy weather performance, while their roll resistance enhances it.   We 
don't need no steeenking bow/stern thrusters, paravanes, flopper 
stoppers, active or passive stabilizer fins.  The small engines provide 
excellent range and superior fuel economy.   Overall the rough water 
tolerance is similar to that of the better planing hull's, and the 
comfort levels significantly better.  Designers recognize that these 
boats are also poorly suited to cross oceans, and provide fuel storage 
suited for coastal cruising.   They too are excellent coastal cruisers, 
with differences of speeds available, boat motion, and such, and are 
generally used in this way -- more cruising and less fishing.   They 
have also achieved long coastal type passages, and are arguably better 
suited to do so.  Only the largest power catamarans of this second type 
can easilly manage long ocean passages.   Among today's designs I feel 
that there are rather few of these second sort of power catamarans under 
fifty feet or so in length, and that virtually all the power catamarans 
over fifty feet conform to this approach.

check out part two in my next message