In the beginning of October, a few days after the Boat Show of Genoa, the first official presentation of the new Suzuki DF 350A outboard engine took place in Spain, at the Barcelona Boat Show, where we had the opportunity to test the Tempest 38΄ on which two Suzuki DF 350A engines were mounted.
Tempest 38 is a 12 meter mega-rib which stands out for its large bow sundeck, the spacious aft lounge and its particularly comfortable cabin, while retaining the character of a very beautiful open rib.
It is a real boat of pleasure that provides all the facilities to those who want to spend a lot of time on board, overnight with comfort as well as to set long destinations on the nautical map.
In this article we intend to focus on the performance of Tempest 38΄ with the new Suzuki 350hp engines since the description of its deck has already been made in a relevant article in this section.
Although we had almost flat conditions during our test, that did not prevent us from evaluating the Tempest 38.
It is a very large and heavy rigid inflatable boat that gives you a great sense of safety and stability even when you drive it to its limits.
It’s obvious that it can comfortably cover long distances offering delightful trips to its passengers.
Crossing our wakes, sometimes even at high speed, Tempest 38’ showed its robust construction with no pounding and any kinds of disturbing vibrations. Every impact of the waves on the hull was perfectly soft while maintaining a very good cruising angle even at low speeds, causing us to push constantly the throttles to the WOT.
The good weather conditions on that day gave us the opportunity to focus our attention on the combination of the Tempest 38 with the new Suzuki engines and to evaluate in detail the performance of its hull, which for the first time would be combined with two lower units, the shafts of which were turning twin propellers.
With a gear ratio of 2.29: 1, the two 350hp Suzuki outboard engines were spinning the 15 ½" x 22.5" dual propellers at 6100 rpm (WOT) when we brought the trim up to 70%, recording a top speed of 48 knots with a fuel consumption of 225 liters per hour. Calculating the theoretical speed at 6100 rpm, which is 49.3 knots, we see that the percentage of slip is only 2.6%, a number that indicates a very good set-up for the particular loads.
We chose this pitch of the propellers because our aim was not to get the best possible top speed but to see the behavior of the engines when carrying a total estimated weight of 4.5 tons and to record the performance of dual propellers in relation to heavy loads.
And indeed the contra-rotating propellers managed to make the mega-rib coming up on plane at very low rpm and standing easily out of the water, indifferent to the heavy loads.
With 500 liters of fuel in our tanks and 4 persons aboard, the 12-meter rib was on plane at 2200 rpm keeping a speed of about12 knots. This speed was also the most economical, with both engines consuming 2.1 liters per nautical mile.
At 3500 and 4000 rpm, the cruise speeds were 25 and 30 knots and the fuel consumption 2.6 and 2.8 liters respectively per each nautical mile. The above numbers are the best we've seen so far, considering the length and weight of Tempest 38΄, giving a range of 179 to 192 nautical miles.
Still, what impressed us the most was the amazing acceleration, despite the long pitch of the propellers, which was noticeably felt throughout the whole rpm range as well as the very low numbers of slip despite our heavy loads.
We all know very well that as loads increase, the acceleration decreases while slip-values climb to high levels, resulting in poor performance and increased fuel consumption.
This is where Suzuki's great success is achieved, which, with the application of dual and inversely rotating propellers, provide great accelerations and low slip numbers while allowing us to load our boats as much as we want without sacrificing performance. And of course, we refer in terms of handling, planing time, acceleration, cruise speed, fuel consumption as well as top speed.
It is worthwhile looking at the table below to see the excellent slip values, the percentage of which is gradually decreasing as the engine speed increases. At medium rpm the slip rate reaches 6.3%, a really impressive number, while at WOT the slip is only 2.6%, which simply means that we get almost the maximum speed the propellers can give.
The above figures clearly show that it is worthwhile raising the engines one hole higher, which is mainly due to the Suzuki’s dual-prop system, and fitting longer propellers of 24".
In this case it is almost certain that we will see even better numbers in each field, not only because of the increase of the pitch but mainly because of the less drag of the two lower units, especially when using the Tempest 38΄ with less loads.
Cantieri Capelli S.r.l.
Via delle industrie, 19
26020 Spinadesco - Cremona (Italy)
Tel. +39 0372 491399 Fax + 39 0372 492115