The idea of this adventure is to experience battle at sea and in the air. This is best arrained perhaps by having the party fight against pirates in order to retreve the Arms which is the body part they seek.
This adventure is not going to be programmed in any way. The party will have numerous choices at every step and will only be committed to a specific set of actions when they have set actions rolling in a particular way.

It will not be eazy to seperate the players from their helm ship. Despite the (perhaps) overwhelming force of three ships to one, they will want the fastest movement possible. This could most easily be done by having the pirates destroy or severly disable the ship when the party does their flyby investigating the island. Even if the dragonship is sunk, they might willingly suspend action against the pirates so they can raise the sunken ship, retreive the helm and mount it in a new ship. If so, they will be waiting for a while. The unusual nature ships design and the unknown qualities of the woods their ship is made from freaks out the builders. Remember this is Isis where a suspicion of being an outsider is apt to send the native into a towering rage! This means the repairs will take twice as long as normal. It will cost 3 times normal to speed to work up to normal levels. The time for repairs will vary from nation to nation as well as the state of ship building techniques varies considerable from place to place in the Seren Sea.

The pirates have a large stake in keeping their helm ships secret. The political workings of the nations that might band together are delicate. There is far more to fear from an enemy that can fly over your castle walls than from one confined to the sea. The pirates have been keeping them quiet and resorting to murder and blackmail to do so. Although they can't stop the rumors, envoys and plants of the pirates have managed to destroy their credability in minds of the leaders.
 
 

Some cool events to work in maybe:
Locate the arms in natural sea caves protected by dozens of deadly traps.
It would be cool to have the only way to get the arms is to become the king of the pirates. This would create problems for the party as they would find it difficult for the Good members of the party to condone killing and robbing innocent people. This could be overcome in several ways:
They could attack the pirates and become the Pirate of the Pirates. This would become a battle of attrition where the players would wait for their actions to weaken the pirates and make them easier to overcome. They might be forced into this option if the kindoms being raided are not sure they are sincere in their offers of help to wipe out the pirates.
They could choose to attack ships from countries that are helping the pirates or themselves preying on their neighbors. This option would involve political role-playing and could result in no action being taken by the kingdom(s) contacted.
They might attempt to subdue all abord and take only company held things (no personal possessions stolen). Probably
They could become Privateers sanctioned by one of the costal kingdoms to eleminate the pirates and perhaps the ships of other countries that have been attacking the kingdoms ships. This sounds good!

What would prevent them from simply waltzing into the location of the arms and kicking butt?
First, the island has been attacked numerous times by parties of all sizes from 2 man assassin crews to joint 2 country armies, and all have failed. The main fortress is said to be impregnable.
The sea around the island is filled with the best crewed ships in the world; all spoiling for treasure and glory.
The treasure of the Pirate King is protected by Battle Beasts that are large in number and powerful in the extreme.
The only entrance to the treasure room is by going through the basement of the impregnable fortress.
All of the gold of almost all the pirates is stored inside the same underground area as the Kings treasure, so they will all fight as hard as they can to prevent their own gold from being stolen!

The pirates are the only ones with ships that fly in the air. Although these are wild space ships, they use them only close to the ground as they are ignorant of many features of the ships, nervous about going to high and disdaneful of what they might find beyond the atmosphere. The existance of these ships is a huge secret. They are only used when necessary (the pirates don't trust mages, but love magic items) and usually under cover of night. If they are used in the light, they make sure there is no one left to tell the tale. There are 3 of these ships. 2 of them are of the flying galleon nature, but the last (and most seldom used) is the Deathglory ship captured from some unfortunate, trusting gnomes.

It would be too cool if it could be set up so the last battle takes place with ships in the sea, troops on the ground and their ship in the air! Even better if a small party went ashore in the chaos of battle to try to take the King of the Pirates alive and offguard as well.

