So my players are planning on starting up a fleet.

I know you can buy a ship worth ship points up to your profit factor but what sort of limitations should I let them buy ships and are there any mechanics you guys use to limit how often players can buy ships?

Buying ships is subject to the same limitations as Acquisition tests, at the very least. I'd add additional limitations depending on what (and whether) there are any ships for sale where and when they are (obviously, that's a matter for you to decide, but I'd personally say no more than 2 "small" ships (Transports, raiders, frigates) available at Footfall at any one time, if that, while there'd be a larger selection at the Breaking Yards at SR-651).

While it's unclear in the RAW what that gets them aside from the hull, the consensus is at least the "default" Essential Components (nothing that costs SP points). All I'd say is if they want more than a bare hull and the "default" Essential Components, then they'd have to start combining Acquisitions, and I'd personally make sure that the Hull Modifier is always applied to the check (even if there is something that would work out "rarer", meaning a fully fitted out ship has a -5 modifier for every Supplemental Component, or if you're feeling really mean, every Component). So at a minimum, their Acquisition roll is going to have a negative penalty equal to the ship hull's SP cost. From there it's a matter of Acquiring Components as standard for upgrading the ship they have, and then fitting them (Battlefleet Koronus has some very useful rule concerning that).

Another way of dealing with it is to allow them to buy a fully fitted out ship- with its' full SP cost as a modifier to the check.

In all cases, remember that Scale Modifiers don't apply when Acquiring starships or Components ("But we're only buying one …" doesn't cut it when that one thing is a multi million tonne starship!).

The next thing to remember is that the group's only so big, and as useful as the NPC and Squadron rules are (Battlefleet Koronus, pp112-115), there's only so far they can stretch without splitting the party (and hence the likely better stats and ability for extended actions) over several ships, which limits their effectiveness. If they want to do that, feel free to let them. It may wind up a little more of a headache for you, but unless you've got a collection of old-school Battlefleet Gothic players, it's going to throw them off even more (I'd recommend it as a teaching aid for the GM as well, actually, as we tend to have more ships in play at any one time, and it's a good way to learn how to handle multiple ships. Fun, too, and the rules are now free, which is a bonus).

If they're after a support/non-combat fleet for background Endeavours and so on- let them. It's more things that can require an Upkeep roll, or can be hit with a Mishap. Don't punish them for it, but remember there's plenty of story hooks in saving your fleet from [being lost in the warp/pirates/xenos/Imperial bureaucracy/whatever].

Hope this helps.

Having more then one ship is handy, but you don't need a fleet to deal with most Endeavers. So if I were you, I would restrict them to their flagship, perhaps one or two escorts/transports to tag along and a few escort class vessels and transports to keep all those juicy trade lines open and running.It is what i am planning to do as my pc's now get their third vessel. No less then a light cruiser they had salvaged from the processional of the damned.

i would do it is they could make acquisition tests to hire ships to serve under them, they cant fly 4 ships themself even if they had them, it is done by officers under them.

to do so they just dont hire 100 ships i would make an upkeep for everyship, say for example a light crusier got an upkeep of 4 profit factor. this profit factor can not be used as long as the ship serve under them

Their going to be going to war with another rogue trader who has a war fleet of 13 ships so they need the fleet.

Their going to be going to war with another rogue trader who has a war fleet of 13 ships so they need the fleet.

Unlucky them. Is this enemy Rogue Trader so powerful that 13 ships is only a fragment of his power, or is he pulling everything out of the field, cutting short numerous background endeavors, to declare war on one enemy who has attracted his ire? Is it likely to be a single, big fleet engagement, or several stretched fights, and you have an idea of the enemy's total power?

For the Acquisition, I'd say once you get past the hard part, "are ANY ships available?" and "are those worth a toss?" I would think most ships are just the hull and essential components. This could benefit, if it had a nice option, like a Command Bridge, or an Ancient Life-Sustainer, but I believe Supplemental Components would have all been pulled, installed on one of the seller's other ships, or sold separately, in order to make more money. If you were buying the ship from an ally, you might be able to discuss with them if all the extra bells and whistles are included, but from a ship yard, I'd guess no.

I wish you the best of luck with your war against the other RT.