The following is based on a very readable article by Henry Lewin Cannon written in 1912, "The Battle of Sandwich and Eustace the Monk" published in the English Historical Review (vol 27, p649-670) and available online.
From being virtual lord of England in 1216, and proclaimed King, the French Dauphin Louis was now faced with most of his English allies defecting to Henry III. More and more, he was relying on French troops and supplies, all of which had to be shipped across the Channel. Control of the sea was vital.
Eustace the Monk
The French fleet was led by Eustace the Monk, an infamous character then and for many years afterwards. So infamous infact that it´s hard to get solid information on him. For example, it was believed that he had gone to Toledo in Spain to study and master the Dark Arts, as this was the only way to account for his remarkable abilities. He was credited with the ability to render his ship invisible, a "cloaking device" only revoked if he could be killed.
In fact Eustace was born near Boulogne in 1143, the son of a local lord, and he did actually become a monk, at Samur. But the Count of Boulogne killed his father and later seized all the family land, turning Eustace into an outlaw vowing revenge. When the Count joined the French cause, Eustace joined the English in turn. He must have impressed because in 1206 John sent him with 3 large ships and 5 galleys to recapture the Channel islands, taken by the French in 1204. He did this so successfully that he was awarded the islands. He then raided Nornandy, landing at Harfleur and reaching as far as Pont Aldemer before retiring with the French commander, Cadoc, in hot pursuit. Cadoc made the mistake of attacking him at sea, only to lose 5 ships for his trouble. Eustace was now lauded even more in England, with a "palace" in London and a home in Winchelsea - ironically his local knowledge there was to help the Louis escape from the English 10 years later.
John, for all his many faults, seems to have had a knack for attracting talented men, and keeping their loyalty. With Eustace he went too far. Desperate for allies in France he convinced the Count of Boulogne to switch sides, instantly causing Eustace to do the same. John was well aware of this possibility and tried to capture Eustace, but he escaped with 5 galleys and joined the French, or rather Louis. He raided Folkstone and then returned to the Channel islands, conquering them yet again, and with his brothers setting up as virtual lords of the islands.
The standard ships in the Channel, for trade and war (interchangeably) were cogs. They were clinker built (with overlapping planks), a single sail, and usually a fairly flat bottom which allowed them to settle flat in harbour for unloading. As a defence against pirates they often had high sides. Those caught by pirates were generally killed and thrown overboard so pirates were especially feared and loathed - when they were captured they could expect little mercy. For war, cogs had added towers at each end for archers.
There are also several mentions of galleys, especially in the service of Eustace. The difference, basically, is that galleys have oars though they might use a sail if the wind was right. They were much more manoeuvrable than cogs and could be equipped with crippling rams making them the warship of choice in the Mediterranean. But they were poor in rough seas and as specialised warships they were hopelessly uneconomic. So, in general, they were uncommon in the Channel, but maybe worth the trouble at times like this.
The main source of ships for the English was the Cinque Ports, a confederation set up in 1155. Basically the towns from Hastings to Sandwich on the south coast were obliged to supply 57 ships for 15 days every year. In return they were given freedom from taxes and a measure of self government. Frankly, self government was taken as a licence to smuggle, but it did give England a reasonably cheap and effective navy.
The War at Sea
Control of the sea lanes was obviously essential for Louis. Eustace had been involved from the start, ferrying war machines over to the Barons even before Louis arrived. And it was Eustace who, with his local knowledge, saved Louis at Winchelsea. But Louis now needed what was, in effect, an invasion fleet. The problem was that, technically, King Philip couldn't help his son as Louis had been excommunicated, but what he could do was make generous "gifts" to Louis's wife Lady Blanche, which she used to hire a fleet and assemble it at Calais.
Since January the English coast had been under the authority of Philip d' Aubigny. The war here took the form of repeated attempts by the French to force their way across the Channel, some successful, some not. On May 15h a French fleet attempting to sail to Dover was met by 80 ships under d'Aubigny sailing out from Romney, including 20 "Great Ships" especially equipped for fighting. The French fled back to Calais, but 25 were caught up with and eight captured. The English fleet then stationed outside Dover.
On Monday 29th May 120 ships were seen approaching Dover from Calais. The English set sail to escape the larger fleet, chased by the French, but the English were quicker. However, when the French fleet turned back for Dover the English fell on them from behind, capturing another eight ships before escaping.
In August William Marshall visited the south coast, alarmed by reports from Calais. He met the men of the Cinque Ports and promised they would receive back all the privileges they had lost under John, as well as a substantial share of any loot from captured ships. As he had done with the army, Marshall did with the navy, numbers increased and moral soared. There were raids on Calais, and counter raids, wins and losses, but French supplies to England were severely disrupted.
