Swedish blogger/twitterer Observationsplatsen yesterday posted a video published by Pella JSC of a Project 03160 ‘Raptor’ (ru. “Раптор”) performing trial runs. The trim of the boat during runs is rather more positive than in the case of its Swedish counterpart, something that spurred me to do some number crunching.
As said, what seemed evident from the video was that the boat traveled in a nose-high position, which generally means that the center of gravity is too far aft and/or that the boat has trouble getting through the transient phase and into the planing phase. The original CB 90 does not, meaning that something apparently went wrong for Pella. As said in my original post on the Raptor, reverse-engineering is not as easy as it seems.
A disclaimer to begin with: it is possible that Pella is simply performing their test-drives at half-power. However, I lack any idea of why they would choose to do that, and post a video of it without any notice about the restrictions imposed.
The hull form of the CB 90 is not optimized for planing, being of the semi-displacing kind. This provides better sea-keeping abilities, but it also means that there is less horizontal surface to provide the dynamic lift needed for planing. On the CB 90, this is accentuated by the very narrow chines. According to the three-view plans provided by Dockstavarvet and Pella JSC respectively, the width of the chines doesn’t seem to differ in any remarkable way. The difference in performance must thus come from something else.
According to Dockstavarvet, the CB 90 has a displacement of 18 tons, being powered by two 805 hp engines, giving a thrust to weight ratio of 89 hp/ton. The Raptor is cited by Pella as 23 tons, with two 1133 hp engines (curiously enough, the English version gives the total power as only 2 x 1000 hp). This gives the Raptor a higher thrust to weight ratio of 98 hp/ton, a 10% increase compared to the CB 90. However, this performance increase comes at a price. The original Scania DSI 14 engines of the CB 90 had a dryweight of around 1700 kg apiece. Pella has not given out their engine of choice on their homepage, but a secondary source of uncertain value gives the engines as Caterpillar C18’s. This seems logical, as several of Pella’s tugs have been fitted with Caterpillar diesels. The C18 has a dryweight of 1950 kg, or 400 kg more weight for the pair of marine diesels. Another change is in the type of waterjets employed. The Rolls-Royce FF410 of the CB 90 had a dryweight of 485 kg, and an entrained water volume of 192 liters/kg. I have not found the model or manufacturer of the waterjets of the Raptor stated anywhere. However, if one looks closely at the drawings provided by Pella, it becomes very clear.
That, my dear friends, is a Rolls-Royce Kamewa A3-series water jet, most probably of the 40A3-model. Unlike the axial-flow FF-series, the A3 provides a hybrid-flow design, giving higher performance. However, another major difference is in the material. Where the FF-series is made of aluminium, the A3 is of an all-steel construction. This raises the dryweight to 850 kg apiece, with an entrained water volume of 186 litres/kg. In total, a simplified calculation of the weight of the drive package (not counting liquids, turbocharger, transmission, shaftes, …) of the Raptor gives a total weight of 5970 kg, with the corresponding value of the CB 90 being 4755 kg. This is a 25% increase in the mass of the drive train (compared to a total increase in displacement of 28%). In fact, the increase in weight of the engine and water jet accounts for roughly 24% of the total increase in displacement.
The important part, however, is the fact that the drive train is placed at the stern of the boat. This gives any increase in weight in the drive train a large impact due to the long lever. As the increase in weight of the jet and engine is in line with (in fact slightly under) the general increase in weight of the boat, this should not affect the trim of the boat if all added mass was distributed evenly.
Why then does the Raptor travel around with its nose in the air? I have no definite answer. It is possible that the added ballistic protection or an increase in engine room insulation has caused a shift in the centre of gravity aft. Another possibility is that the lengthening of the hull (the LoA of the Raptor at 16,9 m is two meter longer than the 14,9 m of the CB 90) has caused a similar shift. The fuel tank is also probably situated slightly aft of the centre of gravity, meaning that an empty vessel with full tank(s) would be trimmed more nose-high than a loaded vessel with empty tanks. The hard numbers available indicates that the boat should be able to attain the 48 knots specified. However, until Pella releases a new video, I will stand by my opinion that the Raptor does look more like a 35+ kts boat than a 48 kts.