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Power Catamarans 

Here's where we get more technical

BOTTOM LINE: More comfort with less motion. Better efficiency (lower fuel consumption for a given speed). Power cats outshine conventional Trawlers. They give you the range, comfort and higher speeds.

Here you'll find both simple and technical explanations of why a Cat Trawler offers advantages over a monohull trawler (or so called fast trawler).

Some important stuff 

A lot's been learned recently--some stuff didn't work.

What didn't work-the solutions...

1. Setting up an aft-cabin, mid cockpit boat. Not enough headroom over the island berth, too many ups and downs--stairs, and trip ledges everywhere.

2. Open bridge deck between the hulls forward. Spray guards on the hull don't work. What happens is that at once you start exceeding 8-10 knots, the bow wave on the inside of the hulls allows the increased wind pressure from the higher speed to create a venturi effect. This causes fine droplets of water to be swept up onto the fly-bridge where the occupants experience a fine mist that ultimately soaks them! (The solution is a tight weave spray cover, or solid decking led further forward.)

3. Narrow hulls. Not enough load carrying and/or they submerge with extra power. (How can you tell? A simple test. If the berths are over the bridge deck, instead of down in the hulls--the hulls are too narrow for load carrying and higher power applications.) Other disadvantages of berths up high, over the hulls: High and difficult to get in and out of. In summer or southern waters--this is where the heat is--up high!

4. Unprotected running gear. This is a problem on virtually all trawlers. Hit a sand bar, and you take out the shaft, strut and probably the rudders. Not only is this expensive, but typically it costs you 3 or more months waiting for everything to be made right--get the insurance company involved and it probably takes longer! The better solution is to build in a structural skeg. All of the Fountaine Pajot models incorporate this. 

5. This entire category of cruiser is new. Many players will come and go. Do you want to get stuck with a one-of-a-kind, with no warranty if the Manufacturer goes under? Not me. Fountain Pajot is the largest builder of Catamarans in the world!!! They build power, sail and commercial catamarans. They are financially strong and well capitalized. They've been around for over 25 years and will be there when you need them. This will also protect your resale value.

6. Keeping the heat out. A great design (left-the Fountaine Pajot 37) uses more vertical windows with a substantial roof overhang that keeps the high, afternoon sun out of the living quarters. Many boats, like the one to the right, below, have little or no overhang and sloped windows. You end up having to put covers on the windows which defeats the entire purpose of the large, eye level windows. You restrict the wonderful outside view! (Also makes it very difficult to clean the windows from the salt crud, etc. that accumulates behind the covers.)

Also, note on the boat at the right that the narrower, W.L. beam may have created the need to extend the transom aft to gain more buoyancy for load handling and/or to give more planning surface for larger engines--not a good solution. . 

Now, continue on...

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There is none!

The phenomenon of power catamarans is very recent. Sailing catamarans and Trimarans are steeped in deep history, however, dating back to the early days of the Micronesian and Melanesian migration.

  BYA has been selling sailboats and Trawlers since the early 70’s. 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.

1. In the last 6 years, we have been involved in the planning and developing of catamaran sailboats with big engines that cruised up to 17 knots under power.
2. In the late 80’s we sold a line of Trawlers, but were unhappy with the performance—especially their rolling in a seaway or from wakes from passing boats in the inter-coastal, but also with the inefficiency when you tried to push these behemoths over 10 knots or so.
3. We have continuously been involved in customizing stock boat designs for long distance, live-aboard cruising.
4. Finally, we were recently greeted with the happy fact that Fountaine Pajot, the largest builder of long-distance cruising catamarans in the world, had recently committed to the Catamaran Trawler market. (In addition to offering successful designs in sailing models, they have been building 200’ plus commercial catamaran passenger and freight carriers.)

 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.

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There’s the simple answer. For more details read on…

Is there a negative? Perhaps. The cost may appear to be higher for a given length boat—and that’s 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. (It’s more like you’re 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 there’s the CSES (Comfort/Stability/Economy/Speed). Is it worth it? Will the world discover this concept—I think so!

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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

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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!

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, it’s 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 don’t make sense for cruising!)

At around 12 knots the planing vessel is using 4 times as much fuel. I.e. it’s 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 it’s passengers very gently.

For more, go see the Fountaine Pajot range of well designed Catamaran Trawlers, or contact us below for more information.

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Bay Yacht Agency Group

Power/Sail/New/Used/Charters
On the Harbor in Annapolis
326 First Street, Suite 29
Annapolis, MD 21403 (map)
Sail/Cat Trawlers: (410) 263-2311

Power 410-990-1090 or Cat Trawlers .
(410) 263-2964 (fax)
info@bayacht.com

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Let's Go Cruising 410-263-1818

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