The pirates have been controlling the area for 90 years.
They are accustomed to attacking and protecting against attacks from the air and the fortress is specially protected. This protection takes the following forms:
Their is always a lookout scanning the sky.
The few flat areas and less steep grades of the fortress mountain cliff are filled with rusty daggers pointing up.
Some of the seige weapons on the roof are pointing at the sky, and 1/2 have the ability to be modified to point this way. The modification takes about 2 turns, but will be started anytime a flying ship is detected.
The top of the fortress is protected by "clothslines". These are thin but strong (silk) ropes that are attacked to large posts located around the outside of all high fortifications. These are strung at 3' intervals and the rope normally just lies on the ground. When a weight is thrown down, the posts swing up and the ropes are stretched taut. These are designed to trap any flying creature and prevent them coming down into the fort. It will mess with invisible creatures. If the pirates know that a party with flying ability or a dragon are soon to attack, they will place fishing nets

Although the pirates are somewhat uneasy with magic, they like to drink. So anything that comes in a bottle is alright with them. Due to the policy of the fort toward potions found as treasure, they are liberally stocked with them. They are only used in emergencies and kept tightly inventoried. Loss or misuse of a potion is treason and punnishable by death.

There are guardian Aquatic Battle Beasts chained to the rocks surrounding the entrance to the lagoon.

The dunes around the entrance to the lagoon are infested with red ants and desert rats that feed on each other. It is a common punnishment for traitors and murders to be staked out in the dunes for the vermine to kill.

The pirates have a unique way of distributing treasure. The monetary is split as usual (1/4 to the fortress then shares are split for the remaining 3/4. 3 for capt., 2 for officers, 1 for crew). The magical treasure is supposted to go to auction. As part of the celibration, the magical swag is described and offered to all attending the party. Anyone may bid, but bids are final and there are no loans. The only exception to this are potions. They are automatically bought by the fortress at 1/2 the standard rate. This is included in the 1/4 share.

They have developed tactics of using their flying capability to max advantage.
The have a black dragon (medium size/young) that has been subdued by their main mage. This is used both to locate prey and to protect the fort. It may also be used to tow one or two ships much faster than they normally go.
 

Outline of proposed adventure.
They arrive on Isis and Beourn tries to feel the body part
He sences the arms and directs the party to the area where they are located.
They fly over pirate island and are attacked by the 2 galleons in the sky with the Deathglory ship held in reserve.They will be invisible and approach the ship from both sides. With a yell, grappeling hooks are thrown over both sides and the battle is engaged.
Beourn will sense the arms somewhere inside the mountain the fortress sits on.
They will probably damage the ships and get away. In any event, the helm(s?) of downed ships will be recovered and placed into new vessels.
From here, the party will be pretty much on their own. They will know the island is well protected by pirates, but not much else. They will probably want more information. They will go somewhere nearby to see if anything is known about the residents of this island.
They might want to attack again, but Beourn will tell them they will never sneak into the fortress and get out alive. "This would be rash action!"
 

The pirates got their first helm by coercing one of their captives who happened to be from the Rock of Brall. He told them about helm ships and took a small flotilla of pirate ships to the place where his helm shiop was moored. They attacked and captured it by stealth. Once they saw the first helm, one of the men remembered a salvage operation where a plundered ship had one. They salvaged it and installed in in the present ship they are using. The Deathglory they tricked the gnomes into letting them on board by supposedly offering a fortune to take them off planet. When the ship was taking off, the pirates struck and overpowered the gnomes and flew it on to the Pirate Island.

None of the pirates use their given names, so they are quite unlike the other names on Isis. To the Isians, this is the ultimate in bad luck and an indication of the depravity of anyone who is a pirate. The names tend to be colorful, descriptive and dark. A good random program would use monster names, red and black colors, place names as in "of Enland", all synonyms of death, and the like.
 

The fortress is in almost an unassailable location. It is protected by cliffs to the East and South, mountains and marshes to the west and north.