Powercat Fuel Consumption Comparisons
by Malcolm Tennant Multihull Design LTD
Frequently in magazines and on web sites you will find claims of ‘greater speeds’, ‘lower fuel consumption’, ‘longer range’ for a particular design. Here at Malcolm Tennant Design Ltd, we pride ourselves at producing fuel efficient powerboats which give high speeds on displacement hulls with as low as possible horsepower requirements. Our efficient hulls allow for longer ranges with a given amount of fuel. To make sure we are delivering the best possible performance to our customers, when possible, we like to compare our hull performance data with other boats as a validation of our design ethos. When fuel consumption tests are published, it allows everybody to look beyond the advertising blurb and estimated performance figures and get down to what a boat is really achieving.
We recently had the 13.6m Escape launched here in New Zealand and had a fuel test completed on her. We managed to find two other published fuel tests for recently launched (2003) vessels of comparable designs. These other two boats have been designed by reputable designers that are competent in designing power multihulls.
The other results are for a round bottomed with chine semi-displacement powercat and a hard chine planing powercat. The fuel consumption was matched to the displacements of the vessels so that the slightly heavier planing cat would not be penalized. Our vessel, the Escape, is a full displacement hull that is Malcolm Tennant’s signature powerboat hull form.
The results are shown in the following graph. Right across the fuel range the Escape was using less fuel than either of her competitors. Her closest rival below 19 knots 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 of the semi-displacement vessel! Even below 10 knots the Escape is only burning 60% of the semi’s fuel. This means more than 40% more range for the Escape at a given 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 other 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 speed the Escape reached with her 200 Hp motors of 23 knots she is burning only 90% of the fuel of the planing cat. The planing cat then uses an additional 480 HP (total) for another 6.5 knots of speed! So unless very high speeds are required in a boat of this length, the displacement cat is superior.
This figure also shows the second advantage of the displacement cat in that the speed can be reduced to a slower cruise speed for 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! To increase her range, the planing cat could increase her speed to 18 knots. 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 77 hours 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 passage making!
This has confirmed for us that our displacement powercat hull form is the ideal cruising power boat, capable of both 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 cost and more time spent on holiday and less time passage making, which should keep everybody happy!
