Project 03160 ‘Raptor’ Capable of 48 kts?

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.

Raptor
The nose-high running of the Raptor. Source: Pella JSC
Strb 90
The flat running profile of the CB 90. Source: Wikimedia Commons & Norwegian Defence Forces.

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.

The narrow chine of the CB 90. Source: Author.
The narrow chine of the CB 90. Source: Author.

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.

Water jets
The stern with the water jets of the Raptor. Source: Pella JSC.

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.

The Need for Speed

The classic three factors of mobility, firepower and protection are usually associated with AFV’s, but can naturally be employed for a variety of vehicles. With regards to small craft, much of the thinking behind land vehicles holds true, but the emphasize have always been on mobility and firepower instead of protection. “Speed is our armour”, but how good an armour is it really, and what difference does a few knots make?

Båtprofiler

Let’s compare three different military boats, which represent a comprehensive cross-section of the speeds naval crafts normally operates at. The first boat is the Linnakevene (or L100-series). The boat in question is a light water bus-type vessel, the primary mission being to ferry military personnel between naval bases in the archipelago and the Finnish mainland. The Linnakevene has a LOA of around 13 meters, and features a displacement hull. The speed is thus relatively low, at 13 kts (~24 km/h).

The second boat is the Jurmo-class landing craft (also known as the U600-series, Watercat M12, or, rarely, the Uisko 600-series). This is a relatively modern landing craft, featuring twin water jets and a semi-displacement hull, with the capability to ferry 25 marines to shore. LOA is 14 meters, and top speed is stated as over 30 kts (~55 km/h) on the homepages of the Finnish Defence Forces, with the manufacturer Marine Alutech promising “over 35 kts” (~65 km/h). We use the lower estimate here, as it fits nicely in between the other two boats.

The third class is the Super Dvora MKIII, a fast patrol craft of Israeli origin. The boat is the latest in line of the Dvora-series, and combines high speed with a relatively heavy armament and seakeeping abilities. The boat is longer than the other two, at 25 meters LOA, and faster as well, at 45 kts (~83 km/h).

Two of the boats are in frequent use by the Finnish Navy (although the Linnakevene is slowly nearing retirement), and represents the typical operational speeds of displacement and planning crafts respectively. The Dvora is chosen as it is one of the faster operational vessels around, and as such it gets to represent the whole upper side of the spectrum, including such diverse vessels as high-speed RIB’s and super-fast patrol and fast attack crafts.

As a side-note, the Norweigan Skjold-class of small corvettes currently holds the title as fastest operational armed vessel, with a top speed of roughly 60 kts (~110 km/h).

The most important way in which speed enhances direct combat survivability is the ability to move from cover to cover fast enough that the enemy does not have time to respond (i.e. before they sink your boat). However, understanding what practical difference the different top speeds means is not obvious at a glance.

To try and get a picture of what a Jurmo can do that a Linnakevene can’t, let’s create a very simple scenario: An enemy position is situated so that a boat passes on a perpendicular course, passing from behind one island to the next one. The simple question is, how long can the distance between the two islands be, so that our boat still survives?

Need for Speed Setup

Obviously there are a vast number of things that affect the outcome. What is the reaction time of the enemy? Are they standing ready to fire, or do they need to pick up their weapons and load them? What weapons are they using? Is it easier to hit a bigger target going faster, or a smaller going slower? How does the boat firing back change the reaction speed and accuracy? However, we only want to know what difference the differing speeds of the boats gives, so we will assume that the only thing that varies are the speeds themselves, ceteris paribus.

If a low state of readiness is assumed, an estimate is that it would take an infantryman about 30 seconds to 1) spot the boat, 2) react, 3) pull out/man the weapon, 4) ready the weapon, and 5) actually aim and fire an AT-rocket or guided missile. Naturally, this could be off by quite a bit in both directions, and on long distances the flight time alone could easily account for over a third of this value, but it will serve our purpose well enough as it provides a controlled “testing environment”.

A HMG will (usually) not sink a boat with a single hit, but it is generally quite fast to man and open fire with (assuming it is standing loaded at a tripod/pintle mount), and has a high rate of fire, so assuming 30 seconds from spotting to scoring enough hits to disable the boat should be within the realms of reality here too.

Need for Speed Outcome

Thus, in the 30 second window of opportunity we have granted it, the Linnakevene can travel ~200 meters. Any longer distance between concealments, and the boat is at risk. Traveling at slightly more than double the speed, the Jurmo (naturally), also travels twice the length, and reaches over 460 meters in 30 seconds. In the meantime, the fast Super Dvora can almost reach 700 meters.

In practice, finding concealment with 200 meters in between is difficult even in the cluttered Finnish archipelago. Finding islands half a kilometer apart might just be doable, and even if this wouldn’t be possible, it still gives the Jurmo quite a lot less ground to cover compared to displacement crafts, which is why the navy likes to go fast.