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06/06/07

Escape


13.75m LOA Powercat

The Escape 13.6 shows the exceptional fuel consumption that the CS hull form--- 

Category: General
Posted by: tony

full story goes here

            FUEL CONSUMPTION     

 

 

 Frequently in magazines, and on web sites, you will find largely unsubstantiated claims of “greater speeds”, “lower fuel consumption”, “longer range” etc for a particular brand, or type, of boat.  However, when the results of fuel consumption tests are published it allows everybody to look beyond the advertising and estimated performance figures and get down to what a boat is really achieving. We pride ourselves at producing very fuel-efficient powerboat designs that give high speeds on displacement hulls with as low as possible horsepower requirements. We also claim that our efficient “CS” hulls allow for longer ranges and higher speeds with a given amount of fuel. To make sure we can substantiate these claims, when possible, we compare our fuel consumption data with other similar boats. This serves as a continual validation of our design ethos. 

 

We recently had a 13.6m [44.6’] “Escape” displacement power cat launched here in New Zealand and an independent fuel test completed on her.  We managed to find published, professionally conducted, controlled fuel tests for three other types of vessels of a similar size with comparable parameters. These were all professionally built and had been designed by reputable naval architects. These were used for a comparison.

 

            The fuel consumption curves are for: a round bottomed, with an aft chine, “semi-displacement” powercat, a hard chine planing powercat and a “fast” cruising monohull. Our vessel, the “Escape” has the full displacement “CS” hull form that was developed by this office. The fuel consumption was matched to the displacements of the vessels so that the slightly heavier planing cat and monohulled vessel would not be penalized. ie: calculated on the basis of fuel consumption at various speeds per tonne of displacement.

 

            The results are shown in the accompanying graph.  Right across the speed range the “Escape” was using less fuel than any of the other vessels.  Her closest rival below 19knots was the semi-displacement powercat.  However, as can be seen from the graph above 10 knots the “Escape” is using, on average only 65% of the fuel per tonne of the semi-displacement vessel! Even below 10knots the “Escape” is only burning 60% of the semi’s fuel.  This means more than 40% more range for the “Escape” at this speed.

 

            At all speeds compared, the planing cat was using more fuel than the “Escape”.  At the planing cat’s drag hump at around 11 knots the Escape was using only 43% of the horsepower of the other boat! Above this speed the planning boat gets onto the plane and her fuel consumption begins to drop until 18 knots where it begins to rise once more.  At the top measured speed of 23 knots that the Escape reached with her 200Hp motors she is still burning only 90% of the fuel of the planning cat.  The planing cat can go faster than the displacement cat ie: it has a higher top speed.  But it uses an additional 480HP (total) to get another 6.5 knots of speed! This clearly shows the high price exacted by higher speeds. So unless very high speeds are required in a boat of this length the displacement cat is clearly superior. However having said that I should point out that we have produced displacement power catamaran designs that easily achieve more than 30 knots from very modest horsepower. But of course they have to be longer.

 

            This graph also shows the second advantage of the displacement cat. If the speed can be reduced to a slower cruise speed then the result is a greatly increased range.  If we look at 15 knots for example the Escape is only using 50% of the fuel of the planing vessel.  For a given amount of fuel this equates to twice the range at that speed!  To increase her range the planning cat could increase her speed to 18knots, at this point she would still only have 55% the range of the Escape.  The other option for the planing cat would be to reduce her speed to 8 knots to achieve the same range as the Escape achieves at 15 knots.  At 15 knots the Escape has a range of 616 nm (with 10 % reserve) and she would cover this distance in 41 hours.  The planing cat would take 77hours to complete this trip.  This means she would arrive 1.5 days later!  On a return trip cruise you would lose three days out of your holiday just to passagemaking!

 

            These comparisons have confirmed for us that the “CS” displacement powercat hull form that we use would appear to be the ideal for a cruising power boat. It is capable of both relatively high top speeds and extended cruising ranges that cannot be matched by the other conventional hull forms compared here.   To the owner this equates to lower fuel costs and more time spent on holiday and less time passagemaking, which should keep everybody happy!