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From: (no name) (no email)
Date: Wed Oct 25 2006 - 13:41:07 EDT
Wayne Bailey asked:
> Anybody have any experience/opinions about power catamarans?
The Admiral and I own a PDQ 34 power catamaran trawler and absolutely love
it. (Technically our 80 foot sidewheel paddleboat/retirement home is a
power catamaran, but not particularly applicable here). We were the first
PDQ power cat owner on the west coast, and have cruised our boat across
the Great Lakes (delivering it from the factory near Toronto Canada),
round and about the Columbia River system plus coastal runs up around
Washington into Puget Sound and the Canadian waters around Vancouver
Island.
> My wife and
> I live aboard our 54 ft trawler - but friends are strongly considering a
> cat of about the same size.
You didn't mention them living aboard full time -vs- extended cruising
where your regular household goods stay ashore. I'll assume the long
cruising mode, based on your next statement. As liveaboards you doubtless
understand already that it is a whopper of an adjustment to get rid of all
the treasured stuff we accumulate, which causes folks making that
transition to full time live-aboard for the first time to get into rather
larger boats. There are a good number of power catamarans capable of the
west coast long cruise in considerably less than 54 foot lengths. If you
meant space equivalent to a typical 54 foot monohull trawler, you would be
talking about typical catamarans around 40 foot lengths. By virtue of
their shorter, squarer overall layout, they allow more roomy accomodations
to be nearer each other, with great light and visibility for the stuff on
the bridge deck, compared with monohull designs which force every
functioning space to more or less line up on a single axis and be
relatively low to the water.
>They are interested in cruising and fishing
> from Baja to the Northwest.
I'll further assume that you and your friends are familiar with the
weather, sea conditions, harsh coastlines and both the scarcity and the
dangerous entrances to the harbors of refuge that distinguish our Pacific
coast boating from most other places. My point is that nearshore and
offshore west coast boats need to be a lot more hardy and have much longer
ranges than most others. Please caution your friends that not every power
catamaran design meets these needs well.
>High speed is not important - plan to cruise
> in the 10 - 12 knot range
I dissagree, to the extent that I feel that the option of high speed can
be a lifesaver. Speeds of twenty knots and sometimes considerably more
are readilly available in power cats, and offer a broad choice in going
fast at reasonable fuel cost or going slow at amazing fuel economy. The
ability to outrun nasty sea and weather conditions, and the power and
maneuverability needed to safely thread some of our more 'adventerous'
harbor and river bar entrances is essential hereabouts.
>but is considering some higher speed cats with
> the plan of cruising slower (single engine op possible ???)
Yes, and despite the props on my boat being 12 feet apart, the single
engine handling is quite reasonable (probably due to having two slender 34
foot hulls acting as keels). Just a little 'windward helm' and it works
fine. I don't do that because with my slender hull design, I can get down
to around 1 gallon per hour with both engines on at maybe six knots,
transitioning smoothly (virtually non-planing) to a fast cruise of four
gallons per hour near sixteen knots (in peacefull water), and I have
kissed twenty two knots a couple of times ('downhill', at damn the
torpedos fuel economy). Weather and waves make a huge difference.
Example; my boat carries 115 gal. in the main and 70 in the
forward/generator tank. Downriver from Portland to Astoria in a leisurely
day, just over 100 miles, topped off tanks around 25 gallons. Next day,
over the bar and slugging up the Washington coast to Neah Bay, about 100
miles again, and a tough 12 hour transit; but with fifteen knot winds and
a knot or so of current on the nose and seas/swells running from six feet
at the south to upwards of fifteen feet at the north end. Again in a day;
fuel burn 125 gallons.
> for fuel
> economy and having the higher speed avail for shorter runs or
>emergencies.
Yeah, but I value very highly a huge fuel capacity so I have the option of
going fast/hard and thirsty or slow and thrifty depending only on my whim
and wallet, and avoid the temptation of counting on extending the range
too far and risk not having a safe fuel reserve for some emergency.
>
> I have applied Keith's Formula to an excell SS. Any thoughts on the
> validity of Keith's to a powercat? Coefficients?
I am unfamiliar with this formula, and I generally suspect that catamaran
hulls are different enough from monohulls that great modifications would
need to be made to it to get realistic predictions.
Two related formulae -- or relationships seem pertinant to me. One is the
familiar 'hull speed -- wave trap computation that dominates monohull
design. Wave trap speed in knots equals one point three four times the
square root of the hull length in feet. Here you calculate the speed
where the trough behind the bow wake crest and the trough ahead of the
stern wake crest appear to move along the side of the hull toward each
other as the speed increases; up to the 'hull speed' where the two troughs
merge and reinforce each other. Thereafter increasing power only seems to
dig a deeper hole in the water.
