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?
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?
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.
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.