I think the most factual account is available there.
To attain his second object, it was necessary to inflict a loss on the government as such. His only method of doing this was to burn the Algerian navy. There was nothing else in the town which would burn (due to adobe construction). But here his difficulties began. His mortars were not weapons of precision. They could hit a town but they could not hit a ship at long range. To destroy the ships, he had to place a man-of-war in the harbour mouth. A single ship was sufficient, but she had to be there and nowhere else because the mole was too high to fire over.
Figs. A, H, C.
THE BATTERIES AT ALGIERS.
To attain his third object, it was necessary to silence some of the batteries. Nothing smaller than a ship of the line could hope to do this. Bomb-vessels protected by frigates could have tortured the town into releasing slaves. Frigates alone could have burnt the Algerian navy at its moorings. But the batteries were a different matter. They were so nearly impregnable that it was exceedingly difficult to prove that they were not impregnable. This object could only be attained in a carefully planned attack by well-trained line of battleships.
Impressing the Algerians with the vulnerability of their port was the essential object of silencing any batteries. The batteries to be dealt with had therefore to be those on the mole. These were, besides, the only ones it was possible to engage at close range. Now, the advantage enjoyed by men defending fortifications is the security, or the feeling of security, derived from the strength of the works. The advantage enjoyed by men attacking fortifications is that they know where the enemy is. By reconnoitring Algiers, Exmouth knew how many guns there were, and he also knew where to find them. Guns which are embedded in masonry can be relied upon to stay where they are. Exmouth was accordingly able to attack the mole at Algiers as Nelson attacked the French fleet at Aboukir - but with even greater certainty that his opponents would remain in position. He could, in short, throw all his force on one point. As he had a choice in the matter, he naturally chose the weakest point.
The diagrams will give the reader some idea of the general position. The rough sketch may suffice to show how the strength of the mole centred on the lighthouse. Only in the lighthouse battery itself, and in the battery immediately to the south of it, are there three tiers of cannon, as in Fig. C. Throughout the stretch of fortifications on the left, there are only two tiers, as in Fig. B; and this part of the mole is lower than the quarter-deck of a three-decker. The mole batteries are all closed in the rear, but Fig. B shows the system of smoke-vents.
In attacking the mole, it was clearly best to avoid the fire of the lighthouse and the three-tiered battery. Fig. A shows how this was to be done. Here the most formidable works are marked in black, and their arcs of fire indicated by shading. It will be seen that there is a central triangle of water on which the heavy batteries will not bear. The same is true of the area marked Z; but the water there is shallow, and a line of storehouses on the pier prevents firing into the harbour. Therefore, it is at X that the attack must be made. The position at X has several advantages, one being a sufficient depth of water close inshore, and another being that the mole itself is high enough to protect a ship from all the batteries above the town. Exmouth saw that there was only room for three sail of the line in the patch of 'dead-ground' at X. In case these three should fail to overpower the batteries, he arranged to make assurance doubly sure. The southern end of the mole could be outflanked. A fourth line of battleship at V could destroy the upper tier of guns from the flank and rear. A three-decker would be needed for this post. By the fire of these four ships the weaker batteries could be silenced. Exmouth asked for five sail of the line because (451) he wanted a fifth to deal with a fort to the south of the town.
For the destruction of the Algerian navy, it would be necessary to place a heavy frigate at Y, to fire into the harbour. At this point there arises the difficulty of the fish-market battery, and the other batteries along the waterside. These all bear on, and some enfilade, the positions at Y and V. Exmouth provided for this difficulty in the only possible way. To stop the guns in the direction of W firing on Y and V, he had merely to give them something else to fire at. For this purpose, he asked for three or four more frigates. There were several more batteries further to the south, but he relied upon Penrose bringing enough ships to keep these fully occupied. In Penrose's absence, this task was allotted to the Dutch. The sloops were required to act as tenders to the ships of the line.
THE ATTACK - AS PLANNED
Here then, is the explanation of Exmouth's demands on the Admiralty, and his obstinate refusal to take a larger force. The numbers of ships he asked for were the exact numbers which his plan of attack implied. Fig. D represents the (452) attack, as planned. By keeping to the left of the line B - D, the Superb, Impregnable and Albion were to concentrate their fire on the mole between A and B, while the Queen Charlotte raked the fortifications from the flank. The lighthouse battery and the battery at B - C were to be left without a target. The Leander was to be warped still nearer the Queen Charlotte, so as to destroy everything in the harbour. The Glasgow and Severn were to engage the batteries along the waterside. The bomb-vessels were to be far out in the bay towards the north and east. The Dutch were to find occupation for the forts to the south.
One flaw in this scientific scheme, a flaw which was painfully patent to every one in the fleet, was the need for passing through the danger zone on the way in; and again, on the way out. The positions assigned to the various ships were relatively safe. But would the ships ever reach them ? It seemed to many that the ships of the line, at any rate would run a grave risk. They might be dismasted. Indeed, they might be blown out of the water. Exmouth alone denied the probability of this. He had the best of reasons for doing so, but probably kept them to himself. The fact was that Omar Pashaw would not fire the first shot. He had said as much at the end of that stormy interview in June. He may even have sworn it. Exmouth knew him to be a man of his word, and was ready to act accordingly. He did not incidentally, mean to fire the first shot himself. His belief was that the Algerians would eventually lose their heads and fire without orders. He was not worried by the prospect of retiring through the danger zone because his withdrawal was likely to be at night, and because he hoped to leave the town in no condition to speed his departure. There was, for that matter, no need to retire at all until the Dey came to terms -so long as the batteries on the mole were silenced.