BYA has been selling sailboats and Trawlers since the early 70s. We have
concentrated in that time in long distance, offshore capable and live-aboard cruisers.
Several events led us to the catamaran trawler as a logical next step for serious
cruising.
Some of this information was
gleaned from a very knowledgeable source, Courtesy of Capt. Graham Pfister, www.trawlercatmarine.com If you are interested in a customized catamaran trawler, please visit these knowledgeable folks at their site.
Return to index
Why a power cat?
In a word, comfort! The secret is in the twin hulls that track better than a V-hull and
the tunnel between them that compresses air and water that gives you the soft ride. The
narrower hulls partially pierce the waves so the pitching motion is less. The wide stance
with individually narrower hulls also greatly reduces the rolling motion.
Since the cat does not stop (or slow down) for every wave like conventional monohulls,
it travels faster using less power and thus has better range and/or speed for a given
engine size.
- Comfort
- Stability
- Economy
- Speed
Theres the simple answer. For more details read on
Is there a negative? Perhaps. The cost may appear to be higher for a given length
boatand thats generally the case. With the extra beam and two hulls there is
simply more surface area (translated fiberglass!) to build. Systems must be duplicated in
both hulls. Pumps, engines, electrical and plumbing systems, etc.
Having said this, what you end up with is more usable space for a given length. Think
in terms of the catamaran having the space of a 8 or 10' longer monohull and you get
closer to putting things in perspective. (Its more like youre buying a
47 boat in a 37 length for about the price of a 40-42 conventional
boat). Fuel costs are considerably less. Also consider that the very redundancy that costs
more, also provides insurance on a long passage. Recently on a sea-trial we lost an engine
(turned out to be a loose water hose causing an overheat condition). We shut that engine
down and proceeded at 9+ knots under one engine with hardly any difference in helm feel.
The autopilot still handled the boat just fine while Chris jumped down in the engine room
to sort out the problem. What would you be doing if you were in a single engine monohull
as you sit rolling in the seas?
And then theres the CSES (Comfort/Stability/Economy/Speed). Is it worth it? Will
the world discover this conceptI think so!
Return to index
More on stability
(And motion comfort)
A typical trawler cat sits flat and does not roll. It enjoys the inherent stability
that comes from two widely spaced hulls that allow the wave pressure that normally causes
a single hull to roll from side to side to pass under one hull at a time creating no more
than a quick up and down motion as it sidesteps over the wave. This is the single most
expressed delight of those who tend towards motion sickness. This rather quick and
unpronounced motion does not produce the same sea-sickening effect that a long, slow
rolling motion does.
Courtesy of Capt. Graham Pfister, www.trawlercatmarine.com
Return to index
More on safety (without getting too technical)
Tank testing was recently carried out by world-renowned Australian catamaran designer
Lock Crowther. It proved conclusively that the power catamaran configuration was superior
in stability to a conventional monohull power vessel. In simulated 52 breaking beam
seas (the largest the facility could produce), the power catamaran could not be rolled
over but the similar sized monohull power vessel could.
In addition, in following seas, the relatively narrow hulls of a
catamaran track straight and true with none of the broaching tendencies of their monohull
brethren. In fact, in most any type of seas from head to abeam to aft--the catamaran is
easier to handle and more comfortable for her guests. Perhaps one of the biggest safety
factors is that the easier motion makes for a less tired crew which is less likely to make
mistakes!
Hundreds of sailing catamarans cross oceans regularly. We, personally deliver about 12
trans Atlantic every year. These vessels make fast, comfortable passages with short handed
crews all of the time. Perhaps the real proof of the pudding is that the insurance rates
for passages in a catamaran are the same or less than those for a monohull for passage
making voyages!
Courtesy of Capt. Graham Pfister, www.trawlercatmarine.com
Return to index
Getting more technical--Performance
Lets look at what makes a power catamaran more efficient. Why do we get more range and
speed for a given sized engine? (Comments and curves are the courtesy of an article in Power
Multihull magazine, Planing vs. Displacement Comparison, by Malcolm Tennant.)
The catamaran may initially require more horsepower than the monohull because of its
lesser planing surface but there is the same initial high drag hump which requires lots of
horsepower to overcome. Once this hump is scaled there is a marked decrease in the
hulls resistance to motion which only gradually begins to climb again as speed
increases.
Although the displacement catamaran will have a wave resistance-determined hull speed
just like the monohull, this hull speed will be very much higher. The speed of the
displacement catamaran is largely a function of the fineness ratio of the hulls. At low
speeds, see the curve, all the factors are similar and, in fact, its very difficult
to get the displacement catamaran to go that slow (we found that at times we have to run
on one engine to meet low speed limit requirements!)
Realistically, the upper speed range for a displacement catamaran hull is about 20-24
knots. This comes about because of the balance ultimately struck between the beam of the
individual hulls, comfort considerations, the size of the engine that must be fitted in a
relatively narrow hull and the desire for reasonable (or spectacular!) range required of a
serious cruising boat. (In other words, further increases in the fineness ratio just
dont make sense for cruising!)
Looking at the curves
Displacement vs planing catamaran with the same displacement.
At around 12 knots the planing vessel is using 4 times as much fuel. I.e. its
hull resistance is 3 times as great and it needs 3 times as much horsepower to move at
this speed. At 20 knots the planing boat is still using twice as much fuel but the curves
are converging. At 24 knots, things even out. The planing vessel will continue to gain,
though it too will eventually reach a point where ridiculous amounts of horsepower will be
required for continual gains.
The
reason for the ultimate limits rests on the fact that for a vessel to plane, the loading
or weight on the planing surfaces of the bottom must not exceed so many kgs/sq.cm of
surface area. If the loading is greater than this then the vessel will not plane and it
will function as a rather inefficient displacement craft. This is the reason why the
larger monohulled pleasure boats are displacement vessels. This principle is even more
important with catamarans, which may have limited planing surfaces in the first place.
The trick, obviously, is to increase the fineness ratio (achieve the proportions) that
move the resistance curve for the displacement boat further to the right, ie: it can be
made to have a higher hull speed by making the hulls longer. It is advantageous in terms
of fuel economy/horsepower requirements to keep your top speed, and particularly your
cruising speed, as far down into the flat part of the resistance curve as is practically
possible (in this case around 15-20 knots). (Going much slower than this does not reduce
the fuel consumption by much.) Going longer increases the cost of the hull and power
plants required.
Juggling the variables, inevitably, seems to mean coming out with
a fuel efficient, semi-displacement catamaran hull which achieves cruising speeds in the
15-20 knot range while treating its passengers very gently.
For more, go
see the Fountaine Pajot range of well designed Catamaran
Trawlers, or contact us below for more information.
Return to index