Much has been made of the distance between the boarding of the merchant vessel by the 15 sailors and marines (who are now back in the UK) and HMS Cornwall. This is either by the heroes of the right wing's 6th Keyboard Fleet who assign incompetence or the tinfoil tinklers on here who assume the 15 were the victims of a conspiracy to dangle a tasty morsel to bait the Iranians into precipitating war. The reality is rather more mundane and simple.
Before I detail the geography of the area etc over the flip, let me explain it in simple words. Any nearer would have put HMS Cornwall of danger of running aground or in the navigable channel that forms the border between Iran and Iraq.
A word about HMS Cornwall. Simply looking up the Royal Navy pages on Wikipedia will take you to a page describing the Type 22 Broadsword class frigates The type was built in three batches and only the last batch of four remain in Royal Navy service. Each batch became effectively a sub-class and the last four are named Cornwall class after the first of the batch. This is how they are described:
The Type 22 was designed to be a specialist anti-submarine warfare vessel as part of the Royal Navy's contribution to NATO. Since then they have evolved into a general purpose frigate with weapons for use against other surface ships, aircraft and submarines.
...
The ships have enhanced command, control and co-ordination facilities that results in them often being used as deployment flagships.
The other ships in the class have been sold to other navies, scrapped or used as targets. They are in fact the smallest ships in the RN to be able to carry helicopters and inflatables for the sorts of boarding and inspection operations needed off Iraq.
The important information is in the boxes to the right of the page. The later batches have a draught of 21 feet or 3.5 fathoms. For safety, you do not go into waters shown as having this depth on your charts as you need clearance and to take account of tides and inaccuracies in your charts. Now let's have a look at where the operations took place.
Unfortunately, I have not been able to find on line charts showing the surveyed depths for the area. There are however satellite pictures on Google that explain the position. If you would care to open this link in a tab you will see the position. To the West we have Kuwait and further east the Shatt al Arab with the line of the presumed border. The picture quite dramatically illustrates the problem. All the silt carried down the Tigris and Euphrates ends up being deposited in the sea off the mouth of the Shatt. Just like the area in the seas outside New Orleans as the Mississippi dumps its silt. The fuzzy light areas in the sea are not clouds but these depositions.
(Update : although there is no scale, the map of the operational area of CTF 158 clearly shows that the Cornwall stayed outside the shallower areas shown in dark blue while the boarding was in even shallower waters marked green. In addition, HMS Cornwall is the operational centre of the operation, the principle mission of which is to protect the oil transfer platforms. This provides even more reason for the Cornwall to remain on station near them rather than following the patrol. Protection should have been provided by the helicopter:
Although not immediately obvious, the Lynx is the most flexible weapon system on the ship, maintained by a small but highly skilled team of air engineers. The Lynx can deliver Stingray torpedoes and depth charges against submarines, but can also engage surface targets with the Sea Skua missile. The helicopter can conduct many other missions including surface surveillance...
I hope this provides some more clarity.)
The constant deposit and movement means that the Shatt and the navigable route to it, which are the borders between Iraq and Iran, are constantly shifting. Equally, the charts for the area will only be accurate when they were last surveyed. They cannot be relied on so you cannot go charging about the area if your ship is not to be stuck on sandbanks. While I have not been able to find a chart, the armchair sailors have already drawn their existence to our attention. The much abused map from the Ministry of Defence which I pushed to Imageshack before an error on the key could be corrected and which everyone seems to have appropriated before an error on the key could be corrected (should read 5 NM, not 1NM) demonstrates that the "12 mile limit" is not 12 nautical miles from the shore,f but far further out. This means that sandbanks or rocks become exposed at low tide. The territorial waters are then measured from those.
The part of Iraq sandwiched between the two sea borders with Kuwait and Iran is known as the Al Faw peninsular. At lot of it is marshy and criss-crossed by minor waterways in the delta. The Shatt eventually leads up river to Basra but that is not the main shipping route because of the shallow and changing waters. The main route, especially for oil tankers, is up the western channel with the border with Kuwait to the port of Umm Qasr. The eastern route is used by smaller merchant vessels with much shallower draughts. Even then, you may have noted from the reports that smuggled cars are often off-loaded at sea onto barges, with much shallower draughts, for the final run.
The reason for the tactics of sending out RIBs, accompanied by a helicopter to do spotting, some distance from the main vessel is therefore not unusual and is in line with the conditions in the area. Can we now move on from the speculation about British motives for them.