The bow wake crest is literally all the water that had to be shoved aside
to accomodate the hull as it progresses. The attendant trough is the
result of the inertia of the water sloshing outward away from the hull and
then sloshing back to form a second crest, etc. in a harmonic pendulum
like way. The size of the crest, and therefore of the trough is dependent
on how much water had to be shoved aside: the frontal cross sectional
area of the immersed portion of the hull -- and the speed with which it
was shoved aside. The shorter, wider and deeper the hull, the more water
must be shoved out of the way. The faster you go, the more energy you
invest in moving this water. When the bow wave system and the stern wave
system troughs get close to each other, they link up and reinforce each
other, where a strictly non planing hull is caught in it's wave trap.
Planing hulls of course use lots of power and a plenty of lift from the
flat shape of the bottom (working kinda like a water ski) to climb up and
over the bow wake crest and then fly at speed 'on the plane' that is with
very little displacement and all lift. This works great, although it
takes several times more power, depends on a suitably flat bottom, and
it's performance is only acceptable at certain speeds, well below the
planing threshhold and well above it.
If the frontal area could be dramatically diminished (for the same
length), the depth of the resultant wave trap would also shrink. A
sufficiently powerful long slender hull, still in displacement mode could
power it's way up to and over the top of the bow crest. This is sometimes
called 'slender ship theory' and it works quite well. Think of the WWI
'four stack' destroyers, 430 feet long and maybe 25 feet beam. With tiny
displacement and lotsa coal fired steam power and they would go into the
high thirty knot speeds, maybe even the forties (I count on my naval
history expert listmates to correct any of those recollected but
unconfirmed details)!
Catamarans can by virtue of having two hulls to support the displacement
load have much less beam of each hull (the portion actually in the water).
Catamarans can also employ planing hull shapes and more powerful engines,
and these are often called tunnel hull cats, with what appears to be a
regular planing hull split down the middle with a relatively narrow
tunnel.
There is a wide selection of approaches, although all designs utilize both
schemes to some extent.
Many of the power cats around today lean toward the planing tunnel hull
approach, with wide individual hulls and total power aboard well over 300
hp. They have fine high speeds and behave much like any other planing
hull with some refinement in boat motion, mostly underway.
The PDQ power cats (check out the brand new 42 footer at their website:
pdqyachts.com!), the hull designs of Malcolm Tennant, Moretti and Melvin,
and a few others (as well as virtually all sailing catamarans -- naturally
low powered) lean heavilly toward the slender, non-planing hulls with low
power and therefore thrifty operation.
There are an increasing number of really intermediary designs, with hull
length to hull beam ratios appearing to be in the 5:1 to maybe 8:1. These
seem to still cling to the large engines and fast planing speeds, but the
lower power and slimmer hull design seems to me to be a slow trend here.
My PDQ 34 has a beam for each 34 foot hull of about 34 inches. Powered by
a pair of 75 hp engines, there is no regular wave trap or step like
planing behavior, and speeds can be selected anywhere up to top speed with
an attendantly smooth power and fuel economy curves, topping at around
five gallons per hour at wide open throttle speeds around 19 kts (in
relatively flat water and without wind). There is some planing shape to
the aft half of the hulls, added to remedy the stern drop you get when the
merged 'wave trap' trough reaches the stern around twice hull speed. The
net result is that my boat's bow raises about five degrees at full speed,
so I never have to leave my comfy lower helm station to see over the bow,
and need no trim tabs. Fine weather and fancy docking alone tempt me to
the flying bridge. With two hulls weighing four tons each seventeen feet
apart there can be no pendular roll (the only roll happens when a swell
raises one hull before getting to the other (very mild, slow and
graceful); so no need for stabilizers or paravanes underway or stationary.
The props are twelve feet apart so the boat can even pivot around a
center about even with the lower helm; no great need for bow or stern
thrusters.
Clearly I really love these boats and could go on swooning about mine for
days, but will instead close by suggesting that your friends consider
chartering a PDQ 34 from Sunsail in Vancouver Canada, or if they like they
are welcome to visit me in Portland for extended discussions and
boatrides. They are also welcome to call as well, email me offline for
address and phone numbers. PDQ has also arrainged for Swiftsure Yachts
in Victoria BC to be a stocking dealer, so anyone interested could reach
them there as well.
I have no financial interest in this beyond being a very happy and
gregarious owner.
>
> Thanks for your help.
>
> Wayne Bailey
> Bermuda
> King Harbor Redondo Beach, CA
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