A little bit off the topic of "Desnair raiding strategy" but since the thread was going this direction anyhow...
Seems to me that Charis' initial moves are to destroy Desnair's commerce raiders and the facilities and ports where they can be built. As someone stated, this is a somewhat short-term solution because commerce raiders can be built pretty much anywhere (albeit it would be a bit more difficult if you don't have a real "shipyard" to build them in).
So I think that if Charis can "prune them back" for a while - until there are more steam-powered, ocean-going ironclads available for use as escorts - then that will be more than satisfactory.
IMHO, a single cargo galleon is at risk unless it has an escort. A convoy would also be at risk unless it has a large enough escort (of conventional galleons or schooners) to screen what I think would be a pretty spread out formation. If the screen isn't large enough, then there's the risk of raiders pressing in and "cutting out" a vessel before the sail-powered escort can respond... Just doing enough damage to slow it to where it can't keep up with the rest of the convoy.
With a steam-powered escort the situation is different... The screening vessel is A) faster; B) no longer at the whim of "wind and wave"; and C) sufficiently armed to drive off / cripple a raider.
Probably not good employment for a King Haarahld, but I envision a better designed version of the original ironclads that is ocean-worthy and functions perhaps like a Destroyer or Cruiser??
And given the mobility of the vessels, far fewer would be required because they could respond much, much faster. The first vessel in the convoy that sees a raider signals the screen. Even if the screening vessel is on the other side of the convoy, it still has plenty of time to respond before the wind-powered raider can approach the convoy. And given its speed, it has plenty of mobility to chase down the raider and destroy it if it chooses to "run away." Not saying you should try to protect convoys with a single vessel as the screen (what if the bad guys coordinate simultaneous attacks from two sides?) but you SHOULD need a lot FEWER screening vessels.