Catamarans

December  17, 2006.

These are my thought of what an oceangoing catamaran should be.

Safety

A boat that can take a husband and wife crew across an ocean with safety and dispatch, as Robert Beebe wrote in his classic book, "Voyaging under power".

The covers of the first and third edition of Robert Beebe's classic.

A must for anybody who wants to cross oceans on a powerboat.  

 

A catamaran that is not intentionally steered across bad whether, but that is not afraid of it when it happens.

I do not think that a catamaran under 20 meters can safely cross an ocean without what Malcolm Tennant calls "anti slam nacelle" in the middle of the cat, between the two hulls.

The anti slam nacelle of Wild Wind IV, designed by Malcolm Tennant.

Notice how there are no flat, horizontal surfaces against which the waves can slam.

Ask anybody that has motored a sailing catamaran against the waves what a nerve racking experience the slamming of the flat underside against the waves is.  Of course sailing cats do not go against the grain, so really do not need them, but power boats do.  I may be wrong and ready to be proven wrong, but to me in the "small" catamaran world the anti slam nacelle separates real cats from pussies.  

With a well designed oceangoing cat you will be doing 10 to 12 knots, if not more, for a considerable amount of time, day and night.  Unfortunately today the oceans are littered with nasties like oil drums, big tree logs and even containers that show only a few centimeters above the water.  There is nothing that you can do to avoid these obstacles, like you cannot avoid being hit by lightning if you are in the wrong spot at the wrong time.  Radar will not see these objects if there are waves and you cannot man a searchlight all night and try to see what lies ahead of you.  Your only recourse, if you don't want to stay close to shore and navigate in daytime only, like planning yachts do, is to try to minimize the damage should the unthinkable happen.  This means that the hulls must have a watertight bow compartment that hopefully absorbs all the damage.  The propellers and rudders must be protected.  Riding over a log must not automatically bend or shear your propeller shaft and tear away a rudder.  I am also very suspicious of any appendices like fins or wings, since it is very difficult to forecast if the wing will break away rather than tear a hole in the hull.   

 

Length

I limit the maximum length to 20 m (app. 66'), even though I know that in the catamaran world maximum length is not equal to size, but somewhere  I have to draw the arbitrary line and for us Europeans (and Australians, New Zealander's etc. etc)  20 meters is a nice, round number.  20 meters is also about the maximum size a couple can handle, not so much for maneuvering, but for maintenance and daily household chores without becoming slaves of the boat.   20 meters also keeps the cost of a cat within a reasonable sum, comparable to a similar sized trawler yacht with two engines.  

Range

An oceangoing catamaran must carry enough fuel to cross an ocean.  Let's take the Atlantic ocean as an example. 

 

The longest leg going westwards, that is from Europe to the Americas, is from the Cape Verde Islands to Barbados, 1900 miles or if you take the canonical trade wind crossing from the Canary Islands to Barbados 2700 miles.  2700 miles are a lot, but don't forget that the whole trip is going downhill.

Going eastwards the longest leg is going from Bermuda to the Azores, 1820 miles.

I want to cross the ocean at a reasonable speed, both for safety and to avoid boredom.  Don't believe all those stories of wonderful crossings with always something pleasant to do, wonderful food and restful night sleeps.  Reality is more like keeping watch at ungodly hours, eating cold sandwiches and constantly moving about, which can be very tiring.  So the sooner you are there the sooner you can drop anchor in a protected tropical bay and enjoy a beer, a barbecue and a good nights sleep.

For an 18 m (LWL) long displacement trawler the hull speed (S/L ratio of 1.34) is approximately 10 knots, which is good going for an ocean crossing, if it can be kept the whole time.  So I want my oceangoing cat to be able to keep a speed of at least10 knots for a long time.

 

2006-12-17
cat_beebe.jpg, cat_atlantic crossing.jpg
cat_oceangoing cats.htm