Well first off Strand's so-called "yacht" is rather a bit of a slouch if all it can do is 20 knots full out, give me a break, this isn't some lake-going pontoon barge here, most yachts are capable of speeds far more impressive than that... The other thing is, if the faster vessel is on Strand's radar then his yacht is on theirs because few sea-going vessels can do these speeds that don't sport radar, once again we're not talking about some indigenous fishing boat with an outboard. Which brings me to my next point, 25 knots is awful close to a military speed such as a type of flank throttle setting. I don't think these are pirates, more than likely it's another branch of the military (such as Navy) they have dispatched to take care of "the plague" by killing everything, live and dead.
That's who (or what) I think is approaching, a military craft.