The Battle of Sandwich
Bartholomew´s Day, Thursday 24th August 1217, was bright and clear in the Channel. The French fleet left Calais "so dense and in such good order it was like a pitched battle". In the lead was Eustace, as guide, but not as commander. This role fell to Robert de Courteney, the French queens uncle. He had selected for himself the "Great Ship of Bayonne" and loaded it with not only all the gold for Louis, but also horses, a trebuchet and many other supplies. In fact it was so over-loaded that water nearly reached the gunwales - it is hard to believe that Eustace would have been so careless if he had been in charge. To protect all this there were 36 knights as well as other soldiers. In fact 3 other ships were guarded by knights, under Mikiius de Harnes, William of St Omer, and the Mayor of Boulogne, about 100 knights in total. As well there there were 6 other Great Ships with men at arms, and about 70 small transports.
The English fleet was led by Hubert de Burgh, Marshall having reluctantly agreed to stay on land. He had 16 ships "well fitted out" and about 20 smaller ones. De Burgh chose the best ship as his own and took a small number of knights from the Dover garrison, as well as a crack crew from the Cinque Ports. Next we have Richard FitzJohn, another illegitimate son of John, and a nephew of the Earl of Warren, who supplied a crew of knights and men at arms. The third ship that caught attention was especially large and rode high in the water as it was lightly loaded. It was crewed by Marshall's men at arms. Roger of Wendover records that there also galleys in the fleet, equipped with iron prows.
Cannon compares the fleets as follows. The French had a large advantage in both ships and knights, as well as, crucially, the wind behind them. All they had to do was make it to port to win. On the other hand, they were very heavily laden and slow. The English had more freedom of manoeuvre. Also, though both sides had excellent leaders in Eustace and de Burgh, Eustace had to defer to de Courteney, which was to prove decisive.
To have any chance the English had to be to windward, so De Burgh let the French pass Sandwich and then followed them, sailing ahead of his fleet as if to attack and then feinting away. The French by this time were nearly at Thanet, and in good order, all they had to do was keep on going. But de Courteney saw Burgh's ship apparently fleeing and with only a few ships in company, and decided this was easy meat. Ignoring Eustace he turned and bore down on the English alone.
It was, for the French, a disaster. De Courteney's ship struck the 2nd of the English column under FitzJohn and boarded, but the English put up a fierce fight, and soon 3 more ships arrived and assailed the French ship from all sides. The French fleet meanwhile, was all the time being blown further away. De Courteney had given no signal to follow, and when they realised what was happening it was extremely difficult for the French to change course and intervene.
The final straw was the arrival of the massive cog full of Marshalls men. With a height advantage they fired at will into the French ranks, but that was not all. With the wind behind them someone had the idea to throw jars of lime onto the deck, the powder blowing into the eyes of the French. Until now things had been fairly equal, the elite French crew holding their own, but blinded they stood no chance and English troops poured onto the ship. Most of the French knights at least were kept alive for ransom, but for Eustace the situation was different. Both loathed and feared by the English sailors he was found hiding in the hold and executed
The loss of their leaders panicked the rest of French fleet and they bolted for Calais, the English falling on them like wolves. Ships were rammed or boarded, and the crews slaughtered. Nine or ten Great Ships made it back to Calais, but most of the rest, the transports, were sunk or captured.
With part of their share of the spoils the people of Sandwich erected a hospital for the aged poor, as thanks for their deliverance, and named it in honour of St Bartholomew.
Louis was ruined and De Courteney was given leave to go to London and negotiate a surrender. Louis prevaricated, but in the end he signed the treaty of Lambeth. The terms were actually fairly generous - Marshall calming some of the more blood thirsty of his army. In return for free passage home Louis denounced his claim to the throne and returned the Channel Island back to English care. That was it, he was actually given a payment of 10,000 marks as compensation, although that was to be paid by his former supporters in England.
William Marshall's reputation was now even higher than ever, few men were ever as respected in England. He continued as Regent to Henry but, unfortunately, the old Earl, who had served four kings, died in Caversham in 1219 at the age of 72.
After Marshall's death Hubert de Burgh was made Regent of the young Henry III. He married John's widow Isabel until the marriage was annulled, and then the daughter of the King of Scotland. He stayed influential until 1232 when his enemies discredited him and he was imprisoned. From then on he alternated between the Kings favour and his earldom, and prison, but by his death in 1243 he was free.
William of Cassingham was granted a Crown pension and made Lord of the Weald and Sergeant of the Peace, a post he kept until his death 40 years later in 1257.