Cruisers on small slow yachts will never be able to withstand a pirate attack any more than the average couple can withstand a carjacking. The problem might be mitigated somewhat if international law allowed the traditional piracy response of arming
commercial shipping. It is interesting to see all the high-tech methods of using expensive sound and laser devices when old technology would be sufficient.
At the highest point on a
commercial ship a special purpose weapon could be allowed and inspected for compliance - perhaps a maximum of two. This would be a recoil-less rifle for which the specifications are well known and there are many on the surplus market. Such a recoil-less rifle could be mounted so that its field of fire is restricted so that it could only engage
boats within a specified range - perhaps from 50 meters to 5000 meters. The weapon could be mounted on an open tower on the superstructure which could be relatively light weight and (they could be mounted and operated from a small jeep) and have minimal windage. It could be kept, covered, with the muzzle locked in a vertical position so it could be seen to be harmless in peaceful waters by craft having
inspection or law enforcement duties. In pirate waters it could be unlocked at the direction of the
captain and prepared for use quickly upon the approach of suspected pirate craft.
The recoil-less rifle would only use special ammunition and fuzing optimized for the destruction of small
boats. Since the recoil-less rifle would only be fired down, the back blast would be directed up away from the commercial ship. Probably the simplest fuze to use would be an update of the proximity fuze developed in WW II. As it neared the
water surface at an optimum distance the fuze would function without the crew needing to make any setting changes. The shell would use two failsafes: 1) the thermal
battery operating the fuze would only operate for enough time for the shell to travel the maximum 5 km set range and would then function; and 2) a separate 'burn through' fuze would also function the munition at its maximum range.
The munition would mainly be filled with inert
steel disks which were pre-cut into quarter inch cubes (except for the filler pieces near circumference). A base charge of high explosive would be detonated by the fuze at the appropriate distance which would propel the steel shrapnel out the front of the rapidly spinning shell into a wide dispersal pattern (think of a flying shotgun with a wide dispersion pattern). The amount of high explosive used will also stop the larger shell body in its forward flight so it will drop into the sea without further
danger to the crew of the commercial ship.
One now needs a simple method of targeting a fast
boat from a relatively stable larger ship. Some years ago the USMC fielded a light anti-tank tracked vehicle called an 'Ontos' which mounted six 106mm recoil-less rifles. To aim this array they had a modified .50 caliber Browning heavy machine gun which fired a round which matched the ballistics of the recoil-less rifle. The crew was then able to fire the smaller round until they could see that they were on target and then fire the larger weapon. One could simply update this system by using one of the
current .50 caliber sniper rifles that use this bullet and load it to match the ballistics - likely using a bright tracer. Such rifles typically come with optical scopes, infrared, and thermal imaging sights to make it useful day or night. As an added advantage, the spotting rounds would act as a 'warning' shot to encourage pirate attack craft to disengage prior to using lethal force.
So there is a relatively inexpensive system, usable by minimally trained commercial crew, which could be rapidly retrofitted to most commercial craft. Once commercial ships were able to protect themselves, much of the money would fall out of the piracy enterprise and the then, being unprofitable, the piracy problem would likely fade away as it has done several times over the last several hundred years. Not optimum perhaps but doable if international law could be modified to allow it to happen.