in many books there is info about some planets that can build a raider in 1 year, my players have read this and want to build a fleet, is there any rules or tips on how I could allow this to them?

the other problem of mine is that they want to make a fleet, no matter how, getting pirate ship via force or buying them, but I feel that many ships will ruin the RPG they want to play, but the books give them infos that its the right thing to do for an RT... but I dont want it to be a Gothis statergy, I want it to stay an RPG

any good advices on the subject?

Simplest method of reining in the use of a fleet in Rogue Trader is AP costs. If you say every ship after the first requires an upkeep of 100AP per Endeavour (per objective if you're feeling evil), you'll find that your players are more likely to keep the scale of their fleet small, in order to increase their profits further, or embark on larger Endeavours that both mitigate that loss of profit and provide more reason for the ships coming along in the first place.

It can actually be useful for the players to have multiple ships, as it encourages them to spread out across the fleet instead of clustering all their bonuses into one vessel.

As to creating new vessels, an Imperial world with the blessing of the Mechanicus, sufficient resources and the requisite technical base can build a starship in a year, yes. This is giving over the entire development of the planet to the project, however. In most cases, the construction and consecration of a new vessel takes decades.

well... I dont really use the point system, I have a house rule for the points... I give them +1 sucsess for every 50 points given from ship bonuses... this way they get something, but we dont need to count points every time, can have a more natural flow of things this way...

I was thinking about making the ships cost actual PF to mirror its huge costs, and if I let them make a ship, it will be for PF only... and as one of my players told me, I could say that an ork fleet is massing so the navy takes all ships, and to sell it to an RT is a huge risk nowdays, thats why the PF is spend permanently...

BTW, they have a huge Star Galleon now, a Star Clipper and a Fregate, but the captain wants to have another Fregate and another smaller trader too...

IN the rule book theres rule for purchasing ships some where either in the armoury or the starchip sections.

Building new ships can take a decade or two mabey a few years if built up from a mostly intact ship. Buying newly made ships or ships being decomissioned fro the imperial navy seems to be the best way to built up a fleet.

I ran a game before and the PCs looted a Battlecruiser after a battle and claimed several small raiders mainly from a space hulk because they wanted to go Empire building. They decided to keep leaving their fleet at the edge of the system because with a crew rating of 30 the Rogue Trader considered the Battlecruiser without any PCs on it a liability. They just kept it as a back up in case their Grand Cruiser got decimated by something. This might happen in your game after a space combat or two, ships aren't nearly as deadly without high level characters manning them.

Secondly why do they want multiple ships, even when you have a small fleet many rogue traders have their ships performing other endeavours, so suggest they send the ships off to perform NPC endeavours and earn them more profit factor. If they ever plan on tackling the Orks of Undred Undred Teef they have a good reason to assemble their fleet but most of the time they'd just be moving about doing nothing while the command crew run around on a planet... as you said they make a big impact on space combat.

My players managed to get themselves a fleet. I think they are up to about 7 or 8 vessels now. I've found that many of their vessels are quite ineffective since they only have average crews (skill 30). They tend to all stay aboard their flagship which with their skills is more deadly than the rest of the fleet combined.

I've also found that its quite easy to wittle the fleet down by spreading their obligations thin. As they build allies and get more connected, you can force more obligations on them that can be solved with the attentions of a ship. For instance, in the course of formenting a crusade, my players sent one ship off with huge amounts of capital and a load of priests to set up some religious groups, another ship was hauling cargo of interest (i.e. sneaky stuff they didn't want to farm out to a free captain).

While not have an inquisitor ask to borrow a ship for a mission, joint operations with the navy around some of the RTs territory.

Stuff like that.

Of course, every now and then a good fleet battle that allow them to really show off their naval might is good too.